Radiation effects from the Fukushima Daiichi nuclear disaster

The 2011 Fukushima Daiichi nuclear disaster, the worst nuclear accident in 25 years, displaced 50,000 households after radiation leaked into the air, soil and sea. Radiation checks led to bans on some shipments of vegetables and fish.

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

The radiation effects from the Fukushima Daiichi nuclear disaster are the results of release of radioactive isotopes from the crippled Fukushima Daiichi Nuclear Power Plant after the 2011 Tōhoku earthquake and tsunami. The total amount of iodine-131 and caesium-137 released into the atmosphere has been estimated to exceed 10% of the emissions from the Chernobyl disaster. Large amounts of radioactive isotopes have also been released into the Pacific Ocean. The accidents were rated at level 7 rating on the International Nuclear Event Scale.

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”.

Summarised daily events

See also: Timeline of the Fukushima I nuclear accidents

Radiation releases during ventings, hydrogen explosions and fires at Fukushima

• On 11 March, Japanese authorities reported that there had been no "release of radiation" from any of the power plants.
• On 12 March, the day after the earthquake, increased levels of iodine-131 and caesium-137 were reported near Unit 1 on the plant site.
• On 13 March, venting to release pressure started at several reactors resulting in the release of radioactive material.
• From 12 to 15 March the people of Namie was evacuated by the local officials to a place in the north of the town. This may have been in an area directly affected by a cloud of radioactive materials from the plants. There are conflicting reports about whether or not the government knew at the time the extent of the danger, or even how much danger there was.
• Chief Cabinet Secretary Yukio Edano announced on 15 March 2011 that radiation rates had been measured as high as 30 mSv/h on the site of the plant between units 2 and 3, as high as 400 mSv/h near unit 3, between it and unit 4, and 100 mSv/h near unit 4. He said, "there is no doubt that unlike in the past, the figures are the level at which human health can be affected." Prime Minister Naoto Kan urged people living between 20 and 30 kilometers of the plant to stay indoors, "The danger of further radiation leaks (from the plant) is increasing," Kan warned the public at a press conference, while asking people to "act calmly". A spokesman for Japan's nuclear safety agency said TEPCO had told it that radiation levels in Ibaraki, between Fukushima and Tokyo, had risen but did not pose a health risk. Edano reported that the average radiation dose rate over the whole day was 0.109 μSv/h. 23 out of 150 tested persons living close to the plant were decontaminated

• On 16 March power plant staff were briefly evacuated after smoke rose above the plant and radiation levels measured at the gate increased to 10 mSv/h. Media reported 1,000 mSv/h close to the leaking reactor, with radiation levels subsequently dropping back to 800–600 mSv. Japan's defence ministry criticized the nuclear safety agency and TEPCO after some of its troops were possibly exposed to radiation when working on the site. Japan's ministry of science (MEXT) measured radiation levels of up to 0.33 mSv/h 20 kilometers northwest of the power plant. Japan's Nuclear Safety Commission recommended local authorities to instruct evacuees leaving the 20-kilometre area to ingest stable (not radioactive) iodine.

• On 17 March IAEA radiation monitoring over 47 cities, showed that levels of radiation in Tokyo had not risen. Although at some locations around 30 km from the Fukushima plant, the dose rates had risen significantly in the preceding 24 hours (in one location from 80 to 170 μSv/h and in another from 26 to 95 μSv/h), levels varied according to the direction from the plant. The level of radiation on spinach grown in open air around 75 kilometers south of the nuclear plant had elevated levels of radioactive iodine and caesium

• On 18 March IAEA clarified that, contrary to several news reports, the IAEA had not received any notification from the Japanese authorities of people sickened by radiation contamination.

• On 19 March MEXT said trace amount of radioactive substances was detected in tap water in Tokyo, as well as Tochigi, Gunma, Chiba and Saitama prefectures. The Japanese Ministry of Health, Labour and Welfare announced that radiation levels exceeding legal limits had been detected in milk produced in the Fukushima area and in certain vegetables in Ibaraki. Measurements made by Japan in a number of locations have shown the presence of radionuclides such as iodine-131 (I-131) and caesium-137 (Cs-137) on the ground.

• On 23 March, MEXT released new environmental data. Radioactivity readings for soil and pond samples were highest at one location 40 km northwest of the plant. On 19 March, upland soil there contained 28.1 kBq/kg of Cs-137 and 300 kBq/kg of I-131. One day later, these same figures were 163 kBq/kg of Cs-137 and 1,170 kBq/kg of I-131. Cs-137 of 163 kBq/kg is equal to 3,260 kBq/m².

• On 24 March, three workers were exposed to high levels of radiation which caused two of them to require hospital treatment after radioactive water seeped through their protective clothes while working in unit 3. It rained in Tokyo from 21 March morning to 24. The rain brought radioactive fallout there. In Shinjuku, on the research by Tokyo Metropolitan Institute of Public Health, 83900 Bq/m² of I-131, 6310 Bq/m² of Cs-134, and 6350 Bq/m² of Cs-137 were detected for these four days in total as radioactive fallout's, including 24 hours from 20 9:00 AM to 21 9:00 AM.

• On 25 March the German Ministry of the Environment announced that small amounts of radioactive iodine had been observed in three places within the German atmosphere.

• On 26 March, Japan's nuclear safety agency said that radiation for iodine-131 in seawater near the discharge had increased to 1,850 times the limit.

• 27 March: Levels of "over 1000" (the upper limit of the measuring device) and 750 mSv/h were reported from water within unit 2 (but outside the containment structure) and 3 respectively. A statement that this level was "ten million times the normal level" in unit 2 was later retracted and attributed to attributing radioactivity to iodine-134 rather than to a longer living element. Japan's Nuclear and Industrial Safety Agency indicated that "The level of radiation is greater than 1,000 millisieverts. It is certain that it comes from atomic fission [...] But we are not sure how it came from the reactor."

• 29 March: iodine-131 levels in seawater 330m south of a key discharge outlet had reached 138 Bq/ml (3,355 times the legal limit)

• 30 March: iodine-131 concentrations in seawater had reached 180 Bq/ml at a location 330m south of a plant discharge, 4,385 times the legal limit. Tests indicating 3.7 MBq/m² of Cs-137 caused the IAEA to state that its criteria for evacuation was exceeded in the village of Iitate, Fukushima, outside the existing 30 kilometres (19 miles) radiation exclusion zone.

• On 31 March, IAEA corrected the value of iodine-131 that had been detected in the Iitate village to 20 million Bq/m². The value that had been announced at a press interview was about 2 million Bq/m².

• On 1 April, besides leafy vegetables and parsley, also beef with iodine-131 and/or caesium-134 and caesium-137 levels above the regulatory limit were reported.

• 3 April: Health officials report radioactive substances higher than the legal limits were found in mushrooms. The Japanese government publicly stated that it expected ongoing radioactive materials releases for "months" assuming normal containment measures are used.

• 4 April to 10 April TEPCO announced it began dumping 9,100 tons of water that was 100 times the contamination limit from a wastewater treatment plant, dumping would take 6 days.

