Tepco admits two nuke reactors had control rod malfunctions

Tokyo Electric Power Co. on Tuesday joined the list of power utilities revealing their nuclear reactors had accidents involving control rods but did not report them.

The nation's largest power firm said two control rods fell off at the Fukushima No. 2 nuclear plant's No. 3 reactor in Fukushima Prefecture in June 1993 and at the Kashiwazaki-Kariwa plant's No. 1 reactor in Niigata Prefecture in April 2000 during routine checks. Operations were suspended for the checks.

As in the control-rod problems at Tohoku Electric Power Co. and Chubu Electric Power Co., neither of the Tepco incidents resulted in a self-sustaining nuclear chain reaction, or criticality, so the utility was not legally required to report them to the government, Tepco said.

In both Tepco cases, an emergency system that immediately inserts control rods into the reactor core failed to function, it said.

Hokuriku Electric Power Co. disclosed on March 15 that a 15-minute criticality accident had resulted from three of the 89 control rods coming off at its reactor in Ishikawa Prefecture in 1999. It neglected to report the incident to the government.

On Monday, Tohoku Electric announced that reactor control rods fell off in 1988 at its reactor in Miyagi Prefecture and Chubu Electric said rods came off in 1991 in its Shizuoka Prefecture reactor.

All these incidents were in boiling-water reactors and were caused by the mishandling of valves in the system that controls the water pressure to move reactor control rods. No one was harmed and their was no contamination of the environment, the utilities and the government claimed.