TOKYO (AP) -- Japan's nuclear watchdog on Wednesday endorsed a panel's conclusion that a seismic fault running underneath one of two reactors at an atomic plant in western Japan is active, making the reactor's restart virtually impossible.
The Nuclear Regulation Authority said it agreed with the panel of experts that the fault underneath the Tsuruga No. 2 reactor could trigger an earthquake and lead to an accident.
Japanese regulations prohibit reactors from sitting above active faults. Tsuruga's No. 2 reactor now faces indefinite stoppage or likely decommissioning unless its operator provides new data overriding the watchdog's decision.
It was the first time Japanese regulators had officially recognized an active fault underneath an existing reactor, virtually acknowledging that the risk at Tsuruga had been overlooked for decades by both the operator and regulators despite warnings by some experts. The watchdog is also investigating five other plants around the country over suspected active faults there.
The case is a crucial test for the Nuclear Regulation Authority to prove if it can resist industry pressure just as Japan's pro-nuclear government moves to restart reactors suspended since the 2011 Fukushima disaster.
All but two of Japan's 50 workable reactors have been offline since the March 2011 earthquake and tsunami triggered multiple meltdowns at the Fukushima Dai-ichi plant. The disaster boosted anti-nuclear sentiment and led to the establishment of a more independent watchdog. But after taking office in December, Prime Minister Shinzo Abe quickly reversed the previous government's nuclear phase-out plan, repeatedly saying that he plans to restart reactors considered safe.
Japanese nuclear plant operators are now rushing to fix and upgrade safety at their reactors so they can apply for mandatory safety inspections as soon as new regulatory requirements take effect in July.
Even before Wednesday's announcement, watchdog chairman Shunichi Tanaka had already hinted that his agency would not open a safety review for Tsuruga's No. 2 reactor if its operator applied for an inspection ahead of a possible restart.
He said Wednesday that there was no change to that plan, and that the watchdog stands by its conclusion about the fault.
"Under the safety guidelines, we say a reactor should not be built on an active fault. It's self-explanatory," Tanaka told a news conference. "We stand by our decision, no matter what kind of pressure we get from outside."
He said the watchdog had also decided to order the operator to study the potential risk of the spent fuel storage pool at Tsuruga's No. 2 reactor in case of a major earthquake.
Tsuruga's operator, Japan Atomic Power Co., rejected the watchdog's decision about the fault and said it would continue its own probe in hopes of overturning the assessment. It has commissioned experts from around the world to conduct a study to challenge the watchdog's findings.
The company has said the panel's findings have serious consequences to its financial status and that it might take legal action. Company officials have also sent a harshly worded protest letter to each of the five experts on the panel.
Tanaka said it was up to the operator to decide what to do with the reactor, since the watchdog does not have the authority to order that it be decommissioned.
The fate of the Tsuruga reactor has also rattled the country's power industry, as nine regional utility companies — including Tokyo Electric Power Co., which operates the crippled Fukushima Dai-ichi plant — have stakes in Japan Atomic Power. They fear that a decommissioning of the relatively new Tsuruga No. 2 reactor would be a huge financial burden and may cause the company to go bankrupt.
Japan Atomic Power operates only three reactors, and its two others are aging and face uncertainty — the No. 1 reactor at the Tsuruga plant is already nearing retirement after operating for 43 years, while the 35-year-old Tokai No. 2 plant in eastern Japan faces a tough anti-nuclear neighborhood that opposes a restart.