Originally published March 15, 2012 at 8:31 a.m., updated March 15, 2012 at 1:19 p.m.
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."
The Unit 3 reactor at the San Onofre Nuclear Generating Station, located about 45 miles north of San Diego, was shut down as a precaution on Jan. 31 after a water leak from another tube in a massive steam generator. Traces of radiation escaped, but officials say there was no danger to workers or neighbors.
Since then, investigators have been looking into 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. The two huge steam generators at Unit 2, each containing 9,700 tubes, were replaced in fall 2009, and a year later in Unit 3 as part of a $670 million overhaul.
The NRC dispatched a special, five-member team to Unit 3 on Thursday after pressure tests showed three of the metal-alloy 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. Inside a steam generator, hot, pressurized water flowing through the hive 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 did not immediately respond with an email seeking comment. 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.