> among Hiroshima bomb survivors the incidence of birth defects was about 0.9%, or about 500 children in total
From what I understand, this is because the blast spread the radioactive material very far and thin, reducing its concentration in any given area.
Just to give you an actual statistic rather than anecdote, among Hiroshima bomb survivors the incidence of birth defects was about 0.9%, or about 500 children in total. I'm not entirely sure how trustworthy this story is.