Westinghouse recently announced a mini-nuclear reactor capable of powering several hundred homes (https://info.westinghousenuclear.com/news/first-canadian-evinc

Westinghouse recently announced a mini-nuclear reactor capable of powering several hundred homes (https://info.westinghousenuclear.com/news/first-canadian-evinci-microreactor-targeted-for-saskatchewan https://info.westinghousenuclear.com/news/first-canadian-evi...). The reactor itself is the size of small apartment and would fit aboard a spaceship (the now defunct Space Shuttle) and is light enough that a large rocket (like the Saturn) could carry it into space. The reactor plant would be constructed around the reactor at its final destination (in this particular case, Saskatchewan Canada, but for purposes of this discussion, Mars). As a side benefit of the design, it uses the heat of the reactor to provide heating, which a Mars colony would need a lot of.