HOUSTON (Reuters) - The U.S. Nuclear Regulatory Commission on Tuesday said it will not allow Southern California Edison's San Onofre nuclear generator in California to restart until the agency is sure that the company has addressed premature degradation of tubes in the plant's steam generators.
Both reactors at the 2,150-megawatt plant near San Diego have been shut since January due to the discovery of premature wear on tubes inside giant steam generators made by Mitsubishi Heavy Industries and installed in 2010 and 2011.
Accelerated wear, which thins the tube walls, in new steam generators is unusual, NRC officials have said. A special NRC inspection team has been at the site since March 19.
"San Onofre is now under the spotlight," said Shaun Burnie of Friends of the Earth, a watchdog group that opposes nuclear power and welcomed the NRC action.
A report from nuclear engineer Arne Gundersen, commissioned by Friends of the Earth, showed that the safety issues raised at San Onofre could lead to a larger radiation release if the units were allowed to restart.
Gundersen said four "significant" design changes in the new Mitsubishi steam generators may be to blame for the tube degradation and must be fully examined.
Officials at Southern California Edison (SCE), a unit of Edison International which owns 78 percent of the station, said they are committed to working with the NRC.
So far, workers have plugged 193 tubes, or nearly 1 percent, in unit 2 which shut in early January for refueling and replacement of the reactor vessel head.
Eight tubes have failed a pressure testing process at unit 3, which was shut in late January after a tube leak released a small amount of radioactive gas. The NRC said only one tube in unit 2 underwent pressure testing.
Failure of the pressure test "is an indication that, for certain design basis events, such as a main steam line break, these steam generator tubes may not be able to maintain design structural integrity," Elmo Collins, the NRC's regional administrator, said in Tuesday's confirmatory action letter sent to Peter Dietrich, SCE's chief nuclear officer.
The letter was included in the Tuesday's NRC statement.
The letter spells out specific action SCE must take to "ensure that the cause of the tube wear in both steam generators is understood and appropriately addressed."
SCE will be required to plug all tubes in each steam generator when testing indicates wear in excess of industry guidelines as well as plugging of tubes that might be experience early wear due to their location.
The NRC said the utility has blamed the premature degradation on vibration and tubes rubbing against adjacent tubes and against support structures inside the steam generators. SCE is still working to determine a cause.
The NRC wants SCE to determine the cause and to establish a process to inspect both steam generators in between scheduled refueling outages.
"Until we are satisfied that has been done, the plant will not be permitted to restart," Collins said in the statement.
The NRC said additional inspections should "minimize the progression of tube wear, and ensure that any tube wear does not progress to the point where it compromises tube integrity."
SCE President Ron Litzinger said the company's priority has always been safety of its employees and the public.
"The utility will only bring the units on line when we and the NRC are satisfied that it is safe to do so," Litzinger said in a release.
The California Independent System Operator said last week it was taking steps to prepare the state grid for a prolonged outage at San Onofre that may last into the summer when electric power use climbs.
Sempra Energy's San Diego Gas & Electric unit owns 20 percent of the San Onofre station and the City of Riverside, California, has less than a 2 percent stake.
(Reporting By Eileen O'Grady; Editing by David Gregorio and Bob Burgdorfer)