Japan’s Fukushima nuclear crisis was a preventable disaster resulting from “collusion” among the government, regulators and the plant operator, an expert panel said on Thursday, wrapping up an inquiry into the worst nuclear accident in 25 years.

Damage from the huge March 11, 2011, earthquake, and not just the ensuing tsunami, could not be ruled out as a cause of the accident, the panel added, a finding that could have serious implications as Japan seeks to bring idled reactors back on line.

The panel also criticised the response of plant operator Tokyo Electric Power Co, regulators and then Prime Minister Naoto Kan, who quit last year after criticism of his handling of a natural disaster that developed into a man-made crisis.

“The … Fukushima Nuclear Power Plant accident was the result of collusion between the government, the regulators and Tepco, and the lack of governance by said parties,” the panel said in an English summary of a 641-page Japanese report.

Regulators, it said, had been reluctant to adopt global safety standards that could have helped prevent the disaster in which reactors melted down, spewing radiation and forcing about 150,000 people from their homes, many of whom will never return.

“Across the board, the Commission found ignorance and arrogance unforgivable for anyone or any organisation that deals with nuclear power. We found a disregard for global trends and a disregard for public safety,” the panel said.

The panel’s finding that seismic damage may well have played a role could affect the restart of reactors that were taken offline, mostly for maintenance and safety checks, in the months since Fukushima.

“We have proved that it cannot be said that there would have been no crisis without the tsunami,” Katsuhiko Ishibashi, a seismologist and panel member, said in the report.

Experts have said that an active fault may lie under Kansai Electric Power Co’s Ohi plant in western Japan, whose No. 3 unit began supplying electricity to the grid early on Thursday. Ohi’s No. 4 unit will come on line later this month after the government approved the restarts to avoid a power shortage.

“This means that all of Japan’s reactors are vulnerable and require retro-fitting, calling into question the hasty decision of the (Prime Minister Yoshihiko) Noda cabinet to restart reactors before getting the lessons of Fukushima,” said Jeffrey Kingston, Asia studies director at Temple University in Tokyo.

The report by the experts – one of three panels looking into the Fukushima disaster – follows a six-month investigation involving more than 900 hours of hearings and interviews with more than 1,100 people, the first such inquiry of its kind.

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