Tubes inside a new steam generator at San Onofre Nuclear Generating Station are showing unusual and potentially dangerous signs of deterioration, federal regulators announced on Thursday.
"The amount of wear that we are seeing on these tubes is unusual for a new steam generator," said Nuclear Regulatory Commission spokesman Victor Dricks.
More than 800 tubes of the San Diego County plant showed a 10 percent thinning in the tube wall, officials said. 69 others had at least 20 percent thinning. And two tubes needed to be plugged and taken out of service because a third of the wall was worn away.
"If you have that kind of thinning anywhere along the length of the tube, you have a problem because it degrades the integrity of the tube, which can contribute to leaks," explained Dricks.
Safety implications could be "very severe," warned Joram Hopenfeld, a retired NRC engineer and researcher, as the tubes are one of the primary barriers to radioactivity
Trouble in the steam generator was discovered during an inspection that took place when the plant, owned by Southern California Edison, San Diego Gas & Electric and the City of Riverside, was off-line for maintenance and refueling.
This news comes two days after operators powered down the reactor following a tube leak at the plant's other unit.
A tiny amount of radiation may have escaped in the shut down, but it posed no threat to public safety, said regulators and officials Wednesday.
The leaked radioactive gas was vented into a building containing auxiliary equipment and detected by monitors, according to the NRC.
Unlike the sealed structure that houses the reactor, people come and go through the doors of the auxiliary building and it is possible that radiation escaped into the atmosphere.
The steam generator manufacturer, Japan-based Mitsubishi Heavy Industries, has not yet commented on the finding.