Fukushima nuclear accident

"Fukushima nuclear disaster" redirects here. For the incidents at Fukushima Daini (Fukushima II), see Fukushima Daini Nuclear Power Plant.

Fukushima nuclear accident

Satellite image on 16 March 2011 of the four damaged reactor buildings
Date	11 March 2011 (2011-03-11)
Location	Ōkuma, Fukushima, Japan
Coordinates	37°25′17″N 141°1′57″E
Outcome	INES Level 7 (ratings by Japanese authorities as of 11 April)
Non-fatal injuries	37 with physical injuries, 2 workers taken to hospital with radiation burns
File:Japan Nuclear power plants map.gif

External videos
24 hours live camera for Fukushima Daiichi nuclear disaster on YouTube, certified by Tokyo Electric Power Co. Inc.

The Fukushima Daiichi nuclear disaster (福島第一原子力発電所事故, Fukushima Dai-ichi () genshiryoku hatsudensho jiko) is a series of equipment failures, nuclear meltdowns, and releases of radioactive materials at the Fukushima I Nuclear Power Plant, following the Tōhoku earthquake and tsunami on 11 March 2011. It is the largest nuclear disaster since the Chernobyl disaster of 1986.

The plant comprises six separate boiling water reactors originally designed by General Electric (GE), and maintained by the Tokyo Electric Power Company (TEPCO). At the time of the quake, Reactor 4 had been de-fuelled while 5 and 6 were in cold shutdown for planned maintenance. The remaining reactors shut down automatically after the earthquake, and emergency generators came online to control electronics and coolant systems. The tsunami broke the reactors' connection to the power grid and also resulted in flooding of the rooms containing the emergency generators. Consequently those generators ceased working, causing eventual power loss to the pumps that circulate coolant water in the reactor. The pumps then stopped working, causing the reactors to overheat due to the high decay heat that normally continues for a short time, even after a nuclear reactor shut down. The flooding and earthquake damage hindered external assistance.

In the hours and days that followed, reactors 1, 2 and 3 experienced full meltdown. As workers struggled to cool and shut down the reactors, several hydrogen-air chemical explosions occurred. The hydrogen gas was produced by high heat in the reactors causing a hydrogen-producing reaction between the nuclear fuel metal cladding and the water surrounding them. The government ordered that seawater be used to attempt to cool the reactors—this had the effect of ruining the reactors entirely. As the water levels in the fuel rods pools dropped, they began to overheat. Fears of radioactivity releases led to a 20 km (12 mi)-radius evacuation around the plant. During the early days of the accident workers were temporarily evacuated at various times for radiation safety reasons. Electrical power was slowly restored for some of the reactors, allowing for automated cooling.

Japanese officials initially assessed the accident as Level 4 on the International Nuclear Event Scale (INES) despite the views of other international agencies that it should be higher. The level was successively raised to 5 and eventually to 7, the maximum scale value. The Japanese government and TEPCO have been criticized in the foreign press for poor communication with the public and improvised cleanup efforts. On 20 March, the Chief Cabinet Secretary Yukio Edano announced that the plant would be decommissioned once the crisis was over.

The Japanese government estimates the total amount of radioactivity released into the atmosphere was approximately one-tenth as much as was released during the Chernobyl disaster. Significant amounts of radioactive material have also been released into ground and ocean waters. Measurements taken by the Japanese government 30–50 km from the plant showed caesium-137 levels high enough to cause concern, leading the government to ban the sale of food grown in the area. Tokyo officials temporarily recommended that tap water should not be used to prepare food for infants. In May 2012, TEPCO reported that at least 900 PBq had been released "into the atmosphere in March last year [2011] alone" although it has been said staff may have been told to lie, and give false readings to try and cover up true levels of radiation.

A few of the plant's workers were severely injured or killed by the disaster conditions resulting from the earthquake. There were no immediate deaths due to direct radiation exposures, but at least six workers have exceeded lifetime legal limits for radiation and more than 300 have received significant radiation doses. Predicted future cancer deaths due to accumulated radiation exposures in the population living near Fukushima have ranged from none to 100 to a non-peer-reviewed "guesstimate" of 1,000. Fear of ionizing radiation could have long-term psychological effects on a large portion of the population in the contaminated areas. On 16 December 2011 Japanese authorities declared the plant to be stable, although it would take decades to decontaminate the surrounding areas and to decommission the plant altogether.On July 5, 2012 The Fukushima Nuclear Accident Independent Investigation Commission (NAIIC) submitted its inquiry report to the Japanese parliament.

Fukushima I Nuclear Power Plant

Simplified cross-section sketch of a typical BWR Mark I containment, as used in Units 1 to 5. Key: DW, dry well enclosing reactor pressure vessel; WW, Torus-shaped all around the base enclosing steam suppression pool. Excess steam from the dry well enters the wetwell water pool via downcomer pipes; SFP, spent fuel pool area; RPV, Reactor Pressure Vessel; SCSW, Secondary Concrete Shield Wall.

Main article: Fukushima I Nuclear Power Plant

The Fukushima I Nuclear Power Plant consists of six light water, boiling water reactors (BWR) designed by General Electric driving electrical generators with a combined power of 4.7 gigawatts, making Fukushima I one of the 25 largest nuclear power stations in the world. Fukushima I was the first GE designed nuclear plant to be constructed and run entirely by the Tokyo Electric Power Company (TEPCO).

Unit 1 is a 439 MWe type (BWR3) reactor constructed in July 1967. It commenced commercial electrical production on 26 March 1971. It was designed for a peak ground acceleration of 0.18 g (1.74 m/s²) and a response spectrum based on the 1952 Kern County earthquake. Units 2 and 3 are both 784 MWe type BWR-4 reactors, Unit 2 commenced operating in July 1974 and Unit 3 in March 1976. The earthquake design basis for all units ranged from 0.42 g (4.12 m/s²) to 0.46 g (4.52 m/s²). All units were inspected after the 1978 Miyagi earthquake when the ground acceleration was 0.125 g (1.22 m/s²) for 30 seconds, but no damage to the critical parts of the reactor was discovered.

Units 1–5 have a Mark 1 type (light bulb torus) containment structure, Unit 6 has Mark 2 type (over/under) containment structure. From September 2010, Unit 3 has been partially fuelled by mixed-oxide (MOX) fuel.

At the time of the accident, the units and central storage facility contained the following numbers of fuel assemblies:

Location	Unit 1	Unit 2	Unit 3	Unit 4	Unit 5	Unit 6	Central Storage
Reactor Fuel Assemblies	400	548	548	0	548	764	0
Spent Fuel Assemblies	292	587	514	1331	946	876	6375
Fuel	UOx	UOx	UO2/MOX	UOx	UOx	UOx	UO2/MOX
New Fuel Assemblies	100	28	52	204	48	64	N/A

Cooling requirements

Diagrammatic representation of the cooling systems of a BWR.

See also: Decay heat – Power reactors in shutdown and Nuclear reactor safety systems

Power reactors work by splitting atoms, typically uranium, in a chain reaction. The reactor continues to generate heat after the chain reaction is stopped because of the radioactive decay of unstable isotopes, fission products, created by this process. This decay of unstable isotopes, and the decay heat that results, cannot be stopped. Immediately after shutdown, this decay heat amounts to approximately 6% of full thermal heat production of the reactor. The decay heat in the reactor core decreases over several days before reaching cold shutdown levels. Nuclear fuel rods that have reached cold shutdown temperatures typically require another several years of water cooling in a spent fuel pool before decay heat production reduces to the point that they can be safely transferred to dry storage casks.

To safely remove this decay heat, reactor operators must continue to circulate cooling water over fuel rods in the reactor core and spent fuel pond. In the reactor core, circulation is accomplished by use of high pressure systems that pump water through the reactor pressure vessel and into heat exchangers. These systems transfer heat to a secondary heat exchanger via the essential service water system, taking away the heat which is pumped out to the sea or site cooling towers.

To circulate cooling water when the reactor is shut down and not producing electricity, cooling pumps can be powered by other units on-site, by other units off-site through the grid, or by diesel generators. In addition, boiling water reactors have steam-turbine driven emergency core cooling systems that can be directly operated by steam still being produced after a reactor shutdown, which can inject water directly into the reactor. Steam turbines results in less dependence on emergency generators, but steam turbines only operate so long as the reactor is producing steam. Some electrical power, provided by batteries, is needed to operate the valves and monitoring systems.

If the water in the Unit 4 spent fuel pool had been heated to boiling temperature, the decay heat has the capacity to boil off about 70 tonnes of water per day (12 gallons per minute), which puts the requirement for cooling water in context. On 16 April 2011, TEPCO declared that Reactors 1–4's cooling systems were beyond repair and would have to be replaced.

The reason that cooling is so essential for a nuclear reactor, is that many of the internal components and fuel assembly cladding is made from zircaloy. At normal operating temperatures (of approximately 300 degrees Celsius), zircaloy is inert. However, when heated to above 500 degrees celsius in the presence of steam, zircaloy undergoes an exothermic reaction where the zircaloy oxidises, and produces free hydrogen gas. The reaction between the zirconium cladding and the fuel can also lower the melting point of the fuel and thus speed up a core melt.

The reactor's emergency diesel generators and DC batteries, crucial components in powering the reactors' cooling systems in the event of a power loss, were located in the basements of the reactor turbine buildings. The reactor design plans provided by General Electric specified placing the generators and batteries in that location, but mid-level engineers working on the construction of the plant were concerned that this made the back-up power systems vulnerable to flooding. TEPCO elected to strictly follow General Electric's design in the construction of the reactors.

Safety history

1967: Changing the layout of the emergency-cooling system, without reporting it

Fukushima reactor control room.

On 27 February 2012 NISA ordered TEPCO to report by 12 March 2012 about the reasoning to change the layout for the piping for an emergency cooling system from the plans originally registered in 1966 before the reactor was taken in operation.

After the plant was hit by the tsunami, the isolation condenser should have taken over the function of the ordinary cooling pumps, by condensing the steam from the pressure vessel into water to be used for cooling the reactor. But the condenser did not function properly, and TEPCO could not confirm whether a valve was opened.

In the original papers submitted – in July 1966 – for government approval of the plans to set up the reactor, the piping systems for two units in the isolation condenser were separated from each other. But in the application for the construction plan of the reactor – submitted in October 1967 – the piping layout was changed by TEPCO, and the two piping systems were connected outside the reactor. The changes were not reported in violation of all legal regulations.

1976: Falsification of safety records by TEPCO

The Fukushima Daiichi nuclear power complex was central to a falsified-records scandal that led to the departure of a number of senior executives of TEPCO. It also led to disclosures of previously unreported problems at the plant, although testimony by Dale Bridenbaugh, a lead GE designer, purports that General Electric was warned of major design flaws in 1976, resulting in the resignations of several designers who protested GE's negligence.

In 2002, TEPCO admitted it had falsified safety records at the No. 1 reactor at Fukushima Daiichi. As a result of the scandal and a fuel leak at Fukushima Daini, the company had to shut down all of its 17 nuclear reactors to take responsibility. A power board distributing electricity to a reactor's temperature control valves was not examined for 11 years. Inspections did not cover devices related to cooling systems, such as water pump motors and diesel generators.

1991: Back-up generator of reactor nr. 1 flooded

On 30 October 1991 one of two backup generators of reactor nr. 1 did fail, after it was flooded in the basement of the reactor buildings. Seawater used for the cooling of the reactor was leaking into the turbine-building from a corroded pipe at a rate of 20 cubic meters per hour. This was told by former TEPCO employees to the Japan Broadcasting Corporation news-service in December 2011. An engineer told, that he informed his superiors about this accident, and that he mentioned the possibility that a tsunami could inflict damage to the generators in the turbine-buildings near the sea. After this TEPCO did not move the generators to higher grounds, but instead TEPCO installed doors to prevent water leaking into the generator rooms. The Japanese Nuclear Safety Commission commented that it would revise the safety guidelines for designing nuclear plants and would enforce the installation of additional power sources. On 29 December 2011 TEPCO admitted all these facts: its report mentioned, that the emergency power system room was flooded through a door and some holes for cables, but the power supply to the reactor was not cut off by the flooding, and the reactor was stopped for one day. One of the two power sources was completely submerged, but its drive mechanism had remained unaffected.

2006: The Japanese government opposes a court-order

In March 2006 the Japanese government opposed a court order to close a nuclear plant in the west part of the country over doubts about its ability to withstand an earthquake. Japan's Nuclear and Industrial Safety Agency believed it was "safe" and that "all safety analyses were appropriately conducted".

2007: Tsunami-study ignored

In 2007 TEPCO did set up a department to supervise all its nuclear facilities, and until June 2011 its chairman was Masao Yoshida, the chief of the Fukushima Daiichi power plant. An in-house study in 2008 pointed out that there was an immediate need to improve the protection of the power station from flooding by seawater. This study mentioned the possibility of tsunami-waves up to 10.2 meters. Officials of the department at the company's headquarters insisted that such a risk was unrealistic and did not take the prediction seriously.

2008: Seismic-concerns

In addition to concerns from within Japan, the International Atomic Energy Agency (IAEA) has also expressed concern about the ability of Japan's nuclear plants to withstand seismic activity. At a meeting of the G8's Nuclear Safety and Security Group, held in Tokyo in 2008, an IAEA expert warned that a strong earthquake with a magnitude above 7.0 could pose a "serious problem" for Japan's nuclear power stations.

2011: Results of governmental investigations

On request of the Japan Broadcasting Corporation, on 2 October 2011 the Japanese Government released a report of TEPCO to NISA. These papers proved that TEPCO was well aware of the possibility that the plant could be hit by a tsunami with waves far higher than the 5.7 meters which the plant was designed to withstand. Simulations done in 2008, based on the destruction caused by the 1896-earthquake in this area, made it clear that waves between 8.4 and 10.2 meters could overflow the plant. Three years later the report was sent to NISA, where it arrived on the 7 March 2011, just 4 days before the plant was hit by the tsunami. Further studies by scientists and an examination of the plant's tsunami resistance measures were not planned by TEPCO before April 2011, and no further actions were planned to deal with this subject before October 2012. TEPCO official Junichi Matsumoto said that the company did not feel the need to take prompt action on the estimates, which were still tentative calculations in the research stage. An official of NISA said that these results should have been made public by TEPCO, and that the firm should have taken measures right away.

