Transcript: Fission is such an efficient energy source that humans have long tried to harness it. A massive atomic nucleus can be split by a neutron. Since the decay of a massive nucleus can also release a neutron, this raises the possibility of a chain reaction where there’s sufficient density or purity of radioactive atoms that the neutrons released by the decay of one atom always trigger the decay of another atom and so on in a sustaining reaction. Fission occurs in a number of ways in the universe. On Earth fission has even occurred naturally in the ground in an underground mine in Gabon, Africa; 1.7 billion years ago the concentration of uranium was sufficient for energy to be released. Humans have tried to purify uranium and other radioactive elements to create nuclear reactors. The sustaining reaction is moderated by carbon rods which will absorb the neutrons, and in an uncontrolled version of the reaction sufficient mass of uranium or another heavy element creates an uncontrolled reaction and catastrophic energy release.