Although humans first witnessed nuclear reactors in 1942 with the development of the Chicago-Pile by Enrico Fermi, natural fission reactors existed billions of years ago. Energy is released during fission in the form of heat and can be harnessed in fission reactors to produce electricity. In order to produce fission a fission event, an atom of U235 is bombarded by a neutron when this neutron collides with the atom it splits into two fission products and produces heat and more neutrons. In order for a runaway fission reaction to occur produces immense amounts of energy, the ratio of U235to U238 needs to exceed 3%. First, due to the decay rate of uranium 2 billion years ago the natural ratio of uranium was approximately 3 percent.