Before the Great War, Kansas City was a major nuclear launch site. After the city was hit by the bombs, the surviving silo staff were turned into ghouls, and formed a town called Gravestone in the ruins of the city.
Only one nuclear bomb was left intact in the silos, and the ghouls took to worshiping it as a god, Plutonius. Plutonius was later taken by the Eastern Brotherhood of Steel, placed on a nuke carrier, and used to crack open the entrance to Vault 0 in order to assault the Calculator.
Plutonius can be found on an alter in the town of Gravestone.