You might want to have a look at this thread
where there's been plenty of discussion on this topic. My belief (and I'm a journalist not a scientist) is that what contamination there might be is very low level and possibly in hot spots rather than widespread. The effects of long term exposure to that (I'm talking years) may not be good, but for short term exposure (and a holiday/visit is extremely short term) I think the risks are very, very minimal.
Obviously don't go inside the declared 30km exclusion zone around the damaged plant. Remember Tokyo is over 225 kms from the plant. almost 8 years ago