GURPS BattleTech, working out heatsinks and the effects of heat buildup

[Mark Skarr]

The Techmanual has this to say:

Originally Posted by BattleTech Techmanual, pg 35

Fusion and Fusion Fuels
Fusion reactors generate their vast quantities of power by, well, fusing light elements like hydrogen together into heavier elements like helium. Contrast this with nuclear fission, which splits heavy elements, like uranium, into lighter materials. The fuel of choice for modern fusion engines is normal hydrogen, the protium isotope if you want to be fancy.
In the past, other fuels were used by early fusion reactors—from the heavier hydrogen isotopes deuterium and tritium, to the helium-3 isotope and even lithium. But these types gradually lost ground to protium users. It was almost a century after the Terran—sorry, the Western Alliance—harnessed fusion that a reactor capable of burning protium was built. Though other fuels would’ve allowed simpler reactors, and backwater planets continued to use such primitive systems for that reason, militaries are fascinated with the newer reactor technology.
Normal hydrogen is a fairly clean nuclear fuel in terms of radioactive waste, at least compared to fusion with other fuels or fission. In fusion reactors today, this normal hydrogen is easily extracted from any number of sources, especially water. This is why most military fusion engines include a small electrolysis unit to extract hydrogen from water. Those tales you may have heard, of MechWarriors “refueling” their BattleMechs with urine? They aren’t myths.