Accident at San Onofre Blamed on Maintenance, Faulty Valves

Inadequate maintenance of faulty valves at Unit 1 of the San Onofre nuclear power plant led to an accident Nov. 21 that threatened the stability of a safety-related water coolant system, according to a Nuclear Regulatory Commission report released Wednesday.

The failure of five valves, combined with a four-minute power outage, led to what is known as a “water hammer”--the over-pressurization of a 10-inch pipe and a resultant shock wave that cracked the pipe, damaged several pipe supports and released non-radioactive steam into the atmosphere.

The report, issued after a 45-day investigation by a special Nuclear Regulatory Commission team, concluded that maintenance records for the valves were “either missing or lacked specificity,” that testing records were “inconsistent” and that the testing procedure “was not rigorous.”

Some managers investigating the incident for Southern California Edison Co., the plant’s operators, “lacked a sufficiently inquiring attitude,” the report said.

“It appears that (Southern California Edison’s) process for evaluating and following up events may not be sufficiently thorough and systematic to assure that failed components are detected and adequately explained,” the report said.

The incident ended with no injuries, no release of radioactive steam and no danger to the public. The plant was shut down and cannot be restarted without the commission’s permission. The commission is still reviewing Edison’s reaction to the incident. Harold Ray, an Edison vice president and site manager for San Onofre, said he thought the commission’s report overstated the blame due the utility for the incident. He called the event “a learning experience” that brought several lessons not only for San Onofre but for nuclear power plants nationwide that use many of the same procedures.

“We’re all going to learn from this experience, just like many other experiences provide opportunities for learning,” Ray said. “But the implication that there was something deficient in our procedures just is not correct.”

Ray said the valves that failed were not properly designed for the purpose they served. He said the company has since bought new valves designed differently. He said they “are not subject to the kind of failure” that led to the November accident. He said there was no way the utility could have known in advance that the valves would not hold up.