Tepco reveals series of past incidents

27 March 2007

[Japan Times, 21 and 23 March] Tokyo Electric Power Co (Tepco) revealed that five dislodged control rods probably caused a criticality incident that could have lasted up to seven-and-a-half hours at Unit three of its Fukushima-1 nuclear power plant in 1978. An internal investigation by Tepco and the reactor's constructor, Toshiba Corp, revealed that five control rods were found to have slipped from their correct positions during a regular inspection. Akio Komori, department head of Tepco's nuclear power plant management unit, said "Judging from testimony and data still available, the possibility that the reactor reached a critical state is extremely high." Tepco also said the investigation revealed two further cases where a control rod dislodged from a reactor core at the Fukushima-1 plant: at Unit 5 in February 1979 and at Unit 2 in September 1980. In addition, Tepco said two control rods came off at Unit three of its Fukushima-2 plant in June 1993 and at its Kashiwazaki-Kariwa-1 reactor in April 2000 during routine checks. Criticality did not occur in any of these other incidents. Tepco's admissions come after a series of other disclosures of past safety lapses to Japan's Ministry of Economy, Trade and Industry (METI) as part of a purge of records intended to help transform corporate safety culture in all electric operating companies, nuclear and otherwise. The deadline for nuclear operators to make such disclosures is 31 March.

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