The earthquake off the coast of San Diego County on Sunday has prompted an anti-nuclear group and at least one state geologist to call for more study of the chance that a stronger temblor could damage the San Onofre nuclear power plant.
"Putting a nuclear power plant on a major fault line is not wise," said Marion Pack, director of the Orange County chapter of Alliance for Survival, an anti-nuclear group. "This affirms what we've been saying for years."
But officials for Southern California Edison, the plant's majority owner and operator, said Monday that the quake fell far short of damaging the plant and changed nothing in the utility's assessment of the plant's vulnerability to earthquakes.
Michael P. Kennedy, a senior marine geologist with the state Division of Mines and Geology, who has argued in the past that the entire area off Southern California's coast is more prone to quakes than the San Onofre plant's owners are willing to admit, said Sunday's earthquake supports research done by him and others at the division's La Jolla office.
"It's very important to take another look at these faults, and to maybe not take some of the data we've put forward so lightly as some of the consultants did at the time of San Onofre's licensing," Kennedy said.
But Harold Ray, Edison vice president and manager of the San Onofre plant, said the quake, centered 28 miles southwest of Oceanside, was too far from the plant to raise much concern. "We do not see it as having occurred in the area of possible seismic influences at San Onofre," he said.
The earthquake was measured at 5.3 on the Richter scale.
Ray said the plant, just south of San Clemente, is built to withstand a quake of magnitude 7.0 five miles offshore.
Still, any temblor that is felt at the San Onofre site initiates a series of actions by plant operators. Sunday's quake and the one early Tuesday morning centered in the Palm Springs area produced almost the same responses at the plant.
On Sunday, Ray said, an "unusual event" was declared at San Onofre at 7:02 a.m., 16 minutes after the quake was felt in the control room. An unusual event is the lowest form of emergency that can be declared at the plant; it calls for the notification of surrounding cities and counties and the Nuclear Regulatory Commission that there is chance of a "degradation" in plant safety.
When that quake struck, San Onofre Unit 2 was at full power and Unit 3 was at 94%. Unit 1, the oldest reactor on the site and the one considered by critics to be most vulnerable to earthquakes, was shut down for repairs.
As soon as the quake was felt, Ray said, a technician in the control room moved to a panel of red and orange lights that alert operators in case vibrations in the plant approach the levels the reactors were built to withstand. If the movements reach about 90% of those levels, the plant is shut down automatically.
In this case, as in all other quakes felt at the plant, none of the lights flashed, meaning that the vibrations did not reach even half the level that the plant was built to withstand. An engineer later calculated that the strongest movements were between 2% and 3% of the normal force of gravity, Ray said. The plant is designed to withstand movements 30 times stronger.
After checking for vibrations, operators monitored the level of water in the plant's key coolant systems. Decreasing water levels in the pressurizer just outside the reactor core, for example, would be a sign that there might be a serious leak in the plant's most important coolant system; alternatively, water levels rising in sumps at the bottom of the containment area would also indicate that a pipe had burst.