@Anthony Mous: "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."
Chernobyl will remain contaminated for hundreds of thousands of years. But if I was president of China, I would not want to risk the remotest test that could make me sick.