What does stuff refer to here?

This is casual, informal usage. One of the meanings of "stuff" is "material" - uranium in this case.

I love money. The trouble is, I haven't got enough of the stuff.

The meaning in your extract is approximately those of (1) and (2) below.

stuff

noun

1. the material of which anything is made: a hard, crystalline stuff.
2. material to be worked upon or to be used in making something: wood, steel, and other stuff for building.
3. material of some unspecified kind: a cushion filled with some soft stuff.
4. Chiefly British. woven material or fabric, esp. wool.
5. property, as personal belongings or equipment; things.
6. something to be swallowed, as food, drink, or medicine.
7. inward character, qualities, or capabilities: to have good stuff in one.
8. Informal. action or talk of a particular kind: kid stuff; Cut out the rough stuff.
9. worthless things or matter: to clean the stuff out of a closet.
10. worthless or foolish ideas, talk, or writing: a lot of stuff and nonsense.
11. Sports.
a. Baseball. the assortment of pitches that a pitcher uses in a game together with the ability to deliver them in the proper manner at the right speed to the desired spot: He saved his best stuff for the tougher hitters in the lineup.
b. spin or speed imparted to a ball, as by a baseball pitcher, a bowler, or a tennis player: a pitch with plenty of stuff.
12. Informal. journalistic, literary, artistic, dramatic, musical, or other compositions or performances: Bach composed some splendid stuff.
13. Informal. one's trade, skill, field, facts, etc.: She knows her stuff.
14. Slang. any kind of drug, esp. an illicit one.
15. Also called stock. Papermaking. refined and beaten wet pulp ready for spreading on the wire.