4 Answers

....guessing:bombs in Japan were exploded in the air

Check out this most excellent essay style article here > http://www.huffingtonpost.com/patrick-takahashi/why-worry-about-fukushima_b_847250.html

“Today, the background radiation in Hiroshima and Nagasaki is the same as the average amount of natural radiation present anywhere on Earth. It is not enough to affect human health.”

Part of the answer is that these bombs exploded high up in the air and all the radioactive material blew or rained away... somewhere. I guess.... read more at the above link ...

  I saw a   DOCO Chernobyl recently, the forests are starting, along with ground level vegetation, the radioactivity has dropped almost to tolarble levels, so much being inhabitable for a 1000 years, As for the Japanese cities which were almost wiped off the map, no strategist value, to end the war they dropped them, it is called Collateral Damage   In 1945 they were still ignorant  what damage radioactivity does to the human body and it is better to anilise on the enermy, it is good the war was favourible to the allies as we could have committed crimes for mass murder.

soon after those cities were radio active, weeds and plants started growing there,and  deer started appearing.