Volcanic eruptions, especially giant events that occur only once every 10,000 years, can pose a serious threat to nuclear power plants.

Concerned parties, particularly the government’s Nuclear Regulation Authority in charge of safety screenings, should continue discussing how to deal with the risks.

Two different presiding judges at the Hiroshima High Court have given opposite decisions on restarting the No. 3 reactor at Shikoku Electric Power Co.’s Ikata nuclear plant in Ehime Prefecture in light of the risk of a potential eruption of Mount Aso in Kumamoto Prefecture.

The initial decision late last year approved residents’ request for an injunction against restarting the reactor.

But another decision handed down last month in response to Shikoku Electric’s appeal gave the green light to resume operations.

The NRA has internal rules for safety screenings titled a “volcano impact assessment guide.” The document sets procedures for evaluating risks posed by a potential eruption of a volcano within 160 kilometers of a nuclear power plant.

In accordance with the guide, the Hiroshima High Court assumed a catastrophic eruption of the sort that occurred 90,000 years ago at Mount Aso and studied the possibility of a pyroclastic flow reaching the Ikata plant located 130 km away.

The latest decision agreed with the initial decision in saying that no nuclear plant should be located at Ikata under the rules of the NRA’s guide.

The conclusions reached, however, were the opposite because of a difference in attitude toward “conventional wisdom.”

No particular regulations or measures are in place to prepare for a giant eruption in domains other than nuclear safety. The public does not appear to be seriously concerned about a catastrophic eruption.

Therefore, the risk of a giant eruption seems acceptable in light of conventional wisdom, according to the court’s second ruling.

The court’s initial decision showed an understanding toward a similar viewpoint, but it attached more importance to the guide, which is based on the NRA’s scientific and technological knowledge.

The regulatory body was set up following the 2011 Fukushima nuclear disaster.

The latest decision said the guide is unreasonable, partly because volcanic eruptions cannot be easily predicted, and instead derived its conclusion from conventional wisdom.

Question persists about a judicial decision that emphasized conventional wisdom, which has its own ambiguities.

The serious nature of a nuclear disaster is evidenced by the rigorous restrictions on entry into areas contaminated by radioactive fallout from the Fukushima disaster.

In-depth discussions are needed on what represents “conventional wisdom” on the issue of nuclear safety. The NRA should take the initial step.

The NRA in March issued an opinion statement in the name of its secretariat that said the risks from a giant eruption are acceptable in light of conventional wisdom. The Hiroshima High Court’s latest decision also referred to that opinion.

Some, however, have criticized the statement, saying the NRA abandoned its mission of providing scientific assessments.

In response to the court’s decision that said the volcano guide is unreasonable, NRA Chairman Toyoshi Fuketa, at a news conference, acknowledged the guide has legibility problems in parts and mentioned the possibility of revisions.

The revisions should not be limited to wording. The NRA should rethink the issue of nuclear safety related to volcanoes, including the role the watchdog will play.

Concerns about volcanic eruptions have been raised at other nuclear plants across Japan, including Kyushu Electric Power Co.’s Sendai plant in Kagoshima Prefecture.

We hope the NRA will spearhead a national debate on the matter.

--The Asahi Shimbun, Sept. 30