HERE is the meteorological report from Fukushima Airport in Japan. It is a little less than 50 miles inland from Fukushima Nuclear Power Plant. For events which do
not include quickly rising superheated steam (i.e. full meltdown hitting water table), this will give a general idea about where less energetic radioactive gasses may be drifting.
Just like a volcano's ashes during an eruption, radioactive gasses will follow the wind. The bigger the eruption/explosion, the higher the particles will be ejected. If they are ejected high enough, they will wind up in the Jet Stream which can carry them quite a ways away. However, in cases like we saw with Fukushima Reactor No.1, it is less likely that much if any of this material has made it into the jet stream and is likely subject to winds blowing at a lower height- which are reported at the link above.
So, right now at the link above it says:
Wind from the ESE (120 degrees) at 3 MPH (3 KT)
To get an idea exactly what direction that is, you can use this diagram to get a better visualization:
PB