The San Onofre Nuclear Power Plant is seen on April 6, 2012.
The operator of the San Onofre nuclear power plant said Friday that some of the steam generator tube wear is not unexpected.
Southern California Edison said it has sent federal regulators a report on the tube wear problems which have kept the San Onofre Nuclear Generating Station (SONGS) offline since January 31.
In a news release, Edison said their data shows that most of the steam generator tube wear – or “wall thinning” is less than 20 percent. The utility says that is far below the 35 percent wall-thinning limit requiring tubes to be plugged – or taken out of service.
Edison said the majority of the wear is related to support structures, which the company said is not unusual in new steam generators and is part of the equipment settling in.
But a report posted on the Nuclear Regulatory Commission (NRC) website shows the extent of wear from friction and vibration in 3,401 tubes, among a total of nearly 40,000 inside the plant's four steam generators.
The NRC report shows evidence of wear was found in 15,000 places in those tubes at varying degrees. In about 280 spots—virtually all in the Unit 3 reactor—more than 50 percent of the tube wall was worn away.
Gradual wear is common in such tubing, but the rate of erosion at San Onofre startled officials since the equipment is relatively new. The generators were replaced in 2009 and 2010 in a $670 million overhaul.
The NRC report suggested that one or more of the steam generators at the plant might need to be replaced.
A radioactive leak in a steam generator tube forced Unit 3 at SONGS to be shut down January 31. Unit 2 was already offline for planned maintenance.
After the leak in Unit 3, inspectors also found problems with accelerated tube wear in Unit 2.
The company said in Unit 2, 1,595 tubes showed wear of some type and 510 tubes were ultimately plugged — six tubes for showing wear of more than 35 percent and the rest for preventative measures.
Edison said in Unit 3, 1,806 tubes showed wear of some type and 807 tubes were ultimately plugged — 381 tubes for wear of more than 35 percent and the rest for preventative measures.
A report from a nuclear engineering consultant said this week that the steam generators at San Onofre are the worst in the U.S. nuclear industry.
Southern California Edison and the Nuclear Regulatory Commission said there is no timetable for restarting the seaside power plant.