The 1999 event at the Hokuriku Electric Power-owned station resulted in an inadvertent criticality, but was not reported until recently after regulators instructed utilities to examine their records and declare any previously undisclosed incidents.

According to the utility, the accident occurred during a refuelling and during the 15-minute long localised criticality temperatures increased slightly in the 540 MW unit. However, no other consequences arose from the event.

On 15 March, the director-general of the Nuclear and Industrial Safety Agency (NISA) handed the president of Hokuriku Electric Power Co a document ordering the company to submit a report as stipulated by law. NISA also warned other power suppliers to take actions to prevent similar accidents.

Shika-1 has now been shut down, according to Japanese media reports.

The Hokuriku incident has been followed by two similar, though unconfirmed, incidents in which two of 89 control rods at Tohoku Electric’s Onagawa-1 reactor failed in 1988, and three of 185 control rods at Chubu Electric’s Hamaoka-3 were found to be out of position during a 1991 inspection.

Both events were apparently caused by malfunctioning valves which affected water pressure in the control-rod drive systems