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.

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