I understand some of you concern on radioactivity level in Japan, which was exposed from Fukushima Daiichi Power Plant after the earthquake on March 2011. For those people, the linked site is maybe helpful to ease your anxious and to make decision whether and in which part you tavel in Japan.
The original site is in Japanese but can be translated by Google into English and other languages. The site is linked to various data reports related to radioactive contamination such as air, water and food.
I hope that helps you with planning trip to Japan.