"It doesn't sound like we are in meltdown mode," Von Hippel said.

Jim Walsh of MIT's Center of International Studies agreed that these reactors will probably be OK. But there are other facilities in Japan that produce enriched fuel for reactors and manage highly radioactive waste, some of which are in remote areas in the north, "and no one has said 'boo' about them," he said. "It's not inconceivable that some of them have had problems. The story may continue to unfold in the next few weeks."

Long before the problems created by the tsunami, a series of serious incidents in Japan's ambitious civilian nuclear power industry over the last decade have raised concerns about its attention to safety and the role of government regulators.

The history of Japan's nuclear incidents includes a pattern of problems being kept secret or passed off as far less serious than they actually were.

In 2007, several electric utilities admitted covering up accidents, including one that experienced an uncontrolled nuclear reaction inside a reactor while it was shut down for maintenance. The same year, Tokyo Electric Power Co. — which operates the two Fukushima plants — apologized for a radiation leak caused by an earthquake at a plant that had not been built to withstand a quake of that magnitude.

Photos: Scenes from the earthquake

In 2004, five workers at a nuclear plant in western Japan were killed when a corroded pipe burst and sprayed them with boiling water and steam, revealing flaws in safety procedures.

The U.S. Nuclear Regulatory Commission said Friday that American nuclear reactors are not vulnerable to the sequence of events that overtook the Japanese reactors because regulations here take into account the specific vulnerabilities at each plant under the most extreme conditions possible.

In the case of the San Onofre Nuclear Generating Station, operated by Edison International in northwestern San Diego County, a 30-foot seawall protects the plant and its emergency generators from the maximum theoretical tsunami that could hit the site, said regulatory commission spokesman Scott Burnell.

Edison International spokesman Chris Abel said the seawall is just one of several redundant systems meant to ensure power for an orderly shutdown in the event of an emergency.

In Japan, the shutdown of the 11 nuclear reactors affects perhaps 8% of the country's electrical generating capacity. Most of those plants should be able to go back on line within a few days, though it may be longer before repairs can be made to damaged power lines that carry electricity from the plants to consumers.