As far as I understand it, no matter what kind of magic fusion device you come up with, it's going to be throwing off lots of fast neutrons which will be absorb

As far as I understand it, no matter what kind of magic fusion device you come up with, it's going to be throwing off lots of fast neutrons which will be absorbed by the equipment generating the fusion reaction So a fusion reaction will gradually irradiate the generator that is creating the reaction. A molten salt reactor which consumes virtually all of its nuclear fuel as the closest approximation to your desire to have nuclear reactors without nuclear waste. Edit: I recall that helium 3 fusion may actually be pretty neutron free. But you got to get your hands on a lot of helium 3