Today, the French ISRN released some results of its computation of the Fukushima plume over Tokyo. Another simulation shows the plume till March 25th for the whole Globe taking into consideration data from the French meteorological office.
and then for the whole globe:
To give some perspective, as per wikipedia, one Bq is defined as the activity of a quantity of radioactive material in which one nucleus decays per second.. From the TORCH report on how to computer the dose due to Cesium from its Bequerel units::
To sum these properties up
- The dose rate due to external irradiation from uniform contamination of the ground by 137Cs is 1.6 x 10-12 Sv hr-1 Bq-1 m2.
- The internal dose for ingestion is 1.2 x 10-8 Sv Bq-1
- The internal dose for inhalation is 8.7 x 10-9 Sv Bq-1
The maximum for this global computation is 1000 Bq/m^3. All other colors are decreasing by increment of ten from that maximum. This number has then to be multiplied by the low numbers above to obtain a dose. As can be seen from the US EPA detectors, this dose is part of the background radiation level. Another map of interest shows that the plume modeling is different for different research teams. The folks at Texas A&M provide computations showing particles in the plume flying at 2 to 4 km altitude while the french model is restricted to 500m high.