Fussion Reactor

Second oldest power source known to humanity, one that produces energy by atomic fussion. They were the predominant technology during first waves of human expansion, although modernized fission reactors quickly catched up to them later on.
Their biggest advantage is safety, since all you need to is cut off fuel and hold magnetic shield until the reactor won't burn off remaining hydrogen. This doesn't spread to the fuel, as fuel tanks can explode, however with much lesser power than reactor themselves. On the other hand, the fuel tanks are actually needed, which means that ships powered by fussion reactors have to include them in their construction, limiting space usable by for example weaponry.
Outer Colonies and Terran Federation prefer fussion reactors in their ships of the line. With two main causes for that. First is the fact that they are bound to receive lots of attention from enemy, especially from his heaviest artillery. Thus, they should be maximally durable. With few fussion reactors (including at least two backup) they can continue doing it for long, with damaged reactors safely shutdown and exploding fuel tanks being a relatively minor damage while being hit from ~500mm shells launched from MAW.
Second is that actually moving them anywhere is a logistic achievement in itself, so you can as well just sent few tankers filled with hydrogen while mounting on them exactly as much fuel as its needed for a battle and subsequent maneuvres.