[DennisP]

Helium-3 is the waste product of pure deuterium fusion. The YCombinator-funded fusion startup Helion is working on a hybrid D-D/D-He3 reactor, which it says will produce only 6% of its energy as neutron radiation. Deuterium of course is absurdly abundant on Earth.

Another aneutronic reaction is proton-boron. Boron isn't as abundant as deuterium but there's still enough on Earth to last tens of thousands of years. (The reaction use B11, which is 80% of natural boron.) pB11 fusion is especially difficult but several startups are trying it; the biggest is Tri Alpha, with about $500M invested. They attained stable plasma a year ago and just completed a larger reactor.

With D-T, the easiest reaction, there may be ways to engineer around the neutron issue. General Fusion does fusion pulses in the middle of a vat of molten lead and lithium. MIT's ARC design is more conservative, with a compact tokamak and modular construction. The inner wall is 3D-printed and replaced annually; they say after a couple decades the radioactivity will have decayed enough for cheap disposal. They surround the core with a blanket of molten FLiBe salt as coolant and breeding blanket.