This all was in sharp contrast with the events at the Tōkai Nuclear Power Plant where the dike around the plant was raised to 6.1 meters after evaluations showed the possibility of tsunami-waves higher than previously expected. Although the dike was not completely finished at 11 March 2011, the plant was able to ride out the tsunami, even though the external power-sources in Tokai were lost also. With two (of three) functioning sea-water-pumps and the emergency diesel-generator the reactor could be kept safely in cold shutdown.

On 26 November a TEPCO spokesman mentioned that TEPCO would have been better prepared to cope with the tsunami in March 2011, if it had taken the 2008-study more seriously. TEPCO was also willing to use the estimates of renewed study done by a national civil engineering society for its facility management.

Nuclear Safety Commission Chairman Haruki Madarame told a parliamentary inquiry in February 2012 that "Japan's atomic safety rules are inferior to global standards and left the country unprepared for the Fukushima nuclear disaster last March". There were flaws in, and lax enforcement of, the safety rules governing Japanese nuclear power companies, and this included insufficient protection against tsunamis.

After the tsunami

Further information: Timeline of the Fukushima I nuclear accidents and 2011 Tōhoku earthquake and tsunami

Map of Japan's electricity distribution network, showing incompatible systems between regions.

The 9.0 M_W Tōhoku earthquake occurred at 14:46 JST on Friday, 11 March 2011 with epicenter near the island of Honshu. It resulted in maximum ground accelerations of 0.56, 0.52, 0.56 g (5.50, 5.07 and 5.48 m/s²) at Units 2, 3 and 5 respectively, above their designed tolerances of 0.45, 0.45 and 0.46 g (4.38, 4.41 and 4.52 m/s²), but values within the design tolerances at Units 1, 4 and 6. The Fukushima I facility had not initially been designed for a tsunami of the size that struck the plant, nor had the reactors been modified when later concerns were raised in Japan and by the IAEA. When the earthquake occurred, the reactors on Units 1, 2, and 3 were operating, but those on Units 4, 5, and 6 had already been shut down for periodic inspection. Units 1, 2 and 3 underwent an automatic shutdown (called SCRAM) when the earthquake struck.

When the reactors shut down, the plant stopped generating electricity, stopping the normal source of power for the plant. TEPCO reported that one of the two connections to off-site power for Reactors 1–3 also failed so 13 on-site emergency diesel generators began powering the plant's cooling and control systems. There are two emergency diesel generators for each of the Units 1–5 and three for Unit 6.

The earthquake was followed by a 13–15 m (43–49 ft) maximum height tsunami arriving approximately 50 minutes later which topped the plant's 5.7 m (19 ft) seawall, flooding the basement of the Turbine Buildings and disabling the emergency diesel generators located there at approximately 15:41. At this point, TEPCO notified authorities, as required by law, of a "First level emergency". The Fukushima II plant, which was also struck by the tsunami, incorporated design changes which improved its resistance to flooding and it sustained less damage. Generators and related electrical distribution equipment were located in the watertight reactor building, so that power from the grid was being used by midnight. Seawater pumps for cooling were given protection from flooding, and although 3 of 4 failed in the tsunami, they were able to be restored to operation.

In the late 1990s, three additional backup generators for reactors Nos. 2 and 4 were placed in new buildings located higher on the hillside, to comply with new regulatory requirements. All six reactors were given access to these generators, but the switching stations that sent power from these backup generators to the reactors' cooling systems for Units 1 through 5 were still in the poorly protected turbine buildings. All three of the generators added in the late 1990s were operational after the tsunami. If the switching stations had been moved to inside the reactor buildings or to other flood-proof locations, power would have been provided by these generators to the reactors' cooling systems.

After the diesel generators located in the turbine buildings failed, emergency power for control systems was supplied by batteries that were designed to last about eight hours. Further batteries and mobile generators were dispatched to the site, delayed by poor road conditions with the first not arriving until 21:00 JST 11 March, almost six hours after the tsunami struck.

Attempts to connect portable generating equipment to power water pumps were eventually discontinued after numerous attempts, as the connection point in the Turbine Hall basement was flooded and because of difficulties finding suitable cables. TEPCO switched its efforts to installing new lines from the grid to the cooling systems. One plant generator at Unit 6 was restored to operation on 17 March, and external power returned to Units 5 and 6, on 20 March, allowing cooling equipment to be restarted.

Unit 1 Reactor

Details of the core

F. Tanabe has estimated that the core contained the following materials:

• Uranium dioxide 78.3 tons
• Zirconium 32.7 tons
• Steel 12.5 tons
• Boron carbide 590 kilos
• Inconel 1 ton

Cooling problems and first radioactivity release

Unit 1 before the explosion. The joint can be seen between the lower concrete building and upper lighter cladding which was blown away in the explosion.

Unit 1 water levels and reactor pressures from 11 to 14 March

On 11 March at 14:46 JST, Unit 1 scrammed (shut off power-producing nuclear fission chain reactions) successfully in response to the earthquake though evacuated workers reported violent shaking and burst pipes within the reactor building. At 15:37 all generated electrical power was lost following the tsunami leaving only emergency batteries, able to run some of the monitoring and control systems. It was later learned that Unit 1's batteries were damaged and unavailable following the tsunami. At 15:42, TEPCO declared a "Nuclear Emergency Situation" for Units 1 and 2 because "reactor water coolant injection could not be confirmed for the emergency core cooling systems." The alert was temporarily cleared when water level monitoring was restored for Unit 1 but it was reinstated at 17:07 JST. Potentially radioactive steam was released from the primary circuit into the secondary containment area to reduce mounting pressure.

After the loss of site power and reactor shutdown, Unit 1 was initially cooled using the isolation condenser system. About 10 minutes after the earthquake, TEPCO operators removed both of Unit 1's isolation condensers from service, and instead chose to activate the HPCI (High Pressure Coolant Injection) systems to cool the reactor and the core spray system was activated at 15:07 to cool the suppression pool. The core spray system was disabled with AC power loss at 15:37 (The tsunami) and the HPCI system failed following DC (i.e., battery back up) power loss.

Operators were unable to restart the isolation condensers for an extended period of time after the tsunami (greater than 30 minutes). After that, the isolation condensers were operated intermittently, for unknown reasons. The isolation condensers were designed to successfully cool Unit 1 for at least 8 hours, and it is unknown how effective they were. After that, refill would have been required to the isolation condenser tanks which are under atmospheric pressure (low pumping head requirements). By design, isolation condensers would have removed the heat from the reactor transferring it out of the primary containment and into the atmosphere, but with limited and non-existent operation, core and containment cooling was not successful.

In event, for whatever ultimate reason, the isolation condensor apparently did not work. On 27 February 2012 NISA ordered TEPCO to reveal – before 12 March 2012 – why the layout of the isolation condensor was changed. In the papers of the original application (dated from July 1966) for reactor 1 the tubes were separated from each other, But in the building-papers – submitted October 1967 – the two tubes were connected with each other outside the reactor vessel. Without mentioning the change to NISA, all of this in violation with all legal procedures.

By midnight water levels in the reactor were falling and TEPCO gave warnings of the possibility of radioactive releases. In the early hours of 12 March, TEPCO reported that radiation levels were rising in the turbine building for Unit 1 and that it was considering venting some of the mounting pressure into the atmosphere, which could result in the release of some radioactivity. Chief Cabinet Secretary Yukio Edano stated later in the morning the amount of potential radiation would be small and that the prevailing winds were blowing out to sea. At 02:00 JST, the pressure inside the reactor containment was reported to be 600 kPa (6 bar or 87 psi), 200 kPa higher than under normal conditions. At 05:30 JST, the pressure inside Reactor 1 was reported to be 2.1 times normal levels, 820 kPa. Isolation cooling ceased to operate between midnight and 11:00 JST 12 March, at which point TEPCO started relieving pressure and injecting water. One employee working inside Unit 1 at this time received a radiation dose of 106 mSv and was later sent to a hospital to have his condition assessed.

Rising heat within the containment area led to increasing pressure. Electricity was needed for both the cooling water pumps and ventilation fans used to drive gases through heat exchangers within the containment. Releasing gases from the reactor is necessary if pressure becomes too high and has the benefit of cooling the reactor as water boils off, but this also means cooling water is being lost, and must be replaced if the water level is not to fall too low. If there is no damage to the fuel elements, water and water vapor from inside the reactor should be only slightly radioactive.

In a press release at 07:00 JST 12 March, TEPCO stated, "Measurement of radioactive material (iodine-131, etc.) by monitoring car indicates increasing value compared to normal level. One of the monitoring posts is also indicating higher than normal level." Dose rates recorded on the main gate rose from 69 nGy/h (for gamma radiation, equivalent to 69 nSv/h) at 04:00 JST, 12 March, to 866 nGy/h 40 minutes later, before hitting a peak of 0.3855 mSv/h at 10:30 JST. At 13:30 JST, workers detected radioactive caesium-137 and iodine-131 near Reactor 1, which indicated some of the core's fuel had been damaged. Cooling water levels had fallen so much that parts of the nuclear fuel rods were exposed and partial melting might have occurred. Radiation levels at the site boundary exceeded the regulatory limits.

On 14 March, radiation levels had continued to increase on the premises, measuring at 02:20 an intensity of 0.751 mSv/h on one location and at 02:40 an intensity of 0.650 mSv/h at another location on the premises. On 16 March, the maximum readings peaked at 10.850 mSv/h.

Hydrogen explosion

At 07:00 JST on 12 March, Prime Minister Naoto Kan asked Daiichi director Masao Yoshida why his workers were not opening the valves to release rising steam pressure within the reactor. Yoshida answered that they could not open the electrical valves because of the power failure and the radiation was too high to send workers to manually open the valves. Nevertheless, with the pressure and temperatures continuing to rise, at 09:15, TEPCO sent workers to begin manually opening the valves. The high radiation slowed the work and the valves were not opened until 14:30.

At 15:36 JST on 12 March, there was an explosion in the reactor building at Unit 1. The side walls of the upper level were blown away, leaving in place only the vertical steel framed gridworks. The roof collapsed, covering the floor and some machinery on the south side. The walls were relatively intact compared to later explosions at Units 3 and 4. Video of the explosion shows that it was primarily directed sideways.

The roof of the building was designed to provide ordinary weather protection for the areas inside, not to withstand the high pressure of an explosion. In the Fukushima I reactors the primary containment consists of "drywell" and "wetwell" concrete structures below the top level, immediately surrounding the reactor pressure vessel. The secondary containment includes the top floor with water-filled pools for storing fresh or irradiated fuel and for storage of irradiated tools and structures.

Experts soon agreed that the cause was a hydrogen explosion. Almost certainly the hydrogen was formed inside the reactor vessel because of falling water levels exposing zircaloy structures/fuel assembly cladding, which then reacted with steam and produced hydrogen, with the hydrogen subsequently vented into the containment building.. when the hydrogen reached ignition concentration in the air of the secondary containment building, a ignition source such as a spark triggered a hydrogen-oxygen explosion, blowing out the walls of this building from the inside.

Officials indicated that reactor primary containment (the reactor pressure vessel) had remained intact and that there had been no large leaks of radioactive material, although an increase in radiation levels was confirmed following the explosion. The report of the fact-finding commission states that "There is a possibility that the bottom of the RPV [reactor pressure vessel] was damaged and some of the fuel might have dropped and accumulated on the D/W [dry well] floor (lower pedestal)." The Fukushima prefectural government reported radiation dose rates at the plant reaching 1.015 mSv/h. The IAEA stated on 13 March that four workers had been injured by the explosion at the Unit 1 reactor, and that three injuries were reported in other incidents at the site. They also reported one worker was exposed to higher-than-normal radiation levels but the level fell below their guidance for emergency situations.

Seawater used for cooling

At 20:05 JST on 12 March, the Japanese government ordered seawater to be injected into Unit 1 in a new effort to cool the reactor core. The treatment had been held as a last resort since it ruins the reactor. TEPCO started seawater cooling at 20:20, adding boric acid as a neutron absorber to prevent a criticality accident. The water would take five to ten hours to fill the reactor core, after which the reactor would cool down in around ten days. The injection of seawater into the reactor pressure vessel was done by fire department trucks. At 01:10 JST on 14 March, injection of seawater was halted for two hours because all available water in the plant pools had run out (similarly, feed to Unit 3 was halted). NISA news reports stated 70% of the fuel rods had been damaged when uncovered.

On 18 March, a new electrical distribution panel was installed in an office adjacent to Unit 1 to supply power via Unit 2 when it was reconnected to the transmission grid two days later. On 21 March, injection of seawater continued, as did repairs to the control instrumentation. On 23 March, it became possible to inject water into the reactor using the feed water system rather than the fire trucks, raising the flow rate from 2 to 18 m³/h (later reduced to 11m³/h, and even further to reduce the build up of contaminated water); on 24 March, lighting was restored to the central operating room.

As of 24 March, the spent fuel pool was "thought to be fully or partially exposed", according to CNN. Pressure in the reactor had increased owing to the seawater injection, resulting in steam being vented, later alleviated by reducing the water flow. Temperature increases were also reportedly temporary. TEPCO condensed some of the steam to water in the spent fuel pool.

It was estimated that as much as 26 tonnes of sea salt may have accumulated in reactor Unit 1 and twice that amount in Units 2 and 3. As salt clogs cooling pipes and erodes zirconium oxide layer of the fuel rods, switching to the use of freshwater for cooling was a high priority.

The use of seawater has the potential to make uranium chemistry more complex; in pure water the hydrogen peroxide formed by the radiolysis of water can react with uranium dioxide to form a solid peroxide mineral known as studtite. According to Navrotsky et al. this mineral has been found in the fuel storage pond at the plutonium production site at Hanford. Navrotsky et al. report that when alkali metal ions are present, uranium can form nanoparticles (U₆₀ clusters) which may be more mobile than the solid studtite. A review of the research done at the University of Notre Dame on the subject of nanoscale actinyl clusters was published in 2010.

