Two nuclear reactors in Japan are under threat from nearby volcanoes that could hit the reactors and result in a nationwide disaster, a volcanologist said Friday.

Toshitsugu Fujii, who leads a government-commissioned panel on volcanic eruption prediction, disagreed with the Japanese regulators who have already declared the reactors to be safe from volcanic activity, the Associated Press reported.

Prime Minister Shinzo has been urging that two Sendai reactors be restarted along with any of Japan's 46 other reactors. Japan needs nuclear power as an inexpensive and reliable energy source as its economy recovers, he said.

Regulators said last month that the two Sendai reactors will be safe from volcanic activity for the next few decades since they adhere to stricter safety requirements instated after the Fukushima disaster in 2011, but Fujii has countered that the nuclear regulators can't possibly know that.

"It is simply impossible to predict an eruption over the next 30 to 40 years," said Fujii, as quoted by the AP. "The level of predictability is extremely limited."

An eruption would result in ash falling as thick as 4 inches, which would essentially eliminate any vehicle except a tank from operating and cause blackouts by weighing down power lines.

People have become newly worried about volcanic activity after a volcano unexpectedly erupted in central Japan on Sept. 27. According to Fujii, an eruption can be predicted hours or days ahead of time at most.

"Scientifically, they're not safe," he said of the Sendai reactors. "If they still need to be restarted despite uncertainties and risks that remain, it's for political reasons, not because they're safe, and you should be honest about that."

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