The operator of California's troubled San Onofre nuclear power plant must explain why key safety equipment failed before restarting the twin reactors, the Nuclear Regulatory Commission chief said Friday.

NRC Chairman Gregory Jaczko said Southern California Edison must pinpoint why tubes carrying radioactive water were worn down at an excessive rate. The reactors are on the San Diego County coast.

The NRC won't allow the plant to restart until the utility can explain why the tubes were worn down and how the problem will be addressed, Jaczko told reporters after a plant tour.

"In the end, we won't make a decision unless we're satisfied that public health and safety will be protected," he said. "We have to have assurances of safety before we allow the plant to restart."

Sen. Dianne Feinstein (D-Calif.), who chairs the Senate Energy and Water Development Appropriations Subcommittee, and House Oversight and Government Reform Chairman Darrell Issa (R-Calif.) also toured the facility.

The plant was shut down in January after a leak was detected in tubes connected to a steam generator on the Unit 3 reactor.

A twin reactor, Unit 2, was shut down for maintenance and refueling, but was also found to have extensive wear on its tubing (Greenwire, March 27).

Initial investigations have revealed that degradation of tubes on Unit 2 is not as severe as on Unit 3, Jaczko said.

The tubes and generators are crucial because they are used to generate power and serve as a barrier between the reactor's core and the external environment, he said.

Jaczko also said the NRC has not "seen indications of a broader safety issue with the plant."

Mitsubishi Heavy Industries manufactured the tubes specifically for the San Onofre plant, and Jaczko said the problem "appears to be a unique set of design" circumstances.