Ablutions

Ablutions is a side quest in the Fallout 4 add-on Far Harbor.

Detailed walkthrough
For this quest, Sister Mai, at the Nucleus asks the Sole Survivor to acquire a pump regulator to repair the decontamination arches near the entrance. Her directions lead to the Vim! Pop factory which is inhabited by a large number of Super Mutants and their hounds. The regulator can be found in the Vim factory floor area with the two large mixing vats. Take the catwalk to the highest level and walk through the elevated office areas until reaching the eastern side of the room. The regulator is next to an ammo box on a desk.

Upon returning to Mai, questioning her about her "repairs" reveals that she is actually attempting to circumvent the decontamination arches' filter systems in order to have them irradiate instead of decontaminating everyone passing through. She then leaves the honor of installing the pump regulator to the Sole Survivor. Reaching the mechanism at the bottom of the submarine dock involves trudging through the dock's heavily irradiated water, so suitable preparation is recommended. With an Intelligence of 7 or higher, the Sole Survivor can opt to reverse Mai's alterations to the system before installing the new regulator, thus making the arches actually decontaminate as they were meant to do. Return to Mai to claim Radical Conversion, the unique radium rifle as reward regardless of how the quest was resolved.

Bugs
After completing the quest, then leaving and re-entering the Nucleus, the decontamination arches will only spray mist, and not affect the player's rads in any way, regardless of whether or not they chose to reverse the alterations.