The leg of the Gyre closest to Japan – the Kuroshio current – begins right next to Fukushima:
While many people assume that the ocean will dilute the Fukushima radiation, a previously-secret 1955 U.S. government report concluded that the ocean may not adequately dilute radiation from nuclear accidents, and there could be “pockets” and “streams” of highly-concentrated radiation.
Physicians for Social Responsibility notes:
An interesting fact for people living on the US west coast is also included in the UNSCEAR [United Nations Scientific Committee on the Effects of Atomic Radiation] report: only about 5% of the directly discharged radiation was deposited within a radius of 80 km from the Fukushima Dai-ichi nuclear power station. The rest was distributed in the Pacific Ocean. 3-D simulations have been carried out for the Pacific basin, showing that within 5-6 years, the emissions would reach the North American coastline, with uncertain consequences for food safety and health of the local population.