LOS ANGELES — A nuclear reactor on the California coast will remain shut down indefinitely while a team of federal inspectors determines why several relatively new tubes became so frail that tests found they could rupture and release radioactive water, a federal official said Thursday.
"This is a significant issue," said Nuclear Regulatory Commission spokeswoman Lara Uselding. "A tube rupture is really the concern. ... That's what we don't want to happen."
Underscoring concern over the test findings, the NRC dispatched a special team to the Unit 3 reactor at the San Onofre Nuclear Generating Station, located about 45 miles north of San Diego. The plant was shut down as a precaution on Jan. 31, after a radioactive water leak in another tube in a massive steam generator. Traces of radiation escaped, but officials say there was no danger to workers or neighbors.
The NRC said Thursday that pressure tests showed three of the metal-alloy generator tubes had become so degraded that they could rupture under some circumstances. Such ruptures can require a plant to shut down, if spewing water reaches 150 gallons a day.
Investigators have been looking into what federal officials call excessive wear found on steam generator tubes in the seaside plant and its twin, Unit 2, which has been off line for maintenance and refueling. In a $670 million overhaul, two huge steam generators, each containing 9,700 tubes, were replaced in Unit 2 in fall 2009 and a year later in Unit 3.
A spokeswoman for the agency that operates the state's wholesale power system, the California Independent System Operator, said the San Diego and Los Angeles areas could see rotating power outages this summer if both reactors remain off-line. The agency is taking steps to prevent those shortages.
"It's all about balancing supply and demand," said ISO spokeswoman Stephanie McCorkle. "You have to have a certain amount of plant (power) generation where the heavily populated areas of California are."
Inside a steam generator, hot, pressurized water flowing through bundles of tubes heats non-radioactive water surrounding them, and the resulting steam is used to turn turbines to make electricity.
According to the NRC, the tubes have an important safety role because they represent one of the primary barriers between the radioactive and non-radioactive sides of the plant. If a tube breaks, there is the potential that radioactivity from the system that pumps water through the reactor could escape into the atmosphere.
"The integrity of steam generator tubes is important because the tubes provide an additional barrier ... to prevent a radioactive steam release," the NRC said in a statement Thursday.
NRC Administrator Elmo E. Collins said in the statement that the agency wants "to make sure we understand the cause of the degraded steam generator tubes and take appropriate actions based on our inspection results."
Uselding said no date has been set to restart Unit 3.
The plant is owned by Southern California Edison, San Diego Gas & Electric and the City of Riverside. Southern California Edison serves nearly 14 million residents with electricity in Central and Southern California.
SCE spokeswoman Jennifer Manfre said the company welcomes the expanded investigation and "we don't put a deadline on safety."
David Lochbaum, director of the nuclear safety project for Union of Concerned Scientists, said the risks from tube leaks are two-fold: When serious, they can drain cooling water from the reactor while elevating the chance radiation could escape.
He said such investigation teams are sent out by the NRC only once or twice a year, highlighting concern within the agency.
While gradual wear in steam generator tubes takes place over time, the rate occurring in some tubes at San Onofre would be expected after many years of use, not in recently installed equipment, he said.
San Onofre's Unit 1 reactor operated from 1968 to 1992, when it was shut down and dismantled. The utility's plan to ship the 600-ton reactor vessel on a 15,500-mile voyage around South America to a disposal site in South Carolina was thwarted and it remains at San Onofre, encased in concrete and steel.