Hi, Phil at the safety hub here. Not sure about 'best' time to visit - depends on what you're looking for.
But radiation and safety... I can tell you there was never any problem following last year's tsunami and nuclear meltdown. Too far away and not affected by prevailing winds.
I know there's been some talk of radioactive clouds circling the globe, but levels of anything detected are so low as to be insignificant.
Interestingly this article was posted by CNN today (http://edition.cnn.com/2012/02/28/world/asia/japan-fukushima-nuclear/index.html?hpt=ias_c1 ). It's all about the radiation threat inside or immediately adjacent to the plant (which is inside a 30 kms no-go zone), and despite the alarmist language and headlines, it quotes experts who rate the risk of another radioactive plume or meltdown at the plant as ZERO. They also say the contamination of nearby seawater is very low.
And that's inside the factory fence!
Enjoy your trip. almost 8 years ago