Hokuriku Electric has admitted to a criticality incident almost eight years ago at its Shika 1 BWR.
The 18 June 1999 event was not reported until recently after regulators instructed utilities to examine their records and declare any previously undisclosed incidents. According to the utility, during the 15-minute localised criticality, temperatures increased slightly in the 540MWe unit. However, no other consequences arose from the event.
Following the announcement by Hokuriku, the director general of Japan's Nuclear and Industrial Safety Agency (NISA) handed the president of Hokuriku a document ordering the company to submit a report as stipulated by law. NISA ordered the immediate halt of operations at Shika 1 so that a full safety inspection could be carried out. NISA also warned other power suppliers to take actions to prevent similar accidents.
According to Hokuriku, the incident occurred in the fifth periodic inspection of the BWR after three of the 89 control rods had moved out of position during preparations for a routine test. The reactor reached a state of criticality, setting off the automatic 'stop' signal. However, the control rods were not automatically inserted at that point as the isolation valves were closed for the test. Some 15 minutes later the operators reopened the valves, and the control rods were reinserted.
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.
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