TOKYO - In the three years since the Fukushima disaster, Japan's utilities have pledged $15 billion to harden their nuclear plants against earthquakes, tsunamis, tornadoes and terrorist attacks.
But as Japan's nuclear safety regulator prepares to rule on whether the first of the country's 48 idled reactors is ready to be come back online, the post-Fukushima debate about how safe is safe enough has turned to a final risk: volcanoes.
The Nuclear Regulation Authority (NRA) has already said the chance of volcanic activity during the lifespan of Kyushu Electric Power's nuclear plant at Sendai was negligible, suggesting it will give it the green light. The plant, some 1,000 km (600 miles) south of Tokyo, lies in a region of active volcanic sites.
Critics, including some scientists who were consulted by the NRA, say that shows regulators are turning a blind eye to the kind of unlikely but potentially devastating chain of events that pushed the Fukushima Daiichi plant into a triple meltdown in 2011 when a tsunami crashed into the facility.
The debate has played out in several months of public hearings in Tokyo by the NRA and could weigh on the last hurdle for restarting nuclear plants - the opinion of local residents - at a time when the costs of keeping reactors shut are mounting.
While reactors have been offline, Japan has had to spend around $87 billion on replacement fuel, utility operators estimate. Before 2011, Japan got about 30 per cent of its electricity from nuclear power.
Kyushu Electric's business plan hinges on getting its two-reactor facility at Sendai restarted. The utility has posted $5.9 billion in losses over the past three years and is seeking a $1 billion bailout in new equity from the Development Bank of Japan.
Critics say the NRA safety review overestimates the power of science to predict future volcanic eruptions.
Japan lies on the "Ring of Fire", a horseshoe-shaped band of fault lines and volcanoes circling the edges of the Pacific Ocean. Japan itself is home to 110 active volcanoes.
Sendai, at the southern end of the island of Kyushu, is 50 km (31 miles) from Sakurajima, an active volcano. Five giant calderas, crater-like depressions formed by past eruptions, are also in the region, the closest one just 40 km (25 miles) from the Sendai plant.
"No-one believes that volcanic risks have been adequately discussed," said Setsuya Nakada, a professor of volcanology at the University of Tokyo, who advised officials when they were forming regulatory guidelines for monitoring volcanoes.
Eruptions that form calderas are devastating, but extremely rare. Scientists believe the odds of a massive caldera-forming eruption happening in Japan are less than 1 in 10,000 in any given year.
Evidence of the most recent mega-eruption in southern Japan is the underwater Kikai caldera, which was formed by a violent eruption around 7,300 years ago. The eruption covered southern Kyushu with more than 60 centimeters of ash.