Fukushima: Radiation Exposure

(Updated February 2016)


Radioactive releases are measured by the amount of (radio)activity in the material, and quoted in Becquerels. Whether this is in the air or settled on the ground, it may expose people to ionizing radiation, and the effect of this is measured in Sieverts, or more typically milliSieverts (mSv). Exposure to ionizing radiation can also be by direct radiation from the plants and fuels themselves, though not released to the environment. This is only a hazard for those on the plant site, and the level diminishes with distance from the radioactive source. It is the chief hazard for the plant workers, who wear film badges so that the dose can be monitored. A short-term dose of 1000 mSv (1 Sv) is about the threshold of acute radiation syndrome (sickness). An instant dose of 100-250 mSv can slightly increase the risk of later developing cancer, but if this dose is spread over time there is less risk of any effect. On 17 March, NISA set 250 mSv as the maximum allowable dose for Fukushima recovery workers, under health physics controls. At the end of October this was reduced to 100 mSv for new workers. The International Commission on Radiological Protection (ICRP) allows up to 500 mSv for workers in emergency rescue operations.

Radioactivity in the cooling water flowing through the core is mainly the activation product nitrogen-16, formed by neutron capture from oxygen. N-16 has a half-life on only 7 seconds but produces high-energy gamma radiation during decay. (It is the reason that access to a BWR turbine hall is restricted during actual operation.) There is also often some leakage from fuel elements of fission products, including noble gases and iodine-131.

Regarding releases to air and water leakage from Fukushima, the main radionuclide from among the many kinds of fission products in the fuel was volatile iodine-131, which has a half-life of 8 days. Iodine-131 decays to inert and stable xenon-131. Iodine is readily taken up by the body and accumulates in the thyroid gland. Three months after the accident (after fission ceased) I-131 had virtually disappeared as a problem.

The other main radionuclide is caesium-137, which has a 30-year half-life, is easily carried in a plume, and when it lands it may contaminate land for some time. It is a strong gamma-emitter in its decay. Cs-134 is also produced and dispersed, it has a 2-year half-life. Caesium is soluble and can be taken into the body, but does not concentrate in any particular organs, and has a biological half-life of about 70 days. In assessing the significance of atmospheric releases, the Cs-137 figure is multiplied by 40 and added to the I-131 number to give an 'iodine-131 equivalent' figure.

Radioactive releases

Supplement to information in main paper:
After the hydrogen explosion in unit 1 on 12 March, some radioactive caesium and iodine were detected in the vicinity of the plant, having been released via the venting. Further I-131 and Cs-137 and Cs-134 were apparently released during the following few days, particularly following the hydrogen explosion at unit 3 on 14th and at unit 4 on 15th. Considerable amounts of xenon-133 and iodine-131 were vented, but most of the caesium-137 (14 out of 15 PBq total) along with most of the Cs-134 apparently came from unit 2 on or after the 15th – the only one of the four units which did not suffer a hydrogen explosion demolishing its superstructure. Also ten times more iodine is attributed to unit 2 than unit 1, while unit 3 produced half as much as unit 1. However, there remains some uncertainty about the exact sources and timings of the radioactive releases.

On 16 March, Japan’s Nuclear Safety Commission recommended local authorities to instruct evacuees under 40 years of age leaving the 20 km zone to ingest stable iodine as a precaution against ingestion (eg via milk) of radioactive iodine-131. The pills and syrup (for children) had been pre-positioned at evacuation centres. The order recommended taking a single dose, with an amount dependent on age. However, it is not clear that this was implemented. On 11 April the government suggested that those outside the 20km zone who were likely to accumulate 20 mSv total dose should move out within a month. Data at the end of May (with most I-131 gone by decay) showed that about half of the 20 km evacuation zone and a similar area to the NW, total about 1000 sq km, would give an annual dose of 20 mSv to March 2012.

France's Institute for Radiological Protection & Nuclear Safety (IRSN) estimated that maximum external doses to people living around the plant were unlikely to exceed 30 mSv/yr in the first year. This was based on airborne measurements between 30 March and 4 April, and appears to be confirmed by the above figures. It compares with natural background levels mostly 2-3 mSv/yr, but ranging up to 50 mSv/yr elsewhere in the world.

