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Simulations of Radiation Leak at Tokai No. 2 Plant Released

Yomiuri Shimbun file photo
Japan Atomic Power Co.’s Tokai No. 2 nuclear plant

MITO (Jiji Press) — The Ibaraki prefectural government published Tuesday the results of simulations showing how radioactive materials would be dispersed if an accident occurred at Japan Atomic Power Co.’s Tokai No. 2 nuclear plant.

The results will be used to examine the effectiveness of evacuation plans for the area within 30 kilometers of the plant in the village of Tokai, which is home to some 920,000 people. It is the most populous among areas inside 30-kilometer radiuses of nuclear plants in Japan.

“The biggest point is that there is no need for 920,000 people to evacuate at the same time,” Ibaraki Gov. Kazuhiko Oigawa said at a press conference. “We hope to complete effective evacuation plans together with surrounding municipalities.”

The simulations were done under two scenarios—one assuming a situation that the Nuclear Regulation Authority deems in its safety reviews to be a serious accident and another in which safety equipment stopped functioning at once and forced evacuations in some areas.

They also assumed the worst weather conditions for preventing the spread of radioactive materials, such as wind blowing in the same direction for a long time.

The simulations showed a total of 22 patterns of air radiation dose rates in areas around the plant 24 hours after the release of radioactive materials.

Both scenarios assumed that residents of the area within 5 kilometers of the nuclear plant, called the Precautionary Action Zone, would evacuate before radioactive materials were released, while residents living between 5 to 30 kilometers of the plant, called the Urgent Protective Action Planning Zone, would retreat indoors.

In the first scenario, the simulations assumed that only 2.2 terabecquerels of radioactive materials would be released thanks to measures such as using vents with filters. In this case, no region saw radiation doses reach the 20-microsieverts-per-hour threshold requiring residents to evacuate within about a week.

In the second scenario, 430 terabecquerels of radioactive materials were assumed to have been released due to failures of safety equipment and damage to the reactor containment vessel. In this case, regions up to 30 kilometers away from the plant saw dose levels requiring residents to evacuate within a week.

Additionally, some regions approximately 6 kilometers from the plant, which are not within the Precautionary Action Zone subject to pre-release evacuation, were simulated to have logged dose levels of over 500 microsieverts per hour, the threshold necessitating immediate evacuation.

Up to around 105,000 people were deemed to have to evacuate within about a week under the second scenario.

However, it is technically unlikely that all of the permanent safety equipment would lose their functions at the same time.

Municipalities within 30 kilometers of nuclear power plants are obliged to draw up evacuation plans based on NRA guidelines. The Ibaraki prefectural government had been asking Japan Atomic Power to conduct simulations of the spread of radioactive substances, to verify the effectiveness such plans.

The prefectural government plans to secure materials, equipment and means of transportation specified in evacuation plans, as well as verify the maintenance of lifelines, based on the simulation results.

Mito District Court ordered Japan Atomic Power in March 2021 to halt operations at the Tokai No. 2 plant, saying there were inadequacies in evacuation plans drawn up by local municipalities. The plant operator is currently appealing the ruling.