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Threat of volcanic ash at 3 nuclear plants in Fukui Pref. greater than expected

From foreground, the No. 3, No. 2 and No. 1 reactors at Kansai Electric Power Co.'s Mihama Nuclear Power Plant in Mihama, Fukui Prefecture, are pictured in this file photo taken in November 2018. (Mainichi/Naohiro Yamada)

Japan's Nuclear Regulation Authority (NRA) on April 17 decided to revise upward a prediction for the amount of ash that could fall within the grounds of three nuclear power plants in Fukui Prefecture on the Sea of Japan in the event of a volcanic eruption.

It was originally predicted that volcanic ash could reach a depth of 10 centimeters within the grounds of the Oi, Mihama, and Takahama nuclear power plants operated by Kansai Electric Power Co. (KEPCO) in the central Japan prefecture.

Seven nuclear reactors at the three power plants have already passed new NRA safety screening standards introduced in the wake of the 2011 Fukushima nuclear disaster, but part of the screening on measures to counter volcanic ash will now be redone. The nuclear watchdog is concerned that if an amount of volcanic ash above the envisaged level falls on the premises of the nuclear plants, it could block the filters of emergency generators, which are needed to cool nuclear fuel to prevent meltdowns.

After the meltdowns at the Fukushima Daiichi Nuclear Power Plant following the March 2011 Great East Japan Earthquake and tsunami, all of Japan's nuclear power plants were subjected to new safety screenings, and Japan was temporarily left with no nuclear plants in operation.

This file photo taken on June 1, 2018 shows Mount Daisen in Tottori Prefecture. (Mainichi/Yoshiyuki Hirakawa)

Reactor Nos. 3 and 4 at the Oi Nuclear Power Plant and Nos. 3 and 4 at the Takahama Nuclear Power Plant have since been restarted. The NRA does not plan to ask that their operations be suspended again in light of its most recent decision, on the grounds that an eruption is not an imminent threat.

Simulations of an eruption of Mount Daisen in Tottori Prefecture in western Japan, about 200 kilometers away from the Fukui Prefecture plants, could result in volcanic ash reaching a depth of 10 centimeters at the plants, and the NRA judged that KEPCO's measures to counter this were appropriate.

New research, however, indicates that when Mount Daisen erupted some 80,000 years ago, volcanic ash reached a depth of 30 centimeters in the city of Kyoto, the capital of Kyoto Prefecture which neighbors Fukui Prefecture. The NRA instructed KEPCO in December last year to conduct a new estimation of the amount of ash that could fall. The power company has submitted documents stating that up to 21.9 centimeters of ash could fall on the grounds of the Takahama plant, 19.3 centimeters at the Oi plant, and 13.5 centimeters at the Mihama plant.

KEPCO had stressed that there was no need for renewed screening as the chance of an eruption of this scale was low. NRA Chairman Toyoshi Fuketa, however, rejected this notion, saying the possibility was not low enough to exclude the possibility from evaluations.

A KEPCO representative said the power company would respond sincerely to the NRA's decision.

(Japanese original by Riki Iwama, Science & Environment News Department)

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