If Fukushima could be stopped spewing fallout 24/7,
and if there were no other nuclear disasters in the future,
I would guess that the topsoil fallout levels would return to acceptable levels in a generation or two of humans.
However a generation or two of humans has already been nuked, not enough to alert most people in the present, but enough to have a significant impact over the next several decades.
Add to that the demographic cultural trend of the US heading for a drastic decrease in population, and I predict dramatic losses of the present day indigenous people
of north America (that is to say mostly "gringos").
Actually in terms North America, small or even moderate rises in radiation levels, as measured on relatively inexpensive geiger counters of open air (radiation network type readings), would not be the primary health threat for areas outside of Fukushima.
The health threat would be more that of radioactive materials that concentrate in certain parts of the body and generally provoke tumors, and increased infertility due to both decreased health levels from secondary causes, and from the radiation itself.
This type of threat can be quantified not with dosimeters or geiger counters, but with more expensive spectrographic instruments, and medical knowledge of how the various isotopes affect the body.
A vacuum in the culture will encourage other cultures to come into the void to occupy, since by that point the radiation may be not so much a deterrent to the new culture moving in.
They will be the clean-up crew, the vultures, the consolidators, and more than that they well be the new land barons, since there will be fewer of them to divide things up amongst.