TOKYO — Seismologists said Wednesday that a nuclear reactor in Tsuruga, in western Japan, stands above an active seismic fault, a finding that could lead to the first permanent shutdown of a reactor since the Fukushima crisis two years ago.

Japan’s newly installed Nuclear Regulation Authority also said Wednesday that it would issue an order that would effectively keep a separate unit, the Monju experimental fast breeder reactor, closed until its operator overhauled safety measures at the site.

Any move to decommission the Tsuruga commercial reactor would deal a blow to Prime Minister Shinzo Abe’s efforts to get the country’s nuclear program back online. So would yet another delay in restarting the troubled Monju experimental reactor, which is a major part of Japan’s longstanding goal of creating a system to recycle nuclear fuel.

But those steps would show that the country’s new nuclear regulator, put in place to bolster oversight of the nuclear industry after the 2011 disaster, has teeth. Its predecessor was criticized as having a lax approach to safety and close industry ties.

“It is really just a matter of luck that there hasn’t been an accident” at Tsuruga, said Kunihiko Shimazaki, the head of a panel of seismologists assessing the fault that runs beneath it, according to Reuters.

Still, both the Tsuruga and Monju plants in Fukui Prefecture, even if not operative, could continue to face risks from a possible earthquake along the active fault.

Almost 3,000 spent uranium fuel rods are stored at the Tsuruga plant, while the Monju site stores plutonium fuel. Some experts suspect that Monju also sits atop an active fault.

All of Japan’s 50 commercial reactors were closed for inspections after the multiple meltdowns at the Fukushima Daiichi station, which forced the evacuation of more than 100,000 people. Since then, only two have been restarted.

The fate of the country’s reactors depends on safety assessments being carried out by the new regulator agency, which has been studying the safety risks at those reactors.

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