Well, I wasn't going to mention that the damage is genetic, and that Japan has a lot lot less room to go round

DAMAGE CAUSED BY THE ATOMIC BOMB EXPLOSION
Levelled Area...................6.7 million square meters
Damaged Houses:
Completely Burned ------11,574
Completely Destroyed-----1,326
Badly Damaged------------5,509
Total-------------------18,409
Casualties
Killed------73,884
Injured-----74,909
Total------148,793
(Large numbers of people died in the following years from the effects)
Considering that Japan only has a land area of 375,000 km ^ 2, which is slightly smaller than California for the entire country, that's a pretty weighty chunk contaminated.