Here's an interesting simulation of the Japanese nuke radiation path across the globe:
http://www.irsn.fr/FR/popup/Pages/irsn-meteo-france_19mars.aspx
The report I read said, "The radioactivity is a thousand times less than that spewed out by the Chernobyl explosion in April 1986. The details were kept secret from the French at the time, but reached levels of between one and 10 becquerels per cubic metre of air. The Fukushima cloud is estimated at a one-hundredth to a thousandth of a becquerel/m3."