• 5 April: Fish caught 50 miles off the coast of Japan had radioactivity exceeding safe levels.

• 15 April: Iodine-131 in seawater was measured at 6,500 times the legal limit, while levels of caesium-134 and caesium-137 rose nearly fourfold, possibly due to installation of steel plates meant to reduce the possibility of water into the ocean.

• 18 April: High levels of radioactive strontium-90 were discovered in soil at the plant, prompting the government to begin regularly testing for the element.

• 22 April: The Japanese government asked residents to leave Iitate and four other villages within a month due to radiation levels.

Total emissions

The primary releases of radioactive nuclides have been iodine and caesium; however, strontium and plutonium have also been found. These elements have been released into the air via steam; and into the water leaking into groundwater or the ocean. The expert who prepared a frequently cited Austrian Meteorological Service report asserted that the "Chernobyl accident emitted much more radioactivity and a wider diversity of radioactive elements than Fukushima Daiichi has so far, but it was iodine and caesium that caused most of the health risk – especially outside the immediate area of the Chernobyl plant." Iodine-131 has a half-life of 8 days while caesium-137 has a half-life of over 30 years. The IAEA has developed a method that weighs the "radiological equivalence" for different elements. TEPCO has published estimates using a simple sum methodology, and using the IAEA weighting methodology. As of 25 April, TEPCO has not released a total water and air release estimate.

According to a June 2011 report of the International Atomic Energy Agency (IAEA), at that time no confirmed long term health effects to any person had been reported as a result of radiation exposure from the nuclear accident.

Air Releases

A 12 April report prepared by NISA estimated the total air release of iodine-131 and caesium-137 at between 370 PBq and 630 PBq, combining iodine and caesium with IAEA methodology. On 23 April the NSC updated its release estimates, but it did not reestimate the total release, instead giving indicating that 154 TBq of air release were occurring daily as on 5 April.

Iodine-131

A widely cited Austrian Meteorological Service report estimated the total amount of I-131 radiation released into the air as of 19 March based on extrapolating data from several days of ideal observation at a handful of worldwide CTBTO radionuclide measuring facilities (Freiburg, Germany; Stockholm, Sweden; Takasaki, Japan and Sacramento, USA) during the first 10 days of the accident. The report's estimates of total I-131 emissions based on these worldwide measuring stations ranged from 10 PBq to 700 PBq. This estimate was 1% to 40% of the 1760 PBq of I-131 estimated to be release at Chernobyl.

A later, 12 April, NISA and NSC estimated the total air release of iodine-131 at 130 PBq and 150 PBq, respectively. However, on 23 April, the NSC revised its original estimates of iodine-131 released. The NSC did not estimate the total release size based upon these updated numbers, but estimated a release of 0.14 TBq per hour on 5 April."

Caesium-137

On 24 March, the Austrian Meteorological Service report estimated the total amount of caesium-137 radiation released into the air as of 19 March based on extrapolating data from several days of ideal observation at a handful of worldwide CTBTO radionuclide measuring facilities. The agency estimated an average being 5,000 TBq daily. Over the course of the disaster, Chernobyl put out a total of 85,000 TBq of caesium-137. However, later reporting on 12 April estimated total caesium releases at 6,100 TBq to 12,000 TBq, respectively by NISA and NSC. On 23 April, NSC updated this number to 0.14 TBq per hour of cesium-137 on 5 April, but did not recalculate the entire release estimate.

Water Releases

On 21 April, TEPCO estimated that 520 tons radioactive water leaked into the sea before leaks in a pit in unit 2 were plugged, releasing 4,700 TBq of total water release (calculated by simple sum, which is inconsistent with the IAEA methodology for mixed nuclide releases) (20,000 times facility's annual limit). TEPCO's detailed estimates were 2,800 TBq of I-131, 940 TBq of Cs-134, 940 TBq of Cs-137.

Another 300,000 tons of relatively less radioactive water had already been reported to have leaked or purposefully pumped into the sea to free room for storage of highly radioactively contaminated water. TEPCO had attempted to contain contaminated water in the harbor near the plant by installing "curtains" to prevent outflow, but now believes this effort was unsuccessful.

Radiation at the plant site

Normal radiation dose rates at the Fukushima I site as established by the stream of monitoring post readings in the 3 months preceding the accident. (03/01=1 March 2011, 1 Gray= 1 Sv for gamma radiation)

Radiation fluctuated widely on the site after the tsunami and often correlated to fires and explosions on site. Radiation dose rates at one location between reactor units 3 and 4 was measured at 400 mSv/h at 10:22 JST, 13 March, causing experts to urge rapid rotation of emergency crews as a method of limiting exposure to radiation. Dose rates of 1,000 mSv/h were reported (but not confirmed by the IAEA) close to the certain reactor units on 16 March, prompting a temporary evacuation of plant workers, with radiation levels subsequently dropping back to 800–600 mSv/h. At times, radiation monitoring was hampered by a belief that some radiation levels may be higher than 1 Sv/h, but that "authorities say 1,000 millisieverts [per hour] is the upper limit of their measuring devices."

Exposure of workers

Prior to the accident, the maximum permissible dose for Japanese nuclear workers was 100 mSv per year, but on 15 March 2011, the Japanese Health and Labor Ministry increased that annual limit to 250 mSv, for emergency situations. This level is below the 500 mSv/year considered acceptable for emergency work by the World Health Organization. Some contract companies working for TEPCO have opted not to use the higher limit. On 15 March, TEPCO decided to work with a skeleton crew (in the media called the Fukushima 50) in order to minimize the number of people exposed to radiation.

On 17 March, IAEA reported 17 persons to have suffered deposition of radioactive material on their face; the levels of exposure were too low to warrant hospital treatment. On 22 March, World Nuclear News reported that one worker had received over 100 mSv during "venting work" at Unit 3. An additional 6 had received over 100 mSv, of which for 1 a level of over 150 mSv was reported for unspecified activities on site. On 24 March, three workers were exposed to high levels of radiation which caused two of them to require hospital treatment after radioactive water seeped through their protective clothes while working in unit 3. Based on the dosimeter values, exposures of 170 mSv were estimated, the injuries indicated exposure to 2000 to 6000 mSv around their ankles. They were not wearing protective boots, as their employing firm's safety manuals "did not assume a scenario in which its employees would carry out work standing in water at a nuclear power plant". The amount of the radioactivity of the water was about 3.9 M Bq per cubic centimetre.

As of 24 March, 19:30 (JST), 17 workers (of which 14 were from plant operator TEPCO) had been exposed to levels of over 100 mSv. By 29 March, the number of workers reported to have been exposed to levels of over 100 mSv had increased to 19. An American physician reported Japanese doctors have considered banking blood for future treatment of workers exposed to radiation. Tepco has started a re-assessment of the approximately 8300 workers and emergency personnel who have been involved in responding to the incident, which has revealed that by 13 July, of the approximately 6700 personnel tested so far, 88 personnel have received between 100 and 150 mSv, 14 have received between 150 and 200 mSv, 3 have received between 200 and 250 mSv, and 6 have received above 250 mSv.

