Perhaps the "rounds" consist of a plastic matrix containing lithium and a few other (highly classified) materials, which are compressed using energy from the reactor to produce a very small fusion-point? The majority of the energy from the reactor is actually used to maintain the magnetic "bottle" that contains the reaction and prevents it from encompassing the entire ship. The plasma cannon can't simply power itself from the reactor; though I suppose some kind of "gigantic fire hose" reaction would be possible using the energy from the reactor plus hydrogen from the fuel tank. (Actually, a "fire lance" weapon would be a ton of fun) PS - the tiny bit of beryllium/plutonium in the core of the plastic matrix is why the rounds are fairly expensive. And you should wear the provided lead gloves while reloading.