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.
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.
Seismic experts, meanwhile, concluded Monday that the temblor should not be viewed as a precursor to the major quake expected to rock the region sometime in the future.
San Diego County authorities said the earthquake, centered in the Pacific about 25 miles west of Solana Beach, caused about $720,000 in damages, but no major structural damage was reported. The number of earthquake-related injuries climbed from 15 to 29 Monday, the most serious being an 87-year-old man who was trapped for 11 hours under a mound of books in his downtown San Diego residential hotel room.
Despite the lack of damage at the San Onofre power plant, anti-nuclear activists were quick to voice their belief that a larger quake could mean disaster at the seaside plant just south of San Clemente.
"This affirms what we've been saying for years," said Marion Pack, director of the Orange County Chapter of Alliance for Survival. "Putting a nuclear power plant on a major fault line is not wise."
Michael P. Kennedy, a senior marine geologist with the state Division of Mines and Geology, has argued in the past that the area off Southern California's coast is more prone to quakes than the San Onofre plant's owners are willing to admit. He said Sunday's earthquake supports research done 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, Southern California Edison vice president and manager of the San Onofre plant, said the quake 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," Ray said.
The earthquake was measured at 5.3 on the Richter scale, which measures ground movement. Ray said the plant is built to withstand a quake of magnitude 7.0 just five miles offshore. An increase of one point in the scale reflects a tenfold rise in quake intensity.
Any temblor felt at the San Onofre site initiates a series of actions by plant operators. Sunday's quake and the one early last 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, the lowest form of emergency that can be declared at the plant, 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 the 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 it has been since Nov. 21.
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 ever felt at the plant, none of the lights flashed, meaning that the vibrations did not reach even half the level for which the plant was built. An engineer called in after the quake later calculated that the strongest movements were between 2% and 3% of the normal force of gravity. 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. If, for example, water levels were decreasing in the pressurizer just outside the reactor core, it would be a sign that there might be a serious leak in the plant's most important coolant system. Water levels rising in sumps at the bottom of the containment area would also indicate that a pipe had burst.
On Sunday, all water levels remained normal and no leaks were detected. Checks for radioactivity released from the core also turned up nothing.
The last step in the inspection calls for workers to enter the plant and check seismic supports for any damage. No damage was found Sunday.
The unusual event was called off at 8:01 a.m., Ray said.
"The plant responded exactly as we would expect it to," he said.