The deadly volcanic eruption of Mount Ontake over the weekend may strengthen the argument of activists campaigning to keep the country’s 48 reactors shut.

Japan’s atomic plants are offline for safety checks as a result of the earthquake and tsunami that caused the meltdown of three reactors in Fukushima more than three years ago.

The Nuclear Regulation Authority has said two reactors at a plant run by Kyushu Electric Power Co. meet new safety standards, indicating they are closest to restarting.

Kyushu Electric’s Sendai plant is about 50 km (31 miles) from another active volcano called Sakurajima. It’s also not far from a cluster of calderas, volcanic craters caused by past eruptions.

“The NRA has been criticized for not taking the elevated risk of volcanic eruption into account,” Stephen Church, a Tokyo-based partner at equity researcher JI Asia, said in a note Monday.

“The Ontake eruption, if it were to become major, may cause a delay in the nuclear reactor restart program.”

The NRA sees no need to change its policy toward the Sendai reactors because it has already determined they could have withstood a known eruption at Sakurajima 10,000 years ago that was far worse than the Ontake event, said Masaru Kobayashi, who directs the agency’s earthquake and tsunami risk division.

The Ontake eruption, which is in a different part of Japan from Kyushu, won’t cause changes to safety measures for the Sendai units, Kyushu Electric spokesman Yuki Hirano said by phone.

Activists who oppose the Sendai reactor restarts had earlier cited volcano risk when the NRA was assessing safety at the facility.

Kyushu Electric argued to the NRA it could use seismic sensors and global positioning technology to predict eruptions that may threaten its reactors. The NRA approved the safety report on Sept. 10.

Seismologists, however, were unable to forecast the Saturday eruption at Ontake, which killed at least 31 people, mostly hikers in the area who were allowed onto the mountain as sensors didn’t indicate any threat from volcanic activity.

“The event has damaged NRA’s arguments about being able to predict eruptions,” Aileen Mioko Smith, director of the Kyoto-based anti-nuclear group Green Action, said in an email. “They said GPS monitoring could do it. In the case of Ontake, it could not.”

With newspaper opinion polls showing the majority of Japan’s public against nuclear plant restarts, the Ontake eruption may further bolster opposition as there are 110 active volcanoes scattered throughout the country.

Hokkaido Electric Power Co.’s Tomari plant in the far north, which has applied for NRA safety checks at three reactors, is also near an active caldera, Charles Connor, a geophysicist specializing in volcanoes at the University of South Florida, said by phone.

Larger volcanic eruptions can be predicted before they occur, which would give plant operators some time to prepare, said Connor.

Chief Cabinet Secretary Yoshihide Suga told reporters Sunday that he does not expect the Ontake eruption to have any impact on nuclear restarts.

Still, the proximity of Sakurajima to the Sendai plant has been raised as an issue by those that don’t support restarts, Tom O’Sullivan, the founder of Tokyo-based energy consultant Mathyos, said by phone. “I think it casts a question mark over the restarts yet again.”

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