Tritium

Tritium is a rod that can be used as either fuel or in recipes.

It can be found as a bonus while smelting Deuterium in the Arc Smelter or as an uncommon item while centrifuging Deuterium in the Gas Centrifuge as well as a rarest item while centrifuging Toxic Waste.

Tritium can also be obtained as byproduct from generating power within a Fission Reactor, albeit in small quantities.