The Meteorological Research Institute said its computer simulations calculated that radioactive materials, including cesium-137, were blown northeastward beginning March 11 toward Russia’s Far East and Alaska before mainly falling into the Pacific.
It estimates that what minute amounts of contamination that remained in the atmosphere blew over the Pacific coast of the United States around March 17.
The diluted amounts of radioactive particles stll aloft after that are believed to have completed their first round-the-globe trip by March 24, the institute said.
In a related development, Japan's science ministry says 8 percent of the country's land surface has been contaminated by radiation from the crippled Fukushima nuclear plant.
Many Japanese have purchased their own Geiger counters to monitor radiation levels around them.
The Japanese government last week banned rice shipments from contaminated Fukushima farms after tests revealed they contained unsafe levels of radioactive cesium.
Video Still: NHK
Map Graphic: NOAA