Natural nuclear fission reactor

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 first such fossil reactor was first discovered in 1972 in Oklo, Gabon, by researchers from the French Alternative Energies and Atomic Energy Commission (CEA) when chemists performing quality control for the French nuclear industry noticed sharp depletions of fissile 235U in gaseous uranium made from Gabonese ore. Oklo is the only location where this phenomenon is known to have occurred, and consists of 16 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.

Before the planetary-scale production of oxygen by the early photosynthesizers groundwater-moderated natural nuclear reactors are not thought to have been possible.

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 235U isotope.

[5] 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.

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.

Fission of uranium normally produces five known isotopes of the fission-product gas xenon; all five have been found trapped in the remnants of the natural reactor, in varying concentrations.

The concentrations of xenon isotopes, found trapped in mineral formations 2 billion years later, make it possible to calculate the specific time intervals of reactor operation: approximately 30 minutes of criticality followed by 2 hours and 30 minutes of cooling down (exponentially decreasing residual decay heat) to complete a 3-hour cycle.

Another factor which probably contributed to the start of the Oklo natural nuclear reactor at 2 billion years, rather than earlier, was the increasing oxygen content in the Earth's atmosphere.

[4] Uranium is naturally present in the rocks of the earth, and the abundance of fissile 235U was at least 3% or higher at all times prior to reactor startup.

It is estimated that nuclear reactions in the uranium in centimeter- to meter-sized veins consumed about five tons of 235U and elevated temperatures to a few hundred degrees Celsius.

The natural reactor of Oklo has been used to check if the atomic fine-structure constant α might have changed over the past 2 billion years.

Several studies have analysed the relative concentrations of radioactive isotopes left behind at Oklo, and most have concluded that nuclear reactions then were much the same as they are today, which implies that α was the same too.