MISSION: COUNTDOWN
Quest
Mission: Countdown Alpha, Mission: Countdown Bravo, and Mission: Countdown Charlie are repeatable main quests in Fallout 76 which trigger a nuke launch upon a map location selected by the player, creating a Blast Zone.
From “Mission: Countdown” on The Fallout Wiki, CC BY-SA 4.0, adapted (formatting).
WHERE
No map location in the game files (it may happen anywhere, or in an instanced area).
From the quest's location aliases; a quest that travels may list where it starts.
REWARDS (113)
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Gameplay Reward records, leveled lists at max level; chances are estimates. Rewards a script hands out aren't listed.