I do not believe they are at risk at all, unless they are close to the actual Fukushima exclusion zone
It seems radiation levels there are actually lower than the normal background radiation found in many major cities worldwide. And certainly for short visits, you will be exposed to a lot more radiation while flying at high altitude
on your way to Japan, than during your stay...!
Furthermore, thanks to the general ignorance/hysteria/paranoia surrounding anything to with radiation, everything in the region is being constantly tested - making it quite possibly
safer than other regions where testing is not so frequent or rigourous (or not done at all), and problems/leaks/hotspots go undetected...
Interesting Bloomberg article - "people just outside Fukushima exlusion zone, as safe as someone living in London"