Officials investigate mishap at Diablo

Plant operators and federal safety inspectors are investigating how two switches on a safety system at Diablo Canyon nuclear power plant were improperly set, potentially impairing operators’ ability to respond in the event of a severe loss of cooling water.

The improperly set switches were discovered late last week during maintenance and testing conducted in a refueling shutdown of the reactor, said Emily Christensen Archer, plant spokeswoman.

The switches were set in such a way that they would have prevented operators from remotely opening two valves that feed cooling water back into the reactor after it had been spilled. Operators would have had to be dispatched to manually open the valves, a procedure operators are trained to perform, Christensen Archer said.

A loss-of-coolant accident like this would be an extreme emergency because a breach of some kind would cause radioactive water to drain from the nuclear reactor. A massive earthquake or terrorist attack could cause such a scenario.

The spilled water would collect in a sump where it would be available for recirculation. Valves on this sump were the ones that were improperly set.

Operators last week reset the switches to their correct configuration and tested them to confirm that they were operating properly. The same switches on the plant’s other reactor were also tested to confirm that they are correctly set, Christensen Archer said.

The federal Nuclear Regulatory Commission was notified and is doing its own investigation, said Michael Peck, senior resident inspector. That investigation is not complete.

The NRC will determine how much risk the improperly set switches posed. The level of the risk will determine the agency’s response, Peck said.

One aspect the inspectors are looking at is how much radiation the operators sent to manually open the valves would be exposed to. The operators would not have to enter the containment dome and would avoid direct exposure to the spilled coolant, Peck said.

However, they could receive an additional dose from the radioactive water flowing through the recirculation pipes, Peck said.

Loss-of-coolant accidents are serious because the nuclear core continues to produce heat for a time even after the reactor is shut down, and the core can melt if it overheats. In addition to recirculating lost coolant, the plant has water storage tanks that can provide emergency coolant water.

Reach David Sneed at 781-7930.