Gregory Jaczko took a first-hand look at the problems that have been keeping the plant offline since the end of January, raising fears of power shortages this summer. He was accompanied by Democratic Sen. Dianne Feinstein and Republican Rep. Darrell Issa.
Jaczko said the plant would remain offline indefinitely until the cause of unusual tube wear in the plant's steam generators is determined. He says there is no timetable to restart the plants but left open the possibility that Unit 2 could be restarted more quickly.
Investigators can't figure out what's causing unusual wear on the tubes that carry radioactive water. Southern California Edison, which operates the plant, says it's cooperating with the NRC investigation.
Jaczko inspected San Onofre's sea wall and safety improvements made since an earthquake and tsunami damaged the nuclear plant at Fukushima, Japan.
San Onofre is owned by Southern California Edison, San Diego Gas & Electric and the city of Riverside.
The Associated Press contributed to this report.