A pump malfunction prompted operators of the San Onofre nuclear power plant to shut down the Unit 2 reactor Thursday.
Officials of Southern California Edison, operator of the plant, said the shutdown did not result in the release of radiation nor was it a hazard to workers or the public.
Two pumps that feed water to the reactor apparently stopped working when a valve incorrectly closed because of a "false signal" emitted by an electrical system that controls the device, a plant spokesman said.
The reactor was operating at 75% of full power when the shutdown occurred. Officials could not say how long repairs would take, but said the shutdown will not affect service to the utility's customers.