Helium 3 Tank - sswelm/KSPInterstellar GitHub Wiki
This small tank stores 250 units of Helium-3.
He-3 is used only by fusion reactors. It is extremely difficult to obtain and, like antimatter, cannot be included in a newly-built ship. The only sources for He-3 are radiactive decay of Tritium or mining it from Jool's atmosphere.
Tritium has a half-life of 12.32 years and, as of KSPInterstellar 0.11, Tritium does not decay when the ship is not focused.
Atmosphere scoops can slowly mine trace amounts while flying deep in the atmosphere. A small radial atmospheric scoop flying at a 40k altitude in Jool will mine about 0.1 units per hour.