Japan’s nuclear regulators considered exactly the sort of total power loss that helped trigger the Fukushima disaster more than 18 years before it occurred, but decided not to do anything about it apparently because nuclear operators said it would never happen.
Documents belatedly released by the Nuclear Safety Commission of Japan (NSC) on June 4 show that one of its working groups considered taking measures to deal with total power losses, but eventually dismissed the idea in 1993 after facing resistance from power firms.
The working group was set up in 1991 and comprised five experts and outside representatives, including employees of Tokyo Electric Power Co. (TEPCO) and Kansai Electric Power Co.
The NSC secretariat instructed utilities in October 1992 to produce written depositions to explain why there was no need to consider total alternating-current (AC) power losses lasting 30 minutes or more.
TEPCO said in its reply: "Sufficient safety can be guaranteed if appropriate management operations are implemented.”
Utility officials were quoted as saying: "The risks due to total losses of AC power are quite low, and it is too much to reflect them in the safety design guidelines."
The report from the working group in 1993 concluded there was little likelihood of a serious problem and decided there was no need to amend a statement in the safety design screening guidelines that said: "It is not necessary to consider sustained, total losses of AC power, because it is possible to count on transmission lines being restored or on emergency AC power facilities being repaired."
All AC power was lost at the Fukushima No. 1 nuclear power plant when it was flooded by a tsunami spawned by the Great East Japan Earthquake on March 11 last year. That led to a loss of cooling in the nuclear reactors and to the worst civil nuclear disaster in Japan's history.
The documents revealing the deliberations of the working group were not immediately released after a decision last August to disclose all the minutes and materials presented at previous NSC meetings, but officials denied a coverup.
"We forgot to disclose them," an NSC secretariat official said. "We did not conceal them on purpose."
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