Natural nuclear fission reactor

A natural nuclear fission reactor is a uranium deposit where self-sustaining nuclear chain reactions occur. The idea of a nuclear reactor existing in situ within an ore body moderated by groundwater was briefly explored by Paul Kuroda in 1956. The existence of an extinct, or fossil, nuclear fission reactor, where self-sustaining nuclear reactions occurred in the past, was established by analysis of isotope ratios of uranium and of the fission products (and the stable daughter nuclides of those fission products). The first discovery of such a reactor happened in 1972 in Oklo, Gabon, by researchers from the French Atomic Energy Commission (CEA) when chemists performing quality control for the French nuclear industry noticed sharp depletions of fissile ²³⁵
U in gaseous uranium hexafluoride made from Gabonese ore.

Oklo is the only location where this phenomenon is known to have occurred, consisting of sixteen sites with patches of centimeter-sized ore layers. There, self-sustaining nuclear fission reactions are thought to have taken place approximately 1.7 billion years ago, during the Statherian period of the Paleoproterozoic. Fission in the ore at Oklo continued off and on for a few hundred thousand years and probably never exceeded 100 kW of thermal power. Life on Earth at this time consisted largely of sea-bound algae and the first eukaryotes, living under a 2% oxygen atmosphere. However, even this meager oxygen was likely essential to the concentration of uranium into fissionable ore bodies, as uranium dissolves in water only in the presence of oxygen. Before the planetary-scale production of oxygen by the early photosynthesizers, groundwater-moderated natural nuclear reactors are not thought to have been possible.

Discovery of the Oklo fossil reactors

In May 1972, at the Tricastin uranium enrichment site at Pierrelatte, France, routine mass spectrometry comparing UF₆ samples from the Oklo mine showed a discrepancy in the amount of the ²³⁵
U isotope. Where the usual concentrations of ²³⁵
U were 0.72% the Oklo samples showed only 0.60%. This was a significant difference—the samples bore 17% less ²³⁵
U than expected. This discrepancy required explanation, as all civilian uranium handling facilities must meticulously account for all fissionable isotopes to ensure that none are diverted into the construction of unsanctioned nuclear weapons. Further, as fissile material is the reason for mining uranium in the first place, the missing 17% was also of direct economic concern.

Thus, the French Atomic Energy Commission (CEA) began an investigation. A series of measurements of the relative abundances of the two most significant isotopes of uranium mined at Oklo showed anomalous results compared to those obtained for uranium from other mines. Further investigations into this uranium deposit discovered uranium ore with a ²³⁵
U concentration as low as 0.44% (almost 40% below the normal value). Subsequent examination of isotopes of fission products such as neodymium and ruthenium also showed anomalies, as described in more detail below. However, the trace radioisotope ²³⁴
U did not deviate significantly in its concentration from other natural samples. Both depleted uranium and reprocessed uranium will usually have ²³⁴
U concentrations significantly different from the secular equilibrium of 55 ppm ²³⁴
U relative to ²³⁸
U. This is due to ²³⁴
U being enriched together with ²³⁵
U and due to it being both consumed by neutron capture and produced from ²³⁵
U by fast-neutron-induced (n,2n) reactions in nuclear reactors. In Oklo, any possible deviation of ²³⁴
U concentration present at the time the reactor was active would have long since decayed away. ²³⁶
U must have also been present in higher-than-usual ratios during the time the reactor was operating, but due to its half-life of 2.348×10⁷ years being almost two orders of magnitude shorter than the time elapsed since the reactor operated, it has decayed to roughly 1.4×10⁻²² its original value and below any abilities of current equipment to detect.

This loss in ²³⁵
U is exactly what happens in a nuclear reactor. A possible explanation was that the uranium ore had operated as a natural fission reactor in the distant geological past. Other observations led to the same conclusion, and on 25 September 1972, the CEA announced their finding that self-sustaining nuclear chain reactions had occurred on Earth about 2 billion years ago. Later, other natural nuclear fission reactors were discovered in the region.