You'd need to enrich it or use a breeder reactor if you wanted to do that all on Mars. Nuclear is so energy dense you'd be better off just including shipments of reactor fuel every 780 days on regular resupply missions than trying to come up with a way of enriching it on Mars. If you were considering a breeder reactor, Thorium appears to be more advantageous than Uranium on the Moon and Mars. Thorium is more plentiful and should be easier to extract in usable quantities and breeding Thorium-232 to Uranium-233 in a small reactor would seem to be more practical than breeding Uranium-238 to Plutonium-239 in i.e. a sodium cooled fast reactor.