VIENNA - Minuscule amounts of radioactive particles believed to have come from Japan's Fukushima nuclear power plant have been detected on the US west coast, two diplomatic sources said on Friday.

The level of radiation was far too low to cause any harm to humans, they said. One diplomat, citing information from a network of international monitoring stations, described the material as "ever so slight", consisting of only a few particles.

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"They are irrelevant," the diplomat added.

Another diplomatic source also said the level was "very low."

The Comprehensive Test Ban Treaty Organisation (CTBTO), a Vienna-based independent body for monitoring possible breaches of the test ban, has more than 60 stations around the world, including one in Sacramento in California.

They can pick up very small amounts of radioactive particles such as iodine isotopes.

"Even a single radioactive atom can cause them to measure something and this is more or less what we have seen in the Sacramento station," said the first diplomat, who declined to be named.

Asked if they were believed to originate from the Fukushima nuclear plant in Japan, which has leaked radioactivity since being damaged by last week's massive earthquake and tsunami, he said: "That is the obvious assumption."

The CTBTO continuously provides data to its member states, but does not make the details public.

A Swedish official, also citing CTBTO data, told Reuters on Thursday that low concentrations of radioactive particles were heading eastwards and expected to reach North America in days.

Also on Thursday, the US Nuclear Regulatory Commission (NRC) said radioactivity would disperse over the long distance and it did not expect any harmful amounts to reach the country.


The New York Times earlier said a CTBTO forecast of the possible movement of the radioactive plume showed it churning across the Pacific, and touching the Aleutian Islands on Thursday before hitting southern California late on Friday.

Radiation from the Chernobyl disaster in 1986 spread around the globe and reached the west coast of the United States in 10 days, its levels measurable but minuscule.

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