The most read contribution on this site is this one:
A long and detailed discussion of the merits and safety of travel to Japan.
In a nutshell: if you're going for a short time, are careful about eating produce not originating from Fukushima prefecture, and do not go within 80 km of the damaged nuclear power plant you will be very safe indeed. over 6 years ago
An update for you all from March 2013
http://safety.worldnomads.com/japan-radiation-study over 5 years ago