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Revision as of 22:13, 19 August 2011

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.

Reactors 5 and 6 were also shut down 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.

Unit 4 Reactor

A Fukushima spent fuel pool in the year 1999 (the view is obstructed by the reflection of the loading crane on the water surface).

At the time of the earthquake, Unit 4 had been shut down for shroud replacement and refueling since 29 November 2010.[1][2] All 548 fuel rods had been transferred in December 2010 from the reactor to the spent fuel pool on an upper floor of the reactor building[3] where they were held in racks containing boron to damp down any nuclear reaction.[4] The pool is used to store rods for some time after removal from the reactor and now contains 1,479 rods.[5] Recently active fuel rods produce more decay heat than older ones.[6] At 04:00 JST on Monday 14 March, water in the pool had reached a temperature of 84 °C compared to a normal value of 40–50 °C.[4] The IAEA was advised that the temperature value remained 84 °C at 19:00 JST on 15 March, but as of 18 March, no further information was reported.[7][8] On 11 April, a fire broke out at Unit 4.

Explosion

At approximately 06:00 JST on 15 March, an explosion damaged the 4th floor rooftop area of the Unit 4 reactor as well as part of the adjacent Unit 3.[9][10] The explosion is thought to be caused by the ignition of hydrogen that had accumulated near the spent fuel pond, the hydrogen was initially thought to have come from the stored fuel rods, but later, TEPCO believed the hydrogen came from Unit 3.[11] Later reports from the US Nuclear Regulatory Commission speculated that fuel could have been ejected from the Unit 4 spent fuel pond during this explosion.[12] Later on the morning of 15 March, at 09:40, the Unit 4 spent fuel pool caught fire, likely releasing radioactive contamination from the fuel stored there.[13][14] TEPCO said workers extinguished the fire by 12:00.[15][16] As radiation levels rose, some of the employees still at the plant were evacuated.[17] On the morning of 15 March, Secretary Edano announced that according to the TEPCO, radiation dose equivalent rates measured from the Unit 4 reached 100 mSv/h.[18][19] Edano said there was no continued release of "high radiation".[20]

Japan's nuclear safety agency NISA reported two holes, each 8 meters square, or 64 m² (690 sq ft), in a wall of the outer building of Unit 4 after the explosion.[21] At 17:48 it was reported that water in the spent fuel pool might be boiling.[22][23] By 21:13 on 15 March, radiation inside the Unit 4 control room prevented workers from staying there permanently.[24] Seventy staff remained at the plant, while 800 had been evacuated.[25] By 22:30, TEPCO was reportedly unable to pour water into the spent fuel pool.[4] By 22:50, the company was considering using helicopters to drop water,[25][26] but this was postponed because of concerns over safety and effectiveness, and the use of high-pressure fire hoses was considered instead.[27]

A fire was discovered at 05:45 JST on 16 March in the northwest corner of the reactor building by a worker taking batteries to the central control room of Unit 4.[28][29] This was reported to the authorities, but on further inspection at 06:15 no fire was found. Other reports stated that the fire was under control.[30] At 11:57, TEPCO released a photograph showing "a large portion of the building's outer wall has collapsed".[31] Technicians considered spraying boric acid on the building from a helicopter.[32][33]

Spent fuel pool

On 16 March, the chairman of United States Nuclear Regulatory Commission (NRC), Gregory Jaczko, said in Congressional testimony that the NRC believed all of the water in the spent fuel pool had boiled dry.[34][35] Japanese nuclear authorities and TEPCO contradicted this report, but later in the day Jaczko stood by his claim saying it had been confirmed by sources in Japan.[36] At 13:00 TEPCO claimed that helicopter observation indicated that the pool had not boiled off.[37] The French Institut de Radioprotection et de Sûreté Nucléaire (IRSN) agreed, stating that helicopter crews diverted planned water dumps to Unit 3 on the basis of their visual inspection of Unit 4.[38]