Reactor stabilization

Because of saltwater corrosion problems and pipes clogging by salt, fresh cooling water is transported by barge to Fukushima.

By 24 March, electrical power (initially from temporary sources, but off-site power used from 3 April) was being restored to parts of the unit, with the Main Control Room lighting being restored.

On 25 March, fresh water became available again to be added to the reactor instead of salt water, while work continued to repair the unit's cooling systems. A volume of 1890 m³ (500,000 USgal) of fresh water was brought to the plant by a barge provided by the US Navy. On 29 March, the fire trucks which had been used to inject water into the reactor were replaced by electrical pumps.

On 28 March, pumping began to remove water contaminated with radioactive ¹³⁷Cs and ¹³¹I from basement areas, storing it in the condenser system. By 29 March, pumping was halted because condensate reservoirs were almost full and plans were being considered to transfer water to the suppression pool surge tanks.

On 7 April, TEPCO began injecting nitrogen into the containment vessel, which was expected to reduce the likelihood of further hydrogen explosions. The injection has been ongoing since then and has been repeated on the other units at Fukushima. On 7 April, before a large aftershock, temperatures in the reactor core unexpectedly "surged in temperature to 260 °C"; the cause was unknown, but the temperature dropped to 246 °C by 8 April. On 27 April, TEPCO revised its estimate of damaged fuel in Unit 1 from 55% to 70%.

On 17 April, remote control robot was used to enter the Reactor Building and performed a series of inspections, which confirmed on 29 April that there was no significant water leakage coming from the containment vessel.

On 23 and 26 April, concerns that Unit 1 fuel rods may be exposed to air caused TEPCO to consider filling the "containment vessel with water to cool the reactor" despite concerns for building integrity. Efforts were slowed by Unit 1 radiation measurements "as high as 1,120 mSv/hr". On 13 May, TEPCO announced it would proceed with a plan to fill the containment vessel despite the possibility of holes caused by melting fuel elements in the pressure vessel. TEPCO had expected to increase the amount of water pumped to Unit 1 to compensate for any leakage from the holes but decided on 15 May to abandon the plan after finding the Unit 1 basement was already half flooded.

On 5 May, ventilation systems were installed in the Reactor Building, to clean the highly radioactive air encapsulated in it.

On 12 May, the water level gauge for the reactor was calibrated, and it was subsequently identified that the water level was lower than previously thought (as the water level went off the lower side of the gauge).

On 13 May, preparatory work started on the installation of the Reactor Building covers. Construction work started on 28 June.

On 20 May, staff entered the Reactor Building, confirming reactor water level and radioactivity.

Since 2 July, the reactor has been cooled using fresh water from the on-site water treatment plant.

On 21 August, TEPCO reported that all of the temperature sensors of Unit 1 were recording temperatures less than 100 degrees Celsius on Friday 19 August. Once other goals are met, Unit 1 will have achieved cold shutdown state.

On 28 October, TEPCO reported the completion of cover construction at reactor building of Unit 1 of Fukushima Daiichi nuclear power station.

On 19 January 2012 the interior of the primary containment vessel of reactor 2 was inspected, by TEPCO for the first time after the accident, with an industrial endoscope. With this device photos were taken and the temperature was measured at this spot and from the cooling-water inside, in an attempt to calibrate the existing temperature-measurements that could have an error margin of 20°C (36°F). The procedure lasted 70 minutes. The photos showed parts of the walls and pipes inside the containment vessel, but they were unclear and blurred, most likely due to water vapors and the radiation inside. According to TEPCO the photos showed no serious damage. The temperature measured inside was 44.7 °C (112.5 °F) and did not differ much from the 42.6 °C (108.7 °F) measured outside the vessel.

Possibility of criticality

Reports of 13 observations of neutron beams 1.5 km "southwest of the plant's No. 1 and 2 reactors" from 13 to 16 March raised the possibility that nuclear chain reactions could have occurred after the initial SCRAMing of the reactors at Fukushima Daiichi. 16 March reports that fuel rods in the spent fuel pool at Unit 4 could have been exposed to air appeared to indicate that uncontrolled fission may have occurred in that fuel pool. Later reports of exceptionally high iodine-134 levels appeared to confirm this theory because very high levels of iodine-134 would be indicative of criticality. The same report also showed high measurements of chlorine-38, which some nuclear experts used to calculate that self-propagating fission must be occurring in Unit 1. Despite TEPCO suggesting the iodine-134 report was inaccurate, the IAEA appeared to accept the chlorine-based analysis as a valid theory suggesting criticality when it stated at a press conference that "melted fuel in the No. 1 reactor building may be causing isolated, uncontrolled nuclear chain reactions". TEPCO confirmed its concern about the accuracy of the high iodine and chlorine report by formally retracting the report on 21 April, which eliminated both the exceptionally high iodine-134 and chlorine-38 levels as proof of criticality. TEPCO did not appear to comment on the criticality concern when withdrawing its report, but the IAEA has not withdrawn its comments, and some off-site experts find the currently measured iodine-134 levels higher than expected.

Meltdown

On 12 May, TEPCO engineers confirmed that a meltdown occurred, with molten fuel having fallen to the bottom of the reactor's pressure vessel, or RPV. The utility said that fuel rods of the No. 1 reactor are fully exposed, with the water level 1 meter (3.3 feet) below the base of the fuel assembly. According to a Japanese press report, there are holes in the base of the pressure vessel – these holes were meant for the control-rods. After the fuel had melted, it produced holes in the bottom of the RPV, and then escaped into the containment vessel. In November 2011 TEPCO did not know the shape or porosity of the fuel mass, which is at the bottom of the containment vessel. As a result it is impossible to know exactly how far the fuel mass would have eroded the concrete floor, but TEPCO estimate that no more than 70 cm of a 7.6 meter concrete slab was eroded away by the hot fuel. The production of heat and steam in unit 1 has decreased, as suggested by both radioactive decay calculations and photographic evidence (same source from TEPCO).

TEPCO estimates the nuclear fuel was exposed to the air less than five hours after the earthquake struck. Fuel rods melted away rapidly as the temperature inside the core reached 2,800 °C within six hours. In less than 16 hours, the reactor core melted and dropped to the bottom of the pressure vessel, burning a hole through the vessel. By that time, water was pumped into the reactor in an effort to prevent the worst-case scenario – overheating fuel melting its way through the containment and discharging large amounts of radionuclides in the environment. In June the Japanese government confirmed that Unit 1 reactor vessel containment was breached, and pumped cooling water continues to leak months after the disaster.

Spent fuel pool of reactor 1

From 31 March, additional sea water was added to the spent fuel pool, initially by using a concrete pump. Fresh water was used from 14 May. By 29 May water was able to be injected using a temporary pump and the Spent Fuel Pool Cooling (FPC) line.

On 10 August the spent fuel pool was switched from the water-injection system – that functioned some 5 months – to a circulatory cooling system. For the first time since the 11 March disaster, all four damaged reactors at the plant were using circulatory cooling systems with heat-exchangers.

Unit 2 reactor

Failure of both Unit 2 diesel generators

Aerial view of the plant area before the accidents, showing separation between Units 5 & 6, and the majority of the complex

Radiation measurements from Fukushima Prefecture, March 2011

Seawater-contamination along coast with caesium-137, from 21 March until 5 May (Source: GRS)

Measured dosis at atomic plant border area from 12. up to 17. March

Unit 2 was operating at the time of the earthquake and experienced the same controlled initial shutdown as the other units. As with unit 1, the reactor scrammed following the earthquake. The two diesel generators came online and initially all cooling systems were available. Initially the high pressure coolant injection (HPCI) system was primary cooling the core and at 15:00 operators activated the residual heat removal system main pump and the containment vessel spray pump at 15:07 to cool the suppression pool; all these systems failed following both AC power and DC power loss after the tsunami as the diesel generators and other systems failed when the tsunami overran the plant. The reactor core isolation cooling (RCIC) system was manually activated by operators at 15:39 following power loss, but by midnight the status of the reactor was unclear; some monitoring equipment was still operating on temporary power. The coolant level was stable and preparations were underway to reduce pressure in the reactor containment vessel should it become necessary, though TEPCO did not state in press releases what these preparations were, and the government had been advised that this might happen. The RCIC was reported by TEPCO to have shut down around 19:00 JST on 12 March, but reported to be operating again as of 09:00 JST 13 March. The pressure reduction of the reactor containment vessel commenced before midnight on 12 March although the IAEA reported that as of 13:15 JST 14 March, that according to information supplied to them, no venting had taken place at the plant. A report in The New York Times suggested that plant officials initially concentrated efforts on a damaged fuel storage pool at Unit 2, diverting attention from problems arising at the other reactors, but that incident was not reported in official press releases. The IAEA reported that on 14 March at 09:30, the RCIC was still operating and that power was being provided by a mobile generator. By midday on 19 March grid power had been connected to the existing transformer at Unit 2 and work continued to connect the transformer to the new distribution panel installed in a nearby building. Outside electricity became available at 15:46 JST on 20 March, but equipment still had to be repaired and reconnected.

Cooling problems

On 14 March, TEPCO reported the shutdown of the RCIC system presumably due to low reactor pressure. Operators had for days taken measures to prevent the reactor pressure from dropping below the level at which the RCIC can operate to keep it running as long as possible. The system was never designed to be used for an extended period. Fuel rods had been fully exposed for 140 minutes and there was a risk of a core meltdown. Reactor water level indicators were reported to be showing minimum-possible values at 19:30 JST on 14 March.

At 22:29 JST, workers had succeeded in refilling half the reactor with water but parts of the rods were still exposed, and technicians could not rule out the possibility that some had melted. It was hoped that holes blown in the walls of reactor building 2 by the earlier blast from Unit 3 would allow the escape of hydrogen vented from the reactor and prevent a similar explosion. At 21:37 JST, the measured dose rates at the gate of the plant reached a maximum of 3.13 mSv/h, which was enough to reach the annual limit for non-nuclear workers in twenty minutes, but had fallen back to 0.326 mSv/h by 22:35.

It was believed that around 23:00 JST, the 4 m long fuel rods in the reactor were fully exposed for the second time. At 00:30 JST on 15 March, NHK ran a live press conference with TEPCO stating that the water level had sunk under the rods once again and pressure in the vessel was raised. The utility said that the hydrogen explosion at Unit 3 might have caused a glitch in the cooling system of Unit 2: Four out of five water pumps being used to cool the Unit 2 reactor had failed after the explosion at Unit 3. In addition, the last pump had briefly stopped working when its fuel ran out. To replenish the water, the contained pressure would have to be lowered first by opening a valve of the vessel. The unit's air flow gauge was accidentally turned off and, with the gauge turned off, flow of water into the reactor was blocked leading to full exposure of the rods. As of 04:11 JST on 15 March, water was being pumped into the reactor of Unit 2 again.

At Thursday 23 June Tepco-workers entered the building of reactor 2, to install a provisional gauge for measuring the water level inside the reactor. The original device was damaged in March. Next Saturday 25 June Tepco reported, that it was still not possible to obtain accurate data on the water level and pressure of this reactor. The temperature near the containment vessel is very high, because of this the gauge did not function properly: the water inside the tubes of the gauge was evaporated.

It was later revealed that workers were minutes from restoring power to the standby liquid control (SLC) system pumps in unit 2 as a way to inject borated water once the RCIC shut down and had spent hours laying cable from a generator truck to the unit 2 power center when the unit 1 explosion occurred. This damaged the cable preventing this method from being used. It is possible this system could have prevented a complete meltdown as it took hours after the explosion until injection using fire trucks could be started.

Explosion

An explosion was heard after 06:14 JST on 15 March in Unit 2, possibly damaging the pressure-suppression system, which is at the bottom part of the containment vessel. The radiation level was reported to exceed the legal limit and the plant's operator started to evacuate all non-essential workers from the plant. Only a minimum crew of 50 men, also referred to as the Fukushima 50, was left at the site. Soon after, radiation equivalent dose rates had risen to 8.2 mSv/h around two hours after the explosion and again down to 2.4 mSv/h, shortly after. Three hours after the explosion, the rates had risen to 11.9 mSv/h.

While admitting that the suppression pool at the bottom of the containment vessel had been damaged in the explosion, causing a drop of pressure there, Japanese nuclear authorities emphasized that the containment had not been breached as a result of the explosion and contained no obvious holes. In a news conference on 15 March the director general of the IAEA, Yukiya Amano, said that there was a "possibility of core damage" at Unit 2 of less than 5%. Japan's Nuclear and Industrial Safety Agency (NISA) stated 33% of the fuel rods were damaged, in news reports the morning of 16 March. On 30 March, NISA reiterated concerns about a possible Unit 2 breach at either the suppression pool, or the reactor vessel. NHK World reported the NISA's concerns as "air may be leaking", very probably through "weakened valves, pipes and openings under the reactors where the control rods are inserted", but that "there is no indication of large cracks or holes in the reactor vessels".

On 8 November workers did enter reactor-building no. 4, and inspected the place to determine the cause of the hydrogen-blast on 15 March 2011. They found the 5th floor more severely damaged compared with the 4th floor, where the spent fuel pool was located. The fuel itself was found undamaged. The workers also found a severely damaged air conditioning duct on floor 5. These findings did not support earlier assumptions that the hydrogen in the blast originated from the spent fuel pool of reactor 4, but instead proved that the explosion was caused by hydrogen from the number 3 reactor, after the valves were opened. The hydrogen reached the fifth floor of reactor building 4 through the air-condition pipe.

Spent fuel pool

From 20 March, seawater was added to the spent fuel pool via the Fuel Pool Cooling (FPC) line. Fresh water was used from 29 March.

On 31 May, the spent fuel pool was switched from the water-injection system, to a circulatory cooling system.

