Dr. Tsubokura's Radiation Lecture Vol.102

Author: Masaharu Tsubokura

Editors: Akihiko Ozaki M.D., Yuki Senoo

103. Circumstances vary after nuclear accidents

　　　　Nuclear accidents are rated according to the International Nuclear Events Scale from Level 0 to Level 7, depending on their impact and intensity. Among global nuclear accidents, those at the Chernobyl power plant in 1986 and Fukushima Daiichi nuclear power plant in 2011 were the only accidents rated Level 7. Although the classification of these two accidents is the same, the circumstances and subsequent countermeasures were not.

　　　　First, the type of radioactive materials and their levels were different at Chernobyl and Fukushima. The levels of iodine-131, cesium-137, strontium-90, and plutonium-239 emitted after the nuclear accident at Fukushima were about 1/10th, 1/6th, 1/50th, and 1/10,000th, respectively, of those after the Chernobyl accident.

In particular, levels of strontium and plutonium released from the Fukushima Daiichi nuclear power plant accident were lower compared to the Chernobyl accident. Therefore, almost all of the additional radiation exposure recorded in the Fukushima prefecture today is from cesium. On the other hand, during the Chernobyl accident, 90% of the internal exposure was caused by cesium, and 10% was from strontium. For this reason, the countermeasures taken after the Chernobyl disaster were based on a wider variety of radioactive materials than after the Fukushima nuclear disaster.

104. The difference in the levels of thyroid gland exposure between the nuclear accidents in Chernobyl and Fukushima

　　　　The Fukushima Daiichi nuclear power plant accident and the Chernobyl disaster in 1986 are the only nuclear accidents classified as Level 7 according to the International Nuclear Events Scale. Although these two accidents were rated the same, the circumstances and subsequent countermeasures differ.

　　　　One of the main differences between these two nuclear accidents is the levels of thyroid exposure to radioactive iodine. To prevent exposure to radioactive iodine, which has a short half-life of eight days, it was essential to control the circulation of milk and dairy products produced from cows that ate contaminated grass immediately after the 2011 Fukushima accident and to avoid giving contaminated milk to children. This countermeasure was prioritized based on the fact that drinking contaminated milk was the main cause of thyroid exposure to radioactive iodine after the Chernobyl accident. Unfortunately, the reported exposure of thyroid glands was much higher after the Chernobyl nuclear accident than after the Fukushima nuclear accident. One plausible explanation is that early evacuation and food control were carried out in Fukushima. Additionally, in Chernobyl, information regarding the accident was not sufficiently disseminated to the surrounding areas, and the contaminated areas were very broad.

Although there are various estimates regarding the levels of thyroid gland exposure in Fukushima after the nuclear accident, it has been established that levels were low. Furthermore, the average and maximum levels of thyroid gland exposure were about two orders of magnitude smaller than those after the Chernobyl disaster. This is one of the reasons why radiation exposure caused by the nuclear accident is unlikely to be the major cause of current thyroid cancer in the Fukushima prefecture.

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The Japanese version of the manuscript was originally published in Fukushima Minyu, a local newspaper in Fukushima prefecture, Japan, 2nd and 9th December 2018 was reproduced for MRIC Global under the author's permission.