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San Onofre nuclear plant can withstand up to 7.0 quake, is protected by a 25-foot tsunami wall, Edison says

March 14, 2011 | 10:49 am

San Onofre nuclear power plant

Operators of the concrete-domed San Onofre nuclear plant Monday were trying to reassure jittery Southern California residents that the nuclear disaster unfolding in Japan won't happen here.

The 84-acre generating station in the northern corner of San Diego County is built to withstand a magnitude 7.0 earthquake, said Gil Alexander, a spokesman for the generation station's operator, Southern California Edison. That is greater than the 6.5 shaker that scientists predicted could strike the plant before it was built 42 years ago, he said. But it's less than the 8.9 quake that hit Japan last week.

A 25-foot-high "tsunami wall" of reinforced concete was also erected between the plant and the adjacent ocean, a height based on scientists' best estimates of the potential threat, he said. The geological fault most likely to directly threaten San Onofre lies about 5 miles offshore, Alexander said.

The Nuclear Regulatory Commission also sought to allay fears that small releases of radiation from Japan's crippled Fukishima reactors were a threat to the U.S.

Available information indicates that weather conditions have carried any radioactive vapors out to sea and away from the Japanese population. Given the thousands of miles between the two countries, Hawaii, Alaska, the U.S. territories and the U.S. West Coast are not expected to experience any harmful levels of radioactivity, the commission said Monday.

San Onofre's three domed units were built in layered shells, like Russian nesting dolls. The outer shell is made of reinforced concrete that is four-feet thick, and is designed to capture any unexpected release of radiation. The inner steel casing housing the reactor is 8 inches thick.

Inside the reactor, fuel rods and control rods that make up the nuclear core are surrounded by pressurized water. In Japan, units of at least two nuclear plants were damaged and lost electrial power after the massive earthquake and tsunami, making it difficult to continue cooling the cores.

Two explosions have occurred in separate units but authorities so far have been able to contain radiation within the steel containment vessels surrounding the fuel rods. Alexander said San Onofre has multiple safety systems should the Southern California plant find itself in a similar situation.

In addition to diesel generators, the plant has a battery system and a gravity-driven emergency cooling system, Alexander said. The utility's operators were watching the Japan situation closely and will take any lessons learned from it to heart, he said.

"We will comb through the details of their emergency very carefully and whatever lessons can and should be applied here will be noted,'' he said. "That process will unfold very vigorously."

The Union of Concerned Scientists, a group that has been critical of nuclear energy, said it was also watching the Japanese crisis unfolding. It was planning a news conference Monday to release its findings.

San Onofre had three operating domes when built but Unit 1 was retired in 1992. Spent fuel road are stored there.

Annual energy output at the plant is the equivalent of that produced by 20 million to 25 million barrels of oil, Southern California Edison officials say. The plant generates 2,200 megawatts of electricity, about 20% of Southern California's usage, or enough to power 1.4 million homes.



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-- Catherine Saillant

Photo: San Onofre nuclear power plant. Credit: Los Angeles Times file. Illustration: How the plant works. Source: Southern California Edison