Containment damage

Unit 2 was considered the most likely unit to have a damaged reactor containment vessel, as of 24 March. On 27 March, TEPCO reported measurements of very high radiation levels, over 1000 mSv/h, in the basement of the Unit 2 turbine building, which officials reported was 10 million times higher than what would be found in the water of a normally functioning reactor. Hours into the media frenzy, the company retracted its report and stated that the figures were not credible. "because the level was so high the worker taking the reading had to evacuate before confirming it with a second reading." Shortly following the ensuing wave of media retractions that discredited the report worldwide, TEPCO clarified its initial retraction; the radiation from the pool surface in the basement of the Unit 2 turbine building was found to be "more than 1,000 millisieverts per hour", as originally reported, but the concentration of radioactive substances was 100,000 times higher than usual, not 10 million.

Seawater used for cooling

At 20:05 JST on 14 March, the Japanese government ordered seawater to be injected into Unit 2 in a new effort to cool the reactor core. The treatment had been held as a last resort since it ruins the reactor. TEPCO started seawater cooling at 16:34. From 26 March, freshwater was used to cool the core.

Reactor stabilization

By 26 March 2011, electrical power (initially from temporary sources, off-site power starting 3 April) was restored to parts of the Unit, with the Main Control Room lighting being restored.

On 28 March, the Nuclear Safety Commission announced its suspicion that radioactive materials had leaked from Unit 2 into water in trenches connecting Unit 2's buildings, leading TEPCO to reduce the amount of water pumped into the reactor because of fears that the water could leak into the sea. The reduction in water pumping could have raised reactor temperatures.

On 27 March, the IAEA reported temperatures at the bottom of the Reactor Pressure Vessel (RPV) at Unit 2 fell to 97 °C (206.6 °F) from 100 °C (212 °F) on Saturday. Operators attempted to pump water from the turbine hall basement to the condenser, but "both condensers turned out to be full." Therefore, condenser water was first attempted to be pumped to storage tanks, freeing condenser storage for water currently in the basement of Unit 2. The pumps now being used can move 10 to 25 tons of water per hour. On 19 April 2011, TEPCO began transferring excess, radioactive cooling water from the reactor's basement and maintenance tunnels to a waste processing facility.

On 18 April, remote control robot was used to enter the Reactor Building and performed a series of inspections.

On 18 May, staff entered the Reactor Building for the first time since 15 March.

On 11 June, ventilation systems were installed in the Reactor Building, to clean the highly radioactive air encapsulated within the Reactor Building.

On 28 June, TEPCO began injecting nitrogen into the containment vessel, which was expected to reduce the likelihood of further hydrogen explosions.

Since 2 July, the Reactor has been cooled using fresh water treated by the on site water treatment plant.

On 14 September at 11AM (JST) TEPCO began injecting water into the No. 2 reactor using the core spray system piping in addition to the feed water piping already being used as this method seemed to be effective in reducing the temperature in the No. 3 reactor. At that time the temperature at the bottom of the No. 2 reactor was still 114.4 degrees Celsius (237.92 °F), compared to the 84.9 °C degrees(184.82 °F) in the No.1 reactor and the 101.3 °C (214.34 °F) in the No. 3 reactor. The new method has led to some temperature decrease, but not as significant as the decrease that occurred in the No.3 reactor.

After some positive effect was noticed using both the core spray system and feed water piping, TEPCO decided on 16 September to increase the amount of water pumped into the No. 2 reactor by one ton, in an attempt to further lower the temperature in the core, to a total of 7 tons per hour. The same was done for reactor No. 3, where 5 tons were added, bring the total to 12 tons per hour. TEPCO also added that the volume of cooling water into the No. 1 reactor would be increased as necessary.

On 21 September 2011, Masanori Naitoh, director in charge of nuclear safety analysis at the Institute of Applied Energy, an expert commenting on the plan to contain the crisis at the Fukushima Daiichi nuclear plant, mentioned that the interior temperatures of the damaged reactors had to be checked to confirm cold-shutdown. Naitoh said that TEPCO was only measuring temperatures outside the reactors, and that the temperatures inside should be confirmed through simulation to confirm that they had fallen below 100 degrees, and that there were no risks of nuclear reactions recurring.

In the first week of February 2012, temperatures inside reactor No. 2 became unstable. On 7 February, the amount of cooling water was increased from 10.5 tons to 13.5 tons per hour. After an slight initial decrease in temperature, sensor readings again showed the temperature rising at some locations in the bottom of the reactor. On 11 February, temperatures rose once again. On 12 February, the temperature rose to 78.3°C (172.94°F). TEPCO denied the possibility of the core going critical again, because that would produce xenon, which was still below detectable levels. To prevent any possible nuclear criticality, TEPCO planned to dump boric acid into the reactor and to increase the volume of cooling water by 3 tons per hour.

Since only one of the temperature-sensors showed fluctuating readings between 70°C and 90°C, TEPCO and NISA thought this sensor was malfunctioning. The sensor works on the principle of changing resistance between the surface of two different metals as the temperature changes. TEPCO planned measurements on this sensor. Since the radiation around reactor 2 could make it impossible to place new sensors inside the reactor vessel, the situation would become very serious if the other two sensors inside the reactor were to also fail. After that, it would be impossible to monitor the reactor. Kazuhiko Kudo, a professor of nuclear engineering at the university of Kyushu, Japan commented: "Because we haven't been able to grasp how the nuclear fuel in the cores has been distributed, it's impossible to rule out localized high temperature spots. As the high radiation rules out installing new temperature sensors, if the last two sensors fail, the situation will be much more serious indeed." On 26 February, TEPCO sent a report to the Japanese government about the malfunctioning temperature-sensors and has since ceased monitoring that sensor. The other two temperature-sensors and the radiation levels inside the containment vessel would be used to monitor the state of the cold shutdown. The amount of cooling water would be lowered, after NISA's approval.

On 15 April 2012, one of the two remaining temperature-sensors at the bottom of the No.2 reactor gave false readings, and because the electric resistance was found greatly increased, TEPCO concluded that it was broken, leaving only 18 of 36 temperature sensors still functioning. At 11 a.m., the remaining thermometer at this place measured 46.7 degrees Celsius.

On 1 June 2012 TEPCO reported that another thermometer had malfunctioned, resulting in more than half of the temperature sensors, 23 out of 41, now being out of use in reactor No. 2., thus making it more problematic to monitor the state of "cold-shutdown". According to TEPCO, the high humidity in the reactor may be a contributing factor in the failure of the sensors. TEPCO stated that it is currently decontaminating the site and training workers to install new thermometers. The plan is to install new thermometers through pipes that are connected to the reactor. TEPCO stated that it plans to decontaminate the site and install the new thermometers by late July 2012.

On 15 June 2012 TEPCO reported that a robot that was sent into the No. 2 reactor building on 13 June 2012 to take video images and radiation measurements, detected a reading of 880mSv (millisieverts) per hour of radiation on the fifth floor, which one floor (4.5 meters) directly above the reactor containment vessel. TEPCO suspects that during initial accident in March 2011, that radioactive substances leaked from the No. 2 reactor moved trough the building, but after analyzing the images taken by the robot it could not find the exact route the radioactive substances traveled, and images taken by the robot found no major damage on the fifth floor. During the March 2011 nuclear accident the No. 2 reactor is believed to have released the largest amount of radioactive substances. But the overall route the radioactive material traveled has yet to be determined. TEPCO needs to find and repair the damaged parts of the reactor to recover melted nuclear fuel before TEPCO can begin the process of decommissioning the reactor. However high radiation often stops workers from entering the building. This scenario means it will take a long time to find the problems in the containment vessel.

Pressure vessel damage

On 15 May, TEPCO revealed that the pressure vessel that holds nuclear fuel "is likely to be damaged and leaking water at Units 2 and 3", which means most of the thousands of tons of water pumped into the reactors had leaked.

Meltdown

On 29 March, Richard Lahey, former head of safety research for boiling-water reactors at General Electric, speculated that the reactor core may have melted through the reactor containment vessel onto a concrete floor, raising concerns of a major release of radioactive material, while failing to divulge the report by Dale G. Bridenbaugh which condemned the design as "unsafe". On 27 April, TEPCO revised its estimate of damaged fuel in Unit 2 from 30% to 35%. TEPCO reported on 23 May that Reactor 2 suffered a meltdown about 100 hours after the earthquake.

Concerns over re-criticality

On 1 November 2011 TEPCO said that xenon-133 and xenon-135 was detected in gas-samples taken from the containment vessel of reactor 2, in a concentration of 6 to 10 (or more) parts per million becquerels per cubic centimeter. Xenon-135 was also detected in gas samples collected on 2 November. These isotopes are the result of nuclear fission-reaction of uranium. Because the short half-lifes of these gases: (Xe-133: 5 days Xe-135: 9 hours), the presence could only mean that nuclear fissions were occurring at some places in the reactor. Boric-acid was poured into the reactor in an attempt to stop the fission-reactions. No significant change in temperature or pressure was found by TEPCO, so there was no sign of large-scale criticality. The reactor-cooling was continued, but TEPCO would examine the situation at reactor 1 and 3 also. Professor Koji Okamoto of the University of Tokyo Graduate School made the comment that localized and temporary fission might still happen, and that the melted fuel could undergo fission, but the fuel was probably scattered around. Neutrons from radioactive materials could react with the uranium fuel and other substances. Self-sustaining chain reactions were unlikely, thanks to the huge amounts of boric acid that were poured into the reactor. According to Okamoto, these neutrons should be closely monitored to make sure fission did not happen, because when the fission-reactions were not controlled, it would be impossible to reach a state of "cold-shutdown". Therefore it was needed to locate all molten fuel in and outside the reactor-vessel.

On 3 November 2011 TEPCO said that the tiny amounts of xenon-135 detected in the reactor's containment vessel atmosphere came from spontaneous nuclear fission with curium-242 and curium-244, substances that were present in the nuclear fuel. A critical fission would have caused much higher concentrations of xenon isotopes. These reactions would occur constantly, and did not lead to criticality in the melted fuel of reactor 2. All assessments would be sent to NISA for reevaluation.

The detection of xenon on the afternoon of 1 November by TEPCO was reported to NISA in the night. The next day, 2 November just past 7 a.m., NISA informed the Prime Minister Yoshihiko Noda's secretary about the possibility of critical reactions in reactor 2. Two hours later at 9 a.m. prime minister Edano learned the news. At a press-conference, the Chief Cabinet Secretary Osamu Fujimura revealed that Minister of Economy, Trade and Industry Yukio Edano sent a strong reprimand to Hiroyuki Fukano, the chief of NISA, because NISA failed to report the incident immediately to both himself and the Prime Minister's Office, and that NISA waited almost a day after the find was done. Fujimura said, "I have been told that NISA decided not to report the incident until the following morning because the agency didn't believe it was a dangerous situation."

Unit 3 Reactor

Reactor Unit 3 (right) and Unit 4 (left) on 16 March.
Three of the reactors at Fukushima Daiichi overheated, causing meltdowns which released large amounts of radioactive material into the air.
Pipes are the direction of the ocean.

Unlike the other five reactor units, reactor 3 ran on mixed core, containing both uranium fuel and mixed uranium and plutonium oxide, or MOX fuel (with the core comprising ~6% MOX fuel), during a loss of cooling accident in a subcritical reactor MOX fuel will not behave differently to UOX fuel. The key difference between plutonium-239 and uranium-235 is that plutonium emits fewer delayed neutrons than uranium when it undergoes fission.

While water-insoluble forms of plutonium such as plutonium dioxide are very harmful to the lungs, this toxicity is not relevant during a Loss Of Coolant Accident (LOCA) because plutonium is very involatile and unlikely to leave the reactor in large amounts. Plutonium dioxide has a very high boiling point. The toxic effect of the plutonium to the public under these conditions is much less than that of iodine-131 and caesium. A key difference between the Fukushima accident and the Chernobyl accident was that the Chernobyl explosion shattered the fuel and flung it out of the reactor building, while at Fukushima there was no steam explosion driven by the release of fission energy. During a loss of cooling accident, the fuel is not subject to such intense mechanical stresses, so the release of radioactivity is controlled by the boiling-point of the different elements present.

Cooling problems

Following the reactor SCRAM, operators activated the reactor core isolation cooling system (RCIC) and the residual heat removal system and core spray systems were made available to cool the suppression pool; whether they were activated prior to the tsunami has not been made clear. The RHRS and CS pumps were knocked out of commission by the tsunami. With DC battery power remaining, the RCIC continued to keep the water level stable, and the operators switched to the high pressure coolant injection (HPCI) system when water level began to drop. On 13 March, the HPCI system failed, the reason for which is not completely clear due to instrumentation not being available. It is believed to be either due to loss of DC power due to depletion of batteries or to reactor pressure dropping below the level at which it can operate. Operators were unable to restart it as batteries were exhausted. After this the operators were unable to start the RCIC system and began injecting seawater. Although it was not clear at the time, some of the fuel in Reactor 3 apparently melted around sixty hours after the earthquake (the night of the 12th to 13th).

Early on 13 March an official of the Japan Nuclear and Industrial Safety Agency (NISA) told at a news conference that the emergency cooling system of Unit 3 had failed, spurring an urgent search for a means to supply cooling water to the reactor vessel to prevent a meltdown of its reactor core. At 05:38 there was no means of adding coolant to the reactor, owing to loss of power. Work to restore power and to vent excessive pressure continued. At one point, the top three meters of the uranium/mixed oxide (MOX) fuel rods were not covered by coolant.

At 07:30 JST, TEPCO prepared to release radioactive steam, indicating that "the amount of radiation to be released would be small and not of a level that would affect human health" and manual venting took place at 08:41 and 09:20. At 09:25 JST on 13 March, operators began injecting water containing boric acid into the primary containment vessel (PCV) via the pump of a fire truck. When water levels continued to fall and pressure to rise, the injected water was switched to seawater at 13:12. By 15:00 it was noted that despite adding water the level in the reactor did not rise and radiation had increased. A rise was eventually recorded but the level stuck at 2 m below the top of reactor core. Other readings suggested that this could not be the case and the gauge was malfunctioning.

