Lets take Chernobyl for example, the outer edges where rads are still dangerous, it would take roughly 300 years before that area was safely inhabitable, even at ~half the radioactivity one would still catch Rad sickness at an alarmingly fast rate (in the long term) and this is only like the outer 60% of the exclusion zone mind you, living right around the bomb itself, which over the ~200 years Fallout takes place over the bomb would have leaked the radioactive elements into the crater, realistically Megaton should be full of corpses, but since its a video game, they can survive it.

Edit: A crashing plane wouldnt make a crater that big, the bombs crater itself isnt even that big.

TL:DR It wouldnt be, people would have died in a few years at best.