The main concentration of radioactive pollution stretches northwest from the plant, and levels of Cs-137 reached over 3 MBq/m2 in soil here, out to 35km away.  In mid-May about 15,000 residents in a contaminated area 20-40 km northwest of the plant were evacuated, making a total of over 100,000 displaced persons.

The IAEA reported on 19 March that airborne radiation levels had spiked three times since the earthquake, notably early on 15th (400 mSv/hr near unit 3), but had stabilized since 16th at levels significantly higher than the normal levels, but within the range that allows workers to continue on-site recovery measures.

NISA estimated that about 130 PBq of iodine-131 was released from the reactors, mostly around 15 March and the two days following – 0.16% of the total inventory. In 32 days this released iodine would have diminished to one-sixteenth of original activity – 8 PBq. NISA's report to IAEA said that this 130 PBq of I-131 together with 6 PBq of caesium-137* released gave an "iodine-131 equivalent" figure of 370 PBq, which resulted in the re-rating of the accident to INES level 7. NISA in June increased this estimate to 770 PBq (I-131 eq.), being 160 PBq of I-131 and 15 PBq of Cs-137.  Japan's Nuclear Safety Commission (NSC, a policy body) estimated that 12 PBq of Cs-137 had been released, giving an "iodine-131 equivalent" figure of 630 PBq to 5 April, but in August lowered this estimate to 570 PBq.

The 770 PBq figure is about 15% of the Chernobyl release of 5200 PBq iodine-131 equivalent. The NSC said that most radioactive material was released from the unit 2 suppression chamber during two days from its apparent rupture early on 15 March.  It said that about 154 TBq/day was being released on 5 April, but that this had dropped to about 24 TBq/d over three weeks to 26 April and to about 24 GBq/d in mid July. In mid-August 2011 the estimate from all three reactors together was about 5 GBq/d. In 2014 Fukushima University’s Institute of Environmental Radioactivity said that the total amount of Cs-137 released was 20.5 PBq, 17 PBq to the air, and of the total, 12 to 15 PBq ended up in the Pacific Ocean. The 17 PBq to air, coupled with the I-131, would give 810 PBq (I-131 eq).

* The Cs-137 figure is multiplied by 40 in arriving at an "iodine-131 equivalent" figure, due to its much longer half-life. Cs-134 is multiplied by 4.

Tepco estimates published in May 2012 showed a total of about 1020 PBq released to the atmosphere over 12-31 March 2011 (after which very little was released).  Apart from noble gases this comprised 500 PBq iodine-131, 10 PBq Cs-137 and 10 PBq Cs-134. In iodine-131 equivalent terms this comes to 500 + 400 + 40 = 940 PBq iodine-131 eq released to atmosphere. In addition, 500 PBq noble gases was estimated, mainly xenon-133.  This is normally disregarded since is not biologically active and has only a 5-day half-life. Of the total releases, about 20% came from Unit 1, 40% from Unit 2 (peak on 15 March), and 40% from Unit 3 (peak on 16 March). Releases to the ocean over 26 March to 30 September were about 11 PBq iodine-13, 3.5 PBq Cs-134, 3.6 PBq Cs-137, total 18.1 PBq (or 169 PBq I-131 eq) apart from atmospheric fallout.  Relatively little radioactive material was released by the active venting of pressure inside the reactor vessels (routing steam through water and releasing it through the exhaust stacks) or by the hydrogen explosions.

Radiation effects

Supplement to information in main paper:
No harmful health effects were found in 195,345 residents living in the vicinity of the plant who were screened by the end of May 2011. All the 1,080 children tested for thyroid gland exposure showed results within safe limits, according to the report submitted to IAEA in June. By December, government health checks of some 1700 residents who were evacuated from three municipalities showed that two-thirds received an external radiation dose within the normal international limit of 1 mSv/yr, 98% were below 5 mSv/yr, and ten people were exposed to more than 10 mSv. So while the was no major public exposure, let alone deaths from radiation, there were reportedly 761 victims of "disaster-related death", especially old people uprooted from homes and hospital because of forced evacuation and other nuclear-related measures. The psychological trauma of evacuation was a bigger health risk for most than any likely exposure from early return to homes, according to some local authorities.*

* eg Dr Shunichi Yamashita, VP Fukushima Medical University.

