Hokuriku Electric Power covered up nuclear reactor reaching criticality

Hokuriku Electric Power covered up nuclear reactor reaching criticality

KANAZAWA -- Hokuriku Electric Power Co. covered up that a nuclear reactor reached criticality in 1999 after three of its control rods accidentally dropped out of position while offline, company officials announced Thursday.

The government's Nuclear and Industrial Safety Agency has summoned Isao Nagahara, president of the power supplier, and instructed him to order that the reactor be stopped and given a thorough safety inspection.

The accident occurred in the No. 1 reactor at the company's Shika Nuclear Power Plant on June 18, 1999, company officials said. Three of the 89 control rods -- which prevent a nuclear reaction from occurring -- dropped out of the reactor while it was offline for inspections. The remaining rods were insufficient to stop the reaction process, which then reached criticality.

Since the emergency shutdown system was turned off for the inspection, workers were forced to manually insert the control rods back into the reactor, shutting down the reactor without any radiation leakage 15 minutes after the accident.

The covers of the reactor pressure vessel (the main part of the reactor) and the containment vessel (which prevents radiation leakage) were open for inspections at the time.

"It was beyond the scope of our assumptions and a grave problem that the reactor reached criticality while the covers were open," an agency official said.

He also pointed out that workers should have kept the emergency shutdown system on while nuclear fuel was still in the reactor, and will investigate to see if it was illegal to turn off the system.

The agency suspects that three control rods dropped out of the reactor because workers erroneously operated their control valves.

Failure to report any emergency stop of nuclear reactors to the government regulator constitutes a violation of the law. However, the three-year statute of limitations has already run out on the Hokuriku Electric Power Co. case. (Mainichi)