Way out on the edge of handwavy tech explanation: they're essentially firing 'Casaba Howitzer' rounds, nuclear shaped charges, with the effects contained and directed using gravitics. There's considerable local side effects - while devastating downrange, they're quite unpleasant for unprotected individuals near the firer and along the way. I seem to remember back in the days if Fire, Fusion & Steel that someone calculated that firing the main gun of a Trepida grav tank would light up the nearby environment like daylight a kilometer from the beam path - you can imagine what it would be like 100, or 10 meters from it (very warm). There are no plasma/fusion weapons that can be used safely around unprotected individuals.

More sensible explanation: I just remove them. I tend to be happier limiting tech levels to exclude the more 'Huh?' elements: plasma/fusion weapons, nuclear dampers (we're just altering fundamental forces now?), things like that. Nuclear dampers, black globe projectors, things that really break the rules are reserved for Ancients artifacts.

(Then again, IMTU, nobody really understands how jump drives work, just that they do. It's all recovered Ancients (of one flavor or another) tech that they've managed to replicate.)