Modern (fission-)fusion bombs are cleaner and more efficient than old fission bombs.
Low efficiency means that the nuclear reaction doesn't end with the explosion. The remaining heavy nucleus decay at their normal rate and radiate.
The biggest one, Tsar Bomba was luckily very efficient.
"It has been estimated that detonating the original 100 Mt design would have released fallout amounting to about 25 percent of all fallout emitted since the invention of nuclear weapons."
http://en.wikipedia.org/wiki/Tsar_Bomba
Low efficiency means that the nuclear reaction doesn't end with the explosion. The remaining heavy nucleus decay at their normal rate and radiate.
The biggest one, Tsar Bomba was luckily very efficient.
"It has been estimated that detonating the original 100 Mt design would have released fallout amounting to about 25 percent of all fallout emitted since the invention of nuclear weapons." http://en.wikipedia.org/wiki/Tsar_Bomba