History of the accident
The accident at the nuclear complex of Fukushima on March 11th, 2011 resulted in a severe release of around 135 radionuclides (IRSN, 2012). It was the consequence of a high-magnitude earthquake (~9.0), which created several massive tsunamis that struck Japan almost 1 h later (Akahane et al., 2012) causing power loss, disruption of the cooling systems and finally hydrogen explosions. As a result, several radionuclides in gaseous or aerosol form were released at elevated levels and dispersed over long distances following the prevailing meteorological conditions of the following days. Air leakages from the FDNPP stopped around 42 d after (Stohl et al., 2012).
Regarding releases to air and also 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. 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 two-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.
As cooling failed on the first day, evacuations were progressively ordered, due to uncertainty about what was happening inside the reactors and the possible effects. By the evening of Saturday 12 March the evacuation zone had been extended to 20 km from the plant. From 20 to 30 km from the plant, the criterion of 20 mSv/yr dose rate was applied to determine evacuation, and is now the criterion for return being allowed. 20 mSv/yr was also the general limit set for children’s dose rate related to outdoor activities, but there were calls to reduce this.In areas with 20-50 mSv/yr from April 2012 residency is restricted, with remediation action taken.
A significant problem in tracking radioactive release was that 23 out of the 24 radiation monitoring stations on the plant site were disabled by the tsunami.
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 in 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 centers. 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-1 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-1, but ranging up to 50 mSv/yr elswhere.
The main concentration of radioactive pollution stretches northwest from the plant, and levels of Cs-137 reached over 3 MBq m-2 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-1 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-1 in mid-July. In mid-August 2011 the estimate from all three reactors together was about 5 GBq d-1. 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 five-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.
Tepco sprayed a dust-suppressing polymer resin around the plant to ensure that fallout from mid-March was not mobilized by wind or rain. In addition it removed a lot of rubble with remote control front-end loaders, and this further reduced ambient radiation levels, halving them near unit 1. The highest radiation levels on site came from debris left on the ground after the explosions at units 3&4.
Radioactivity, primarily from caesium-137, in the evacuation zone and other areas beyond it has been reported in terms of kBq/kg (compared with kBq/m2 around Chernobyl). However the main measure has been presumed doses in mSv/yr. The government has adopted 20 mSv/yr as its goal for the evacuation zone and more contaminated areas outside it, and supports municipal government work to reduce levels below that. The total area under consideration for attention is 13,000 km2.