Five dislodged control rods probably caused a 7 1/2-hour criticality accident at a Fukushima nuclear plant run by Tokyo Electric Power Co. in 1978, an internal probe by the utility and the reactor's builder, Toshiba Corp., revealed Thursday.
A probe also uncovered two more cases where a control rod dislodged from a reactor core at the No. 1 plant in Fukushima Prefecture -- the plant's No. 5 reactor in February 1979 and its No. 2 reactor in September 1980. None of the cases was apparently reported to the government, Japan's largest electric power firm said.
On Tuesday, Tepco said two control rods came off at its Fukushima No. 2 nuclear plant's No. 3 reactor in June 1993 and at the Kashiwazaki-Kariwa plant's No. 1 reactor in Niigata Prefecture in April 2000 during suspensions for routine checks.