Using the supercomputer program called SPRINTARS, researchers at Kyushu University and Tokyo University created the simulation of how radioactive materials from Fukushima I Nuclear Power Plant may have dispersed throughout the northern hemisphere. The researcher say their simulation fit the actual measurements.
It was published in the Scientific Online Letters on the Atmosphere (SOLA) under the title "A numerical simulation of global transport of atmospheric particles emitted from the Fukushima Daiichi Nuclear Power Plant" in June.
You can read the paper at this link (PDF file).
You can also view the animation, here, and the press release in Japanese here.
Their simulation also shows, like France's CEREA, radioactive materials from March 14/15 release reached the west coast of North America on March 18. The researchers attribute the rapid dispersion of radioactive materials from Fukushima to the unusually strong jet stream. Also, on March 14/15, there was a low pressure on the east cost of Japan, which created a strong updraft that lifted the radioactive materials to the jet stream. more