TEPCO has been criticized in providing safety equipment for its workers. After NISA warned TEPCO that workers were sharing dosimeters, since most of the devices were lost in the disaster, the utility sent more to the plant. Japanese media has reported that that workers indicate that standard decontamination procedures are not being observed. Others reports suggest that contract workers are given more dangerous work than TEPCO employees. TEPCO is also seeking workers willing to risk high radiation levels for short periods of time in exchange for high pay. Confidential documents acquired by the Japanese Asahi newspaper suggest that TEPCO hid high levels of radioactive contamination from employees in the days following the accident. In particular, the Asahi reported that radiation levels of 300 mSv/h were detected at least twice on 13 March, but that "the workers who were trying to bring the situation under control at the plant were not informed of the levels."

Radiation within the primary containment of the reactors

Within the primary containment of reactors 1, 2, 3 and 4 strongly varying levels of radiation were reported:

time (JST)	Reactor 1 (Sv/h)		Reactor 2 (Sv/h)		Reactor 3 (Sv/h)
	Dry Well	Wet Well (torus)	Dry Well	Wet Well (torus)	Dry Well	Wet Well (torus)
17 March 2011, 12:50	0.00410	31.6	84.4	2.43	---	---
18 March 2011, 7:55–12:35	0.00375	46.9	78.0	2.37	105	5.90
20 March 2011, 15:00–16:00	12.0	40.0	0.625	2.13	71.7	2.00
23 March 2011, 9:10–14:20	48.0	29.9	50.7	1.67	60.2	1.74
24 March 2011, 17:00	40.9	25.8	47.4	1.36	53.3	1.45
25 March 2011, 10:00	38.9	24.9	45.6	1.54	51.0	1.50
25 March 2011, 14:00–16:30	37.1	24.5	45.2	1.54	38.8	1.31
26 March 2011, 9:30–10:00	35.1	23.6	43.4	1.49	36.1	1.40

Radiation outside primary containment of the reactors

Outside the primary containment, plant radiation level measurements have also varied significantly

On 25 March, an analysis of stagnant water in the basement floor of the turbine building of Unit 1 showed heavy contamination.

Nuclide	Concentration (Bq/ml)
38 Cl	1.6×106
74 As	3.9×102
91 Y	5.2×104
131 I	2.1×105
134 Cs	1.6×105
136 Cs	1.7×104
137 Cs	1.8×106
140 La	3.4×102

On 27 March, TEPCO reported stagnant water in the basement of unit 2 (inside the reactor/turbine building complex, but outside the primary containment) was measured at 1000 mSv/h or more, which prompted evacuation. Additional basement and trench area measurements indicated 60 mSv/h in unit 1, "over 1000" mSv/h in unit 2, and 750 mSv/h in unit 3. The report indicated the main source was iodine-134 with a half-life of less than an hour, which resulted in a radioactive iodine concentration 10 million times the normal value in the reactor. TEPCO later retracted its report, stating that the measurements were inaccurate and attributed the error to comparing the isotope responsible, iodine-134, to normal levels of another isotope. Measurements were then corrected stating that the iodine levels were 100,000 times the normal level. On 28 March, the erroneous radiation measurement caused TEPCO to reevaluate the software used in analysis.

Measurements within the reactor/turbine buildings, but not in the basement and trench areas, were made on 18 April. These robotic measurements indicated up to 49 mSv/h in unit 1 and 57 mSv/h in unit 3. This is substantially lower than the basement and trench readings, but still exceeds safe working levels without constant worker rotation. Inside primary containment, levels are much higher.

By 23 March 2011, neutron radiation had been observed outside the reactors 13 times at the Fukushima I site. While this could indicate ongoing fission, a recriticality event was not believed to account for these readings. Based on those readings and TEPCO reports of high levels of chlorine-38, Dr. Ferenc Dalnoki-Veress speculated that transient criticalities may have occurred. However, Edwin Lyman at the Union of Concerned Scientists was skeptical, believing the reports of chlorine-38 to be in error. TEPCO's chlorine-38 report was later retracted. Noting that limited, uncontrolled chain reactions might occur at Fukushima I, a spokesman for the International Atomic Energy Agency (IAEA) “emphasized that the nuclear reactors won’t explode.”

On 15 April, TEPCO reported that nuclear fuel had melted and fallen to the lower containment sections of three of the Fukushima I reactors, including reactor three. The melted material was not expected to breach one of the lower containers, causing a serious radiation release. Instead, the melted fuel was thought to have dispersed uniformly across the lower portions of the containers of reactors No. 1, No. 2 and No. 3, making the resumption of the fission process, known as a "recriticality" most unlikely.

On 19 April, TEPCO estimated that the unit 2 turbine basement contained 25,000 cubic meters of contaminated water. The water was measured to have 3 MBq/cm3 of Cs-137 and 13 MBq/cm3 of I-131, TEPCO characterized this level of contamination as "extremely high." To attempt to prevent leakage to the sea, TEPCO planned to pump the water from the basement to the Centralized Radiation Waste Treatment Facility.

A suspected hole from the melting of fuel in unit 1 has allowed water to leak in an unknown path from unit 1 which has exhibited radiation measurements "as high as 1,120 mSv/h." Radiation measurements of the water in the unit 3 spent fuel pool were reported at 140 kBq of radioactive cesium-134 per cubic centimeter, 150 kBq of cesium-137 per cubic centimeter, and 11 kBq per cubic centimeter of iodine-131 on 10 May.

Site contamination

Soil

TEPCO have reported at at three sites 500 meters from the reactors that the cesium-134 and cesium-137 levels in the soil are between 7.1 kBq and 530 kBq per kilo of undried soil.

The Fukushima soil contamination compared with the chernboyl release for a series of key isotopes. The two data sets have been adjusted to make the cesium-137 bars the same size to allow the isotope signatures to be compared with ease

Small traces of plutonium have been found in the soil near the stricken reactors, repeated examinations of the soil suggest that the plutonium level is similar to the background level caused by atomic bomb tests. As the isotope signature of the plutonium is closer to that of power reactor plutonium, TEPCO suggested that "two samples out of five may be the direct result of the recent incident." The more important thing to look at is the curium level in the soil, the soil does contain a short-lived isotope (curium-242) which shows that some alpha emitters have been released in small amounts by the accident. It is important to bear in mind that the release of the beta/gamma emitters such as cesium-137 has been far greater. In the short and medium term the effects of the iodine and the cesium release will dominate the effect of the accident on farming and the general public. In common with almost all soils the soil at the reactor site contains uranium but the concentration of uranium and the isotope signature is suggesting that the uranium is the normal natural uranium in the soil.

Radioactive strontium-89 and strontium-90 were discovered in soil at the plant on 18 April, amounts detected in soil one-half kilometer from the facility ranged from 3.4 to 4400 Bq/kg of dry soil. Strontium remains in soil from above-ground nuclear testing, however, the amounts measured at the facility are approximately 130 times greater than the amount typically associated with previous nuclear testing.

The isotope signature of the release looks very different to that of the Chernboyl accident, the Japanese accident has released much less of the involatile plutonium, minor actinides and fission products than Chernobyl did.

