Uranium

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

Industrialcraft 2
Unlike most ores, Uranium cannot 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 (except as a hot source for the Thermoelectric Generator).

NuclearCraft
NuclearCraft adds Uranium as a base radioactive material, and can be processed by Industrial Foregoing, Tinker's Construct, Mekanism (up to 2x), etc.