Injection of seawater into the primary containment vessel (PCV) was discontinued at 01:10 on 14 March because all the water in the reserve pool had been used up. Supplies were restored by 03:20 and injection of water resumed. On the morning of 15 March, Secretary Edano announced that according to TEPCO, at one location near reactor Units 3 and 4, radiation at an equivalent dose rate of 400 mSv/h was detected. This might have been due to debris from the explosion in Unit 4.

Explosion

At 12:33 JST on 13 March, the chief spokesman of the Japanese government, Yukio Edano said hydrogen gas was building up inside the outer building of Unit 3 just as had occurred in Unit 1, threatening the same kind of explosion. At 11:15 JST on 14 March, the envisaged explosion of the building surrounding Reactor 3 of Fukushima 1 occurred, owing to the ignition of built up hydrogen gas. The Nuclear and Industrial Safety Agency of Japan (NISA) reported, as with Unit 1, the top section of the reactor building was blown apart, but the inner containment vessel was not breached. The explosion was larger than that in Unit 1 and felt 40 kilometers away. Pressure readings within the reactor remained steady at around 380 kPa at 11:13 and 360 kPa at 11:55 compared to nominal levels of 400 kPa and a maximum recorded of 840 kPa. Water injection continued. Dose rates of 0.05 mSv/h were recorded in the service hall and of 0.02 mSv/h at the plant entrance.

Eleven people were reported injured in the blast. TEPCO and NISA announced that four TEPCO employees, three subcontractor employees, and four Self-Defence-Force soldiers were injured. Six military personnel from the Ground Self Defense Force's Central Nuclear Biological Chemical Weapon Defense Unit, led by Colonel Shinji Iwakuma, had just arrived outside the reactor to spray it with water and were exiting their vehicles when the explosion occurred. Iwakuma later said that TEPCO had not informed them that there was a danger of a hydrogen explosion in the reactor, adding, "Tokyo Electric was desperate to stabilize (the plant), so I am not angry at them. If there is a possibility of an explosion, I would be reluctant to send my men there."

Possibility of criticality in the spent fuel pool

TEPCO claimed that there was a small but non-zero probability that the exposed fuel assemblies could reach criticality. The BBC commented that criticality would never mean a nuclear explosion, but could cause a sustained release of radioactive materials. Criticality is usually considered highly unlikely, owing to the low enrichment level used in light water reactors. American nuclear engineer Arnold Gundersen, noting the much greater power and vertical debris ejection compared to the Unit 1 hydrogen blast, has theorized that the Unit 3 explosion involved a prompt criticality in the spent fuel pool material, triggered by the mechanical disruption of an initial, smaller hydrogen gas explosion in the building.

On 11 May, TEPCO released underwater robotic video from the spent fuel pool. The video appears to show large amounts of debris contaminating the pool. Based on water samples analysed, unnamed experts and TEPCO reported that the fuel rods were left "largely undamaged", and that it appears that the Unit 3 explosion was entirely related to hydrogen buildup within the building from venting of the reactor.

Cooling efforts

Around 10:00 JST on 16 March, NHK helicopters flying 30 km away videotaped white fumes rising from the Fukushima I facility. Officials suggested that the Reactor 3 building was the most likely source, and said that its containment systems may have been breached. The control room for Reactors 3 and 4 was evacuated at 10:45 JST but staff were cleared to return and resume water injection into the reactor at 11:30 JST. At 16:12 JST, Self Defence Force (SDF) Chinook helicopters were preparing to pour water on Unit 3, where white fumes rising from the building was believed to be water boiling away from the fuel rod cooling pond on the top floor of the reactor building, and on Unit 4 where the cooling pool was also short of water. The mission was cancelled when helicopter measurements reported radiation levels of 50 mSv. At 21:06 pm JST, the government reported that major damage to Reactor 3 was unlikely but that it nonetheless remained their highest priority.

Early on 17 March, TEPCO requested another attempt by the military to put water on the reactor using a helicopter and four helicopter drops of seawater took place around 10:00 JST. The riot police used a water cannon to spray water onto the top of the reactor building and then were replaced by members of the SDF with spray vehicles. On 18 March, a crew of firemen took over the task with six fire engines each spraying 6 tons of water in 40 minutes. 30 further hyper rescue vehicles were involved in spraying operations. Spraying continued each day to 23 March because of concerns the explosion in Unit 3 may have damaged the pool (total 3,742 tonnes of water sprayed up to 22 March) with changing crews to minimise radiation exposure. Lighting in the control room was restored on 22 March after a connection was made to a new grid power supply and by 24 March it was possible to add 35 tonnes of seawater to the spent fuel pool using the cooling and purification system. On 21 March grey smoke was reported to be rising from the southeast corner of Unit 3 – where the spent fuel pool is located. Workers were evacuated from the area. TEPCO claimed no significant change in radiation levels and the smoke subsided later the same day.

On 23 March, black smoke billowed from Unit 3, prompting another evacuation of workers from the plant, though Tokyo Electric Power Co. officials said there had been no corresponding spike in radiation at the plant. "We don't know the reason for the smoke", Hidehiko Nishiyama of the Nuclear Safety Agency said.

On 24 March, three workers entered the basement of the turbine building and were exposed to radiation when they stepped into contaminated water. Two of them were not wearing high boots and received beta ray burns. They were hospitalized but their injuries were not life-threatening.

From 25 March, the source of water being injected into the core was switched from seawater to freshwater.

In August, TEPCO began considering changing the core injection method for the no. 3 reactor as it was requiring a much larger quantity of water to cool and the temperatures remained relatively high compared to the nos. 1 and 2 reactors which required far less water. TEPCO has hypothesized that this may be because some fuel is still present above the core support plate inside the pressure vessel of the no. 3 reactor in addition to the fuel that has fallen to the bottom of the pressure vessel. The fuel on the bottom would be easily cooled by the existing method, but as the pressure vessel is leaking, any fuel located on the support plate was likely only being cooled due to the steam generated by the cooling of the melted fuel at the bottom. TEPCO began considering utilizing the reactor's core spray system pipes as an additional path of water injection and then reduce the amount of water through the existing feedwater piping system. A team of workers were sent inside the reactor building to inspect the core spray system pipes and it was found that the piping was undamaged. Hoses were then run from the temporary injection pumps located outside the building and connected to the core spray system piping. On 1 September, TEPCO began injecting water using the new route. The new injection method has been considerably effective in lowering the temperature of the reactor to below 100 degrees celsius. As of 27 September, most of the no. 3 reactor's temperature readings are between 70–80 degrees celsius. Later, TEPCO began utilizing the same method in the no. 2 reactor; it has not had as significant effect on the no. 2 reactor as it did on the no. 3.

Further developments

Unit 3 reactor temperatures, 19 March to 28 May

On 25 March, officials announced the reactor vessel might be breached and leaking radioactive material. High radiation levels from contaminated water prevented work. Japan Nuclear and Industrial Safety Agency (NISA) reiterated concerns about a Unit 3 breach on 30 March. NHK World reported the NISA's concerns as "air may be leaking", very probably through "weakened valves, pipes and openings under the reactors where the control rods are inserted", but that "there is no indication of large cracks or holes in the reactor vessels". As with the other reactors, water was transferred from condenser reservoirs to the suppression pool surge tanks so that condensers could be used to hold radioactive water pumped from the basement.

On 17 April, remote control robots were used to enter the Reactor Building and performed a series of inspections.

On 27 April, TEPCO revised its estimate of damaged fuel in Unit 3 from 25% to 30%. Radiation measurements of the water in the Unit 3 spent fuel pool were reported at 140 kBq of radioactive caesium-134 per cubic centimeter, 150 kBq of caesium-137 per cubic centimeter, and 11 kBq per cubic centimeter of iodine-131 on 10 May.

On 15 May, TEPCO revealed that the pressure vessel that holds nuclear fuel "is likely to be damaged and leaking water at Units 2 and 3", which means most of the thousands of tons of water pumped into the reactors had leaked.

On 23 May, TEPCO reported that Reactor 3 had suffered a meltdown some sixty hours after the earthquake.

On 9 June, staff entered the Reactor Building to conduct radiation surveying.

On 25 June and the following day boric acid dissolved in 90 tons of water was pumped into the spent fuel pool of Reactor 3. Concrete debris from the March hydrogen explosion of the reactor building have been detected in the spent fuel pool. In June TEPCO discovered that the water in the pool was strongly alkaline: the pH had reached a value of 11.2. Leaching of calcium hydroxide (portlandite) or calcium silicate hydrate (CSH) from the concrete could have caused this. The alkaline water could accelerate the corrosion of the aluminium racks holding the spent fuel rods. If the fuel assemblies would fall, this could lead to re-criticality. In the mean time preparative works began to install a recirculation cooling system at the fuel pool, that should be operational in the first weeks of July.

On 14 July, TEPCO began injecting nitrogen into the containment vessel, which was expected to reduce the likelihood of further hydrogen explosions.

On 1 July, the spent fuel pool was switched from the water-injection cooling system, to a circulatory cooling system.

Since 2 July, the Reactor has been cooled using fresh water treated by the on site water treatment plant.

On 11 January 2012, radioactive contaminated water was found in two underground tunnels. On 12 January, TEPCO admitted that around 300 cubic meter water had accummilated in an underground tunnel near reactor No.3, with electric cables. Radioactive caesium was measured in concentrations varying from 49 to 69 becquerels per cubic centimeter. Smaller amounts of contaminated water with lower concentrations caesium was found in a tunnel near reactor no.1. How the water could accumulate at these places was under examination.

Units 4, 5 and 6

Main article: Fukushima Daiichi units 4, 5 and 6

When the Fukushima Daiichi nuclear disaster began on 11 March 2011, reactor unit 4 was shut down and all fuel rods had been transferred to the spent fuel pool on an upper floor of the reactor building. On 15 March, an explosion damaged the fourth floor rooftop area of the unit 4 reactor. Japan's nuclear safety agency NISA reported two large holes in a wall of the outer building of unit 4 after the explosion. It was reported that water in the spent fuel pool might be boiling. Radiation inside the unit 4 control room prevented workers from staying there permanently. Visual inspection of the spent fuel pool of reactor 4 on 30 April showed that there was no significant visible damage to the fuel rods in the pool. A radiochemical examination of the water from the pond confirms that little of the fuel in the pond has been damaged.

Reactors 5 and 6 were also not operating when the earthquake struck although, unlike reactor 4, they were still fueled. The reactors have been closely monitored, as cooling processes were not functioning well.

Central fuel storage areas

Used fuel assemblies taken from reactors are initially stored for at least 18 months in the pools adjacent to their reactors. They can then be transferred to the central fuel storage pond. This contains 6375 fuel assemblies and was reported "secured" with a temperature of 55 °C. After further cooling, fuel can be transferred to dry cask storage, which has shown no signs of abnormalities. On 21 March, temperatures in the fuel pond had risen slightly, to 61 °C and water was sprayed over the pool. Power was restored to cooling systems on 24 March and by 28 March temperatures were reported down to 35 °C.

Cascade of failures

Government agencies and Tepco were thoroughly unprepared for the "cascading nuclear disaster" which was largely caused by a public myth of "absolute safety" that nuclear power proponents had nurtured over decades. The tsunami that "began the nuclear disaster could and should have been anticipated and that ambiguity about the roles of public and private institutions in such a crisis was a factor in the poor response at Fukushima". In March 2012, Prime Minister Yoshihiko Noda said that the government shared the blame for the Fukushima disaster, saying that officials had been blinded by a false belief in the country's "technological infallibility", and were taken in by a "safety myth". Mr. Noda said "Everybody must share the pain of responsibility".

According to Naoto Kan, Japan's former prime minister, the country was totally unprepared for the Fukushima disaster, and the crippled Fukushima plant should not have been built so close to the ocean on a tsunami-prone coast. Kan has acknowledged flaws in authorities' handling of the crisis, including poor communication and coordination between nuclear regulators, utility officials and the government. He said the disaster "laid bare a host of an even bigger man-made vulnerabilities in Japan's nuclear industry and regulation, from inadequate safety guidelines to crisis management, all of which he said need to be overhauled".

A national program to develop robots for use in nuclear emergencies was terminated in midstream because it "smacked too much of underlying danger". Japan, supposedly a leader in robotics, had none to send in to Fukushima when the crisis began. Similarly, Japan’s Nuclear Safety Commission said in its safety guidelines for light-water nuclear facilities that "the potential for extended loss of power need not be considered." But just such an extended loss of power contributed to the Fukushima meltdowns.

Physicist Amory Lovins has said: Japan’s "rigid bureaucratic structures, reluctance to send bad news upwards, need to save face, weak development of policy alternatives, eagerness to preserve nuclear power’s public acceptance, and politically fragile government, along with TEPCO’s very hierarchical management culture, also contributed to the way the accident unfolded. Moreover, the information Japanese people receive about nuclear energy and its alternatives has long been tightly controlled by both TEPCO and the government".

Poor communication and delays

The Japanese government has admitted it did not keep records of key meetings during the Fukushima nuclear crisis, even though such detailed notes are considered a key component of disaster management. Data from SPEEDI (System for Prediction of Environmental Emergency Dose Information) were sent by email to the government of the Fukushima prefecture, but not shared with others. The data of five crucial days, from 12 March 2011 11:54 p.m. tp 16 March 9 a.m – holding vital information for evacuation and health advisories – the emails from NISA to Fukushima stayed unread and were deleted afterwards. All was revealed more than a year later, on 21 March 2012. The data were not used, because the disaster countermeasure office did regard the data "useless because the predicted amount of released radiation is unrealistic."