On 31 March, TEPCO reported that it had measured radioactivity in the plant site groundwater which was 10,000 times the government limit. The company did not think that this radiation had spread to drinking water. NISA questioned the radioactivity measurement and TEPCO is re-evaluating it. Some debris around the plant has been found to be highly radioactive, including a concrete fragment emanating 900 mSv/h.

Air and direct radiation

Air outside, but near, unit 3 was reported at 70 mSv/h on 26 April. This was down from radiation levels as high as 130 mSv/h near units 1 and 3 in late March. Removal of debris reduced the radiation measurements from localized highs of up to 900 mSv/h to less than 100 mSv/h at all exterior locations near the reactors; however, readings of 160 mSv/h were still measured at the waste treatment facility.

Discharge to seawater and contaminated sealife

Results revealed on 22 March from a sample taken by TEPCO about 100 m south of the discharge channel of units 1–4 showed elevated levels of Cs-137, caesium-134 (Cs-134) and I-131. A sample of seawater taken on 22 March 330m south of the discharge channel (30 kilometers off the coast line) had elevated levels of I-131 and Cs-137. Also, north of the plant elevated levels of these ions were found on 22 March (as well as Cs-134, tellurium-129 and tellurium-129m (Te-129m)) although the levels were lower. Samples taken on the 23 March and/or 24 March contained about 80 Bq/mL of iodine-131 (1850 times the statutory limit) and 26 Bq/mL and caesium-137, most likely caused by atmospheric deposition. By 26 and 27 March this level had decreased to 50 Bq/mL (11) iodine-131 and 7 Bq/mL (2.9) caesium-137 (80 times the limit). Hidehiko Nishiyama, a senior NISA official, stated that radionuclides would "be very diluted by the time it gets consumed by fish and seaweed." Above the seawater, IAEA reported "consistently low" dose rates of 0.04–0.1 μSv/h on 27 March.

By 29 March iodine-131 levels in seawater 330 m south of a key discharge outlet had reached 138 Bq/ml (3,355 times the legal limit) and by 30 March, iodine-131 concentrations had reached 180 Bq/ml at the same location near the Fukushima Daiichi nuclear plant, 4,385 times the legal limit. The high levels could be linked to a feared overflow of highly radioactive water that appeared to have leaked from the unit 2 turbine building. On 15 April, I-131 radiation levels were 6,500 times the legal limits. On 16 April, TEPCO began dumping zeolite, a mineral "that absorbs radioactive substances, aiming to slow down contamination of the ocean."

Seawater radionuclide concentration on 29 March 2011:

Nuclide	Concentration (Bq/cm3)	Regulatory limit (Bq/cm3)	Concentration / Regulatory Limit
99m Tc	0.16	40	.0004
131 I	130	0.04	3250
134 Cs	31	0.06	517
136 Cs	2.8	0.3	9.3
137 Cs	32	0.09	356
140 Ba	5.0	0.3	17
140 La	2.5	0.4	6.3

On 4 April, it was reported that the "operators of Japan's crippled power plant say they will release more than 10,000 tons of contaminated water into the ocean to make room in their storage tanks for water that is even more radioactive." Measurements taken on 21 April indicated 186 Bq/l measured 34 km from the Fukushima plant, Japanese media reported this level of seawater contamination second only to the Sellafield nuclear accident.

On 11 May, TEPCO announced it believed it had sealed a leak from unit 3 to sea, TEPCO did not immediately announce the amount of radiation released by the leak. On 13 May, Greenpeace announced that 10 of the 22 seaweed samples it had collected near the plant showed 10,000 Bq/Kg or higher, five times the Japanese standard for food of 2,000 Bq/Kg for iodine-131 and 500 Bq/kg for radioactive cesium.

In addition to the large releases of contaminated water (520 tons and 4,700 TBq) believed to have leaked from unit 2 from mid-March until early April; another release of radioactive water is believed to have contaminated the sea from unit 3, because on 16 May TEPCO announced seawater measurements of 200 Bq per cubic centimeter of cesium-134, 220 Bq per cubic centimeter of cesium-137, and unspecified high levels of iodine shortly after discovering a unit 3 leak.

At two locations 20 kilometer north and south 3 kilometers from the coast TEPCO found strontium-89 and strontium-90 in the seabed soil. The samples were taken on June 2. Up to 44 becquerels per kilogram of strontium-90 were detected, which has a half-life of 29 years. These isotopes were also found in soil and in seawater immediately after the accident. Samples taken from fish and seafood caught off the coast of Ibaraki and Chiba, did not contain radioactive stontium.

Radiation and Nuclide Detection in Japan

Periodic overall reports of the situation in Japan are provided by the United States Department of Energy.

In April, 2011, the United States Department of Energy published projections of the radiation risks over the next year (that is, for the future) 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. However, 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.

Air exposure within 30 kilometers

Dose rates for locations in the Fukushima Prefecture and neighboring prefectures. *Iitate, Fukushima *Minamisōma, Fukushima *Iwaki, Fukushima *Tamura, Fukushima

File:Fukushima area 01.png

Fukushima area detector readings

The zone within 20 km from the plant was evacuated on 12 March, while residents on a distance up to 30 km were advised to stay indoors and were from 25 March subject to voluntary evacuation.

IAEA reported on 14 March that about 150 people in the vicinity of the plant "received monitoring for radiation levels"; 23 of these people were also decontaminated.

At a distance of 30 km (19 miles) from the site, radiation of 0.003–0.170 mSv/h was measured to the north-west on 17 March, while it was 0.001–0.005 mSv/h in other directions. Experts said exposure to this amount of radiation for 6 to 7 hours would result in absorption of the maximum level considered safe for one year. On 16 March Japan's ministry of science measured radiation levels of up to 0.33 mSv/h 20 kilometers northwest of the power plant. At some locations around 30 km from the Fukushima plant, the dose rates rose significantly in 24 hours on 16–17 March. In one location from 0.080 to 0.170 mSv/h and in another from 0.026 to 0.095 mSv/h. The levels varied according to the direction from the plant.

Normal background radiation varies from place to place but delivers a dose equivalent in the vicinity of 2.4 mSv/year, or about 0.3 µSv/h. For comparison, one chest x-ray is about 0.02 mSv and an abdominal CT scan is supposed to be less than 10 mSv (but it has been reported that some abdominal CT scans can deliver as much as 90 mSv). People can mitigate their exposure to radiation through a variety of protection techniques.

• 
  DOE and NNSA aerial survey of the area surrounding the plant 17–19 March 2011. NW dispersion of plume obvious. Source: [1]
• 
  Same survey single day flight revision on 24 March 2011.
• 
  Same survey performed 24 and 26 March.
• 
  Broader area survey focusing on outside the 24 mile radius, from 27 and 28 March.
• 
  Combination of measurements made from 30 March to 3 April

Ground and water contamination within 30 kilometers

The unrecovered bodies of approximately 1,000 quake and tsunami victims within the plant's evacuation zone are believed to be contaminated with dangerous levels of radiation.

