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.

Originally Posted by Mark Skarr (Post 2407202)

The Techmanual has this to say:

Originally Posted by Fred Brackin (Post 2407205)

Ah, that's.......remarkably disconnected from science as I understand it. I think I'll ignore it.

The bit about He3 reactions being dirty particularly stands out. The part about refuelling in the field is a more common mistake. I did the numbers for a Traveller starship once and got a fuel consumption number measred in liters per year.

Originally Posted by Fred Brackin (Post 2407205)

Ah, that's.......remarkably disconnected from science as I understand it. I think I'll ignore it.

The bit about He3 reactions being dirty particularly stands out. The part about refuelling in the field is a more common mistake. I did the numbers for a Traveller starship once and got a fuel consumption number measred in liters per year.

Originally Posted by warellis (Post 2407222)

IIRC D+He3 reactors are relatively "clean" regarding neutrons compared to D+D or D+T reactors, right?

Originally Posted by Fred Brackin (Post 2407230)

Yes, D+D and D+T put out 80% of their energy in the form of neutrons and even shielding from those is a major project. Absorbing and utilizing them is another. D+He3 makes no neutrons (only gamma rays) and is easier on both those fronts though of course it's harder to initiate.