Japan's response to the crisis at Fukushima Daiichi was flawed by "poor communication and delays in releasing data on dangerous radiation leaks at the facility", a government-appointed investigative panel has found. The panel was led by University of Tokyo Professor Yotaro Hatamura and the panel's Investigation Committee on the Accident at the Fukushima Nuclear Power Stations of Tokyo Electric Power Company report attaches blame to Japan's central government as well as Tokyo Electric Power Co., "depicting a scene of harried officials incapable of making decisions to stem radiation leaks as the situation at the coastal plant worsened in the days and weeks following the disaster". The 507-page interim report, which resulted from hundreds of interviews with utility workers and government officials, said poor planning also worsened the disaster response, noting that authorities had "grossly underestimated tsunami risks" that followed the magnitude 9.0 earthquake. The 40-foot-high tsunami that struck the plant was twice as tall as the highest wave predicted by officials, and the erroneous assumption that the plant's cooling system continued to work after the tsunami struck worsened the disaster. "Plant workers had no clear instructions on how to respond to such a disaster, causing miscommunication, especially when the disaster destroyed backup generators. Ultimately, the series of failures led to the worst nuclear catastrophe since Chernobyl".

In February 2012, an independent investigation into the accident by the Rebuild Japan Initiative Foundation described how Japan's response was hindered at times by a loss of trust between the major actors: Naoto Kan, the Tokyo headquarters of Tepco, and the manager at the stricken plant. The report said that these conflicts "produced confused flows of sometimes contradictory information in the early days of the crisis". According to the report, Kan delayed the cooling of the reactors by questioning the use of seawater instead of fresh water. Kan further hindered the response to the crisis by micromanaging disaster management efforts and appointing his own nominees to a small, closed, decision-making staff. The report stated that the Japanese government was also slow to accept assistance from U.S. nuclear experts.

A 2012 report in The Economist said: "The reactors at Fukushima were of an old design. The risks they faced had not been well analysed. The operating company was poorly regulated and did not know what was going on. The operators made mistakes. The representatives of the safety inspectorate fled. Some of the equipment failed. The establishment repeatedly played down the risks and suppressed information about the movement of the radioactive plume, so some people were evacuated from more lightly to more heavily contaminated places".

From 17 to 19 March 2011, US military aircraft, on behalf of the US Department of Energy, measured the radiation within a 45-km radius of the reactors. The data recorded 125 microsieverts per hour of radiation as far as 25 km northwest of the plant. The US provided the data, illustrated on detailed maps, to the Japanese Ministry of Economy, Trade, and Industry on 18 March and to the Ministry of Education, Culture, Sports, Science, and Technology two days later. Japanese government officials did not act on the information provided by the maps.

The data were not forwarded to the prime minister's office and the Nuclear Safety Commission, and subsequently not used to direct the evacuation of the people living around the plant. Because a substantial portion of radioactive materials released from the plant went northwest and was fallen onto the ground, and some residents were "evacuated" into this direction, these people could have avoided unnecessary exposure to radiation when the data would have been published directly. According to Tetsuya Yamamoto, chief nuclear safety officer of the Nuclear Safety Agency, "It was very regrettable that we didn't share and utilize the information." But an official of the Science and Technology Policy Bureau of the technology ministry, Itaru Watanabe, said it was not Japan, but more appropriate for the United States to release the data.

After the Americans had published their map on 23 March, Japan felt itself forced to publish, and the fallout-maps, compiled from measurents on the ground and the predictions of the computer-simulations of SPEEDI, were released the same day. On 19 June 2012 science minister Hirofumi Hirano defended the decision not to publish, with the remark, that his "job was only to measure radiation levels on land", and that the government would study whether disclosure of the maps could have helped in the evacuation efforts.

Regulation

Regulatory capture may have contributed to the cascade of failures which were revealed after the tsunami receded. Regulatory capture may have also contributed to the current situation. Critics argue that the government shares blame with regulatory agency for not heeding warnings, for not ensuring the independence of the nuclear industry's oversight while encouraging the expansion of nuclear energy domestically and internationally. World media have argued that the Japanese nuclear regulatory system tends to side with and promote the nuclear industry because of amakudari (roughly translated as descent from heaven), in which senior regulators accept high paying jobs at the companies they once oversaw. To protect their potential future position in the industry, regulators seek to avoid taking positions that upset or embarrass the utilities they regulate. TEPCO's position as the largest electrical utility in Japan led it to be the most desirable position for retiring regulators, typically the "most senior officials went to work at Tepco, while those of lower ranks ended up at smaller utilities" according to the New York Times.

In August 2011, several top energy officials were fired by the Japanese government; affected positions included the Vice-minister for Economy, Trade and Industry; the head of the Nuclear and Industrial Safety Agency, and the head of the Agency for Natural Resources and Energy.

Accident rating

Comparison of radiation levels for different nuclear events.

The severity of the nuclear accident is provisionally rated 7 on the International Nuclear Event Scale (INES). This scale runs from 0, indicating an abnormal situation with no safety consequences, to 7, indicating an accident causing widespread contamination with serious health and environmental effects. Prior to Fukushima, the Chernobyl disaster was the only level 7 accident on record, while the Three Mile Island accident was a level 5 accident. Arnold Gundersen, a former nuclear power industry executive who served as an expert witness in the investigation of the Three Mile Island accident, said that "Fukushima is the biggest industrial catastrophe in the history of mankind," although current estimates of the total amount of radioactivity released from the 3 Fukushima Daiichi reactors is only 10% that from Chernobyl.

The Japan Atomic Energy Agency initially rated the situation at Unit 1 below both of these previous accidents; on 13 March it announced it was classifying the event at level 4, an "accident with local consequences". On 18 March it raised its rating on Units 1, 2 and 3 to Level 5, an "accident with wider consequences". It classified the situation at Unit 4 as a level 3 "serious incident".

Several parties disputed the Japanese classifications, arguing that the situation was more severe than they were admitting at the time. On 14 March, three Russian experts stated that the nuclear accident should be classified at Level 5, perhaps even Level 6. One day later, the French nuclear safety authority ASN said that the Fukushima plant could be classified as a Level 6. as of 18 March, the French nuclear authority—and as of 15 March, the Finnish nuclear safety authority—estimated the accidents at Fukushima to be at Level 6 on the INES. On 24 March, a scientific consultant for noted anti-nuclear environmental group Greenpeace, working with data from the Austrian ZAMG and French IRSN, prepared an analysis in which he rated the total Fukushima accident at INES level 7.

Radiation releases during the initial hydrogen explosions.

The Asahi Shimbun newspaper reported on 26 March that the accident might warrant level 6, based on its calculations. The Wall Street Journal stated that Japan's NISA would make any decision on raising the level. INES level 6, or "serious accident", had only been applied to the Kyshtym disaster (Soviet Union, 1957), while the only level 7 was Chernobyl (Soviet Union, 1986). Previous level 5 accidents included the Windscale fire (United Kingdom, 1957); the Lucens reactor (Switzerland, 1969); Three Mile Island (United States, 1979); and the Goiânia accident (Brazil, 1987).

Assessing "seriousness" as partial or full meltdown at a civilian plant, The New York Times reported on 3 April that based on remote sensing, computer "simulations suggest that the number of serious accidents has suddenly doubled, with three of the reactors at the Fukushima Daiichi complex in some stage of meltdown." The Times counted three previous civilian meltdowns, from World Nuclear Association information: Three Mile Island; Saint-Laurent Nuclear Power Plant (France, 1980, INES level 4); and Chernobyl.

On 11 April, the Japanese Nuclear and Industrial Safety Agency (NISA) temporarily raised the disaster at Fukushima Daiichi to Level 7 on the INES scale, by considering the whole event and not considering each reactor as an individual event per se (rated between 3 and 5). This would make Fukushima the second Level 7 "major accident" in the history of the nuclear industry; having said that, radiation released as a result of the events at Fukushima was, as of 12 April, only approximately 10% of that released as a result of the accident at Chernobyl (1986), also rated as INES Level 7. The largest study, as of 21 October 2011, on Fukushima fallout concludes that Fukushima was "the largest radioactive noble gas release in history not related to nuclear bomb testing. The release is a factor of 2.5 higher than the Chernobyl 133Xe source term.", although the "Xenon-133 [main noble gas] does not pose serious health risks because it is not absorbed by the body or the environment." Arnold Gundersen said Fukushima has 20 times the potential to be released than Chernobyl. Hot spots are being found 60 to 70 kilometres away from the reactor (further away than they were found from Chernobyl), and the amount of radiation in many of them is the amount that caused areas to be declared no-man's-land for Chernobyl.

In off-the-record-interviews with Japanese newspapers like the Tokyo Shimbun, Naoto Kan, former premier minister of Japan, revealed that there were moments he believed the disaster could have surpassed Chernobyl, many times. At first Tepco denied that fuel-cores were melted, after all cooling functions were lost. Trade minister, Banri Kaieda, mentioned that Tepco seriously considered pulling away all staff-members from the plant and leaving it abandoned. Kan could not accept this: "Withdrawing from the plant was out of the question, If that had happened, Tokyo would be deserted by now. It was a critical moment for Japan's survival. It could have been a led to leaks of dozens of times more radiation than Chernobyl." That might have "compromised the very existence of the Japanese nation".

Tepco's president at that time, Masataka Shimizu, was never clear in his answers, and TEPCO failed to obey the orders to vent one of the overheating reactors, In an interview to the Asahi Shimbun newspaper. Kan revealed, that he went to the plant itself and inspected the plant from above in a helicopter because: "I felt I had to go there in person and speak to the people in charge or I would never have known what was going on." The American Government was seriously concerned about the Japanese response to the accident: Kan said: "We were not told straight out, but it was obvious that they questioned whether we were really taking this seriously."

Kan did defend his changed attitude to a non-nuclear energy policy with the following remarks: "If there is a risk of accidents that could make half the land mass of our country uninhabitable, then we cannot afford to take that risk."

Casualties

Evacuation flight departs Misawa.

Major news source reporting at least 2 TEPCO employees confirmed dead from "disaster conditions" following the earthquake. "The two workers, aged 21 and 24, sustained multiple external injuries and were believed to have died from blood loss, TEPCO said. Their bodies were decontaminated as radiation has been spewing from the plant for three weeks."

45 patients were reported dead after the evacuation of a hospital in Futaba due to lack of food, water and medical care as evacuation was delayed by three days.

The Associated Press reported that fourteen senior citizens died after being moved from their hospital which was in the Fukushima plant evacuation zone.

On 14 April 2011, it was reported that the oldest resident of Iitate, a 102-year-old, committed suicide rather than to leave following the announcement of his village's evacuation.

According to the Japanese Government, over 160,000 people in the general population were screened in March 2011 for radiation exposure and no case was found which affects health. Thirty workers conducting operations at the plant had exposure levels greater than 100 mSv.

In April 2011, the United States Department of Energy published projections of the radiation risks over the next year for people living in the neighborhood of the plant. Potential exposure could exceed 20 mSv/year (2 rems/year) in some areas up to 50 kilometers from the plant. That is the level at which relocation would be considered in the USA, and it is a level that could cause roughly one extra cancer case in 500 young adults. Natural radiation levels are higher in some part of the world than the projected level mentioned above, and about 4 people out of 10 can be expected to develop cancer without exposure to radiation. Further, the radiation exposure resulting from the accident for most people living in Fukushima is so small compared to background radiation that it may be impossible to find statistically significant evidence of increases in cancer.

As of September 2011, six workers at the Fukushima Daiichi site have exceeded lifetime legal limits for radiation and more than 300 have received significant radiation doses.

Workers on-site now wear full-body radiation protection gear, including masks and helmets covering their entire heads, but it means they have another enemy: heat. As of 19 July 2011, 33 cases of heat stroke had been recorded. In these harsh working conditions, two workers in their 60s have died from heart failure.

Two other worker deaths have been reported to date. By mid-August 2011, a man in his forties who had worked for a week on the Fukushima Daiichi site was hospitalized and died of acute leukemia not long after passing a physical test. It was not caused by occupational exposure, according to Tepco officials, as "it is medically impossible for symptoms of acute leukemia to manifest from occupational radiation exposure from a few weeks ago." In October 2011, another worker died in his 50s for an undisclosed reason which, according to TEPCO, "had nothing to do with exposure to radiation."

As of September 2011, there were no deaths or serious injuries due to direct radiation exposures. Cancer deaths due to accumulated radiation exposures cannot be ruled out, and according to one expert, might be in the order of 100 cases. A May 2012 United Nations committee report stated that none of the six Fukushima workers who had died since the tsunami had died from radiation exposure.

Frank N. von Hippel, a U.S. scientist, has estimated that "on the order of 1,000" people will die from cancer as a result of their exposure to radiation from the Fukushima Daiichi disaster, that is, an increase of 0.1% in the incidence of cancer, and much less than the approximately 20,000 people killed directly by the earthquake and tsunami. Because contaminated milk was "interdicted in Japan" the number of (mostly non-fatal) thyroid cancer cases will probably be less than 1% of similar cases at Chernobyl. Von Hippel added that "fear of ionizing radiation could have long-term psychological effects on a large portion of the population in the contaminated areas".

According to a 2012 Yomiuri Shimbun survey, 573 deaths have been certified as "disaster-related" by 13 municipalities affected by the Fukushima nuclear disaster. These municipalities are in the no-entry, emergency evacuation preparation or expanded evacuation zones around the crippled Fukushima nuclear plant. A disaster-related death certificate is issued when a death is not directly caused by a tragedy, but by "fatigue or the aggravation of a chronic disease due to the disaster".

According to a June 2012 Standford University study, the radiation released could cause 130 deaths from cancer (the lower bound for the estimater being 15 and the upper bound 1100) and 180 cancer cases (the lower bound being 24 and the upper bound 1800), mostly in Japan. Radiation exposure to workers at the plant was projected to result in 2 to 12 deaths. The radiation released was an order of magnitude lower than that released from Chernobyl, and some 80% of the radioactivity from Fukushima was deposited over the Pacific Ocean; preventive actions taken by the Japanese government may have substantially reduced the health impact of the radiation release. An additional approximately 600 deaths have been reported due to non-radiological causes such as mandatory evacuations. Evacuation procedures after the accident may have potentially reduced deaths from radiation by 3 to 245 cases, the best estimate being 28; even the upper bound projection of the lives saved from the evacuation is lower than the number of deaths already caused by the evacuation itself.

