The struggle to avert disaster at a Japanese nuclear power plant has many Californians wondering about the risk of a radiation cloud crossing the Pacific. The Bee talked with experts on Monday about that possibility.

• Is radiation drifting from the Japan reactors to California now?

Probably, but not enough to worry about, said Krzysztof Starosta, an associate professor and nuclear physicist at Simon Fraser University in Vancouver, British Columbia.

Steam has vented from the damaged reactors. This steam contains radioactivity, but not enough to cause health problems even for the local population in Japan, he said.

If it did reach California, it would not be a health risk. A dental X-ray or smoking a cigarette imparts more radiation than what might reach California from this steam venting.

"Unless we see breaching of the containment vessels, there should be no significant releases of radioactivity," Starosta said.

• What about that worst-case situation?

The boiling-water reactor design common in Japan includes double containment vessels: an inner steel container and an outer concrete one. One reactor holds about 140 tons of uranium fuel.

"You'd have to breach both to have a significant release of radioactivity," Starosta said.

It would have to be a large release to get picked up by the jet stream. This river of air six miles above Earth travels west to east across the Pacific and brings California most of its weather.

"Most will stay in the (reactor) buildings," said Lake Barrett, who supervised civilian radioactive waste management at the U.S. Department of Energy until retiring in 2002. "Most will fall out of the sky because there's no energy to blow it up into the jet stream to get it here quickly."

• Would some of that radioactivity reach California?

Yes. But the risks depend on variables that are difficult to predict. This includes the volume of material, nature and duration of the release, and behavior of the jet stream.

Starosta and Barrett said radioactivity would be reduced or eliminated by decay during the ride on the jet stream, or dispersion in the air.

However, research confirms the jet stream routinely delivers common pollutants to California. UC Davis atmospheric scientist Steven Cliff contributed to a recent study that found as much as 29 percent of California's fine particulate pollution arrives on the jet stream from Asia, mainly from China.

Bill Patzert, a climatologist at NASA's Jet Propulsion Laboratory in Pasadena, said the jet stream travels at 200 mph. A particle from Japan could reach California in 25 hours.

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