Dr. Tsubokura's Radiation Lecture Vol.104
Author: Masaharu Tsubokura
Editors: Akihiko Ozaki M.D., Yuki Senoo
107. Differences between two post-nuclear disaster evacuation plans
Among all the nuclear incidents that have occurred in the past, the Fukushima Daiichi nuclear power plant accident and the 1986 Chernobyl disaster are the only such events classified as level 7 accidents according to the International Nuclear Events Scale. Although these two events received the same rating, their circumstances and subsequent countermeasures differed.
In both cases, evacuation plans were formulated based on the estimated annual accumulated air dose rate. However, their estimated levels differed.
Following the Chernobyl accident, the air dose rate criterion for evacuation was a minimum of 100 millisieverts per year for the first year, meaning that it was mandatory for the residents living in areas with annual air dose rates exceeding 100 millisieverts to evacuate. This decreased to 30 millisieverts in the second year, 25 millisieverts in the third and fourth years, 20 millisieverts in the fifth year, and 5 millisieverts in the sixth year and thereafter. On the other hand, after the Fukushima nuclear disaster, this criterion was set at an annual air dose rate of 20 millisieverts from the first year, and it remained at this level in subsequent years.
The levels of radioactive materials remaining in the areas surrounding these nuclear power plants were particularly high during the first year after the accidents. In this respect, the evacuation air dose rate criterion had a significant impact on the residents’ radiation exposure. When comparing the evacuation criteria for the first years after the nuclear accidents, the requirement for evacuation after the Fukushima accident was five times stricter than that imposed after the Chernobyl disaster. Moreover, this led to differences in residents’ radiation exposure levels of after these two nuclear accidents.
108. Different levels of internal radiation exposure due to the type of food consumed
The Fukushima Daiichi nuclear power plant accident and the 1986 Chernobyl disaster are the only nuclear incidents classified as level 7 accidents according to the International Nuclear Events Scale. However, although these two events received the same rating, their circumstances, and subsequent countermeasures differed.
The previous article explained that the thyroid gland radiation exposure levels caused by radioactive iodine after these two accidents differed by almost two orders of magnitude. In addition, the internal radiation exposure levels caused by radioactive cesium also differed.
Internal radiation exposure occurs when a person constantly consumes contaminated food. After the Chernobyl nuclear accident, although the air radiation dose rate gradually decreased with time, the internal exposure level remained high in some areas or even increased, leading long-term health issues.
In contrast, radioactive cesium has not been detected in residents of Fukushima or food produced there for several years. In addition to differences in historical backgrounds, the former Soviet Union and modern Japan differ in terms of their soil quality, food types, supply networks, and food investigation systems. Furthermore, the enormous efforts of the farmers in Fukushima have had a significant effect on preventing internal radiation exposure.
Therefore, any excessive radiation exposure related to the Fukushima nuclear accident is mostly caused by external radiation exposure. This is another difference between the Chernobyl disaster and the Fukushima nuclear accident.