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HOME  > Past issues  > 2007 March 21 - 27  > 7 1/2 hour-long N-plant criticality accident concealed for nearly 30 years
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2007 March 21 - 27 [SOCIAL ISSUES]

7 1/2 hour-long N-plant criticality accident concealed for nearly 30 years

March 23, 2007
Tokyo Electric Power Co. (TEPCO) on March 22 made public that it is highly possible that a 7.5 hour-long nuclear criticality accident had occurred at a TEPCO plant nearly 30 years ago.

According to TEPCO, during the regular inspection at the No. 3 reactor at its Fukushima No. 1 Nuclear Power Station (Fukushima Pref.) on November 2, 1978, five out of 137 control rods of the reactor came off, possibly triggering a self-sustained nuclear fission reaction.

The reactor may have remained in critical state from 3:00 a.m. to 10:30 p.m., said TEPCO.

TEPCO’s internal probe located a memo written by an engineer of Toshiba Corporation, a manufacturer of the plant, that control rods fell off and an increase in neutrons was observed. TEPCO’s duty staff at that time also admitted this.

In the plant’s daybook, however, no record of the accident has been found, and the reasons for the omission is yet unknown, the company said.

TEPCO said that it is investigating whether the records in the daybook were falsified and whether an emergency shutdown had taken place at that time.

TEPCO also announced that control rods fell off at the same power station’s No. 5 reactor in 1979 and No. 2 reactor in 1980, but that in both cases the reactors did not reach criticality. Again, no report on these incidents was filed to the government.

Since the eight-year concealment of the criticality accident at Hokuriku Electric Power Co.’s Shika Nuclear Power Plant (Ishikawa Pref.) was revealed a week ago, past accidents in which control rods came off at boiling water type reactors have been made public one after another.

The government has completely failed to deal with such serious accidents as occurred at Shika and Fukushima No. 1 plants.

Power companies and plant manufacturers have kept concealing the accidents and neglected to take any measures.

This clearly shows that the government is incapable of supervising and regulating nuclear power plants.

Learning from the Chernobyl nuclear disaster, the 1994 Convention on Nuclear Safety emphasized the necessity “to ensure an effective separation between the functions of the regulatory body and those of any other body or organization concerned with the promotion or utilization of nuclear energy.”

Japan must create a fully responsible regulatory body independent of the Ministry of Economy, Trade and Industry which promotes nuclear power generation.
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