On March 15, 2007, Nuclear and Industrial Safety Agency (NISA) received a report from Hokuriku Electric Power Company (HEPCO) on a criticality accident at Shika Nuclear Power Plant unit 1 (BWR). The accident occurred in 1999 during periodic inspection, but was not reported to NISA at that time. The fact was discovered by HEPCO during full check of data falsification being conducted according to NISA’s order issued on November 30, 2006.
The report of the accident was submitted from HEPCO to NISA on March 30 and April 6, 2007.
The report describes that the event occurred during the test of the hydraulic control rod system at the time of reactor shutdown. False steps taken in the test caused the pressure increase in the control rod withdrawal pipes and resulted in a simultaneous drops at 3 control rods that made reactor criticality for approximately fifteen minutes.