Southern California Edison disclosed the latest failures at the San Onofre nuclear plant late Thursday.
Hours earlier, the Nuclear Regulatory Commission said it was sending federal inspectors to the northern San Diego County plant to determine why the relatively new metal-alloy tubes carrying radioactive water were degraded.
The plant was shut down Jan. 31 as a precaution after a water leak in another tube. Traces of radiation escaped, but officials say there was no danger to workers or neighbors.
The NRC had said that tests showed three tubes were frail and could rupture.
The latest tests bring those tube failures to seven.