San Onofre

A leaking valve on an emergency cooling system at the San Onofre nuclear power plant’s Unit 2 reactor prompted operators to shut down the system to make repairs, officials said Monday.

The reactor was shut down at 7 p.m. Sunday and is expected to be operational again today or Wednesday, said David Barron, a spokesman for Southern California Edison, operator of the facility.

Barron said officials elected to shut down the reactor after they discovered a leak in a release valve on one of four tanks that are used for emergency cooling water.

Nuclear Regulatory Commission regulations require all four tanks to be operational while the reactor is in operation.

San Onofre officials also declared an unusual event, the lowest category of emergency conditions at the plant, from 7 p.m. until 8:25 p.m., Barron said. That step is a standard practice whenever a unit is shut down because of a mechanical problem, he said.