The theoretical existence of natural nuclear reactors was first predicted by Japanese-American chemist Paul Kuroda in 1956. To understand how they work, it is first necessary to understand the operation of man-made nuclear reactors. A nuclear chain reaction is initiated when a Uranium-235 atom absorbs a neutron, causing it to split or fission. In a man-made nuclear reactor this is accomplished by bundling together rods of uranium fuel so that neutrons can travel freely between the rods. So if natural Uranium can’t be used as nuclear fuel, how did the Oklo ore bodies sustain a chain reaction?