Shirley this is one of the most asked safety questions, so if you search "Japan" on Ask A Nomad you'll see plenty of information from similar questions.
In a nutshell - The Japanese government has imposed a 30 km exclusion zone around the failed nuclear power station in Fukushima. Many foreign governments recommend you make that 80 kms (no evidence to suggest increased exposure risk between 30 and 80, but they just doubled it plus a bit to be on the safe side).
Due to the continuing pressures on devastated infrastructure I'd think really hard about going to the quake and tsunami affected north-east.
Tokyo: apart from little pockets of unusually high radiation - and I mean pocket-sized bits... usually debris caught in a stormwater drain - radiation levels in Tokyo have been safe and near-normal since that bad 48 hur period on march 16/17.
There have been instances of contaminated food finding its way into the supermarkets... particularly beef and some vegetables. The government has widened a ban on foodstuffs from Fukushima prefecture and also Iwaki prefecture.
A health official said you'd have to eat the contaminated beef at every meal for a year before it got close to unacceptable levels in your body.
There's no evidence of contamination in Tokyo's drinking water, still many locals are sticking to bottled water.
South of Tokyo ie everywhere else in Japan is not affected.
Radiation.... a big dose over a short time is bad. A low dose over a long time is bad. You'll be visiting for a short time, and IF there is any extra radiation around it is pretty low dose, so low dose and a short time - I wouldn't worry if I were you.
Don't forget, Japan has numerous fault lines running through it and earthquakes are common - thankfully massive ones like last march are extremely rare.
Phil over 6 years ago