Nuclear Fission was Invented by 2 Billion Years Ago

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. Fission is the process of breaking apart atoms of heavy elements such as uranium. Energy is released during fission in the form of heat and can be harnessed in fission reactors to produce electricity. Fission is separate from radioactive decay where an atom spontaneously emits some radiation which can be timed by the half life. Fission is instead initiated by a neutron colliding with an atom and splitting it. One fission pathway commonly used is that of U²³⁵, a uranium atom which has an atomic weight of 235 with 92 protons and 143 neutrons. 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. If these neutrons then collide with another U²³⁵ atom this can start a chain reaction, however, we don’t just see nuclear explosions happening in the naturally occurring U²³⁵ found in rocks and seawater. That is because the vast majority of uranium is in the form of U²³⁸ which is a very stable form of uranium and decays very slowly so slow that it vastly outlives U²³⁵ . In order for a runaway fission reaction to occur produces immense amounts of energy, the ratio of U²³⁵to U²³⁸ needs to exceed 3%. In addition, the reaction needs a moderator, something to slow the neutrons. If the neutrons are too fast it is near impossible to have a fission event. 

However, this is a very delicate system and if too many of the neutrons are absorbed by so-called “neutron poisons” which are elements that can absorb neutrons without undergoing fission the reaction will fail. In order for the reaction to reach a critical state, a chain reaction, it must have both a moderator and at least a 3% ratio of U²³⁵ to U²³⁸, and lack any possible neutron poisons. However, the ratio of naturally occurring uranium is currently only 0.72%. And as time passes it gets smaller and smaller asU²³⁵ decays ten times faster than the heavier U²³⁸ . But it decays at a constant rate, it is in fact so constant that it is used to date rocks. 

However, in 1972 workers at a nuclear fuel processing plant in France discovered an unexplainable anomaly. They found an abnormally low ratio of U²³⁵ to U²³⁸ of only 0.717%. Now this may seem like a small difference but in terms of uranium, it is a huge chasm. At the processing plant, they came up short 200 kilograms of U²³⁵ since they had assumed the normal ratio. 200 kilograms of U²³⁵ is equivalent to almost 600 million kilograms of coal and enough to sustain all the electricity needs of 800,000 people in France for a year. How could the ratio be lower? Did it decay faster? Something must have occurred to burn off more U²³⁵ than normal. It was as if it had already been a nuclear reactor.

A natural nuclear reactor.

The ore had come from the Oklo mine in Gabon. The conundrum was dubbed the “Oklo Phenomena” and stumped scientists because the conditions necessary for a nuclear reactor are very specific. Examining the deposits found that surprisingly Oklo mine was able to maintain the delicate balance necessary to sustain a chain reaction 2 billion years ago. First, due to the decay rate of uranium 2 billion years ago the natural ratio of uranium was approximately 3 percent. Since then much of the U²³⁵ has decayed but for a period of time far in the past, the natural levels were enough to produce a reaction. Once this was satisfied, scientists searched for a moderator. The Oklo uranium deposits are hosted in a layer of permeable rocks; the high permeability allowed groundwater to flow through the deposit and act as moderator. Water is a very efficient moderator and is used in many reactors today. The clay minerals around the deposits recorded the heat of the reactor and the original host rock became hydrothermally altered due to the immense heat of fission. The unique structure of the deposit allowed sustained fission to occur. By measuring the different isotope present, scientists developed an operational model for the reactor. Fission started in the pockets of uranium ore when a stray neutron, which is naturally produced and elements decay, hit a U²³⁵ atom. Then this started a chain reaction and water acted as a moderator for approximately 30 minutes. After a time, the water would become so hot that it became steam and escaped through the permeable rock. Then it would take several hours for the reactor to cool then start the cycle again in a process similar to eruptions from the geyser “Old Faithful”. 

 
The reactor had produced radioactive isotopes such as plutonium but since then it has decayed leaving just a strange ratio of isotopes to indicate what had occurred. This phenomenon convinced scientists that similar things might have occurred elsewhere and discovered a similar occurrence at Bangombé, 30 kilometers southeast of Oklo. It could be that 2 billion years ago this phenomenon was common. The Oklo mine provided not only a stumping science question but also a reminder of the immense power of nature.

Further Reading Fujii, Y., Iwamoto, A., Fukahori, T., Ohnuki, T., Nakagawa, M., Hidaka, H., … & Möller, P. (2000). The nuclear interaction at Oklo 2 billion years ago. Nuclear Physics B, 573(1-2), 377-401. Kuroda, P. K. (2012). The origin of the chemical elements and the Oklo phenomenon. Springer Science & Business Media. Cowan, G. A., Bryant, E. A., Daniels, W. R., & Maeck, W. J. (1975). Some United States studies of the Oklo phenomenon. In The Oklo Phenomenon.