Uranium

Uranium is a resource added by Advanced Solars, GregTech, IC2 and MineChem. It's main purpose is fuel in a Nuclear Reactor, but it also has some uses as crafting materiel. It is also required for the Nuke.

Industrialcraft 2
Unlike most ores, Uranium can't be smelted, but needs to be Compressed into ingots or processed through the IC2 ore processing chain. Uranium also has no nugget form. Instead, processed Uranium is split into Uranium-238 and Uranium-235.

Immersive Engineering
Immersive Engineering adds Uranium, but it is currently unused.