Plight of evacuees

A survey by the Iitate, Fukushima local government obtained responses from approximately 1,743 people who have evacuated from the village, which lies within the emergency evacuation zone around the crippled Fukushima Daiichi Plant. It shows that many residents are experiencing growing frustration and instability due to the nuclear crisis and an inability to return to the lives they were living before the disaster. Sixty percent of respondents stated that their health and the health of their families had deteriorated after evacuating, while 39.9% reported feeling more irritated compared to before the disaster.

Summarizing all responses to questions related to evacuees' current family status, one-third of all surveyed families live apart from their children, while 50.1% live away from other family members (including elderly parents) with whom they lived before the disaster. The survey also showed that 34.7% of the evacuees have suffered salary cuts of 50% or more since the outbreak of the nuclear disaster. A total of 36.8% reported a lack of sleep, while 17.9% reported smoking or drinking more than before they evacuated.

Experts on the ground in Japan agree that Mental health challenges are the most significant issue. Stress, such as that caused by dislocation, uncertainty and concern about unseen toxicants, often manifests in physical ailments, such as heart disease. So even if radiation risks are low, people are still concerned and worried. Behavioral changes can follow, including poor dietary choices, lack of exercise and sleep deprivation, all of which can have long-term negative health consequences. People who lost their homes, villages and family members, and even just those who survived the quake, will likely continue to face mental health challenges and the physical ailments that come with stress. Much of the damage was really the psychological stress of not knowing and of being relocated, according to U.C. Berkeley's McKone.

Investigations

File:Nuclear plants Japan in 2011.svg

Position of Japanese atomic plants and spreading of tsunami

On 7 June 2011 a government-appointed committee of 10 people convened to investigate the accident. The panel was headed by Yotaro Hatamura, professor emeritus of the University of Tokyo, and included Yukio Takasu, Michio Furukawa, the mayor of Kawamata, Fukushima, and author Kunio Yanagida, considered an expert on crisis management.

As part of the government inquiry, the House of Representatives of Japan's special science committee directed TEPCO to submit to them its manuals and procedures for dealing with reactor accidents. TEPCO responded by submitting manuals with most of the text blotted out. In response, the Nuclear and Industrial Safety Agency ordered TEPCO to resubmit the manuals by 28 September 2011 without hiding any of the content. TEPCO replied that it would comply with the order.

On 24 October NISA published a large portion of Tokyo Electric Power Company's procedural manuals for nuclear accidents. These were the manuals that the operator of the Fukushima Daiichi nuclear power plant earlier did send to the Lower House with most of the contents blacked out, saying that this information should be kept secret to protect its intellectual property rights, and that disclosure would offer information to possible terrorists. NISA ordered TEPCO to send the manuals without any redaction, as the law orders. 200 pages were released from the accident procedural manuals used for Fukushima Daiichi nuclear power plant. All their contents were published, only the names of individuals were left out.

From these documents could be concluded:

• TEPCO did not make sufficient preparations to cope with critical nuclear accidents.
• After the batteries and power supply boards were inundated on 11 March, almost all electricity sources were lost
• TEPCO did not envision such a power failure or any kind of prolonged power loss.
• TEPCO thought that in a serious incident, venting pressure in the reactor containment vessels or carrying out other safety procedures would still be possible, because emergency power sources would still be available.

The agency said, the decision to publish the manuals was taken, for transparency in the search what caused the nuclear accident in Fukushima and also to establish better safety measures for the future.

On 24 October 2011 the first meeting was held by a group of 6 nuclear energy specialists invited by NISA to discuss the lessons to be learned from the accidents in Fukushima. Their first remarks were:

• Japanese nuclear power plants should have multiple power sources
• plants should be able to maintain electricity during an earthquake or other emergencies
• TEPCO should examine why the equipment failed to work and should take appropriate actions to prevent such failures in the future

According to professor Tadashi Narabayashi of the Hokkaido University Graduate School, plant operators should arrange emergency power supplies with other utilities. These discussion should be completed in March 2012, to be able to implement their conclusions into the new safety-regulations by the new nuclear safety agency to be launched in April 2012.

The Investigation Committee on the Accident at the Fukushima Nuclear Power Stations of Tokyo Electric Power Company was formed 7 June 2011 by the Japanese government as an independent body to investigate the Fukushima Daiichi disaster. The Investigation Committee issued an interim report in December 2011, and is expected to issue its final report summer, 2012. The interim report was "a scathing assessment of the response to the Fukushima disaster", in which the investigative panel "blamed the central government and the Tokyo Electric Power Co., saying both seemed incapable of making decisions to stem radiation leaks as the situation at the coastal plant worsened in the days and weeks after the disaster".

In February 2012, an independent investigation into the accident by the Rebuild Japan Initiative Foundation said that "In the darkest moments of last year's nuclear accident, Japanese leaders did not know the actual extent of damage at the plant and secretly considered the possibility of evacuating Tokyo, even as they tried to play down the risks in public". The government was preparing for the possibility of having to evacuate Tokyo while assuring its millions of residents that everything was under control.

Officials revealed in interviews that they were grappling the possibility of a "demonic chain reaction": If Fukushima collapsed and released enough radiation, it was possible that other nearby nuclear power plants would have to be abandoned and could also collapse, thereby necessitating the evacuation of one of the world's largest cities.

A 2012 report in The Economist said that the response to Fukushima has, so far, been inadequate, as many questions remain. One of the more worrying is how much damage the earthquake did to the reactors:

It is claimed that they weathered the quake, but some experts, such as Masashi Goto, a retired nuclear engineer, argue that there is evidence of significant damage that speeded up the subsequent meltdown. Analysis of the spread of fallout suggests that the first releases came very soon after the tsunami hit, if not before. With quakes a more constant threat than monster tsunamis, these are the sort of lessons that Japan’s "nuclear village" needs to learn.

Oregon's United States Senator Ron Wyden toured the plant and issued a statement that the situation was "worse than reported." He sent a letter to Japanese Ambassador Ichiro Fujisaki urging Japan to seek international help to relocate spent fuel rods stored in unsound structures and prevent leakage of dangerous nuclear material.

TEPCO released its final internal investigation report on 20 June 2012. In the report, TEPCO complained that top politicians, including the prime minister, interfered with recovery efforts during the initial stages of the disaster by making specific requests that were out of touch with what was actually taking place at the plant. TEPCO concluded that the direct cause of the accident was the tsunami which knocked out the reactors' cooling system. TEPCO also admitted that it was at fault in not being prepared for the situation, but said that its workers did the best they could "amid unprecedented circumstances."

On July 5, 2012, the National Diet of Japan Fukushima Nuclear Accident Independent Investigation Commission released an executive summary report of The Fukushima Nuclear Accident. The report "blames Japanese culture for the fundamental causes of the disaster." The panel is due to deliver its final report at the end of July.

Insurance

According to Munich Re, a major reinsurer, the private insurance industry will not be significantly affected by the accidents at the Fukushima nuclear power plant. Swiss Re similarly states "Coverage for nuclear facilities in Japan excludes earthquake shock, fire following earthquake and tsunami, for both physical damage and liability. Swiss Re believes that the incident at the Fukushima nuclear power plant is unlikely to result in a significant direct loss for the property & casualty insurance industry."

Radiation releases

Main article: Radiation effects from Fukushima Daiichi nuclear disaster

Map of contaminated areas around the plant (22 March – 3 April).

Fukushima dose rate comparison to other incidents and standards, with graph of recorded radiation levels and specific accident events from 11 to 30 March.

Radioactive material has been released from the Fukushima containment vessels as the result of deliberate venting to reduce gaseous pressure, deliberate discharge of coolant water into the sea, and accidental or uncontrolled events. Concerns about the possibility of a large scale radiation leak resulted in 20 km exclusion zone being set up around the power plant and people within the 20–30 km zone being advised to stay indoors. Later, the UK, France and some other countries told their nationals to consider leaving Tokyo, in response to fears of spreading radioactive contamination. The Fukushima accident has led to trace amounts of radiation, including iodine-131, caesium-134 and caesium-137, being observed around the world (New York State, Alaska, Hawaii, Oregon, California, Montreal, and Austria). Large amounts of radioactive isotopes have also been released into the Pacific Ocean.

A monitoring system designed to detect nuclear explosions, operated by the Preparatory Commission for the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO), tracked the dispersion of radioactivity from the crippled nuclear reactor on a global scale. Radioactive isotopes originating from Fukushima were picked up by over 40 CTBTO radionuclide monitoring stations. The CTBTO makes its monitoring data and analysis results available to all its 183 Member States. Around 1,200 scientific and academic institutions in 120 countries currently make use of this offer.

On 12 March, radioactive releases first reached a CTBTO monitoring station in Takasaki, Japan, around 200 km away from the troubled power plant. The dispersion of the radioactive isotopes could then be followed to eastern Russia on 14 March and to the west coast of the United States two days later. By day 15, traces of radioactivity were detectable all across the northern hemisphere. Within one month, radioactive particles were also picked up by CTBTO stations in the southern hemisphere, located for example in Australia, Fiji, Malaysia and Papua New Guinea.

According to one expert, the release of radioactivity is about one-tenth that from the Chernobyl disaster and the contaminated area is also about one-tenth that that of Chernobyl. A March 2012 report by the Ministry of Education, Culture, Sports, Science and Technology agreed that radioactive debris from the damaged reactors had dispersed about one-eighth to one-tenth of the distance as those in the Chernobyl disaster. But according to a study conducted by Norwegian Institute for Air Research, the release of caesium-137 was about 40 percent of the total from Chernobyl.

In March 2011, Japanese officials announced that "radioactive iodine-131 exceeding safety limits for infants had been detected at 18 water-purification plants in Tokyo and five other prefectures". As of July 2011, the Japanese government has been unable to control the spread of radioactive material into the nation's food. Radioactive material has been detected in a range of produce, including spinach, tea leaves, milk, fish and beef, up to 200 miles from the nuclear plant. Inside the 12-mile evacuation zone around the plant, all farming has been abandoned.

As of August 2011, the crippled Fukushima nuclear plant is still leaking low levels of radiation and areas surrounding it could remain uninhabitable for decades due to high radiation. It could take "more than 20 years before residents could safely return to areas with current radiation readings of 200 millisieverts per year, and a decade for areas at 100 millisieverts per year".

On 24 August 2011, the Nuclear Safety Commission (NSC) of Japan published the results of the recalculation of the total amount of radioactive materials released into the air during the accident at the Fukushima Daiichi Nuclear Power Station. The total amounts released between 11 March and 5 April were revised downwards to 1.3 × 10¹⁷ Bq for iodine-131 and 1.1 × 10¹⁶ Bq for caesium-137, which is about 11% of Chernobyl emissions. Earlier estimations were 1.5 × 10¹⁷ Bq and 1.2 × 10¹⁶ Bq.

On 8 September 2011 a group of Japanese scientists working for the Japan Atomic Energy Agency, the Kyoto University and other institutes, published the results of a recalculation of the total amount of radioactive material released into the ocean: between late March through April they found a total of 15,000 TBq for the combined amount of iodine-131 and caesium-137. This was more than triple the figure of 4,720 TBq estimated by the plant-owner. TEPCO made only a calculation about the releases from the plant in April and May into the sea. The new calculations were needed because a large portion of the airborne radioactive substances would enter the seawater when it came down as rain.

In the first half of September 2011 the amount of radioactive substances released from the plant was about 200 million becquerels per hour, according to TEPCO, this was approximately one four-millionth of the level of the initial stages of the accident in March. Traces of iodine-131 are still detected in several Japanese prefectures in the months of November and December 2011.

According to a report published in October 2011 by the French Institute for Radiological Protection and Nuclear Safety, between 21 March and mid-July around 2.7 × 10¹⁶ Bq of caesium-137 entered the ocean, about 82 percent having flowed into the sea before 8 April. This emission of radioactivity into the sea represents the most important individual emissions of artificial radioactivity into the sea ever observed. The Fukushima coast has one of the world's strongest currents and this transported the contaminated waters far into the Pacific Ocean, causing a high dispersion of the radioactive elements. The results of measurements of both the seawater and the coastal sediments lead to suppose that the consequences of the accident, for what concerns radioactivity, will be minor for marine life as of late 2011 (weak concentration of radioactivity in the water and limited accumulation in sediments). On the other hand, significant pollution of sea water along the coast near the nuclear plant might persist, because of the continuing arrival of radioactive material transported towards the sea by surface water running over contaminated soil. Further, some coastal areas might have less favorable dilution or sedimentation characteristics than those observed so far. Finally, the possible presence of other persistent radioactive substances, such as strontium-90 or plutonium, has not been sufficiently studied. Recent measurements show persistent contamination of some marine species (mostly fish) caught along the coast of Fukushima district. Organisms that filter water and fish at the top of the food chain are, over time, the most sensitive to caesium pollution. It is thus justified to maintain surveillance of marine life that is fished in the coastal waters off Fukushima.

As of March 2012, there had been no reported cases of Fukushima residents suffering ailments related to radiation exposure. Experts cautioned that insufficient data was available so far to make conclusions on the impact on residents' health. Nevertheless, Michiaki Kai, professor of radiation protection at Oita University of Nursing and Health Sciences, stated, "If the current radiation dose estimates are correct, (cancer-related deaths) likely won't increase."

On 24 May 2012, TEPCO released their estimate of radiation releases due to the Fukushima Daiichi Nuclear Disaster. An estimated 538,100 terabecquerels (TBq) of iodine-131, caesium-134 and caesium-137 was released. 520,000 TBq was released into the atmosphere between March 12 to March 31, 2011 and 18,100 TBq into the ocean from March 26 to September 30, 2011. A total of 511,000 TBq of iodine-131 was released into both the atmosphere and the ocean, 13,500 TBq of caesium-134 and 13,600 TBq of caesium-137.

In May 2012, TEPCO reported that at least 900 PBq had been released "into the atmosphere in March last year [2011] alone".

