TOKYO -- The Nuclear Regulation Authority (NRA) is poised to instruct the operator of three nuclear plants in Fukui Prefecture on the Sea of Japan to strengthen measures to protect the complexes for fear that an unexpected amount of volcanic ash could fall in case of a volcanic eruption in Tottori Prefecture.

The move by the government's nuclear watchdog follows the finding that past eruptions of Mount Daisen, the volcano in question, may have been larger than believed.

The NRA will make a final decision on whether to issue such an instruction after listening to opinions from Kansai Electric Power Co. (KEPCO), which operates the three nuclear complexes -- Takahama, Oi and Mihama nuclear plants.

It will be the first time for the authority to give an instruction to a nuclear plant operator under the so-called back-fitting system set by the Act on the Regulation of Nuclear Source Material, Nuclear Fuel Material and Reactors, which reflects the latest scientific knowledge.

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)

The system was incorporated into the law when it was revised after the March 2011 outbreak of the Fukushima nuclear crisis. Before the introduction of the new system, operators of atomic power stations were not necessarily obligated to implement the most advanced safety measures once the reactors passed safety inspections conducted by the nuclear regulator.

If a massive amount of volcanic ash falls on nuclear plants, the ash could clog filters inside power generators that are used in cooling reactor cores in case of an emergency situation, forcing their operators to take countermeasures such as replacing the filters more frequently than usual.

It was assumed that volcanic ash about 10 centimeters thick could pile up on the premises of the three KEPCO nuclear plants in Fukui Prefecture when they passed safety inspections by the NRA.

However, a new academic paper stating that past eruptions of Mount Daisen were larger than previously believed has been released, forcing the NRA in December 2018 to instruct KEPCO to reassess the impact of volcanic eruptions of Mount Daisen on the three plants. The utility then revised its estimate of volcanic ash that could pile up on the premises of these plants in case of an eruption to 13.5 to 21.9 centimeters thick.

As Mount Daisen is not an active volcano and faces no imminent danger, however, the authority will not demand that KEPCO stop operations at the No. 3 and 4 reactors at Oi nuclear plant and the No. 3 and 4 units at Takahama plant that have been restarted.

KEPCO intends to basically follow the NRA's instructions on safety measures at the utility's nuclear plants.

"We intend to apply for permission for changing the conditions for establishing these plants in an appropriate manner. We'll continue to sincerely respond to inspections by the NRA," said an official of the utility.

(Japanese original by Suzuko Araki, Science & Environment News Department, and Yuhi Sugiyama, Business News Department)