Radioactive particles from the failing Fukushima Daiichi nuclear power station pose little immediate risk to North America, and should fall into the Pacific before reaching western shores.

Using a publicly available modeling system for airborne pollutants developed by the National Oceanic and Atmospheric Administration, Weather Underground’s Jeff Masters has modeled the spread of radioactive plumes. So far, the “great majority of these runs” have seen the plumes float over the Pacific, reaching eastern Siberia and the western coast of North America in about a week.

“Such a long time spent over water will mean that the vast majority of the radioactive particles will settle out of the atmosphere or get caught up in precipitation and rained out,” wrote Masters. “It is highly unlikely that any radiation capable of causing harm to people will be left in the atmosphere after seven days and 2000-plus miles of travel distance.”

A press release issued March 13 by the U.S. Nuclear Regulatory Commission echoed Masters’ speculation. “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,” (pdf) they announced.

The March 11 earthquake disrupted cooling systems that pump fresh water onto fuel rods inside the plant’s reactors. Even when reactors are shut down, the rods continue to produce heat. Without cooling, the rods could melt, releasing radioactive steam inside. (See Nature.com’s Great Beyond blog for an excellent anatomy of the disaster.)

Repair crews are now using fire pumps to flood the plant’s reactors with seawater. It’s a touch-and-go process, however, and steam buildup produced explosions on Saturday and again Monday morning. A partial meltdown is now taking place. A full meltdown is possible but unlikely. In the meantime, steam from the reactors will send radioactive particles airborne.

According to the Pentagon, soldiers aboard the aircraft carrier Ronald Reagan, now sailing the Pacific, were exposed to a month’s worth of radiation in one hour from particles blown out to sea. Winds over Japan blew east across the Pacific last week, and are expected to do so for the next week.

Masters noted that the Chernobyl disaster failed to spread “significant contamination” beyond 1,000 miles, and that disaster was far worse than Fukushima Daiichi has been. This release could, however, continue for months, until the fuel rods have completely spent themselves.

Image: Atmospheric simulation for radioactive particles released March 11. The blue represents particles released about 300 feet into the air, and red about 1,000 feet. The black star is the location of the Fukushima Daiichi plant./Jeff Masters and NOAA.

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