Nuclear fission involves splitting the nucleus of a heavy atom, releasing energy and several neutrons. If these neutrons strike other fissionable nuclei, they cause more splitting events, creating a multiplying cascade known as a chain reaction. While human technology uses fission to power cities, the natural world has almost entirely failed to produce a self-sustaining, large-scale reaction. The specific, rare conditions required for a reaction to start and sustain itself in a geological setting today are almost never met on Earth.
The Necessary Ingredients for a Chain Reaction
A sustained nuclear chain reaction relies on specific physical factors. The fundamental requirement is the presence of fissile material, an isotope capable of undergoing fission after absorbing a neutron. For the reaction to continue, the rate of neutron production must equal or exceed the rate of neutron loss, either through absorption or by escaping the system. This balance is defined by the concept of “criticality.”
The physical size and shape of the fuel mass must be sufficient to achieve critical mass. If the mass is too small or the concentration too dilute, too many neutrons leak out before striking another nucleus. Fertile material, such as uranium-238 (U-238), can absorb neutrons and become other isotopes, but it does not fission easily and suppresses the chain reaction.
The Role of Uranium-235 Concentration
Natural fission is rare today due to the planet’s supply of the necessary fuel isotope. The only naturally occurring fissile material is Uranium-235 (U-235), which makes up only about 0.72% of all natural uranium today. The vast majority of natural uranium is the less-fissionable isotope U-238.
The half-life of U-235 is significantly shorter, about 704 million years, compared to the 4.5-billion-year half-life of U-238. Because U-235 decays faster, its concentration was substantially higher earlier in Earth’s history. Approximately two billion years ago, the U-235 concentration was closer to 3.7%, comparable to the level used in many modern nuclear reactors.
This higher concentration was sufficient to allow a chain reaction to occur under natural conditions. Today, geological processes cannot concentrate uranium ore enough to compensate for the low isotopic percentage. The natural concentration of 0.72% U-235 is too low to reach criticality without the artificial enrichment performed by human technology.
The Need for Neutron Moderation and Control
Even if the U-235 concentration were higher, a natural reaction would still require specific environmental conditions. When a U-235 nucleus fissions, it releases neutrons at very high speeds, known as fast neutrons. These fast neutrons are less effective at causing further fission in other U-235 nuclei.
For the chain reaction to continue efficiently, a neutron moderator is required to slow these fast neutrons down into “thermal” or slow neutrons. Water is an excellent natural moderator because its light hydrogen atoms efficiently absorb the fast neutron’s energy upon collision. However, the geological setting must have a high concentration of uranium ore saturated with water, a geologically uncommon combination.
The presence of certain elements in the Earth’s crust can act as “neutron poisons,” which absorb neutrons without undergoing fission. Elements like Boron and Cadmium are strong neutron absorbers. Any geological setting containing a rich uranium deposit must be relatively free of these neutron-absorbing impurities to allow a sustained chain reaction.
The Oklo Phenomenon: Earth’s Ancient Anomaly
The rarity of natural fission today is highlighted by the single known instance of it ever occurring, in the Oklo region of Gabon, Africa. Discovered in 1972, this site contains evidence of sixteen natural nuclear reactors active about 2 billion years ago. This ancient anomaly proves that the specific combination of necessary conditions did once exist.
The Oklo phenomenon was possible because the U-235 concentration at that time was high enough, around 3.6%. The uranium ore deposit was rich and saturated with groundwater, which served as the moderator to slow the neutrons. The reaction was naturally controlled: as the reaction heated the water, the resulting steam loss stopped the moderation, slowing the reaction until the water flowed back in.
The Oklo sites operated intermittently for hundreds of thousands of years, releasing an estimated total energy of nearly 100 billion kilowatt-hours. This event confirms that both a high concentration of U-235 and the presence of a moderator like water are necessary. The convergence of these time-dependent and geologically specific conditions is why Oklo remains the only known example of a natural, large-scale nuclear fission reaction.