Air exposure outside of 30 kilometers

See also: Radiation monitoring in Japan

Tokyo low-level gamma radiation with comparisons to average annual radiation intake. Based on Geiger counter measurements in Tokyo. Does not show radiation from physically transported sources, i.e. particulate matter transported in food, water, or the atmosphere.

Radiation levels in Tokyo on 15 March were at one point measured at 0.809 μSv/hour although they were later reported to be at "about twice the normal level". Later, on 15 March 2011, Edano reported that radiation levels were lower and the average radiation dose rate over the whole day was 0.109 μSv/h. The wind direction on 15 March dispersed radiatioactivity away from the land and back over the Pacific Ocean. On 16 March, the Japanese radiation warning system, SPEEDI, indicated high levels of radiation would spread further than 30 km from the plant, but Japanese authorities did not relay the information to citizens because "the location or the amount of radioactive leakage was not specified at the time." From 17 March, IAEA received regular updates on radiation from 46 cities and indicated that they had remained stable and were "well below levels which are dangerous to human health". In hourly measurements of these cities until 20 March, no significant changes were reported.

On 30 March 2011, the IAEA stated that its operational criteria for evacuation were exceeded in the village of Iitate, Fukushima, 39 kilometres (24 miles) north-west of Fukushima I, outside the existing 30 kilometres (19 miles) radiation exclusion zone. The IAEA advised the Japanese authorities to carefully assess the situation there. Experts from Kyoto University and Hiroshima University released research, on 11 April, that "soil samples has revealed that as much as 400 times the normal levels of radiation" which "could remain in communities beyond a 30-kilometer radius from the Fukushima" site.

Urine samples taken from 10 children in the capital of Fukushima prefecture, were analyzed in a French laboratory. All of them contained cesium-134. The sample of an eight year old girl contained 1.13 becquerel/liter. The kids were living up to 60 kilometers away from the troubled nuclear power plant. The Fukushima Network for Saving Children urged the Japanese government to check the children in Fukushima. The Japanese non-profit Radiation Effects Research Foundation said that people should not overreact, because there are no reports known of health problems with these levels of radiation.

Ground, water and sewage contamination outside of 30 kilometers

Tests concluded between 10 and 20 April revealed radioactive cesium in amounts of 2.0 and 3.2 kBq/kg in soil from the Tokyo districts of Chiyoda and Koto, respectively On 5 May, government officials announced that radiation levels in Tokyo sewage had spiked in late March. Simple sum measurements of all radioactive isotopes in sewage burned at a Tokyo treatment plant measured 170,000 Bq/kg "in the immediate wake of the Fukushima nuclear crisis. The government announced that the reason for the spike was unclear, but suspected rainwater. The 5 May announcement further clarified that as of 28 April, the radiation level in Tokyo sewage was 16,000 Bq/kg.

A detailed map of ground radiation contamination within 80 kilometers of the plant, the joint product of the U.S. Department of Energy and the Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT), was released on 6 May. The map showed that a belt of contamination, with radiation from 3 to 14.7 MBq caesium-137 per square meter, spread to the northwest of the nuclear plant. For comparison, areas with activity levels with more than 0.55 MBq cesium-137 per square meter were abandoned after the 1986 Chernobyl accident. The village of Iitate and the town of Namie are impacted. Similar data was used to establish a map that would calculate the amount of radiation a person would be exposed to if a person were to stay outdoors for 8 hours per day through March 11th, 2012. Scientists preparing this map, as well as earlier maps, targeted a 20 mSv/a dosage target for evacuation. The government's 20 mSv/a target led to the resignation of Toshiso Kosako, a senior nuclear adviser to Japanese Prime Minister Naoto Kan, who stated "I cannot allow this as a scholar," and argued that the target is too high, especially for children; he also criticized the increased limit for plant workers. In response, parents-groups and schools in some smaller towns and cities in Fukushima Prefecture have organized decontamination of soil surrounding schools defying orders from Tokyo asserting that the schools are safe. Eventually, the Fukushima education board plans to replace the soil at 26 schools with the highest radiation levels.

Anomalous "hot spots" have been discovered in areas far beyond the adjacent region. For example, experts cannot explain how radioactive cesium from the reactors at Fukushima end up in Kanagawa more than 300 kilometers (180 miles) to the south.

Deposition of radioactivity and effect on agricultural products

Radiation monitoring in all 47 prefectures showed strong variation, but an upward trend in 10 of them on 23 March. No deposition could be determined in 28 of them until 25 March The highest value obtained was in Ibaraki (480 Bq/m² on 25 March) and Yamagata (750 Bq/m² on 26 March) for iodine-13. For caesium-137, the highest value were in Yamagata at 150 and 1200 Bq/m² respectively.

Measurements made by Japan in a number of locations have shown the presence of radionuclides in the ground. On 19 March, upland soil levels of 8,100 Bq/kg of Cs-137 and 300,000 Bq/kg of I-131 were reported. One day later, the measured levels were 163,000 Bq/kg of Cs-137 and 1,170,000 Bq/kg of I-131.

Summary of restrictions imposed by the Japanese govt as of April 25, 2011

Item	Shipping restrictions					Consumption restrictions
	Fukushima	Ibaraki	Tochigi	Gunma	Chiba	Fukushima
Raw milk	3/21 – 4/8: Kitakata, Bandai, Inawashiro, Mishima,　Aizumisato, Shimogou, Minami-aizu 3/21 – 4/16: Fukushima, Nihonmatsu, Date, Motomiya, Kunimi, Ootama, Kooriyama, Sukagawa, Tamura　(excl. former Miyakoji), Miharu, Ono, Kagamiishi, Ishikawa, Asakawa, Hirata, Furudono, Shirakawa, Yabuki, Izumizaki, Nakajima, Nishigou, Samegawa, Hanawa, Yamatsuri, Iwaki 3/21 – 4/21: Souma, Shinchi 3/21 – ongoing: All other areas	3/23 – 4/10: All areas
Spinach	3/21 – ongoing: All areas	3/21 – 4/17: All areas except Kita-ibaraki, Takahagi 3/21 – ongoing: Kita-ibaraki, Takahagi	3/21 – 4/21: Nasushiobara, Shioya 3/21 – ongoing: All other areas	3/21 – 4/8: All areas	4/4 – 4/22: Asahi, Katori, Tako	3/23 – ongoing: All areas
Kakina	3/21 – ongoing: All areas	3/21 – 4/17: All areas	3/21 – 4/14: All areas	3/21 – 4/8: All areas		3/23 – ongoing: All areas
Chrysanthemum	3/23 – ongoing: All areas				4/4 – 4/22: Asahi	3/23 – ongoing: All areas
Bok choi	3/23 – ongoing: All areas				4/4 – 4/22: Asahi	3/23 – ongoing: All areas
Korean lettuce	3/23 – ongoing: All areas				4/4 – 4/22: Asahi	3/23 – ongoing: All areas
Other non-round leafy vegetables	3/23 – ongoing: All areas					3/23 – ongoing: All areas
Round leafy vegetables (such as cabbage)	3/23 – ongoing: All areas					3/23 – ongoing: All areas
Brassicaceae buds (broccoli, cauliflower, etc.)	3/23 – ongoing: All areas					3/23 – ongoing: All areas
Turnip	3/23 – ongoing: All areas
Parsley		3/23 – 4/17: All areas			4/4 – 4/22: Asahi
Celery					4/4 – 4/22: Asahi
Shiitake	4/13 – 4/25: Iwaki 4/13 – ongoing: Shinchi, Date, Iitate, Souma, Minami-souma, Namie, Futaba, Ookuma, Tomioka, Naraha, Hirono, Kawamata, Katsurao, Tamura, Kawauchi 4/18 – ongoing: Fukushima 4/25 – ongoing: Motomiya					4/13 – ongoing: Iitate
Sand lance young	4/20 – ongoing: All areas					4/20 – ongoing: All areas

