A failure to conduct proper testing caused two safety valves to be misaligned for more than a year at Diablo Canyon nuclear power plant.
That’s the conclusion of a special inspection conducted by the Nuclear Regulatory Commission just after Thanksgiving. The inspectors and other agency officials met with plant operators Tuesday to discuss the results at a public meeting.
In February 2008 during a previous refueling outage, operators at the plant made several modifications to valves that would be used to recirculate cooling water lost from the reactor resulting from a broken pipe or other severe accident, said Michael Peck, senior resident NRC inspector at the plant.
The modifications prevented two valves from being opened remotely, but this was not discovered at the time because of a lack of testing. Jim Becker, top executive at the plant, said procedures are now in place to ensure that needed testing will be done.
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“We take this issue very seriously,” he said.
The misaligned valves were discovered Oct. 23 during a subsequent refueling outage. The NRC sent a team of inspectors to the plant to look into the problem.
Initially, the agency thought the mistake posed a heightened safety risk because operators would have to manually open the valves in the event of an emergency. This would have exposed them to additional radiation.
The inspection determined that operators would have had at least 70 minutes to manually open the valves and could have performed the task in 20 minutes. They also determined that additional radiation exposure would have been within allowed limits.
As a result, the safety significance of the mistake was determined to be very low.