Japan's nuclear watchdog said Wednesday that one reactor was sitting directly above an active tectonic fault, effectively ruling out a restart forever.
The Nuclear Regulation Authority (NRA) said it had approved a report from experts which found a crack in the Earth's crust lying underneath the reactor at a plant in Tsuruga, western Japan, was active.
"There is a need for us to take the report seriously," NRA chairman Shunichi Tanaka said.
It is the first time the newly-minted NRA has made such a ruling. It is still investigating possibly-active faults under five other reactors. A second reactor at Tsuruga, which sits 300 metres (328 yards) away, is not one of this number.
The final decision on a restart rests with the government, who are expected to be asked by plant operator Japan Atomic Power to overrule the watchdog.
Observers say despite its pro-nuclear stance, Prime Minister Shinzo Abe's administration is unlikely to risk public ire by backing the operator, meaning the reactor would become the first to be permanently shuttered since the Fukushima disaster.
Only two of Japan's 50 nuclear reactors have been restarted after being switched off for safety checks in the aftermath of the tsunami-sparked catastrophe in March 2011, where reactors went into meltdown.
Government rules ban nuclear reactors and other facilities with important safety functions from being located directly above "active faults", which are currently defined as those that have moved in the last 120,000-130,000 years.
The NRA experts spent more than five months looking at the bedrock under the reactor and said they could not say with certainty that the fault was not active.
Japan Atomic Power, which maintains the fault is not active, said it would continue conducting its own investigations.
Since Abe became prime minister last December, observers have speculated it is just a matter of time before idled reactors are restarted, with industry pleading for a cheaper electricity supply than that coming from fossil fuel plants in use now.
The Abe government has also resumed Japan's drive to export nuclear plants. On May 3, Japan signed a deal to build a plant on Turkey's Black Sea coast.
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