The plant operator replaced all the steam generators at the plant in December 2010 with the steam generators (SG) made by Mitsubishi Heavy Industries. Unit 3 re-started in February 2011. Unit 2 is under regular maintenance.
From The Orange County Register (2/2/2012):
As workers began inspecting a leaky tube in one of the San Onofre nuclear plant's reactors Thursday, federal regulators said more than 800 tubes in a second, offline reactor showed wear and thinning, although they are less than two years old.
And plant officials confirmed that sensors showed a tiny amount of radioactive gas may have leaked out of a building next to the first reactor before the reactor was shut down late Tuesday.
All four of the plant's steam generators and their tubes are less than two years old, installed after being delivered to the West Coast by the Japanese manufacturer of the generators, Mitsubishi Heavy Industries.
Some 9,700 tubes carry water from the reactor and through each generator.
"They have inspected 80 percent of the tubes in one of the steam generators at unit 2," said Victor Dricks, spokesman for the Nuclear Regulatory Commission. "Two of the tubes have thinning so extensive that they need to be plugged and taken out of service. Sixty nine other tubes have thinning greater than 20 percent of the wall thickness, and a larger number have thinning greater than 10 percent of wall thickness."
The tubes with 10 percent thinning number more than 800, he said.
Gil Alexander, a spokesman for plant operator Southern California Edison, said he could not confirm Dricks's numbers Thursday because an assessment of the unit 2 tubes is continuing.
Dricks and Alexander, however, said that highly sensitive alarms were tripped in an building next to San Onofre's unit 3 reactor after the leak was detected about 5:30 p.m. Tuesday, indicating that at least some radioactive gas was present.
The building houses the turbine that generates electricity.
And while workers were able to begin inspecting the unit 3 reactor, which had cooled enough to allow them to enter, they had not yet been able to characterize the size or nature of the leak, he said.
Mitsubishi officials were on scene, he said, and he had no estimate of when the reactor might be started up again, though it will likely be several days.
“Mitsubishi is aware of the issue reported at the San Onofre Nuclear Generating Station and has been in contact with the customer, Southern California Edison," the company said in a statement released to the press. "The investigation of the incident is being conducted by our customer. However, as the manufacturer of the steam generators, we will do whatever we can to support our customer in resolving the issue.”
New details also emerged about an unrelated accident on Friday, when a contract worker slipped into a pool above the unit 2 reactor, now offline for maintenance, while trying to retrieve a flashlight.
The man, whose name Edison declined to release, was not injured, and did not suffer harmful radiation exposure, but might have ingested the mildly radioactive water, Alexander said. No internal contamination was found.
Normally, water that is heated by the reactor and flows through the tubes is kept separate from another loop of water, from another source, inside the steam generators.
Instead, the tubes are immersed in the water inside the steam generators, heating the water, which turns to steam that powers the plant's turbines to produce electricity.
But a small leak in one of the tubes could have allowed radioactive water circulating from the reactor to mix with the water in the steam generator.
If so, it could have resulted in the escape of a small amount of radioactive gas.
(Full article at the link)
From Enformable (2/2/2012):
It’s not clear what caused the pipe to fail, or whether the company was facing an isolated break in a single alloy tube or a manufacturing defect that might be at issue elsewhere in the massive plant tube system.
This might mean there is a now a worry about the possibility that San Onofre has purchased 4 faulty steam generators from Mitsubishi, and the financial costs to replace them might prevent the plant from restarting.
Southern California Edison said a manufacturing defect was reported by Mitsubishi before delivery of two generators from Japan — but that the equipment was retested by the manufacturer and plant operator and determined to be fine before installation at the southern reactor.