If Megaton is destroyed, Deputy Weld's faceplate will survive the atomic explosion and continue to welcome people to Megaton, albeit in a garbled manner, and state that the bomb is safe.
Deputy Weld can be deactivated by hacking a computer terminal in the Megaton armory. A skill of 75 Science is required to hack it and deactivate Deputy Steel. If the Robotics Expert perk is used to sneak-deactivate him, he still says his phrases and later on in the game will return to normal.
Deputy Weld will respawn after a while if he is killed.
pcps3 In Broken Steel, if Deputy Weld is killed in the firefight between Megaton settlers and the Brotherhood of Steel soldiers accompanying the water caravan, he may later be found wandering the wastes in the vicinity of Megaton, still saying "Welcome to Megaton."