LOS ANGELES — Federal regulators Thursday disclosed the most detailed information to date on damage at California's idled San Onofre nuclear power plant, where scores of tubes that carry radioactive water have eroded at an alarming rate.
The detailed data, posted obscurely on the Nuclear Regulatory Commission website, has implications for the future of the seaside plant that has been offline since a tube break in January released traces of radiation.
The records provide the first close portrait of how heavily damaged some tubes have become in a short time, and hint at the challenge faced by operator Southern California Edison to get the reactors running again. Edison officials had no immediate comment.
The steam generators, which resemble massive steel fire hydrants, are one of the central pieces of equipment in a nuclear plant. At San Onofre, each one stands 65 feet high, weighs 1.3 million pounds, with 9,727 U-shaped tubes inside, each three-quarters of an inch in diameter.
The alloy tubes represent a critical safety barrier – if one breaks, there is the potential that radioactivity could escape into the atmosphere. Also, serious leaks can drain protective cooling water from a reactor.
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 a $670 million overhaul and began operating in April 2010 in Unit 2 and February 2011 in Unit 3.
Tubes have to be taken out of service if 35 percent – roughly a third – of the wall wears away, and each of the four generators at the plant is designed to operate with a maximum of 778 retired tubes.
In one troubled generator in Unit 3, 420 tubes have been retired. The records show another 197 tubes in that generator have between 20 percent and 34 percent wear, meaning they are close to reaching the point when they would be at risk of breaking.
More than 500 others in that generator have between 10 percent and 19 percent wear in the tube wall.
"The new data reveal that there are thousands of damaged tubes in both Units 2 and 3, raising serious questions whether either unit should ever be restarted," said Daniel Hirsch, a lecturer on nuclear policy at the University of California, Santa Cruz, who is a critic of the industry. "The problem is vastly larger than has been disclosed to date."
Overall, the records show investigators found wear from friction and vibration in 15,000 places, in varying degrees, in 3,401 tubes inside the four generators.
In about 280 spots – virtually all in the Unit 3 reactor – more than 50 percent of the tube wall was worn away.
The company has said previously that 1,300 tubes will be taken out of service at the plant, either because of heavy wear or as a precaution, although the number is well within the margin to allow the generators to keep operating.
The generators function something like a car radiator, which controls heat in a vehicle's engine. The generator tubes circulate hot, radioactive water from the reactors, which heat non-radioactive water surrounding them. That makes steam, which is used to turn turbines to make electricity.
The trouble began to unfold in January, when the Unit 3 reactor was shut down as a precaution after a tube break released traces of radiation. That began a spiral of events that led to a months-long federal probe.
Last month the NRC blamed a botched computer analysis for creating excessive vibration inside the generators that damaged tubes, with agency officials saying last month it's not known how the generators can be fixed.
The NRC left open the possibility that one or more of the huge machines might have to be replaced.
Decaying generator tubes helped push San Onofre's Unit 1 reactor into retirement in 1992, even though it was designed to run until 2004. The following year, the Trojan nuclear plant, near Portland, Ore., was shuttered because of microscopic cracks in steam generator tubes, cutting years off its expected lifespan.
Also on Thursday, Friends of the Earth, a group critical of the nuclear industry, released a report saying the tube damage at San Onofre far exceeded anything witnessed in the industry. The report, written by Vermont-based nuclear consultants Fairewinds Associates, said the plant has retired four times as many tubes as the combined total at other U.S. nuclear power plants where generators have been replaced.
Regulators and plant owners say the reactors won't be restarted until all safety issues are addressed. Costs continue to mount and scrutiny has intensified.
The state Public Utilities Commission plans to vote on an order next month requiring plant owners to disclose the potential economic hit for ratepayers, ranging from a relatively quick restart to a permanent shutdown of the twin reactors.
About 7.4 million Californians live within 50 miles of San Onofre, which can power 1.4 million homes.
The plant is owned by SCE, San Diego Gas & Electric and the city of Riverside. The Unit 1 reactor operated from 1968 to 1992, when it was shut down and dismantled.
Follow Michael R. Blood at . http://twitter.com/MichaelRBloodAP