Nuclear fission is only the first stepping stone for an interstellar civilization. Fusion, antimatter-matter destruction, and artificial Penrose processes all offer much greater yields.
An ancient but safe, simple, and reliable power source with many applications.
Violently combining antimatter and matter, these reactors have high yields but can have undesirable explosive properties if their fuel storage is damaged.
The singularity core, a descendant of the Penrose sphere, is a favored method of high-yield starship/megastructure power generation.