Edison Blamed in Onofre Accident : NRC Report on November Mishap Cites Poor Valve Maintenance, Testing

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 4-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, the product of a 45-day investigation by a special five-person NRC team, concluded that maintenance records for the valves were “either missing or lacked specificity,” that testing records were “inconsistent,” and that the testing procedure used on the valves “was not rigorous.”

Some managers investigating the incident for Southern California Edison Co., the plant’s operator, “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. Unit 1 was shut down and cannot be re-started without the NRC’s permission. The commission is still reviewing Edison’s reaction to the incident. The plant has two other operable units.

Harold Ray, a Southern California Edison vice president and site manager for San Onofre, said he thought the NRC 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.”

The report included a detailed, minute-by-minute account of the incident that provides a rare glimpse into the closed world inside a nuclear power plant in crisis, where alarms sometimes ring in error, workers mistake steam for smoke from a fire, and communications between plant operators and federal regulators are strained.

At one point during the incident, a special NRC “red phone” rang simultaneously in the power plant’s control room and at NRC headquarters in Bethesda, Md., for reasons no one has yet to determine. The scene that followed when both parties picked up the phone thinking the other had called could be comical if it hadn’t occurred in a real-life, pressure-packed situation.

“The exchange of questions and answers that ensued between the Unit 1 control room and the NRC duty officer was a mixture of miscues and incomplete communication,” the report said. “Finally, the shift superintendent implied to the duty officer that he was too busy to talk, told him that he would call him back, and told him the unit was stable.”

The incident began 10 minutes before midnight Nov. 20 when, just after a shift change in the control room, an alarm sounded, indicating an electrical problem in a conductor carrying power to safety-related water pumps.

For several hours, plant operators tried without success to isolate the problem. At one point, according to the report, they “improvised a trouble-shooting method” that would allow them to keep power flowing to certain parts of the plant so they could avoid conditions that, under federal regulations, might later require a complete shutdown.

At 4:51 a.m., two electrical technicians were inspecting a transformer in an attempt to locate the electrical ground that was causing the problem.

“It was while they were inspecting the switchgear that they heard a loud ‘boom,’ ” the report said. “At the same time, a security guard saw a flash of light from the vicinity of the top of the transformer near the technicians.”

In the control room, a supervisor quickly realized that power had been lost in a vital part of the plant, and began the process to shut down the nuclear reactor--called a “reactor trip.”

During the power blackout, which lasted four minutes, two key pumps shut down. The pumps send water into three steam generators, where intense heat coming from the nuclear reactor is transferred to cooler water. The result is steam that runs the turbines, producing electrical power, and a constant removal of heat from the reactor.

When power was resumed and the electricity flowed again to one of the pumps, the failed valves caused water to move backward through the system, emptying water from three 10-inch feedwater pipes and partially filling those pipes with steam. When cold water was then brought into those lines, the water condensed the steam and created a low-pressure area. This “water slug” then moved rapidly and with great force from areas of higher to lower pressure until it hit an elbow in the pipe. The force of that blow is known as a “water hammer.”

“The forces from the water hammer displaced the 10-inch-diameter feedwater piping, distorted its original configuration, caused an 80-inch (long) crack, and damaged pipe hangers,” the report said. “In seconds, the one-half-inch-thick piping was irreversibly damaged--the 80-inch crack, 30% through the (pipe) wall at places, indicates how close the pipe had come to splitting open.”

Ray, the Southern California Edison vice president, 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, which “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 wouldn’t hold up.