Right, the nuclear reaction itself does not produce thrust. It merely generates energy to power generation of electricity and waste heat. What I figure is that that the waste heat is somehow harnessed and used to superheat propellant, which is fed into a chamber and superheated, then expelled out of the exhausts to produce thrust.
I was thinking "air" being used as the reaction mass in atmosphere for the Valkyries, as the intakes for the engines seem to be open/uncovered in F and G modes, when the engines are providing thrust. The advantage being, that as long as the fusion turbines are generating heat, there is an unlimited amount of propellant available, within the confines of the atmospheric service ceiling. When I think about it, the air that's being drawn in is probably getting compressed in the process--possibly to a state that is dense enough for superheating to work efficiently... In B mode, the fusion turbines are functioning primarily to generate electricity, and the intakes get covered--unless the foot thrusters are being used.
In space, there must be some sort of material that is injected into the chamber for superheating and expulsion to generate thrust... That would be the reaction mass carried in the FAST packs.
I guess that the Valkyrie has an internal "fuel" supply, but that is fuel for the fusion reaction only. The FAST packs may contain their own fusion reactors and fusion fuel supplies as well (IIRC there were radiation symbols on the FAST packs during the launch scene at the start of DYRL?)...
Does this make sense, or am I thinking too much about it?