I received an inquiry from a friend about traveling to Japan this July. This is the relevant part of my response. I know there are a lot of opinions about this out there, but this is my latest opinion on the matter. Read on if you are interested.
Tokyo is relatively safe. My mom tells me that there is no problem getting bottled water anymore and groceries are fine as well. I think once you are there you might be surprised at how normal things might seem. I am sure there is an underlying tension and you might notice less foreigners, but I am getting the sense from my friends (who are mostly in Kanto and in and around Tokyo) that people are trying really hard to live life as it has been–in other words, trying to act out normalcy as much as possible.
There are two things I would caution you about. The first is that it might be hotter in buildings because of the effort to save energy and because of planned blackouts during the day. The second is that there might be another big earthquake, but I don’t think that there is much point in trying to avoid this. That would mean simply not traveling to Japan again. One thing I would try to make sure of is to carry around a small and light emergency kit (along the lines of your ID, flashlight, some bread, candy, and water) and to stay in good enough shape that you can walk for many hours if you have to.
Radiation doesn’t seem to be a big problem at the moment, but it’s in the atmosphere so you should probably avoid getting rained on. The amount of radiation included in fresh produce doesn’t seem to be of much concern because where there is any danger or concern, the food is not sold. (Pretty tough on the farmers.)
The only people I would warn against going are people who are or will be traveling with people who cannot take care of themselves in an emergency–like extremely unfit people, elderly people, young children, etc.