In the world of Francis's mind, we're remembering a religious verse with references to nuclear weapons and radioactive isotopes.
Outside in the really-real world, Francis comes across a door labeled "INNER HATCH"… and hopes the Fallout Demon is safely trapped inside.
He also finds a desk, some lockers, and a skull with a gold tooth.
Eventually, he overcomes his initial fear of whatever might be contained in this shelter and hopes the desk and lockers might actually contain Memorabilia—precious documents written before the Age of Simplification (we'll learn more about that bit of future "history" in Chapter 6).
He explores the contents carefully, remembering the incident of the Venerable Boedullus. This infamous incident involved a search for Memorabilia, the discovery of a "launching pad," and the creation of a new lake (2.14).
In a box, Francis finds Memorabilia including a memo, shopping list, and a circuit design. He notices the circuit design is signed in the name of Leibowitz, his order's founding member.
He's ecstatic, believing Heaven has sent him "a token of his vocation" (2.37). For those non-monks out there, imagine acquiring a vintage copy of the White Album, in mint condition, signed by all of the Beatles. Then you might begin to understand Francis's excitement.
That night, Francis wonders what the Fallout Shelter will mean for the Leibowitz Abbey and his future as a monk. He falls asleep and awakes alone with the box. Well, except for the wolves prowling around the shelter. Eep.