Uranium is the most common fuel in nuclear reactors. Naturaly occuring Uranium consist of about 99,2% U-238 and 0,7% of U-235. It is the U-235 which is intressting. Because it is fissible. Uranium has to be enriched to be used in the most coherences. Under 20% U-235 it is called LEU (Low Enriched Uranium), which is used in the most nucler reactors. HEU stand for High Enriched Uranium, which is used in some atomic-submarines and in nuclear weapons.
But in this project we are not going to use U-235. Naturally Uranium (with 0,7% U-235), can be used directly in heavy water-reactors. But we are going our from U-238, depleted Uranium.
If a neutron is catched up in the U-238 nuclear, it transmute to U-239 with T½ = 22,3 min, which than decays into Pa-239, and the half-life of that is about 27 days. Then the Protactinium decays to the fissible isotope Plutonium-239.
Then the isotope of Uranium, U-233 is fissible, and that is the type we use with the Thorium-project.
An Uranium glass-marble I have. The green color comes from Uranium.