The accident at the TEPCO Fukushima Daiichi Nuclear Power Plant: Workers and the evacuated/relocated populations

*Makoto Akashi, National Institute of Radiological Sciences 

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A huge earthquake struck the Pacific coast of eastern Japan at 14:46 on March 11, 2011. This earthquake was named The Great East Japan Earthquake. It triggered a tremendous tsunami. Unfortunately, it was then followed by another quake almost an hour later, and numerous sustained aftershocks followed. The number of aftershocks during March 2011 was 427; aftershocks were observed at frequencies of 678, until December 8 of the same year. The earthquake and tsunami resulted in 15,854 deaths and 3,155 missing (as of March 14, 2012, National Police Agency) (http://www.npa.go.jp/archive/keibi/biki/higaijokyo.pdf). The earthquake caused damage to the Fukushima Daiichi Nuclear Power Plant (NPP) of Tokyo Electric Power Co. (TEPCO). Reactors generate heat after the chain reaction is stopped because of radioactive decay of unstable isotopes and fission products. After shutdown, the temperature of the reactor core is still high before reaching a cold shutdown level. Nuclear fuel rods that have reached a temperature of cold shutdown typically require another several years of water-cooling in a spent fuel pool before they can be safely transferred to dry storage casks. Thus, the cooling system is extremely important. At the time of the earthquake on March 11, the operating units automatically shut down. Soon after, the emergency generator system came online to control the cooling systems. However, one of the two connections to off-site power for these Units failed due to the earthquake. There were two emergency diesel generators for each of Unit. Unfortunately, the earthquake was followed by a huge tsunami (13 to 15 m height) arriving approximately 50 minutes later, which topped the plant’s 5.7 m seawall, flooded the basement of the turbine buildings and disabled the emergency diesel generators located there, at approximately 15:41. As a result, approximately 1.8 x 1016 Bq of cesium-134, 1.5×1016 Bq of cesium-137, and 1.6×1017 Bq of iodine-131 were released into the atmosphere. More than 80,000 local residents living within a 20km radius of the plant and also other areas with high-dose rates have been evacuated from their homes. A "stay indoors" policy was also taken in the area 20-30 km from the NPP. The Fukushima prefecture started the program of health management for residents and announced the preliminary results in estimation of external doses for residents on February 20, 2012. This dose assessment was based on the questionnaires and the interview forms for all residents of Fukushima prefecture in collaboration with NIRS, and the accumulating doses during March11and July 11 2011were calculated using a software developed by NIRS. Results were presented in the website of Fukushima prefecture on February 24, 2012 (http://www.pref.fukushima.jp/imu/kenkoukanri/240220siryo.pdf in Japanese). Kawamata-town and Iitate-village are located over than 20-30 km from the NPP, whereas Namie-town is located within a 20km radius of the NPP and a no-entry zone. Since dose-rates in these areas were relatively higher than others, the dose estimation was first performed in 9,747 residents of these three areas. The accumulated doses in 94.6 % of residents (except workers) were lower than 5mSv and those over than 15 mSv were 0.1%. The maximum dose in this investigation was 23.0 mSv. Moreover, the assessment for internal dose was performed for 15,408 residents in collaboration with NIRS, the Japan Atomic Energy Agency (JAEA) and others. Radionuclides in the body were quantified by a whole body counter (WBC) on June 27- Jan 31 2012 and internal doses were estimated as committed effective doses. Since the physical half-life of iodine-131 was as short as 8 days, main radionuclides were cesium-134 and -137. Results were shown in the website of Fukushima prefecture (http://www.pref.fukushima.jp/imu/wbc/20120223WBC_joukyou.pdf in Japanese). The doses of 15,383 residents (99.8 %) were less than 1mSv and the maximum dose was 3 mSv (2 residents). A number of workers have been involved in the recovery or emergency works everyday at the NPP since March 11, 2011. The Japanese government revised the dose limit for emergency workers from 100 mSv to 250 mSv at the end of March 2011. This revision was applied to only emergency works at the TEPCO Fukushima NPP. The TEPCO performed the dose assessment for almost all workers of TEPCO and contractors involved in emergency works at this NPP. The assessment of dose was based on values of personal dosimeter for external dose and results of WBC for internal dose (Mach 11, 2011 to January 31, 2012). Results of 20,115 workers were shown at the TEPCO website (http://www.tepco.co.jp/en/press/corp-com/release/betu12_e/images/120131e5.pdf). The total accumulative exposure doses of 6 workers (0.03%) were higher than 250 mSv, and the maximum and the mean doses were 678.80 and 11.90 mSv, respectively. The number of workers whose doses were over than 100 mSv was 167 (0.83 %) and doses for 94.2 % of workers were less than 100 mSv. On the other hand, there was no worker with doses over than 100 mSv since Aril 2011. A hydrogen explosion occurred at the reactor building at 11:01 on March 14, and 7 workers and 4 Japan Self Defense (JSD) personnel were injured in this explosion. Four JSD personnel were brought to the local headquarters; all of them showed external heavy contamination. Radiation at a significant dose rate was detected at 10 cm from the surface. After removal of their protective gear, decontamination and taking a shower, the remaining contamination on the face was observed. One JSD member was transferred to the Fukushima Medical University Hospital, the secondary emergency hospital in Fukushima prefecture, by ambulance. The patient had a contaminated wound on the right thigh. He was then transferred to NIRS by JSD helicopter. At NIRS he was diagnosed as internal contamination with radioactive iodine and cesium. Fortunately, treatment was not required for him and there has been no patient requiring treatment in terms of radiation exposure and/or contamination with raionuclides in the Fukushima NPP accident. One of the most important lessons learned from past accidents is that misunderstandings and misconceptions cause excess anxiety, leading to psychological consequences. Radiation can cause medical and environmental effects. However, “radiation accidents” induce psychological and economic problems. Scientifically correct information about health issues is a key for the prevention of psychological consequences, and explanation of radiation risks and any countermeasures in plain language is a vital part of an effective risk communication process for not only the general public but also for emergency responders. However, radiation cannot be seen, heard, or felt, and knowledge of radiation exposure is not imparted in schools. This earthquake caused not only death and life-threatening injuries, but also had a tremendous impact on the infrastructures of the public and NPP. The complexity of disasters including damage to nuclear or radiation facilities, particularly in the case of earthquakes, has increased the need for multidisciplinary medical experts as critical assets in disaster responses. In our efforts for recovery from the damages, reconstruction of the medical system for radiation emergency in the affected areas has to be accelerated, since many workers are still involved in recovery work, with high risks of radiation exposure at the NPP site.