A nuclear reactor on the Southern 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 [on March 15]. 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. 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 offline 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 Nuclear Regulatory Commission dispatched a special, five-member team to Unit 3 ... 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. 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, radioactivity from the system that pumps water through the reactor could escape into the atmosphere.