A court ruling cast serious doubt over the credibility of safety assessments by the Nuclear Regulation Authority with regard to the operations of nuclear reactors.

The ruling called into question the safety of reactors restarted with NRA approval after being shut down in the aftermath of the 2011 Fukushima nuclear disaster.

It also underscored an urgent need for a sweeping review of the nuclear regulation system as a whole.

The Osaka District Court on Dec. 3 struck down the NRA’s endorsement of safety measures for the No. 3 and No. 4 reactors at the Oi nuclear power plant in Fukui Prefecture.

It invalidated the green light the nuclear safety watchdog gave in 2017 to Kansai Electric Power Co.’s plan to restart the two reactors.

The court said the NRA’s safety assessment was not fully in accordance with new tougher nuclear safety standards introduced after the catastrophic accident at the Fukushima No. 1 nuclear plant and screening guidelines based on those standards.

The ruling labeled the NRA’s decision as “unreasonable,” asserting there were “errors and deficiencies that cannot be overlooked” in the process of examining and approving anti-earthquake measures the electric utility adopted for the reactors.

In designing measures to protect a reactor against major earthquakes, the operator estimates the maximum possible ground motion generated by an earthquake around the reactor, called “reference ground motion.” It develops steps to ensure the safety of the reactor based on this estimate and requests for NRA approval for restarting the reactor.

The NRA examines the plan and determines whether the estimate is appropriate and the proposed safety measures are sufficient. It grants approval and authorization if it decides the plan meets the new safety standards.

Kansai Electric Power determined the reference ground motion by calculating the magnitude of the maximum credible earthquake based on its own assumptions concerning the length and width of faults around the reactors.

But residents of Fukui and six other prefectures filed a lawsuit to question the utility’s estimate of the reference ground motion. They argued that the utility’s estimate only represents an “average” for the spectrum of possible quakes, meaning that the safety measures are not based on the maximum strength of a possible earthquake in the area.

They cited a newly included provision in the NRA’s screening guidelines that says consideration should be given for the “variability” that arises due to the calculation methods used.

The plaintiffs claimed the NRA’s approval of the anti-quake measures was illegal because it was based on the utility’s questionable reference ground motion figure.

The government countered this argument by saying the utility’s calculation has a sufficient margin of error that makes it unnecessary to consider variability. But the court sided with the plaintiffs.

The ruling puts weight on the reasons for the NRA’s own decision to introduce the “variability” provision into the guidelines and demands that the screening process strictly follow strictly the established procedures.

The NRA should respond to the ruling by first reviewing the process of the safety screening of the two reactors at the Oi plant. It is possible that the screening of other reactors was similarly flawed. The ruling is likely to arouse anxiety among residents living in the vicinity of reactors that have been brought back online. The NRA should make a sincere and convincing response to the court decision.

The No. 3 and No. 4 reactors at the Oi plant are currently offline for regular maintenance. Debate is unnecessary in stating that the utility must not rush to restart the reactors.

Even without the triple meltdown at the Fukushima plant, it is amply obvious that this nation could be hit by unexpectedly severe natural disasters, such as earthquakes, tsunami and volcanic eruptions, at any time.

That makes it all the more important to establish nuclear power standards based on the principle of erring on the side of safety and ensure that safety screening and regulation are strictly based on the standards.

The government, which seems to be bent on restarting reactors, should take this imperative to heart.

--The Asahi Shimbun, Dec. 5