The Hiroshima High Court on Friday revoked a lower court decision and ordered Shikoku Electric Power Co. to suspend its only operable nuclear reactor in Ehime Prefecture because its preparations for a potential eruption of Mount Aso are inadequate.

The utility has previously claimed the reactor is safe to run because it would have enough advance warning of an eruption to take safety measures.

The high court also said the Nuclear Regulatory Agency’s regulations were unreasonable.

The ruling marks the second time the high court has ordered a halt of the No. 3 reactor at the Ikata nuclear power plant.

The reactor had been shut for regular maintenance work since late December and was likely to restart within a couple of months, but now must remain idled pending an appeal. Shares in the company, which didn’t disclose the court’s reason for issuing the order, plunged on the news, ending the day down 6 percent at ¥957.

The move is the latest in a series of setbacks for an industry still struggling to recover from the Fukushima nuclear disaster nearly nine years ago, with less than a fifth of the nation’s reactors having received approval to operate.

Residents near reactors have been filing numerous lawsuits against nuclear power operations in recent years, leading to some temporary closures. Utilities have generally been successful in getting rulings against them overturned on appeal.

In a statement, Shikoku Electric said the decision by the Hiroshima High Court is “extremely regrettable” and pledged to “promptly file an appeal so that the order can be revoked as soon as possible.”

In making its decision, the court considered whether the operator and the NRA’s regulations and risk estimates for a potential eruption at the caldera of Mount Aso, about 130 km away, were adequate.

Last March, three residents of nearby Yamaguchi Prefecture who had lost a case against the Ikata reactor in the Yamaguchi District Court were appealing the decision made by the Iwakuni branch. The lower court ruled the Ikata plant could continue operating because the probability of a big eruption occurring during the reactor’s life span was low, and the NRA’s safety standards were adequate.

The reactor is currently idled for scheduled inspections and the removal of spent mixed-oxide fuel was completed on Wednesday. It is expected to be restarted on April 27.

A previous order forcing a halt in operations was issued by the Hiroshima High Court in December 2017, citing the risk of Mount Aso erupting. The same court then overturned the decision in September 2018 on appeal, and Shikoku Electric restarted the reactor a month later.