In July 2012 a Hirosaki University study reported on I-131 activity in the thyroid of 46 out of the 62 residents and evacuees subject to detailed investigation in April 2011. The median thyroid equivalent dose was estimated to be 4.2 mSv and 3.5 mSv for children and adults respectively, much smaller than the mean thyroid dose in the Chernobyl accident (490 mSv in evacuees). Maximum thyroid equivalent doses for children and adults were 23 mSv and 33 mSv, respectively.  This is consistent with health authorities' screening tests on 1149 children under 15 in March 2011.  Working from these data to estimated maximum doses in the worst-exposed areas in the first week after the accident it was estimated that some children could have received more than 50 mSv dose, still only about one tenth of Chernobyl evacuees.*

* Shinji Tokonami et al., Thyroid doses for evacuees from the Fukushima nuclear accident, Scientific Reports, 2:507, DOI: 10.1038/srep00507

The residents of Minamisoma town, on the coast 23 km north of Fukushima Daiichi, were found to have very low levels of radiation contamination. In a study of internal radiation dose, measurements were taken of the full-body contamination from caesium exposure of 9498 residents who had returned to the town and stayed there between September 2011 and March 2012. The study found that two-thirds of the residents had no detectable levels of caesium. Of the rest, only one appeared to have received an equivalent dose more than 1 mSv, and that was 1.07 mSv. The current ambient dose rate in the town is about 3 mSv/yr from external sources, well within the government's 20 mSv/yr limit for returnees. Some 1500 of the town's 70,000 residents lost their lives in the tsunami. The internal dose results were published in the Journal of the American Medical Association.

In October 2012 the new Nuclear Regulatory Authority (NRA) released new emergency preparedness guidelines. Its new emergency planning zones, in line with International Atomic Energy Agency standards, call for 'precautionary action zones' 5 kilometers around nuclear energy facilities and 'urgent protective action planning zones' 30 km around the plants. NRA then drew up specific evacuation criteria, which local municipalities will use to formulate emergency response plans.

Japan's health ministry set up a special office to monitor the health of workers at the plant. The new office compiles data on radiation exposure for workers for long-term monitoring purposes, and inspects daily work schedules in advance. To March 2013 Tepco has employed some 25,837 workers at the site since the accident, keeping records of their radiation exposure as clean-up and remediation proceeded. Of these, over 95% received less than 50 mSv during the 25 month period; 4% received 50-100 mSv and fewer than 1% received over 100 mSv.

Return of evacuees

Supplement to information in main paper:
From April 2012 part of the 20-km radius area and the portion of Minami Soma city extending north from it, were recategorised by the Environment Ministry: below 20 mSv/yr, evacuation called off; 20-50 mSv/yr "restrict residency", allowing entry for specific purposes with no protective equipment required and remediation action to be completed in March 2014 (now 2017); and over 50 mSv/yr "difficulty of return", with restricted entry and remediation deferred. These restricted areas, comprising about half of the 20-km radius evacuation zone, are not expected to drop below 20 mSv/yr before about March 2016. Such areas add to those devastated by the tsunami, where rebuilding is very uncertain.

In July 2012 this zoning was extended by METI to Iitate Village, an area 28-45 km northwest of the Daiichi plant and contiguous with the northern part of Minami Soma city. Most of Iitate’s residents can now return without protective gear or monitoring but not stay overnight, since annual exposure on a continuous basis would be no more than 20 mSv in some parts and 20-50 mSv in others. However, one area of Iitate (about 10 sq km), and a part of Minami Soma joining it and extending into the 20-km radius, remain fully evacuated. In August the Naraha town area south of the plant was added to the minimally-restricted category, below 20 mSv if continually occupied. In December parts of Okuma-machi southwest of the plant and close to it were de-restricted on the same basis. Thus more than half of the original evacuated area (20 km radius plus Iitate and Minami Soma) was then accessible without protective gear or monitoring. The area of Namie town and also Katsurao village outside the 20 km radius remain uncategorised. In March 2013 the government reopened Tomioka and Namie towns, making nine areas to secure redesignation.

