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

[Anthony]

Originally Posted by Fred Brackin

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.

Well, it doesn't directly make neutrons. There's a tendency to have D-D side reactions in the fuel which produce neutrons as normal. You need to go to p-B11 before you get something that doesn't produce any significant amount of neutrons. p-p actually produces almost as many neutrons as D-D, because the basic reaction is that protons fuse to produce deuterium, and then the deuterium fuses normally.