'Kilopower' Could Power a Mars Colony and Deep Space Missions
https://www.realclearscience.com/articles/2018/04/14/kilopower_could_power_a_mars_colony_and_deep_space_missions.html
Relatively abundant my@ss. <1% naturally occurring and a small mountain of tech and chemical separation to mine, process, and purify usable U-235. Thorium is more plentiful.