From July 2012 to August 2013 this zoning was extended by METI to all municipalities affected by radioactive fallout, both within the 20 km zone and extending beyond it to about 45 km northwest. The extended area includes Iitate, Minami Soma, Namie, Tomioka, Katsurao and Kawamata. Within the 20 km zone, Futaba, close to the plant, was the only municipality still closed to any return of evacuees, but the government relaxed restrictions for the town in May 2013. Most of the houses were destroyed in the tsunami, so return of evacuees is limited by that. Of the total evacuees from around the Fukushima Daiichi plant, some 72,800 lived in the towns and villages of Futaba district. The Futaba district was heavily dependent economically on the plant, with much of its industry geared towards the power sector.

Japan's Reconstruction Agency < http://www.reconstruction.go.jp/english/> reviewed the human toll of the evacuation and reported in August 2012. The stresses of personal involvement in the evacuation, management and clean-up emerged as the biggest factors in ill health for the people affected. Transfer trauma - the mental or physical burden of the forced move from their homes was the cause of 34 early deaths, almost all elderly, reported then. This figure compares with 1916 people from Fukushima, Iwate and Miyagi prefectures that died during evacuation from areas hit by the tsunami and the earthquake. (The figure is additional to the 19,000 that died in the actual tsunami.) The leading causes of most of those early deaths were disruption to the smooth operation of hospitals, the exacerbation of pre-existing health problems, and the transfer trauma or general 'mental fatigue' from dramatic changes in life situation.
Evacuees dying after over a year of fatigue and despair, Japan Daily Press, 13 July 2012

Managing contaminated water, marine effects

Supplement to information in main paper:
A four-year international survey assessing radiological pollution of the marine environment near the plant commenced in July 2011, under IAEA auspices and led by Australia, South Korea and Indonesia. In September 2011, researchers at the Japan Atomic Energy Agency, Kyoto University and other institutes estimated that about 15 PBq of radioactivity (I-131 and Cs-137) had been released into the sea from late March through April, including substantial airborne fallout. In August 2013 Tepco estimated that 20 to 40 TBq of tritium might have leaked into the sea over 28 months since May 2011, which it compared with 22 TBq/yr discharge limit from the six-unit plant normally. The nine-month estimated releases from December 2012 for Sr-90 and Cs-137 were 0.7 and 1.0 TBq respectively, compared with 0.22 TBq/yr combined discharge limit. This is going into the 30 hectare inner harbour area, which is barricaded from the open sea. The radioactive contamination in the sea adjacent to the plant has remained much the same since early 2012, however, at harmless levels. Beyond the barricaded inner harbour, sampling out to 15km has indicated no detectable contamination since December 2011. (In 2013, Cs-134 levels of 0.9 Bq/m3 from Fukushima – 8000 times less than drinking water standards – were detected offshore Vancouver, enabling helpful study of ocean currents.) Radioactive Isotopes from Fukushima Meltdown Detected near Vancouver, Scientific American, 25 February 2014

In July 2014 Tepco began coating 18 ha of sea bed in the plant’s port using cement with bentonite or sand to prevent contaminated sediments moving.

In August 2013 a leak of partly-treated water was discovered and rectified. The water concerned in the puddle on the ground (concentrated by evaporation in hot weather) had 80 MBq/L, and it was initially considered that about 300 m3 (24 TBq) had leaked into the soil in the immediate vicinity of the tank and some possibly moved further. Tepco said that it was the most serious event at the plant since the March 2011 accident, and that "There is a possibility that contaminated earth and sand flowed into the drainage. We cannot rule out the possibility that part of the contaminated water flowed into the sea." The NRA classified the incident as a Level 1 ‘Anomaly’ on INES scale, but a week later raised that to provisional Level 3. At the end of June 2014 Tepco reported to the NRA that it had recovered 80% of the leaked radionuclides by removing contaminated soil, and the other 20% remained deeper but in situ. Tepco said that the leaked water had contained about 45 TBq of strontium activity.


You may also be interested in