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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, 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. 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.
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.
The neodymium found at Oklo has a different isotopic composition to that of natural neodymium: the latter contains 27% 142
Nd, while that of Oklo contains less than 6%. The 142
Nd is not produced by fission; the ore contains both fission-produced and natural neodymium. From this 142
Nd content, we can subtract the natural neodymium and gain access to the isotopic composition of neodymium produced by the fission of 235
U. The two isotopes 143
Nd and 145
Nd lead to the formation of 144
Nd and 146
Nd by neutron capture. This excess must be corrected (see above) to obtain agreement between this corrected isotopic composition and that deduced from fission yields.
Similar investigations into the isotopic ratios of ruthenium at Oklo found a much higher 99
Ru concentration than otherwise naturally occurring (27–30% vs. 12.7%). This anomaly could be explained by the decay of 99
Tc to 99
Ru. In the bar chart, the normal natural isotope signature of ruthenium is compared with that for fission product ruthenium which is the result of the fission of 235
U with thermal neutrons. The fission ruthenium has a different isotope signature. The level of 100
Ru in the fission product mixture is low because fission produces neutron rich isotopes which subsequently beta decay and 100
Ru would only be produced in appreciable quantities by double beta decay of the very long-lived (half-life 7.1×1018 years) molybdenum isotope 100
Mo. On the timescale of when the reactors were in operation, very little (about 0.17 ppb) decay to 100
Ru will have occurred. Other pathways of 100
Ru production like neutron capture in 99
Ru or 99
Tc (quickly followed by beta decay) can only have occurred during high neutron flux and thus ceased when the fission chain reaction stopped.
The natural nuclear reactor at Oklo formed when a uranium-rich mineral deposit became inundated with groundwater, which could act as a moderator for the neutrons produced by nuclear fission. A chain reaction took place, producing heat that caused the groundwater to boil away; without a moderator that could slow the neutrons, however, the reaction slowed or stopped. The reactor thus had a negative void coefficient of reactivity, something employed as a safety mechanism in human-made light water reactors. After cooling of the mineral deposit, the water returned, and the reaction restarted, completing a full cycle every 3 hours. The fission reaction cycles continued for hundreds of thousands of years and ended when the ever-decreasing fissile materials, coupled with the build-up of neutron poisons, no longer could sustain a chain reaction.
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Natural nuclear fission reactor AI simulator
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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, 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. 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.
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.
The neodymium found at Oklo has a different isotopic composition to that of natural neodymium: the latter contains 27% 142
Nd, while that of Oklo contains less than 6%. The 142
Nd is not produced by fission; the ore contains both fission-produced and natural neodymium. From this 142
Nd content, we can subtract the natural neodymium and gain access to the isotopic composition of neodymium produced by the fission of 235
U. The two isotopes 143
Nd and 145
Nd lead to the formation of 144
Nd and 146
Nd by neutron capture. This excess must be corrected (see above) to obtain agreement between this corrected isotopic composition and that deduced from fission yields.
Similar investigations into the isotopic ratios of ruthenium at Oklo found a much higher 99
Ru concentration than otherwise naturally occurring (27–30% vs. 12.7%). This anomaly could be explained by the decay of 99
Tc to 99
Ru. In the bar chart, the normal natural isotope signature of ruthenium is compared with that for fission product ruthenium which is the result of the fission of 235
U with thermal neutrons. The fission ruthenium has a different isotope signature. The level of 100
Ru in the fission product mixture is low because fission produces neutron rich isotopes which subsequently beta decay and 100
Ru would only be produced in appreciable quantities by double beta decay of the very long-lived (half-life 7.1×1018 years) molybdenum isotope 100
Mo. On the timescale of when the reactors were in operation, very little (about 0.17 ppb) decay to 100
Ru will have occurred. Other pathways of 100
Ru production like neutron capture in 99
Ru or 99
Tc (quickly followed by beta decay) can only have occurred during high neutron flux and thus ceased when the fission chain reaction stopped.
The natural nuclear reactor at Oklo formed when a uranium-rich mineral deposit became inundated with groundwater, which could act as a moderator for the neutrons produced by nuclear fission. A chain reaction took place, producing heat that caused the groundwater to boil away; without a moderator that could slow the neutrons, however, the reaction slowed or stopped. The reactor thus had a negative void coefficient of reactivity, something employed as a safety mechanism in human-made light water reactors. After cooling of the mineral deposit, the water returned, and the reaction restarted, completing a full cycle every 3 hours. The fission reaction cycles continued for hundreds of thousands of years and ended when the ever-decreasing fissile materials, coupled with the build-up of neutron poisons, no longer could sustain a chain reaction.