LOS ANGELES -- An ammonia leak that caused an emergency alert at the San Onofre nuclear plant in November was caused by employees who failed to recognize degraded equipment and fix it, federal regulators said Friday.
The Nuclear Regulatory Commission report concluded the problem had "very low safety significance," and faulted plant operator Southern California Edison for failing to follow its own procedures at the twin-reactor site, about 45 miles from San Diego.
The company conducted a parallel investigation and found the same contributing factors as the NRC, according to a statement. The utility said it made changes to address the findings.
The report came as the company continues to investigate a separate leak in a relatively new steam generator tube that prompted the shut-down of one reactor as a precaution.
The other reactor is off line for maintenance, and inspectors there are continuing to study what the NRC described as unusual wear found in hundreds of similar tubes that carry radioactive water.
Watchdog and environmental groups have criticized the utility for not alerting the public for more than an hour after the Nov. 1 ammonia leak started in a storage tank.
The NRC report found workers "failed to adequately identify, evaluate, and correct a problem" in a water purification system, which led to the leak.
No one was injured and there was no danger to the public, although some workers were evacuated as a precaution. The report said the leak made areas of a building that houses turbines inaccessible.
"The failure to take adequate corrective actions for degraded plant equipment was a performance deficiency. The performance deficiency is more than minor because" it resulted in an emergency alert, the report said.
The report also highlighted confusion among workers.
It found that guidance was not provided to operations personnel to fix the tank once a problem was detected. Meanwhile, the report said, an equipment operator believed the problem was being corrected when, in fact, it was allowed to degrade, eventually triggering an alert.
Exposure to high levels of ammonia can cause irritation, serious burns, lung damage and even death. It is used at the plant to treat water that is turned into steam, which runs the turbines that produce electricity. The treated water also is used to remove heat from the reactor's cooling system.
The plant is jointly owned by Edison, San Diego Gas and Electric and the city of Riverside.