At approximately 14:30 on 16 March, TEPCO announced that the storage pool, located outside the Unit 4 containment area,[39] might be boiling. Around 20:00 JST it was then planned to use a police water cannon to spray water on Unit 4.[40]

On 18 March, it was reported that water sprayed into the spent fuel pool was disappearing faster than evaporation could explain, suggesting leakage.[41][42] SDF military trucks sprayed water onto the building to try to replenish the pool on 20 March.[43] On 22 March, the Australian military flew in Bechtel-owned robotic equipment for remote spraying and viewing of the pool. The Australian reported this would give the first clear view of the pool in the "most dangerous" of the reactor buildings.[44]

The IAEA reported, "From 22 March to 25 March 130 to 150 tonnes of seawater were poured into the spent fuel pool each day using a concrete pump equipped with a long articulated arm. Seawater was also poured in through spent fuel cooling system from 21:05 UTC 24 March to 01:20 25 March. White smoke was still being observed coming from the reactor building as of 23:00 UTC 25 March."[45] On 29 March, the seawater was changed to fresh water.[46]

Cumulative amount of cooling water, 20 March to 28 May

Analysis of spent fuel pool water collected on 12 April suggests that while some of the 1535 fuel assemblies stored there may have been damaged, the majority of the stored fuel assemblies are intact based on measured radiation levels.[47] TEPCO further stated that "the fuel rods in the Unit 4 pool had released caesium-134 and −137 in the process of being damaged", and that TEPCO would "need to continue monitoring it".[48] On 13 April, TEPCO reported that the temperature of the spent fuel pool had increased to 90 °C, and that the radiation level 6 meters above the pool had reached 84 mSv/h.[48] The spike was later attributed to a failure to properly keep the SFP covered in water.[49] As of 25 April, TEPCO was still pumping between 70 and 210 tons of water into the pool, varying the amounts depending on the temperature in the pool. TEPCO also reported that it was attempting to minimize the amount of water added to the pool for fear "the weight of the water could weaken the reactor building".[50] On 28 April, TEPCO announced it believed that water was not leaking from the pool but only evaporating. TEPCO based its belief on calculations that the heat generated by the spent fuel stored in the pool would be expected to evaporate 140 to 210 tons of water daily, in line with the amount of replacement water it adds.[51] On 9 May, TEPCO began work to install a supporting structure for the Unit 4 spent fuel pool, due to the concerns that explosions could have weakened the structure.[52]

On 11 June, it was discovered, that the waterlevel in the spent fuel pool was only one third of normal, and only part of the fuel rods were covered with water. This was probably the cause of the high radiation-levels measured. This pool has also been used to dump equipment. Sunday 19 June the pool was refilled, to minimize the radiation and making it possible to work again at this place.[53] On 21 June the first stage of strengthening at the second floor in the building under the pool was finished: 32 steel columns 8 meters long with a weight of 40 tons each were placed at the second floor. The plan was to cover the columns with concrete. At this moment the temperature in the pool was too high at 86 grade Celsius. The work went on with the installation of a new closed circuit cooling system like already was done at Reactor 2.[54]

Possibility of criticality in the spent fuel pool

Boric acid is airlifted to Fukushima for addition to cooling water.

Boron injection

Officials have considered insertion or targeted aerial dropping of boric acid, boronated plastic beads or boron carbide pellets into the spent fuel pools to absorb neutrons.[32][55] France flew 95 tonnes of boron to Japan on 17 March 2011[56] and the US has provided 9 tons.[57] Neutron absorbing boric acid has been injected into the reactor cores, but is unclear if boron was included with the spraying of spent fuel pools (SFP)s.[58]

Visual inspection

Visual inspection of the spent fuel rod pool on Reactor 4 on 30 April has shown that there is no significant visible damage to the fuel rods in the pool.[59]

Units 5 and 6

Unit 5&6 connection to the 500 kV Futaba Line (双葉線)

Both reactors were offline at the time the earthquake struck (Reactor 5 had been shut down on 3 January 2011 and Reactor 6 on 14 August 2010), although they were still fueled, unlike Reactor 4 where the fuel rods had been removed prior to the earthquake.[5]

