Notice the circuit and the cell in the diagram? Guess what they do...
generate more waste heat.
Also notice the big writing 'heat emitted' - where does this waste heat go?
(It's probably worth mentioning the diagram is inaccurate with respect to electron flow. Electrons flow from negative to positive but by convention electric current flows from positive to negative, the blue arrows show current direction not electron flow).
In the end, the issue is that the game doesn't explain where the excess heat goes to, since a vacuum doesn't lend it self to that, which is where my personal theory comes in that it has to get collected in a form that can contain it, and then be ejected.
There's enough magic technology involved in Traveller's starships, and not just when it comes to power generation, to suppose that there's some form of heat dampening involved with the fusion reactor that at least brings it down to manageable levels.
You make a really tiny black hole, see, and the heat's all still there, see, but it's red shifted so far down that its basically cold... sure.
Well, until the black hole evaporates and lets it all out in a flash, but that's some future person's problem, so it's like time travel into the future for heat...