Agricultural products

On 19 March, the Japanese Ministry of Health, Labour and Welfare announced that levels of radioactivity exceeding legal limits had been detected in milk produced in the Fukushima area and in certain vegetables in Ibaraki. On 21 March, IAEA indicated it confirmed that "in some areas, iodine-131 in milk and in freshly grown leafy vegetables, such as spinach and spring onions, is significantly above the levels set by Japan for restricting consumption". One day later, iodine-131 (sometimes above safe levels) and caesium-137 (always at safe levels) detection was reported in Ibaraki prefecture. On 21 March, levels of radioactivity in spinach grown in open air in Kitaibaraki city in Ibaraki, around 75 kilometers south of the nuclear plant, were 24,000 becquerel (Bq)/kg of iodine-131, 12 times more than the limit of 2,000 Bq/kg, and 690 Bq/kg of caesium, 190 Bq/kg above the limit. In four Prefectures (Ibaraki, Totigi, Gunma, Fukushima) distributing of spinach and kakina was restricted as well as milk from Fukushima. On 23 March, similar restrictions were placed on more leafy vegetables (komatsuna, cabbages) and all flowerhead brassicas (like cauliflower) in Fukushima, while parsley and milk distribution was restricted in Ibaraki. On 24 March, IAEA reported that virtually all milk samples and vegetable samples taken in Fukushima and Ibaraki on 18–21 and 16–22 March respectively were above the limit. Samples from Chiba, Ibaraki and Tochigi also had too high levels in celery, parsley, spinach and other leafy vegetables. In addition, certain samples of beef mainly taken on 27–29 March showed concentrations of iodine-131 and/or caesium-134 and caesium-137 above the regulatory levels.

After the detection of radioactive caesium above legal limits in Sand lances caught off the coast of Ibaraki Prefecture, the Prefectural government banned such fishing. On 11 May, caesium levels in tea leaves from a prefecture "just south of Tokyo" were reported to exceed government limits, this was the first agricultural product from Kanagawa Prefecture that exceeded safety limits. In addition to Kanagawa Prefecture, agricultural products from Tochigi and Ibaraki prefectures have also been found to exceed the government limits, for example, pasture grass collected on 5 May, measured 3,480 Bq/kg of radioactive cesium, approximately 11 times the state limit of 300 becquerels. Even into July radioactive beef was found on sale in eleven prefectures, as far away as Kochi and Hokkaido. Authorities explained that until that point testing had only been performed on the skin and exterior of livestock, animal feed and meat cuts had not been checked for radiation previously.

Hay and straw were found contaminated with cesium, 80 kilometers from the reactors, far outside the evacuation-zone. Beef from the prefecture Fukushima was taken from the distribution-channels. But health minister Kohei Otsuka, mentioned on 17 July 2011 that this could be not enough. An inspection was set up, testing the urine of all cows, to be able to return those cows that showed levels of radioactive substances higher than the government-set limit to farms so they could be decontaminated feeding safe hay. According to the minister, the Japanese government should try to buy not-contaminated straw and hay in other parts of the country and offering this to the farmers in the affected areas. All transports of beef raised in the prefecture Fukushima was prohibited after 19 July. The meat of some 132 cows was sold to at least 36 of the 47 prefectures of Japan. In more and more places contaminated meat was found.

As of July 2011, the Japanese government has been unable to control the spread of radioactive material into the nation’s food, and "Japanese agricultural officials say meat from more than 500 cattle that were likely to have been contaminated with radioactive cesium has made its way to supermarkets and restaurants across Japan". On July 22 it became known, that at least 1400 cows were shipped from 76 farms that were fed with contaminated hay and rice-straw, that had been distributed by agents in Miyagi and farmers in the prefectures of Fukushima and Iwate, near the crippled Fukushima Daiichi nuclear power plant. Supermarkets and other stores were asking their customers to return the meat. Farmers were asking for help, and the Japanese government was considering whether it should buy and burn all this suspect meat.

On July 26 the body-count was more than 2,800 cows fed with cesium-contaminated food, shipped to 46 of the 47 prefectures in Japan, only Okinawa remained free. Part of this beef, that had reached the markets, still needed to be tested. In an attempt to ease consumer-concern the Japanese government promised to impose inspections on all this beef, and to buy the meat back when higher-than-permissible cesium-levels were detected during the tests. The government planned to eventually pass on the buy-back costs to TEPCO. The same day the Japanese ministry of agriculture urged farmers and merchants to renounce the use and sale of compost made of manure from cows that may have been fed the contaminated straw. The measure also applied to humus from leaves fallen from trees. After developing guidelines for safety-levels of radioactive cesium in compost and humus, this voluntary ban could be lifted.

On July 28 a ban was imposed on all the shipments on cattle from the prefecture Miyagi. Some 1,031 beasts had been shipped that probably were fed with contaminated rice-straw. After measurements in 6 of them revealed 1,150 becquerels per kilogram, more than twice the governmental set safety level. Because the origins were scattered all over the prefecture, Miyagi became the second prefecture with a ban on all beef cattle shipments. In the year before March 11 about 33,000 cattle were traded from Miyagi.

On the first of August a ban was put on all cattle in the prefecture Iwate, after 6 cows from two villages were found with heavy levels of Cesium. Iwate was the third prefecture where this was decided. Shipments of cattle and meat would only be allowed after examination, and when the level of Cesium was below the regulatory standard. In Iwate some 36,000 cattle was produced in a year. All cattle would be checked for radiation exposure before shipment, and the Japanese government asked the prefecture to temporarily reduce the number of shipments to match its inspection capability.

On 3 August, yet another local government in western Japan, the prefecture Shimane, decided to conduct radiation checks on all beef cattle to ease consumer concerns about food safety. Starting from the second week of August all cattle would be tested. Late July at one farm in this prefecture rice-straw was discovered with radioactive cesium-levels exceeding the government safety guide. Although all other tests of beef cattle found far lower levels of radioactivity than the government standard, prices of beef from Shimane plummeted and wholesalers avoided all cattle from the prefecture. All processed beef would undergo preliminary screening, and meat registering 250 becquerels per kilogram or more of radioactive cesium – half the government safety level – would be tested further.

