The Nuke in Megaton can only be disabled through a high enough explosives rating. If your explosives rating is not above 25, it simply will not work. The quest is definitely worth undertaking however as I recently read on the PS3 Blog that the quest reward is a pretty sizeable one especially for early game. You actually get a house in Megaton along with a cash bonus for completing the quests. This also means that you will have a place to stash all your loot which is the fundamental joy behind most RPGs. I don’t know why this game is so effective at turning people into pack rats!