By Ralph Vartabedian, Los Angeles Times

Small amounts of radioactive isotopes from the crippled Japanese nuclear power plant are being blown toward North America high in the atmosphere over the Pacific Ocean and will reach California as soon as Friday, according to experts.

A network of sensors in the U.S. and around the world is watching for the first signs of that fallout, though experts said they were confident that the amount of radiation would be well within safe limits.

Operated by the Environmental Protection Agency, the U.S. network known as Radnet is a system of 100 radiation monitors that work 24 hours a day, spread across the country in places such as Anaheim, Bakersfield and Eureka. In addition, a network of 63 sensors is operated by the Comprehensive Test Ban Treaty Organization, an international agency allied with the United Nations.

Atmospheric experts said the material should begin showing up on the West Coast as early as Friday, though it could take up to an additional week for the 5,000-mile trip from Japan to Southern California. Although the organization has told its member countries that the first indication of radiation would hit on Friday, the plume from a North Korean nuclear test in 2006 took about two weeks to travel to North America, U.N. officials said.

The Nuclear Regulatory Commission, which oversees the U.S. nuclear industry, said Wednesday that it did not expect dangerous levels of radioactivity to hit the West Coast, Hawaii, Alaska or U.S. territories in the Pacific. But whatever levels reach the U.S. initially are likely to increase in subsequent days, because radioactive emissions from the damaged Fukushima Daiichi nuclear power plant have grown since the disaster began Friday. The NRC sharply raised its warning to American citizens in Japan, urging them to evacuate an area within 50 miles of the Fukushima complex. Japanese authorities have ordered an evacuation within about 12 miles of the plant.

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