The second week of August the prefecture of Fukushima started with a buy-out of all cattle that could not be sold because the high levels of cesium in the meat. The prefecture decided to buy back all beef cattle that had become too old for shipment due to the shipping suspension in place since July. On 2 August a group of farmers agreed with the Fukushima prefectural government to set up a consultative body as early as next week to regulate this process. The prefectural government would provide the subsidies needed. There was some delay, because the farmers and the local government could not agree about the prices for the cattle, further negotiations were needed. The problems for the farmers were growing, because they did not know how to protect their cattle from contamination and did not how to feed their cattle. A senior official of the farmers said that the buy-up plan needed to be implemented without any delay.

In response to calls for more support by farmers the Japanese government unfolded on the 5th of August 2011 a plan to buy up all beef contaminated with radioactive cesium, that had already reached the distribution chains, as an additional measurement to support beef cattle farmers. The plan included:

• the buy-out of about 3,500 head of cattle suspected to have been fed with contaminated rice straw, with cesium in excess of the safety limit.
• regardless the fact that some beef could be within the national safety limits.
• all this meat would be burned, to keep it out of distribution-channels

Other measurements were the expansion of subsidies to beef cattle farmers:

• Farmers who were unable to ship their cattle due to restrictions received 50,000 yen, (~ 630 dollars) per head of cattle regardless of the cattle's age.
• financial support was offered to prefectures that were buying up beef cattle, that had become too old to ship due to the ban.
• The Japanese Government planned to go on to buy all beef containing unsafe levels of radioactive cesium that reached the market through private organizations.

On 13 August 2011, a group of 5 manufacturers of natto, or fermented soybeans, in Mito city, Ibaraki Prefecture, planned to seek damages from TEPCO because their sales had fallen to almost 50 percent of normal. Natto is normally packed in rice-straw and after the discovery of the cesium-contamination of the straw, they had lost a lot of their normal customers. The lost sales from April until August 2011 had risen to around 1.3 million dollars.

On 19 August 2011 was reported, the meat of 4 cows from one Fukushima farm had been found to be contaminated with radioactive cesium in excess of the government-set safety limits. The day after the meat of 5 other cows from this farm was also found to contain radioactive cesium. Because of this the central government on Friday to put off lifting a shipment ban on Fukushima beef. The 9 cows were among a total of over 200 head of cattle shipped from the farm and slaughtered at a facility in Yokohama city between the March 11th nuclear accident and April. The meat of the 9 had been stored by a food producer. The farmer denied feeding the cows contaminated rice straw, in stead he used imported hay that had been stored at another farm.

Drinking water

The regulatory safe level for iodine-131 and caesium-137 in drinking water in Japan are 100 Bq/kg and 200 Bq/kg respectively. The Japanese science ministry said on 20 March that radioactive substances were detected in tap water in Tokyo, as well as Tochigi, Gunma, Chiba and Saitama prefectures. IAEA reported on 24 March that drinking water in Tokyo, Fukushima and Ibaraki had been above regulatory limits between 16 and 21 March. On 26 March, IAEA reported that the values were now within legal limits. On 23 March, Tokyo drinking water exceeded the safe level for infants, prompting the government to distribute bottled water to families with infants. Measured levels were caused by iodine-131 (I-131) and were 103, 137 and 174 Bq/l. On 24 March, iodine-131 was detected in 12 of 47 prefectures, of which the level in Tochigi was the highest at 110 Bq/kg. Caesium-137 was detected in 6 prefectures but always below 10 Bq/kg. On 25 March, tap water was reported to have reduced to 79 Bq/kg and to be safe for infants in Tokyo and Chiba but still exceeded limits in Hitachi and Tokaimura. On 27 April, "radiation in Tokyo’s water supply fell to undetectable levels for the first time since March 18."

The following graphs show Iodine-131 water contaminations measured in water purifying plants From 16 March to 7 April:

• 
  Chiba Prefecture
• 
  Fukushima Prefecture
• 
  Saitama Prefecture
• 
  Tochigi Prefecture
• 
  Tokyo Prefecture

On 2 July samples of tapwater taken in Tokyo Shinjuku ward radioactive cesium-137 was detected for the first time since April. The concentration was 0.14 becquerel per kilogram and none was discovered yesterday, which compares with 0.21 becquerel on April 22, according to the Tokyo Metropolitan Institute of Public Health. No cesium-134 or iodine-131 was detected. The level was below the safety limit set by the government. "This is unlikely to be the result of new radioactive materials being introduced, because no other elements were detected, especially the more sensitive iodine," into the water supply, were the comments of Hironobu Unesaki, a nuclear engineering professor at Kyoto University.

Breast milk

Small amounts of radioactive iodine were found in the breast milk of women living east of Tokyo. However, the levels were below the safety limits for tap water consumption by infants.

Schoolchildren

In a survey by the local and central governments conducted on 1,080 children aged 0 to 15 in Iwaki, Kawamata and Iitate on March 26–30, almost 45 percent of these children had experienced thyroid exposure to radiation with radioactive iodine, although in all cases the amounts of radiation did not warrant further examination, according to the officials of the Nuclear Safety Commission said on Tuesday 5 July.

Citizens groups involved added on 5 July, that a survey of soil at four places in the city of Fukushima taken on June 26 proofed that all samples were contaminated with radioactive cesium, measuring 16,000 to 46,000 becquerels per kilogram and exceeding the legal limit of 10,000 becquerels per kg,

Decontamination efforts

In the last week of August premier minister Naoto Kan informed the governor of Fukushima Prefecture about the plans to build a central storage facility to store and treat nuclear waste including contaminated soil in Fukushima. On August 27 at a meeting in Fukushima City Governor Yuhei Sato spoke out his concern about the sudden proposals, and the implications that this would have for the prefecture and its inhabitants, that already had endured so much from the nuclear accident. Kan said, that the government had no intention to make the plant a final facility, but the request was needed in order to make a start with decontamination.

Distribution outside Japan

Distribution by sea

Seawater containing measurable levels of iodine-131 and caesium-137 were collected by Japan Agency for Marine-Earth Science and Technology (JAMSTEC) on 22–23 March at several points 30 km from the coastline iodine concentrations were "at or above Japanese regulatory limits" while caesium was "well below those limits" according to an IAEA report on 24 March. On 25 March, IAEA indicated that in the long term, caesium-137 (with a half-life of 30 years) would be the most relevant isotope as far as doses was concerned and indicated the possibility "to follow this nuclide over long distances for several years." The organization also said it could take months or years for the isotope to reach "other shores of the Pacific".

Distribution by air

File:Fukushima trajectory animation for days 10 to 21 after the accident

The United Nations predicted that the initial radiation plume from the stricken Japanese reactors would reach the United States by 18 March. Health and nuclear experts emphasized that radioactive isotopes in the plume will be very diluted, and would have extremely minor health consequences in the United States. A simulation by the Belgian Institute for Space Aeronomy indicated that trace amounts of radioactivity would reach California and Mexico around 19 March. These predictions were tested by a worldwide network of highly sensitive radiative isotope measuring equipment, with the resulting data used to assess any potential impact to human health as well as the status of the reactors in Japan. Consequently, by 18 March radioactive fallout including isotopes of iodine-131, iodine-132, tellurium-132, iodine-133, caesium-134 and caesium-137 was detected in air filters at the University of Washington, Seattle, USA.

