As others have said, don't get confused between nuclear fission and fusion; fission is what power plants today use, and it's entirely reasonable to think that a large generator (ie, capship engine) could produce enough power for the vessel. Compared to other options it'd be a cheap and easy power supply - and with a bit of waste management, pretty safe, too.
Fusion, if you want a comparison, is what powers stars. Even a small engine would be capable of providing a huge amount of energy, and given the commodities used/traded regularly (H-fuel, deuterium, etc.) it sounds like it's probably the main energy supply for Sirius. Since it's primarily using hydrogen - the most abundant element in the universe by a hell of a long way - it's silly cheap (although reactors/engines would be much more expensive/complex than for fission) and very efficient, producing substantially more energy per nucleon (but not per reaction, mind) than fission.
From a current standpoint, antimatter is difficult - as people have said, it's hard and expensive to create (and for a sci-fi setting, presumably difficult to "harvest" too, compared to the ease of using fusion). Another hurdle is how to contain it, as by its very nature it'll annihilate on contact with any regular matter. Magnetic confinement is one method which shows promise, and it's entirely possible that in a future, FL-esque setting, antimatter fuel cells would be viable.
There is currently research into antimatter as an ignition for fusion, although I don't know of any ongoing work for antimatter as a power source by itself.
But to be honest, this is Freelancer, and we can pretty much pseudo-science our way into anything. The reason we don't have antimatter power in the mod is that no one's thought about it enough/seen it important enough to bother putting it in.
But hey, here's some nerd reading if you're curious.
Well I think I will be collecting these admin quotes as my signature "Collecting Admin quotes. So I gave him a Moderator one." -Moveit56 "Tractorlancer" -Jansen "I'm higher than aerelm's ping. ~Narc "Some random quote" -mwerte LWB is the bestest! ~ Moveit56 "Will you be my minion? -Mrs. Altejago No, he's mine ~Snoopy Wow, the Team's memory desk in this sig? K. I'm in ~ Dimon
Well, the only mention I ever seen about anitmatter in Freelancer was Nansei Research Station in Kyushu, requiring large solar arrays to power the experiments, and a heavy security to prevent any sabotage or critical failure.
Nansei Research Station Wrote:In 400 A.S., the Kusari government began offering incentives for corporations to move to the Kyushu system and set up offices and business there. Taking advantage of the offer, Samura Industries began construction on Nansei Research Complex, which finished later the same year. The research facility was designed primarily to study antimatter and antimatter based weaponry, using the solar panel arrays along the station to utilise the vast amounts of cheap energy available from the proximity of Kyushu's main star.
Nansei has several kilograms of antimatter in storage for research purposes, and the storage facilities are often a favourite target of attacks by the Blood Dragons. This has led to the Battleship Nagumo being stationed nearby to oversee both the defence of the Tau-29 jump gate and of Nansei itself. The consequences for the Kyushu system were the Blood Dragons ever to succeed in destroying the storage containers would be disastrous, though Samura insists that the antimatter is heavily protected and poses no risks.
Nuclear power plants are mega effective. In most forms of energy producton problem is that most of said energy is dissipating in form of heat. I dont recall precise number but I believe with current (real life) tech, we harvest only 3% of fission reaction products.