Also, if you think this explosion in Fukushima was bad, if a fast breeder reactor like in Iran went critical it could be alot worse. Fast Breeder reactor means as the nuclear reaction proceeds, it produces more and more fissionable fuel, they have to stop the reaction and de-concentrate the plutonium and U235. The u235 and plutonium is then used in nuclear bombs.

Fast breeder reactors are banned by international treaty for two reasons really. 1, they are extremely dangerous, and 2. you can use them to make lots of nuclear bombs quickly.