Evidence exists that a large natural nuclear reactor formed and operated on Mars in the northern Mare Acidalium region of Mars. However, unlike its terrestrial analogs this natural nuclear reactor was apparently much larger, bred 233U off of thorium, and apparently underwent explosive disassembly, ejecting large amounts of radioactive material over Mars' surface.
(See also: Natural nuclear fission reactor. Only, on Mars, all geological features seem to be supersized ... I don't care if this is implausible, it's bloody going in a novel. OK?)