How long did it take for hiroshima/nagasaki to return to normal background radiation levels + Chernobyl?

how many years? and does anyone know what fuel was used in the bomb? i think most likely uranium or plutonium. how is this comparing to Chernobyl. what fuel did they use, and why is it that chernobyl is going to take "tens of thousands" of years to become normal (as quoted by different opinions on BBC news) as opposed to under a century for the WW2 bombs? granted there is more fuel, but wouldnt the half life decay it eventually regardless of how much nuclear fuel is there? besides, its all locked away in a sarcophagus now, shouldnt the radiation outside start clearing up within the next few decades?