The International Atomic Energy Agency’s final report on the March 2011 triple meltdowns at Tokyo Electric Power Co.’s Fukushima No. 1 nuclear power plant puts the main blame on the then prevailing assumption that Japan’s “nuclear power plants were so safe that an accident of this magnitude was simply unthinkable.” Constant monitoring is needed to make sure the government, power companies and nuclear regulatory authorities aren’t falling into the same “safety myth” as they push to reactivate idled reactors that meet what is now touted as the “world’s most stringent” nuclear safety standards.

Last week, Kyushu Electric Power Co. began commercial operation of the No. 1 reactor of its Sendai nuclear power plant in Satsumasendai, Kagoshima Prefecture — a little over a month after it became the first reactor idled since 2011 to be reactivated on the basis of the safety standards that were tightened in response to the Fukushima disaster. The utility plans to restart the plant’s No. 2 reactor as early as next month, and the Abe administration and the power industry are pushing to bring more idled plants back online once they have cleared the Nuclear Regulation Authority’s screening.

The regulatory system for nuclear power generation has been reformed since the 2011 crisis. The old Nuclear and Industrial Safety Agency, which came under fire for the Fukushima debacle, has been replaced by the NRA, and new regulations introduced in 2013 require operators of nuclear power plants to beef up their defense against natural disasters such as major earthquakes and tsunamis. But while the NRA itself states that compliance with the new standards does not guarantee the plants’ safety, the government says the plants are ready for restart because they meet the NRA criteria. No one appears ready to take final responsibility for the plants’ safety.

The IAEA report, compiled by around 180 experts from 42 countries and submitted to an annual general conference of the United Nations nuclear watchdog this week, highlights the “assumption” held by Japan’s nuclear plant operators prior to 2011 that a crisis of that magnitude would not happen, which was never challenged by the government or regulatory authorities, leaving the nation unprepared for a severe accident.

The Fukushima power plant lost its emergency power supply after it was flooded by a 15-meter tsunami triggered by the magnitude-9 quake on March 11, 2011. The loss of power crippled its crucial core-cooling functions and led to the meltdowns in its three operating reactors. Citing Tepco’s failure to take precautionary action against such external hazards despite an estimate prior to the disaster that a powerful quake off Fukushima could cause a tsunami of roughly the same scale that hit the plant site, the report said “there was not sufficient consideration of low probability, high consequence external events,” partly because “of the basic assumption in Japan, reinforced over many decades, that the robustness of the technical design of the nuclear plants would provide sufficient protection against postulated risks.” This assumption led to “a tendency for organizations and their staff not to challenge the level of safety” and “resulted in a situation where safety improvements were not introduced promptly.”

The report also pointed to the deficiencies in Japan’s nuclear regulatory system behind the Fukushima disaster. “The regulation of nuclear safety in Japan at the time of the accident was performed by a number of organizations with different roles and responsibilities and complex interrelationships. It was not fully clear which organizations had the responsibility and authority to issue binding instructions on how to respond to safety issues without delay,” it said. “The regulations, guidelines and procedures in place at the time of the accident were not fully in line with international practice in some key areas, most notably in relation to periodic safety reviews, re-evaluation of hazards, severe accident management and safety culture.”

IAEA Director General Yukiya Amano, in his foreword to the report, says Japan’s regulatory system has since been reformed to meet international standards, with regulators given clearer responsibilities and greater authority. Whether the new plant safety standards are the world’s most stringent or not, plants that meet the standards are supposed to withstand much greater levels of external hazards and be better able to respond to emergencies than before.

Still, complacency under the new standards would risk reviving the same safety myth rebuked in the report. Questioning whether the tightened standards are sufficient could be branded as demanding zero tolerance of risks and thereby unrealistic. However, as the IAEA report points out, it was an “unlikely combination of events” that hit the Tepco plant, and the utility’s unpreparedness for such a situation that resulted in the 2011 disaster.

We need to consider whether the tendency to dismiss low-probability risks as “small enough” — as was, for example, the risk of Kyushu Electric’s Sendai plant being hit by a volcanic eruption when the go-ahead was given for its restart — is acceptable from the viewpoint of preventing severe accidents at nuclear plants in the future.

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