This is a short short I remember reading, part of an anthology, possibly near the front. The main character is a member of an 'atomic priesthood', a monkish order charged in ancient times with keeping people safe from a substance, that they now consider holy. They go around to various roadside shrines and test the rocks people have enshrined there, taking them back if they glow. The place where they live has several levels, called mysteries, like the "Mystery of Comms", near the surface, that no longer works and nobody can remember what it means. Something about gaps in the records. He remembers that there is a giant safe, with the door rusted shut. The back is also rusted and he used something to pull some old papers out that talks about the founding of their Order. There is also a place at the lowest level where there are large tanks of something that glows. Over time the seals have corroded, letting the holy substance out. The members of the order periodically expose themselves to this as a test and as mortification. The narrator is of a select few that don't feel any ill effects from being there.
Towards the end, he says that a new group have come in, taking over the place, sealing off the tanks, wearing protective suits. He goes to one of the tanks and throws himself in.