The nuclear plant has also built a 25-foot-high tsunami wall out of reinforced concrete between the plant and the adjacent ocean.
Alexander said the height of the wall was based on scientists' best estimates of the potential threat and that the geological fault most likely to directly threaten the plant lies about 5 miles offshore of San Onofre.
The U.S. Nuclear Regulatory Commission also wanted to calm resident's fears about the crippled Fukushima reactors in Japan and the small releases of radiation that could threaten the U.S.
The commission said Monday that the available information indicates weather conditions have carried any radioactive vapors out to sea and away from the Japanese population. They said they do not expect, given the thousands of miles between the two countries, Hawaii, Alaska, the U.S. territories and the West Coast to experience any harmful levels of radioactivity.
San Onofre's generating station's three domed units were built in layers. The outer shell of the dome is made of four-feet thick reinforced concrete and was designed to capture any unforeseen release of radiation, while the inside is made of 8-inch thick steel casing.
Inside the reactor, surrounded by pressurized water, there are control and fuel rods that make up the nuclear core.
With the Japan disaster the units of at least two nuclear plants lost electrical power and were damaged after the massive earthquake and tsunami hit, which made it very difficult to keep the cores cool.
Alexander said should the Southern California plant find itself in a similar situation to Japan, the San Onofre plant has multiple safety systems in place.
In Japan, authorities thus far have been able to contain the radiation within the steel containment vessels surrounding the fuel rods after two explosions happened in separate units.
Alexander said the plant has a battery system and a gravity-driven emergency cooling system, in addition to the diesel generators.
The plant's operators were closely watching the situation in Japan and will take lessons to be learned to heart, the spokesman said.