Helium 3

Helium-3 is a rare, non-radioactive isotope of helium that is seldom found on the surface or in the atmosphere of a planet or moon. It is one of the resources that cannot be created in the VAB (the other being antimatter). In KSP Interstellar, Helium-3 (He3) is a product of the decay of tritium and is found in trace amounts in Jool's atmosphere.

Producing He3 via tritium decay requires only a lot of tritium and an even larger amount of time. You will need to produce roughly 30 tritium units before a single unit of He3 is produced but this ratio decreases over time. With five un-upgraded 3.75m fission reactors at 0% utilization and tritium breeding enabled, it took two years to produce 520 units of tritium and 30 units of He3. In the same time period, 550 units of lithium were consumed.

NOTE: He3 production does not occur in the background. In the above example with 5 reactors, if you switch to a different vessel and time warp two years, 520 units of tritium will be produced but no He3 will be produced.