Initiate Vault Decontamination is a quest in Fallout: Brotherhood of Steel. It is found in the Secret Vault Laboratory Level 4.
Get to the emergency decontamination center and activate the vault decontamination sequence to kill giant blob.
Patty failed to activate the emergency decontamination procedure and asks the Initiate to do this. Activate the switch next to Patty and enter the emergency room. The're is a blob mouth here which must be dealt with (Oh Shit! (Part 2)).
Flipping the switch on the machine behind the blob mouth will activate the decontamination sequence (a nuclear device designed to destroy the entire vault and surrounding area). Take the monorail next to the save point to get to Level 1 of the Secret Vault - Laboratories.
You've got 120 seconds to reach the monorail car at the other end of the station before the emergency decontamination procedures begin. Ignore the Mutants and other monsters around as you don't need any experience any more. Just run east until you get to the exit. Take the monorail car that will take you to a distant location in the mountains, miles away from the Secret Vault and watch the explosion.