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.
Neither incident resulted in a self-sustaining nuclear chain reaction, according to the Tokyo-based utility, which had to shut down all of its reactors in 2003 when it was revealed it had covered up flaws.
In the 1978 case in Fukushima, the problem at the No. 3 reactor had “caused criticality,” Tokyo Electric officials said, quoting a source familiar with the case.
The uncontrolled nuclear chain reaction may have lasted up to 7 1/2 hours, according to the investigation.
“We have so far not been able to confirm data that back up the testimony, because it is an incident from a long time ago,” an official of the utility said.
All three incidents that emerged Thursday apparently took place during regular checkups and in the course of work to fasten control rods, utility officials said.
Last week, it was revealed that in 1999 one of Hokuriku Electric Power Co.’s reactors in Ishikawa Prefecture suffered a self-sustaining chain reaction that lasted 15 minutes after control rods fell off.
The accident caused no harm to workers or outside effects.
Since the revelation, a number of electric utilities have come forward with their own cases of control-rod detachments at their nuclear plants.
Meanwhile, critics are questioning the government’s past nuclear plant checks, arguing the recent spate of coverups proves they were ineffective.
Teruyuki Matsushita, 58, a former municipal assembly member in Mihama, a Fukui Prefecture town hosting Kansai Electric Power Co.’s Mihama plant, said locals must stand up against the state’s energy policy, even at the expense of huge local subsidies host towns are annually receiving.