Thorium

Thorium is a metal added by GregTech and NuclearCraft, the main purpose of which is to fuel nuclear reactors.

NuclearCraft
Thorium Ore spawns in the overworld and is easily mined with an Iron Pickaxe. The ore can be smelted into ingots or doubled into dusts using a Manufactory or similar machine. Thorium is the first Fission Reactor fuel for most players.

Thorium can be put into an Isotope Separator to get Thorium-230 and a small amount of Thorium-232 (1 tiny clump, or 1/9th of a normal clump). Thorium-232 has two uses: You can put it next to a Decay Generator to produce a small amount of power and get a Lead Block, or put the clumps in a Decay Hastener to get Lead Dust. Thorium-230 can be put in a Decay Hastener to be turned into Lead Dust as well, but it's normally used to make TBU Fuel instead, the base Fission Reactor fuel (unless you get Uranium first.)

GregTech
Unlike most Metals, Thorium can't be smelted, but needs to be Compressed into ingots. Thorium also has no nugget form. Thorium's main source is Monazit Ore