Southern California Edison, the operator of the San Onofre Nuclear Generating Station, said in a statement that investigators found wear in its Unit 2 generators that is similar to degradation in its sister, Unit 3, though at a lower level.
The new findings come just days after NRC Chairman Gregory Jaczko signaled that Unit 2 might be able to go back into service more quickly than its twin, where tube wear has been more damaging. The twin-reactor plant between San Diego and Los Angeles has been shut down for more than two months.
Gradual wear is common in such tubing, but the rate of wear at San Onofre has been unsettling to officials since the equipment is relatively new. The generators were installed in a multimillion-dollar makeover in 2009 and 2010.
The Nuclear Regulatory Commission has said the plant will remain dark until the company determines the cause of the wear and fixes it. The company has said 321 tubes that were heavily damaged will be plugged and taken out of service at the two reactors, well within the margin to allow them to keep operating.
The company said the reactors will not be restarted until SCE and the NRC are satisfied it is safe to do so.
Last month, a report commissioned by nuclear watchdog Friends of the Earth claimed the company misled the NRC about design changes that it said are the likely culprit in excessive tube wear. The group said in a report Thursday that the new findings confirm its analysis showing hundreds of additional tubes were added to each generator.
"While Unit 2 has experienced more tube degradation and Unit 3 has experienced deeper cracking in fewer tubes, both units require a concurrent root-cause analysis due to their identical specifications and operating conditions," the report said.
The steam generators were manufactured by Japan-based Mitsubishi Heavy Industries, according to company officials.
The troubles began to unfold in late January, when the Unit 3 reactor was shut down as a precaution after a tube break. Traces of radiation escaped, but officials said there was no danger to workers or neighbors.
Unit 2 had been taken offline earlier that month for routine maintenance and refueling, but investigators later found unusual wear on tubing in both units.