I believe God showed mercy on Japan and the radiation after the bombs were dropped seemed to have a fairly mild impact, I believe God can do miracles like that, lessen the impact of the radiation.
As the bible says to his disciples, "if you drink poison, but it shall not affect you" and "It is not what a man eats, but what comes out of his mouth that defiles him". I have found that to be really true, if you worry about your physical health and what you eat, it ends up in frustration but if you worry about your spiritual health, it can be a greater boost to your physical health.