Due to an anticyclone south of Japan, favorable westerly winds were dominant during most of the first week of the accident, depositing most of the radioactive material out to sea and away from population centers, with some unfavorable wind directions depositing radioactive material over Tokyo. Low-pressure area over Eastern Japan gave less favorable wind directions 21–22 March. Wind shift to north takes place Tuesday midnight. After the shift, the plume would again be pushed out to the sea for the next becoming days. Roughly similar prediction results are presented for the next 36 hours by the Finnish Meteorological Institute. In spite of winds blowing towards Tokyo during 21–22 March, he comments, "From what I've been able to gather from official reports of radioactivity releases from the Fukushima plant, Tokyo will not receive levels of radiation dangerous to human health in the coming days, should emissions continue at current levels."

Norwegian Institute for Air Research have continuous forecasts of the radioactive cloud and its movement. These are based on the FLEXPART model, originally designed for forecasting the spread of radioactivity from the Chernobyl disaster.

As of 28 April, the Washington State Department of Health, one of the U.S. states nearest Japan, reported that levels of radioactive material from the Fukushima plant had dropped significantly, and were now often below levels that could be detected with standard tests.

Response in other countries

See also: International reaction to the Fukushima I nuclear accidents

Rush for iodine

Packaged potassium iodide tablets.

Fear of radiation from Japan prompted a global rush for iodine pills, including in the United States, Canada, Russia, Korea, China, Malaysia and Finland. There is a rush for iodized salt in China. A rush for iodine antiseptic solution appeared in Malaysia. WHO warned against consumption of iodine pills without consulting a doctor and also warned against drinking iodine antiseptic solution. The United States Pentagon said troops are receiving potassium iodide before missions to areas where possible radiation exposure is likely.

The World Health Organisation (WHO) says it has received reports of people being admitted to poison centres around the world after taking iodine tablets in response to fears about harmful levels of radiation coming out of the damaged nuclear power plant in Fukushima.

U.S. military

See also: Operation Tomodachi

Humanitarian flight is checked for radiation at Yokota

In Operation Tomodachi, the United States Navy dispatched the aircraft carrier USS Ronald Reagan and other vessels in the Seventh Fleet to fly a series of helicopter operations. A U.S. military spokesperson said that low-level radiation forced a change of course en route to Sendai. The Reagan and sailors aboard were exposed to "a month's worth of natural background radiation from the sun, rocks or soil" in an hour and the carrier was repositioned. Seventeen sailors were decontaminated after they and their three helicopters were found to have been exposed to low levels of radioactivity.

The aircraft carrier USS George Washington was docked for maintenance at Yokosuka Naval Base, about 280 kilometres (170 mi) from the plant, when instruments detected radiation at 07:00 JST on 15 March. Rear Admiral Richard Wren stated that the nuclear crisis in Fukushima, 320 kilometres (200 mi) from Yokosuka, was too distant to warrant a discussion about evacuating the base. Daily monitoring and some precautionary measures were recommended for Yokosuka and Atsugi bases, such as limiting outdoor activities and securing external ventilation systems. As a precaution, the Washington was pulled out of its Yokosuka port later in the week. The Navy also stopped moving its personnel to Japan.

Isotopes of possible concern

See also: Nuclear fission product § Countermeasures against the worst fission products found in accident fallout

The isotope iodine-131 is easily absorbed by the thyroid. Persons exposed to releases of I-131 from any source have a higher risk for developing thyroid cancer or thyroid disease, or both. Iodine-131 has a short half-life at approximately 8 days, and therefore is an issue mostly in the first weeks after the accident. Children are more vulnerable to I-131 than adults. Increased risk for thyroid neoplasm remains elevated for at least 40 years after exposure. Potassium iodide tablets prevent iodine-131 absorption by saturating the thyroid with non-radioactive iodine. Japan's Nuclear Safety Commission recommended local authorities to instruct evacuees leaving the 20-kilometre area to ingest stable (not radioactive) iodine. CBS News reported that the number of doses of potassium iodide available to the public in Japan was inadequate to meet the perceived needs for an extensive radioactive contamination event.

Caesium-137 is also a particular threat because it behaves like potassium and is taken up by cells throughout the body. Additionally, it has a long, 30-year half-life. Cs-137 can cause acute radiation sickness, and increase the risk for cancer because of exposure to high-energy gamma radiation. Internal exposure to Cs-137, through ingestion or inhalation, allows the radioactive material to be distributed in the soft tissues, especially muscle tissue, exposing these tissues to the beta particles and gamma radiation and increasing cancer risk. Prussian blue helps the body excrete caesium-137.

Strontium-90 behaves like calcium, and tends to deposit in bone and blood-forming tissue (bone marrow). 20–30% of ingested Sr-90 is absorbed and deposited in the bone. Internal exposure to Sr-90 is linked to bone cancer, cancer of the soft tissue near the bone, and leukemia. Risk of cancer increases with increased exposure to Sr-90.

Plutonium is also present in the MOX fuel of the Unit 3 reactor and in spent fuel rods. Officials at the International Atomic Energy Agency say the presence of MOX fuel does not add significantly to the dangers. Plutonium-239 is particularly long-lived and toxic with a half-life of 24,000 years and remains hazardous for tens of thousands of years. Experts commented that the long-term risk associated with plutonium toxicity is "highly dependent on the geochemistry of the particular site."

Regulatory levels

An overview for regulatory levels in Japan is shown in the table below:

–	Value	Unit	Reference	Comment
Yearly exposure of workers	250	mSv		Increased from 100
Spinach	2	Bq/g (iodine-131)		possibly a general standard for food
Spinach	0.5	Bq/g (caesium-137)		possibly a general standard for food. Increased from 0.37
Seawater (at discharge)	0.04	Bq/mL (iodine-131)
Seawater (at discharge)	0.09	Bq/mL (caesium-137)
drinking water	0.1	Bq/g (iodine-131)
drinking water	0.2	Bq/g (caesium-137)

See also

• 2011 Japanese nuclear accidents
• Lists of nuclear disasters and radioactive incidents
• Fukushima 50

External links

• TEPCO News Releases, Tokyo Electric Power Company
• NISA Information update, Nuclear and Industrial Safety Agency, the nuclear safety authority of Japan
• IAEA Update on Japan Earthquake, International Atomic Energy Agency
• Navigating Fukushima: Lessons from Chernobyl, Potential Radiation Effects, and Other Health Impacts, Q&A with Dr. Scott Davis about the mechanics of the crisis in Fukushima and how it compares to Chernobyl
• Christodouleas, JP; Forrest, RD; Ainsley, CG; Tochner, Z; Hahn, SM; Glatstein, E (2011). "Short-Term and Long-Term Health Risks of Nuclear-Power-Plant Accidents" (PDF). The New England Journal of Medicine. doi:10.1056/NEJMra1103676. PMID 21506737. {{cite journal}}: Unknown parameter |month= ignored (help)
• Detailed measurements of radiation levels in air at Fukushima I