From The Sydney Morning Herald :
" DILUTED radiation from the crippled Fukushima nuclear plant is expected to reach parts of the United States, including southern California, this afternoon, as Japanese authorities continue their battle to stabilise the plant.
The radioactive reach of Japan's worst nuclear disaster is not expected to be as lethal as that from the 1986 Chernobyl explosion, as the particles were released at a lower altitude and residual radiation reaching the US and parts of Canada will have diluted over the Pacific.
Former chief executive of the Australian Radiation Protection and Nuclear Safety Agency Peter Burns says that once in the atmosphere, radioactive vapours dissipate rapidly.
Outside the 20 to 30-kilometre evacuation zone, the concentration becomes about a million times less than at the point of discharge.
By the time the contaminated air travels the more than 8600 kilometres across the ocean, there would at worst be only minor health concerns for US residents."