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.
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.
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.