A recent court ruling on the operation of a nuclear power plant in western Japan has raised fundamental questions about the safety of nuclear power generation in a country sitting on chains of volcanoes.

The Hiroshima High Court on Dec. 13 issued an injunction banning the operation of the No. 3 reactor at the Ikata nuclear power plant in Ikata, Ehime Prefecture, operated by Shikoku Electric Power Co.

The ruling referred to the possibility that the plant could be hit by a pyroclastic flow of superheated gases and debris produced by a huge eruption of Mount Aso, an active volcano in Kumamoto Prefecture.

The court decision is bound to have massive, wide-ranging repercussions since there are many other nuclear plants located close to a volcano in Japan.

The government’s Nuclear Regulation Authority (NRA), the nuclear safety watchdog, and utilities operating nuclear power plants should take the ruling seriously.

The NRA has developed internal guidelines for assessing safety risks posed to nuclear plants by volcanoes under the new nuclear safety standards introduced after the 2011 Fukushima nuclear disaster. The guidelines say areas within 160 kilometers from a volcano are not suitable for operating a nuclear power plant unless the risks posed by a possible eruption, such as an avalanche of extremely hot ash reaching the plant, are “sufficiently small.”

Many volcanologists argue that it is impossible to predict the timing and scale of large volcanic eruptions. But the NRA has approved plans to operate reactors near volcanoes submitted by utilities on the assumption that there should be telltale signs of a looming major eruption.

The high court based its decision on the view shared by many volcanologists that the possibility of a gigantic eruption of Mount Aso cannot be ruled out.

It is known that a supersized eruption occurred at the volcano some 90,000 years ago. The risk of an Aso eruption of a similar scale causing serious damage to the Ikata nuclear plant, located about 130 km from the mountain, cannot be dismissed as “sufficiently small,” the ruling said. The court judged the NRA’s conclusion that the plant has fulfilled the safety standards to be “not rational.”

In a nutshell, the court decided that the NRA failed to assess the safety of the plant strictly in line with the guidelines.

This is not the first time that a court has questioned the NRA’s approval of a plan to operate a nuclear reactor near a volcano.

In April last year, the Fukuoka High Court’s Miyazaki branch rejected an appeal against a lower court ruling allowing Kyushu Electric Power Co. to continue operating the Nos. 1 and 2 reactors at its Sendai nuclear power plant in Kagoshima Prefecture. Pointing out that huge volcanic eruptions rarely occur, the court argued that social common sense disregards the risk as negligible.

But the ruling nevertheless said the guidelines’ claim that such eruptions are predictable is “irrational.”

The fact that shortcomings in the way the NRA evaluates the risks posed to nuclear plants by volcanoes have been repeatedly pointed out by courts has significant implications.

The NRA should pay more serious attention to what volcanologists say and remake the guidelines from the ground up.

It is indeed difficult to reasonably assess the safety risks from gargantuan volcanic eruptions that occur only once in every tens of thousands of years.

The Hiroshima High Court’s ruling has raised the fundamental question of how society should deal with the threat of natural disasters.

Any catastrophic volcanic eruption would cause enormous damage over a wide swath of the nation.

Some people may say that debate focused on the effects of such an eruption on nuclear plants is of little use.

But the principal lesson we have learned from the catastrophic accident at the Fukushima No. 1 nuclear power plant is that failing to take measures to prepare for disasters that rarely occur could lead to irrecoverable damage.

The government, which has been promoting restarts of offline nuclear reactors, should remember this lesson and seriously consider whether it is really possible to operate nuclear power plants safely in a country dotted with so many volcanoes.

--The Asahi Shimbun, Dec. 15