Fundamentals of CANDU Reactor Physics
Nuclear fission is the splitting of a (large) nucleus, with the release of energy. The nuclei of some heavy elements, such as U-238, do exhibit spontaneous fission in nature. However, the rate of such fissions is extremely low. The half-life of uranium is longer than 100 million years, and most of its decay is by alpha emission, so spontaneous fission is not a practical source of energy. Spontaneous fission is not of much use to us as an energy source!