Government spokesman Edano stated on 15 March that Reactors 5 and 6 were being closely monitored, as cooling processes were not functioning well.[20][60] At 09:16 JST, the removal of roof panels from reactor buildings 5 and 6 was being considered in order to allow any hydrogen build-up to escape.[7] At 21:00 on 15 March, water levels in Unit 5 were reported to be 2 m above fuel rods, but had fallen 40 cm in 5 hours.[7] Published water temperatures on 18 March showed 182 °C inside Reactor 5 and 161 °C in Reactor 6.[61]

On 17 March, Unit 6 was reported to have operational diesel-generated power and this was to be used to power pumps in Unit 5 to run the Make-up Water Condensate System (MUWC) to supply more water.[7] Preparations were made to inject water into the reactor pressure vessel once external power could be restored to the plant, as water levels in the reactors were considered to be declining.[7] NISA reported that connections from the grid to all units was complete 20 March[62] through new cables and transformers.

Information provided to the IAEA indicated that storage pool temperatures at both Units 5 and 6 remained steady around 60–68 °C between 19:00 JST 14 March and 21:00 JST 18 March, though rising slowly.[7] On 18 March reactor water levels remained around 2 m above the top of fuel rods.[8][63] It was confirmed that panels had been removed from the roofs of Units 5 and 6 to allow any hydrogen gas to escape.[7] At 04:22 on 19 March, the second unit of emergency generator A for Unit 6 was restarted which allowed operation of pump C of the residual heat removal system (RHR) in Unit 5 to cool the spent fuel storage pool.[64] Later in the day, Pump B in Unit 6 was also restarted to allow cooling of the spent fuel pool there.[7][65] Temperature at Unit 5 pool decreased to 48 °C on 19 March 18:00 JST,[66] and 37 °C on 20 March when Unit 6 pool temperature had fallen to 41 °C.[43] On 20 March, NISA announced that both reactors had been returned to a condition of cold shutdown.[67][68]

On 23 March, it was reported that the cooling pump at Reactor 5 stopped working when it was transferred from backup power to the grid supply.[69][70] This was repaired and the cooling restarted approximately 24 hours later. RHR cooling in Unit 6 was switched to the permanent power supply on 25 March.[71]

On 28 May, a cooling pump for Reactor 5 stopped, which was discovered by TEPCO at 21 local time. At that time, the temperature in the reactor was 68 °C, and in the spent fuel pool 41 °C.[72] At 11 in the morning the following day the temperatures had risen to 92.2 °C and 45.7 °C.[73] Cooling was restored at 12:49.[74]

On June 24 TEPCO started the Fuel Pool Cooling and Clean-up System for Unit 5. The fuel pool and reactor in Unit 5 are now being cooled simultaneously. In Unit 6, TEPCO continues to cool the reactor and fuel pool alternately.[75]

On 3 July in the morning, a crack in a polyvinyl hose - 30 centimeters long and 7 centimeters wide - was found around the outlet of a temporary pump for seawater. At 10AM the pump was shut off, the cooling system of reactor 5 was halted 15 minutes later. The polyvinyl pipe was replaced, and the coolingsystem was restarted at 13:45PM. The temperature in the reactor rose some 5 degrees Celsius to 47.7 degrees Celsius. Constant tidal movements were probably the cause of the rupture of the pleated flexible tube.[76] On 4 July TEPCO announced that all tubes and hoses used in the heat removal systems will be checked on possible weaknesses. Tubes like these, that were used to transfer highly radioactive water, did not require replacement because they were not bent at extreme angles.[77] At 10:16 AM on 15 July, TEPCO completed repairs to Pump B of the original Residual Heat Removal Seawater System of Unit 5. The original seawater system was severely damaged in the tsunami requiring the temporary seawater pumps and hoses to be installed which have proven to be leak prone. At 2:45 PM on 15 July, TEPCO began pumping seawater through the residual heat removal heat exchanger with the newly repaired pump.[78]

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