Natural Nuclear heat?
Scientific American has an interesting article from 2009 about naturally developing nuclear fission reaction, some 2 billion years ago here on Earth. The researchers claim that there are a few spots in Africa where key conditions of a nuclear fission reactor came naturally together, namely a concentration of Uranium 235 (3%) in a rather small volume (6 ft), a low concentration of Neutron-absorbing matter, and water as moderator to slow Neutrons that would be emitted from the U235, so that a chain reaction could be sustained. This story is apparently in discussion since the 1970s, known as the Oklo Phenomenon, and there have been perhaps more than a dozen of such sites in Africa. The natural reactor was apparently regulated through the temperature: the fission heat removed the surrounding water, and the reaction slowed down, which caused the temperature to go down. Due to that, more water could come come in, igniting the reaction again, perhaps in a 3 hour cycle, over a few hundred thousand years. My own theory is now that one of those reactors went critical and wiped out the dinosaurs, Only the timing does not match by one and a half billion years.