(Reuters) - Workers at Southern California Edison's San Onofre nuclear plant in California have begun plugging tubes in the Unit 2 steam generators, the company said on Wednesday.
Both reactors at SCE's 2,150-megawatt San Onofre station are currently shut to address problems with the steam generators and no return date has been set for either reactor, said a spokesman for SCE, a unit of Edison International.
"Workers have commenced plugging some tubes in Unit 2," SCE said in a statement and inspection of the thousands of tubes in the steam generators at both reactors is progressing.
The final number of tubes to be plugged and taken out of service has not been determined, but "the process is well within the built-in margin for the plugging allowance," according to the statement.
SCE said initial tests have been completed on all 19,454 tubes in the two steam generators in Unit 2. The large generators produce steam that drives the plant's turbines.
Preliminary tests on one steam generator completed earlier this month revealed the walls of two tubes had thinned to the point that the tubes would have to be plugged and taken out of service and hundreds of other tubes showed thinning walls, nuclear regulators said.
The Unit 2 steam generators were replaced in 2010 and the generators in Unit 3 were replaced last year.
Problems surfaced at Unit 2 after it shut to refuel in late January and workers had the first opportunity to inspect the new generators since the 2010 installation.
Officials with the U.S. Nuclear Regulatory Commission said the discovery of tube degradation in Unit 2 was unusual given that new steam generators from Mitsubishi Heavy Industries Ltd had only been running two years.
Tube wear can be common in older steam generators that have operated for many years, the NRC said.
Southern California Edison said the tube-wear findings were not unprecedented in new steam turbines.
The only other U.S. nuclear reactor with replacement Mitsubishi steam generators is the 478-MW Fort Calhoun reactor in Nebraska owned by the Omaha Public Power District.
New steam generators installed at Fort Calhoun in late 2006 have not experienced premature wear, the NRC said.
San Onofre shut Unit 3 January 31 in response to indications of a steam generator tube leak and a small release of radioactive gas near the leak.
Inspection of the steam generator tubes in Unit 3 continues, SCE said.
"Details regarding tube wear will not be available until all inspections are completed and all data is analyzed," the company said. "We are already applying lessons learned from our inspections to work on both units."
"SCE is committed to the safe operation of the San Onofre plant and will not return the units to service until we and the Nuclear Regulatory Commission are satisfied it is safe to do so," the utility said.
SCE operates San Onofre and owns a 78 percent stake; Sempra Energy's San Diego Gas & Electric subsidiary has a 20 percent stake; and the city of Riverside, California, has less than a 2 percent stake.
(Reporting By Eileen O'Grady in Houston; Editing by Lisa Shumaker)