San Onofre's reactors are pressurized water reactors. Unit 3's reactor was made by Combustion Engineering, which was purchased in 2000 by Westinghouse (now Toshiba).
From Market Watch (1/31/2012):
Southern California Edison, operator of the San Onofre Nuclear Generating Station, said in a press release Tuesday it has begun a precautionary shutdown of Unit 3 because sensors detected a possible leak in one of the unit's steam generator tubes.
The company said "the potential leak poses no imminent danger to the public or plant workers. There has been no release to the atmosphere." SCE said personnel will evaluate the cause of the leak and the steps needed to repair it and resume operations.
SCE also said it has ample reserve power to meet customer needs. The station's Unit 2 is currently offline for a planned maintenance outage, the company added in the statement.
SCE is a division of Edison International.