The Nuclear Regulation Authority (NRA) on Dec. 13 released corrections to all of its radiation forecast maps, which show the likely spread of radioactive substances from a serious accident at 16 nuclear power plants across Japan.
The corrections affect all of the nuclear plants under study, which do not include the Fukushima No. 1 nuclear plant, site of the reactor meltdowns following an earthquake and tsunami in March 2011, or the Monju prototype fast-breeder reactor.
The forecast maps show the likely spread of radioactive substances in the event of an accident similar in scale to the Fukushima disaster, and the corrections are expected to affect the ongoing efforts by local governments situated near nuclear plants to draw up emergency evacuation plans.
Errors in the maps have been found on successive occasions since the nuclear regulatory agency initially released them on Oct. 24.
The most significant errors discovered in the recent overhaul concerned the forecast maps for the Tomari plant in Hokkaido, the Genkai plant in Saga Prefecture, and the Sendai plant in Kagoshima Prefecture.
The previous map for the Tomari plant was found to have overestimated the area of likely spread because of errors in weather data input. The maps for the Genkai and Sendai plants had to be turned 180 degrees because Kyushu Electric Power Co., operator of the plants, had mixed up the windward and leeward directions.
Local governments have been urged to draw up evacuation plans for areas where the weekly radiation doses are likely to reach 100 millisieverts, and the Dec. 13 corrections affect five municipalities in that regard.
Kutchan and Iwanai, both in Hokkaido, and Akune, Kagoshima Prefecture, were removed from the list of municipalities with areas of 100-millisievert weekly doses, while Tokamachi, Niigata Prefecture, and Sasebo, Nagasaki Prefecture, were added to the list.
Tokamachi, which was initially on the list but was delisted with the Oct. 29 correction, was put on the list again.
The Oct. 29 corrections included 22.5-degree rotations of the forecast maps for six nuclear plants, either clockwise or counterclockwise. The mix-up of windward and leeward directions for the Genkai and Sendai plants was made public by the NRA on Nov. 6. The NRA said it would release corrected maps for the two plants on Nov. 8, but they were not released on that day because more errors were discovered. The NRA instead decided to conduct the latest overhaul on all forecast maps for the 16 nuclear plants.
The NRA said its predecessor, the Nuclear and Industrial Safety Agency (NISA), gave ambiguous instructions when it commissioned the Japan Nuclear Energy Safety Organization (JNES) in March to create the forecast maps. JNES, which won the contract, failed to double-check the specifications, while the NRA also failed to define the procedures for checking them after it took over the mission from the now-defunct NISA.
The NRA on Dec. 13 verbally admonished three officials in charge of the matter.
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