How Does a Meltdown Compare to an Atomic Bomb Detonating?

for example, Fukushima is is meltdown. Is that better, worse, or not comparable to Hiroshima and Nagasaki? The USA didn't seem to have any worries detonating those bombs for fear of fallout blowing East to the West Coast. Also, Japan seems to have rebuilt and inhabited those areas since then with no harmful effects.