Uranium is produced in a Uranium Reactor.
Its main use is troop training. Uranium is the primary material used in unit production, and it is also used for researching units at Tech Lab and for some building upgrades. It can be obtained via PvE combat from the Helio Legion, who specialise in it's production. You can also earn it by opening Uranium Resource Crates bought at the Store or by converting other resources at Resource Converter.