Naively I'd expect that you would need fusion specifically. Uranium isn't plentiful unless you're on a rocky planet, and isn't very high energy density compared

Naively I'd expect that you would need fusion specifically. Uranium isn't plentiful unless you're on a rocky planet, and isn't very high energy density compared to hydrogen. Hydrogen is everywhere by comparison. The other reasonable option might be solar power near stars, and forwarding the power on to your ships/distant planets using some form of directed energy (lasers, lenses, etc).