The feasibility of thermal neutron fission and fast neutron fission in planetary and protostellar matter is calculated from nuclear reactor theory. Means for concentrating actinide elements and for separating actinide elements from reactor poisons are described. The implications of intermittent or interrupted planetaryscale nuclear fission breeder reactors are discussed in connection with observed changes in the giant outer planets and changes in the geomagnetic field. The concept that thermonuclear fusion reactions in stars are ignited by nuclear fission energy is disclosed. The suggestion is made that dark matter, inferred to exist in the Universe, might be accounted for, at least in part, by the presence of dark stars (not necessarily brown dwarfs) whose protostellar nuclear fission reactors failed to ignite thermonuclear fusion reactions.