Jet streams could carry radioactivity from Japan to the US.
In the second world war Japan used balloons carried by the jet stream
to send bombs to the US.
Some bombs actually made it.
It seems to me, that in the same way, if radioactive substances
get high enough in the atmosphere, those may be carried at high speed towards the US.
Time to buy or build your radiation counter.
http://www.panteltje.com/panteltje/pic/gm_pic/
http://www.panteltje.com/panteltje/pic/sc_pic/