Community reaction

Reaction in Japan and evacuation measures

Main article: Japanese reaction to Fukushima Daiichi nuclear disaster

Japan towns, villages, and cities in and around the Daiichi nuclear plant exclusion zone. The 20 km and 30 km areas had evacuation and sheltering orders, and additional administrative districts that had an evacuation order are highlighted.

A nuclear emergency was declared by the government of Japan on 11 March 2011. Later Prime Minister Naoto Kan issued instructions that people within a 20 km (12 mi) zone around the Fukushima Daiichi nuclear plant must leave, and urged that those living between 20 km and 30 km from the site to stay indoors. The latter groups were also urged to evacuate on 25 March.

Japanese authorities have admitted that lax standards and poor oversight contributed to the nuclear disaster. They have come under fire for their handling of the emergency, and have engaged in a pattern of withholding damaging information and denying facts of the accident. Authorities apparently wanted to "limit the size of costly and disruptive evacuations in land-scarce Japan and to avoid public questioning of the politically powerful nuclear industry". There has been public anger about an "official campaign to play down the scope of the accident and the potential health risks". The accident is the second biggest nuclear accident after the Chernobyl disaster, but more complex as all reactors are involved.

Once a proponent of building more reactors, Prime Minister Naoto Kan took an increasingly anti-nuclear stance in the months following the Fukushima disaster. In May, he ordered the aging Hamaoka Nuclear Power Plant be closed over earthquake and tsunami fears, and said he would freeze plans to build new reactors. In July 2011, Mr. Kan said that "Japan should reduce and eventually eliminate its dependence on nuclear energy ... saying that the Fukushima accident had demonstrated the dangers of the technology".

On 22 August 2011 a spokesman of the Japanese Government mentioned the possibility, that some areas of the evacuation zone around the nuclear plant for "could stay for some decades a forbidden zone". According to the Japanese newspaper Yomiuri Shimbun the Japanese government was planning to buy some properties from civilians to store radioactive waste and materials that had become radioactive after the accidents. Chiaki Takahashi, Japan's foreign minister, criticised foreign medias reports over accidents in Fukushima Daichii as overdone and excessive. But Takahashi added that "he can understand the concerns of foreign countries over recent developments at the nuclear plant, including the radioactive contamination of seawater".

Due to frustration with Tokyo Electric Power Company (TEPCO) and the Japanese government "providing differing, confusing, and at times contradictory, information on critical health issues" a citizen's group called "Safecast" has been recording detailed radiation level data in Japan. The Japanese government "does not consider non-government readings to be authentic". The group uses off-the-shelf Geiger counter equipment. It is important to note that a simple geiger counter is a contamination meter and not a dose rate meter, as the response differs so much between different radioisotopes it is not possible to use a simple GM tube for dose rate measureemnts when more than one radioisotope is present. A thin metal shield is needed around a GM tube to provide energy compensation to enable it to be used for dose rate measurements. For measurements of dose rates due to gamma emitters either an ionization chamber, a gamma spectrometer or an energy compensated GM tube should be used. Members of the Air Monitoring station facility at the Department of Nuclear Engineering at the University of Berkeley, California have been doing extensive tests of environmental samples in Northern California.

International reaction

Main article: International reaction to the Fukushima Daiichi nuclear disaster

The international reaction to the 2011 Fukushima Daiichi nuclear disaster has been diverse and widespread. Many inter-governmental agencies are responding to the Fukushima Daiichi nuclear disaster, often on an ad hoc basis. Responders include International Atomic Energy Agency, World Meteorological Organization and the Preparatory Commission for the Comprehensive Nuclear Test Ban Treaty Organization, which has radiation detection equipment deployed around the world.

Many countries have advised their nationals to leave Tokyo, citing the risk associated with the nuclear plants' ongoing accident. International experts have said that a workforce in the hundreds or even thousands would take years or decades to clean up the area. Stock prices of many energy companies reliant on nuclear sources have dropped.

There has been a significant re-evaluation of existing nuclear power programs in many countries. One poll found that what had been growing acceptance of nuclear power in the United States was eroded sharply following the 2011 Japanese nuclear accidents, with 43% approving and 50% disapproving of building new plants. Worldwide, a study by UBS, reported on 12 April 2011, suggests that around 30 nuclear plants may be closed as a result of Fukushima, with those located in seismic zones or close to national boundaries being the most likely to shut. Events at Fukushima "cast doubt on the idea that even an advanced economy can master nuclear safety". Increased anti-nuclear sentiment has been evident in India, Italy, Germany, Spain, Switzerland, Taiwan, and the United States.

Much of the help and decontamination work could be done by AREVA France with boron acid, shutting down one reactor, protection suits, measurement equipment, generators, filters; by more than 1000 men with own first-hand help and information offered.

Decontaminated water
Week from	Tons	Plant- capacity utilisation
29 June	6380	76 %
6 July	6130	73 %
13 July	4510	54 %
20 July	4870	58 %
27 July	6190	74 %
3 August	6720	80 %
10 August	7420	88 %

Reactor stabilization and cleanup operations

Main article: Fukushima disaster cleanup

The multiple nuclear reactor units involved in the Fukushima Daiichi nuclear disaster were close to one another and this proximity triggered the parallel, chain-reaction accidents that led to hydrogen explosions blowing the roofs off reactor buildings and water draining from open-air spent fuel pools. This situation was potentially more dangerous than the loss of reactor cooling itself. Because of the proximity of the reactors, plant workers were put in the position of trying to cope simultaneously with core meltdowns at three reactors and exposed fuel pools at three units.

On 21 December 2011, the Japanese government released a roadmap for the cleanup activities, which predicted that the full cleanup will take 40 years. On 10 April 2011, Tokyo Electric Power Company (TEPCO) began using remote-controlled, unmanned heavy equipment to remove debris from around nuclear reactors 1–4. TEPCO announced on 17 April that it expected to have the automated cooling systems restored in the damaged reactors in about three months and have the reactors put into cold shutdown status in six months. TEPCO planned to largely empty the basements of the turbine and reactor buildings of units 1–3 of contaminated water by the end of 2011 to allow workers access to the crucial basement areas of both the turbine and reactor buildings.

When the monsoon season began in June 2011, a light fabric cover was used to protect the damaged reactor buildings from storms and heavy rainfall. On 1 August 2011, TEPCO said that very high radiation levels were found outside the building of reactor 1 and 2 from an exhaust-pipe. On 16 August, TEPCO announced the installation of devices in the spent fuel pools of reactor 2, 3 and 4, which used special membranes and electricity to desalinate the water. These pools were cooled with seawater for some time, and TEPCO feared the salt would corrode stainless steel pipes and the pool walls. Burying the reactors in sand and concrete is considered to be a last resort.

In October 2011, Japanese Prime Minister Yoshihiko Noda said the government will spend at least 1 trillion yen ($13 billion) to clean up vast areas contaminated by radiation from the Fukuahima nuclear disaster. Japan "faces the prospect of removing and disposing 29 million cubic meters of soil from a sprawling area in Fukushima, located 240 kilometers (150 miles) northeast of Tokyo, and four nearby prefectures".

Energy policy implications

Anti-nuclear power plant rally on 19 September 2011 at the Meiji Shrine complex in Tokyo.

See also: Nuclear power debate and Renewable energy commercialization

By March 2012, one year after the disaster, all but two of Japan's nuclear reactors had been shut down; some were damaged by the quake and tsunami. Authority to restart the others after scheduled maintenance throughout the year was given to local governments, and in all cases local opposition prevented restarting. The loss of 30% of the country's generating capacity has led to much greater reliance on liquified natural gas and coal. Unusual conservation measures have also been necessary. In the immediate aftermath, nine prefectures served by TEPCO suffered power rationing. The government asked major companies to reduce power consumption by 15%, and some shifted their weekends to weekdays to even out power demand.

According to The Japan Times, the Fukushima nuclear disaster changed the national debate over energy policy almost overnight. "By shattering the government's long-pitched safety myth about nuclear power, the crisis dramatically raised public awareness about energy use and sparked strong anti-nuclear sentiment". A June 2011 Asahi Shimbun poll of 1,980 respondents found that 74% answered "yes" to whether Japan should gradually decommission all 54 reactors and become nuclear free.

An energy white paper, approved by the Japanese Cabinet in October 2011, says "public confidence in safety of nuclear power was greatly damaged" by the Fukushima disaster, and calls for a reduction in the nation's reliance on nuclear power. It also omits a section on nuclear power expansion that was in last year's policy review.

Environmental activists at a 2011 United Nations meeting in Bangkok used the Fukushima disaster as an example to promote accelerated use of renewable energy.

Physicist Amory Lovins has said: "Japan is poor in fuels, but is the richest of all major industrial countries in renewable energy that can meet the entire long-term energy needs of an energy-efficient Japan, at lower cost and risk than current plans. Japanese industry can do it faster than anyone — if Japanese policymakers acknowledge and allow it". Benjamin K. Sovacool has said that, with the benefit of hindsight, the Fukushima disaster was entirely avoidable in that Japan could have chosen to exploit the country's extensive renewable energy base. Japan has a total of "324 GW of achievable potential in the form of onshore and offshore wind turbines (222 GW), geothermal power plants (70  GW), additional hydroelectric capacity (26.5 GW), solar energy (4.8 GW) and agricultural residue (1.1 GW)."

One result of the Fukushima Daiichi nuclear disaster could be renewed public support for the commercialization of renewable energy technologies. In August 2011, the Japanese Government passed a bill to subsidize electricity from renewable energy sources. The legislation will become effective on 1 July 2012, and require utilities to buy electricity generated by renewable sources including solar power, wind power and geothermal energy at above-market rates.

In September 2011, Mycle Schneider said that the Fukushima disaster can be understood as a unique chance "to get it right" on energy policy. "Germany – with its nuclear phase-out decision based on a highly successful renewable energy program – and Japan – having suffered a painful shock but possessing unique technical capacities and societal discipline – can be at the forefront of an authentic paradigm shift toward a truly sustainable, low-carbon and nuclear-free energy policy".

As of September 2011, Japan plans to build a pilot floating wind farm, with six 2-megawatt turbines, off the Fukushima coast. After the evaluation phase is complete in 2016, "Japan plans to build as many as 80 floating wind turbines off Fukushima by 2020."

In 2012, Naoto Kan said the Fukushima disaster made it clear to him that "Japan needs to dramatically reduce its dependence on nuclear power, which supplied 30% of its electricity before the crisis, and has turned him into a believer of renewable energy".

Sales of solar cells in Japan rose 30.7% to 1,296 megawatts in 2011, helped by a government scheme to promote renewable energy. Canadian Solar plans to build a factory in Japan and is currently in negotiations with local governments in Fukushima and Miyagi prefectures. The facility is expected to have a capacity of 150 megawatts of solar panels a year, could go online as soon as 2013.

Fukushima Nuclear Accident Independent Investigation Commission

Main article: National Diet of Japan Fukushima Nuclear Accident Independent Investigation Commission

The Fukushima Nuclear Accident Independent Investigation Commission (NAIIC) is the first independent investigation commission by the National Diet in the 66-year history of Japan’s constitutional government. NAICC was established on December 8, 2011 with the mission to investigate the direct and indirect causes of the Fukushima nuclear incident. NAICC submitted its report to both houses on July 5, 2012. The 10-member commission compiled its report based on more than 1,167 interviews and 900 hours of hearings.

It was a six-month independent investigation, the first of its kind with wide-ranging subpoena powers in Japan's constitutional history, which held public hearings with former Prime Minister Naoto Kan and Tokyo Electric Power Co's former president Masataka Shimizu, who gave conflicting accounts of the disaster response. The commission chairman, Kiyoshi Kurokawa, declared with respect to the Fukushima nuclear incident: “It was a profoundly man-made disaster — that could and should have been foreseen and prevented.” “Across the board, the commission found ignorance and arrogance unforgivable for anyone or any organization that deals with nuclear power,” the NAIIC report said. The report outlines errors and willful negligence at the plant before the 2011 Tōhoku earthquake and tsunami on March 11, 2011 and a flawed response in the hours, days and weeks that followed. It also offers recommendations and encourages Japan's parliament to "thoroughly debate and deliberate" the suggestions.

See also

• 2011 Japanese nuclear accidents
• List of civilian nuclear accidents
• Lists of nuclear disasters and radioactive incidents
• Timeline of the Fukushima Daiichi nuclear disaster
• Comparison of Fukushima and Chernobyl nuclear accidents

External links

• The Fukushima Nuclear Accident Independent Investigation Commission Report website in English
• Executive summary of the Fukushima Nuclear Accident Independent Investigation Commission Report
• Fukushima report: Key points in nuclear disaster report - An outline of key quotes, findings and recommendations from the 88-page executive summary of the Nuclear Accident Independent Investigation Commission's report, as provided by the BBC, 5 July 2012
• Inside Japan's Nuclear Meltdown – PBS Frontline documentation
• Fukushima Aftermath – Wikibooks, open books for an open world
• Webcam Fukushima nuclear power plant I, Unit 1 through Unit 4
• Investigation Committee on the accidents at the Fukushima Nuclear Power Station of Tokyo Electric Power Company
• Schematic drawing of Unit 1 reactor building
• TEPCO News Releases, Tokyo Electric Power Company
• NISA Information update, Nuclear and Industrial Safety Agency, the nuclear safety authority of Japan
• JAIF Information update, Japan Atomic International Forum
• JAEA Information update, Japan Atomic Energy Agency
• IAEA Update on Japan Earthquake, International Atomic Energy Agency
• The Future of Nuclear Energy in Japan, Q&A on the earthquake recovery and impact of the nuclear crisis
• Navigating Fukushima: Lessons from Chernobyl, potential radiation effects, and other health impacts
• Nature Journal – Specials: Japan earthquake and nuclear crisis
• TerraFly Timeline Aerial Imagery of Fukushima Nuclear Reactor after 2011 Tsunami and Earthquake
• Documentary photographs: residential damage within "No Go" Zone
• "Lessons From Fukushima". On Point. 5 March 2012
• Why Fukushima Was Preventable, Carnegie Paper. 6 March 2012
• In graphics: Fukushima nuclear alert, as provided by the BBC, 9 July 2012