Short story: member of 'atomic priesthood' reminisces

Question

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.

Answer 1

Short version: I think this story is almost certainly "Aspic's Mystery" a short story by Arsen Darnay. Originally published in Analog in 1976, it was later anthologised in "Space Mail, Volume II" which is probably where you read it. The text is available online at the Luminist Archives where you can find the September 1976 issue of Analog.

Long version: As I remarked in a comment, the "atomic priesthood" reminded me strongly of a novella by Arsen Darnay entitled "Plutonium" published in Galaxy 1976. This deals with a future history in which the long-term storage of nuclear waste is done by a "priesthood" specifically set up for this purpose - the point being that only institutions like the Roman Catholic church have demonstrated the kind of longevity comparable to the half-life of plutonium (24000 years). The monks are conditioned to seek out and hoard radioactive waste, and in particular they have a holy ritual in which they rub the waste against their skins (their equivalent of communion). To ensure that the order does not die out, some mad-science was employed so that the priests are reincarnated. So the cycle endlessly repeats: they work with radioactive material, the material kills them, they are reborn and return to their lethal work.

This does not fit the other details provided by the OP, but Darnay wrote a number of works set in this universe. In particular "Aspic's Mystery" seems an excellent fit to the question. This deals with the reminiscences of Hamsters Dugout, one of the Plutonium Priests. He is investigating the origin of the order, and has managed to extract some ancient documents from a rusted safe. Deep within the monastery are tanks and vats of "godbod" (the radioactive waste), which gives the monk a type of holy ecstasy when he feels its radiation on his skin.

As the OP recalls, the monastery contains a Mystery (which has been destroyed/gone missing):

The stories had it that the Mystery consisted of three rooms. Each room was named as follows: Power, Signal Acquisition, and Conditioning... Only the Abbot could enter Power (or so the brothers speculated), and that explained why the abbots had so brief a life. Godbod's love called them to an early bliss.

The monastery is in decay, and a new faction has arisen, which wants to cover the sacred godbod with the heretical metal, lead, which blocks its radiation. The monk is furious about this, but does not throw himself in the waste as the question mentions. He is clearly in the last stages of radiation sickness, and just hopes to die before the desecration occurs.