San Onofre

Reactor Unit 2 at the San Onofre nuclear power plant shut itself down Wednesday after an electrical failure cut power to the reactor’s steam turbine, utility officials said.

David Barron, a spokesman for Southern California Edison Co., which operates the plant, said an electrical switch interrupted power to valves that control the amount of steam entering the turbine.

When the power to the valves was cut about 10:30 a.m., they shut themselves off automatically. When the valves closed, the steam turbine shut itself down, and the nuclear reactor followed suit.

Barron said plant operators did not know why the electrical switch cut the power and started the sequence of events shutting down the reactor.

He said the utility plans to keep the reactor shut down for three or four days for maintenance work that had been pending.