A forecast of the likely radiation released in another nuclear accident shows that at four plants a 30-kilometer evacuation zone would be insufficient for public safety, and that more distant residents would need to flee their homes too.
The findings mean additional local authorities may need to draw up contingency plans for evacuations, and power companies may need to seek the approval of those entities for reactor restarts.
In the first published survey of its kind, the Nuclear Regulation Authority on Oct. 24 released the results of a study that used the spill from the Fukushima No. 1 nuclear power plant and modeled likely fallout at plants in 16 other locations, taking into consideration factors such as reactor size and local meteorological data.
The study found that at four plants, radiation doses more than 30 km away would exceed the current safety threshold and trigger evacuations.
It was the first time the central government had assessed the impact of a serious accident at each of Japan's nuclear plants and released the results.
In response to the Fukushima disaster, the Nuclear Regulation Authority plans to define a new zone within which local authorities must draft an evacuation strategy for all residents. It decided to expand the zone from between 8 and 10 km to 30 km, in line with an International Atomic Energy Agency recommendation. Accordingly, local governments will need to compile new contingency plans for nuclear plants by March.
Furthermore, the Nuclear Regulation Authority has adopted the IAEA standard of a radiation dose of 100 millisieverts, acquired cumulatively over the course of a week, in delineating mandatory evacuation zones.
The latest report provides a reference point for prefectural governments as they go about drawing up new evacuation zones around nuclear plants.
The study assumed that a future accident would be similar to that at the Fukushima No. 1 plant, releasing a comparable amount of radioactive materials. The study also considered the number of reactors at each plant and their generating capacity.
Based on those assumptions, the likely accumulated radiation exposure level was found to exceed 100 millisieverts farther than 30 km from four plants: Oi in Fukui Prefecture, Kashiwazaki-Kariwa in Niigata Prefecture, Fukushima No. 2 in Fukushima Prefecture and Hamaoka in Shizuoka Prefecture.
Calculations considered the direction and distance that radioactive materials would spread. Hourly meteorological observations measured at the plant premises, such as wind direction, velocity and rainfall, were applied to all areas around the facilities.
But the calculations had several important limitations. Local topographical features such as mountains were not factored in.
The study found that in the case of the Oi plant, currently Japan's sole operating nuclear plant, radioactive materials would likely spread in a south-southwesterly to southeasterly direction.
Accumulated radiation exposure would exceed 100 millisieverts in a part of Kyoto city which is located 32.2 km south of the plant.
The high concentration of nuclear plants in Fukui Prefecture means that some municipalities there could face high radiation levels from more than one plant.
For example, the city of Tsuruga would be affected by accidents at either the Tsuruga plant operated by Japan Atomic Power Co. or the Mihama plant operated by Kansai Electric Power Co.
Likewise, the municipalities of Obama and Oi in Fukui Prefecture and Nantan in Kyoto Prefecture would face high radiation levels if accidents occurred at either of Kansai Electric's Oi or Takahama plants.
And if there was a major accident at the Takahama plant, the area around the Oi plant would reach levels requiring evacuation. That means that the Oi plant itself would be affected in the event of an accident at its Takahama neighbor.
Moreover, if a massive earthquake or huge tsunami should hit the area, there is the strong possibility that accidents could occur at more than one nuclear plant simultaneously.
In fact, after the March 2011 earthquake and tsunami, it was not only the Fukushima No. 1 plant that faced difficulties, but also the Fukushima No. 2 plant—for a certain amount of time.
If accidents should occur at more than one plant simultaneously, evacuation and disaster management would be complicated by the possible spread of airborne radioactive materials from multiple directions, a factor that plans for a single accident would not necessarily take into consideration.
Meanwhile, the study found that radioactive materials would spread most widely from an accident at the Kashiwazaki-Kariwa plant, reaching likely evacuation levels even in Uonuma, Niigata Prefecture, which lies 40.2 km east-southeast of the plant.
This plant was forecast to release the largest volume of radioactive materials because it has seven reactors and it has the largest total generating capacity of any plant in Japan.
The Nuclear Regulation Authority's planned extension of its prepare-to-evacuate zone to 30 km meant that the number of municipalities required to compile evacuation plans will increase to 135 across 21 prefectures, up from the earlier total of 45 in 15 prefectures.
And the latest findings mean that additional municipalities may need to draw up plans too, faced with possible evacuation-triggering fallout levels outside the 30-km zone.
A key figure used in the forecast was the modeled release of 770 quadrillion becquerels of radioactive materials (a quadrillion is one thousand trillion). This was the size of the atmospheric release during the Fukushima nuclear accident.
In the event of another accident, local weather conditions will affect the spread of such materials. However, the forecast assumed that the wind direction at the time the release began continued for a week. It considered meteorological records in determining the possible direction of a radioactive plume.
The map produced showed which areas, in 16 different points of the compass from the nuclear plant, could end up with accumulated radiation exposure levels of 100 millisieverts.
The wide area of forecasted contamination means local governments will face a greater burden in compiling effective disaster management plans.
However, Shunichi Tanaka, chairman of the Nuclear Regulation Authority, has called such plans a minimum requirement before nuclear plants can be cleared for a restart.
If local governments cannot compile effective disaster management plans, it means the security of local residents is not assured. In such circumstances, not only would it be difficult to resume operations at such plants, but the very existence of those plants could be called into question.
In the past, electric power companies needed to fulfill few conditions before constructing new plants or additional reactors at existing ones, and reactivating them. They needed the approval of the central government and the consent of both the hosting municipal governments and corresponding prefectural governments.
If the number of municipalities that could be affected by a nuclear accident is increased, the utilities will need to take into consideration the greater number of local governments that will have to draw up disaster management plans.
Some local governments have already made known their opposition to reactivating nuclear power plants. That means electric power companies will face even longer delays before obtaining the support of such governments.
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