While officials downplayed any immediate danger, the US Nuclear Regulatory Commission deployed two experts to Japan, where the Fukushima plant, which was rocked by a large explosion earlier in the day in the aftermath of Japan's strongest-ever earthquake.

"At present there is no danger to California. However we are monitoring the situation closely in conjunction with our federal partners," Michael Sicilia, spokesman for California Department of Public Health, told AFP.

"California does have radioactivity monitoring systems in place for air, water and the food supply and can enhance that monitoring if a danger exists," he added.

He was speaking as Japanese authorities moved to calm fears of a meltdown and said a huge explosion Saturday had not ruptured the container surrounding the reactor, although there had already been some radiation leakage prior to the explosion.

Experts have suggested that, if there were a reactor meltdown or major leak at Fukushima, the radioactive cloud would likely be blown out east across the Pacific, towards the US West Coast.

"The wind direction for the time being seems to point the (nuclear) pollution towards the Pacific," said Andre-Claude Lacoste of the French Nuclear Safety Authority, briefing journalists in Paris on the Japanese crisis.

The Nuclear Regulatory Commission meanwhile said it has sent two experts to Japan, and has been in regular contact with Japanese officials about the crisis.

"We have some of the most expert people in this field in the world working for the NRC and we stand ready to assist in any way possible," commission chairman Gregory Jaczko said in a statement announcing the deployment.

He said the pair were experts in boiling water nuclear reactors and are part of a broader US aid team sent to the disaster zone.

Earlier the NRC said it was "examining all available information as part of the effort to analyze the event and understand its implications both for Japan and the United States."

While US nuclear experts acknowledged the seriousness of Japan's reactor crisis, some stressed that taking steps in the United States such as distributing iodine tablets -- which prevent iodine 131 from being absorbed into the body -- would be "vastly premature."

"It's a big ocean. These (radiation) releases are essentially going to be at ground level," said Ken Bergeron, a physicist who has worked on nuclear reactor accident simulation.

"We should not confuse it with health issues in the United States."

Japan is roughly 5,000 miles (8,000 kilometers) from the US West Coast.

earlier related report
US experts fear 'Chernobyl-like' crisis for Japan
Washington (AFP) March 12, 2011 - US nuclear experts warned Saturday that pumping sea water to cool a quake-hit Japanese nuclear reactor was an "act of desperation" that may foreshadow a Chernobyl-like disaster.

Several experts, in a conference call with reporters, also predicted that regardless of the outcome at the Fukushima No. 1 atomic plant crisis, the accident will seriously damage the nuclear power renaissance.

"The situation has become desperate enough that they apparently don't have the capability to deliver fresh water or plain water to cool the reactor and stabilize it, and now, in an act of desperation, are having to resort to diverting and using sea water," said Robert Alvarez, who works on nuclear disarmament at the Institute for Policy Studies.

"I would describe this measure as a 'Hail Mary' pass," added Alvarez, using American football slang for a final effort to win the game as time expires.

An 8.9 magnitude earthquake that struck Japan on Friday set off the emergency at the plant, which was then hit by an explosion Saturday that prompted an evacuation of the surrounding area.

Workers doused the stricken reactor with sea water to try to avert catastrophe, after the quake knocked out power to the cooling system.

What occurred at the plant was a "station blackout," which is the loss of offsite air-conditioning power combined with the failure of onsite power, in this case diesel generators.

"It is considered to be extremely unlikely but the station blackout has been one of the great concerns for decades," said Ken Bergeron, a physicist who has worked on nuclear reactor accident simulation.

"We're in uncharted territory," he said.

The reactor has been shut down but the concern is the heat in the core, which can melt if it's not cooled. If the core melts through the reactor vessel, Bergeron explained, it could flow onto the floor of the containment building. If that happens, the structure likely will fail, the experts said.

"The containment building at this plant is certainly stronger than that at Chernobyl but a lot less strong than at Three Mile Island, so time will tell," he said.

Peter Bradford, former head of the US Nuclear Regulatory Commission, said that if the cooling attempts fail, "at that point it's a Chernobyl-like situation where you start dumping in sand and cement."

The two worst nuclear accidents on record are the 1986 Chernobyl disaster in Ukraine and the partial core meltdown of the Three Mile Island reactor in the US state of Pennsylvania in 1979.

Another expert said the Japanese accident will rank as one of the three worst in history.

"If it continues, if they don't get control of this and... we go from a partial meltdown of the core to a full meltdown, this will be a complete disaster," Joseph Cirincione, the head of the Ploughshares Fund, said in an interview on CNN.

Cirincione faulted Japanese authorities for providing partial and conflicting information about what was happening at the plant.

Cirincione said the presence of radioactive cesium in the atmosphere after the plant was vented indicated that a partial meltdown was underway.

"That told the operators that the fuel rods had been exposed, that the water level had dropped below the fuel rods and the fuel rods were starting to burn, releasing cesium," he said.

Japan's nuclear safety agency rated the Fukushima accident at four on the International Nuclear Event Scale from 0 to 7, meaning an accident "with local consequences," an official said Sunday.

The Three Mile Island accident was rated five while Chernobyl was a seven.

The government declared an atomic emergency and said tens of thousands of people living within 12 miles (20 kilometers) of the plant should leave after an explosion at the nuclear plant Saturday.

"This is obviously a significant setback for the so-called nuclear renaissance," said Bradford, the former NRC commissioner.

"The image of a nuclear power plant blowing up before your eyes on the television screen is a first."

But a spokesman for the World Nuclear Association said in an interview with CBS News that the threat of a full meltdown is minimal.

"I think that possibility is remote at the best of times and is diminishing by the hour as the fuel gets cooler and generates less heat," said Ian Hore-Lacy, spokesman for the industry organization.

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