It is found in Vault 3 in Motor Runner's throne room inside a locker, and is used for repairing the air filtration system in Brotherhood of Steel's hidden bunker in Hidden Valley for the Still in the Dark quest.
xbox360 The reverse pulse cleaner has also been known (according to multiple users on the Bethesda Softworks Forums subcategory Fallout 3 under Xbox 360) to cause the Downloadable Content Error which causes saved game files that have obtained reverse pulse cleaner and have it in their inventory to not load correctly. In some instances as long as the person is in Vault 3, grabs the item and decides to reload regardless if the item is in the inventory or not prior to the save the save file will still give the error. This is one of the many causes to spur the DLC error and while there is no specific reason for this, it is believed that if you grab the reverse pulse cleaner without having the quest Still in the Dark the DLC error will appear. It might happen regardless though. Adding the item through the console outside Vault 3 seems to get you around the bug. [verified]