The predominant winds from Japan are bringing radiation spread into the air, to North America.  The radiation is, so far, lower level isotopes, and the air disperses it over the long distance.

Now there is highly radioactive isotopes spilling into the Pacific Ocean, in higher concentrations, and the currents could eventually bring it to the North American coast.

The Kuroshio Current (Black Current, or Black Tide) runs northward up the Pacific coast of Japan, then turns eastward into the North Pacific Current heading towards the North American coast.