Ancient Nuclear Power
When extracting uranium from natural deposits, you get 3 isotopes; uranium 238, uranium 234 and uranium 235. The latter isotope is the most precious to humanity, as it is the type that can sustain nuclear reactions. It occurs as 0.72% of the total of extracted uranium wherever you look — in the earth’s crust, the moon and even in meteorites.
So when, in a French nuclear power processing plant in 1972, the ratio was off by 0.03%, alarm bells rang. That tiny percentage mismatch in the ore that was being studied amounted to about 200kg of uranium 235; enough uranium to make about 6 nuclear bombs.
All civilian nuclear processing facilities must account for all fissionable isotopes, in order to ensure that none are diverted for weapons, so this tiny discrepancy had to be reported. The Commissariat à l’énergie atomique mounted a full investigation. It turned out that at the particular mine that this uranium had been extracted from was far from typical. In some cases, the uranium extracted showed uranium 235 concentrations as low as 0.42%. One other factor was the differing amounts of other elements from their usual natural amounts.
Neodymium 148 and ruthenium 99 are typically found at 27% and 12.7% respectively. In the mines this ore came from, the percentages were 6% and 27%. The loss of uranium 235 and the increase and decrease of these other elements is almost exactly what you would find in a nuclear reactor, but this ore had just been freshly mined. The explanation for these anomalies lay in a paper from the 1950’s by Dr Paul Kuroda — in the right circumstances, natural uranium deposits can undergo spontaneous and controlled nuclear fission.
Careful investigations into the mine in Okla, Gabon, showed that in about 16 sites in the area, spontaneous nuclear fission had occurred in the distant past, although the scientists studying the phenomena were unsure how the ore seams had not become supercritical and exploded.
It took nearly 30 years for all the answers to come to light, but the answers now explain how nature happened to chance upon a self-sustaining nuclear reactor that operated for over 150 million years, with a 3 hour cycle that output around 100 kilowatts of power.
About 2 billion years ago, the concentrations of uranium 235 in these deposits was far higher, around 3.1%. At that time, the oxygen content of the atmosphere was higher. This may have allowed uranium to be dissolved and transported with groundwater to places where a high enough concentrations could accumulate to form rich uranium ore bodies. Without the new aerobic environment available on Earth at the time, these concentrations probably could not have taken place.
Under normal conditions, atoms like uranium 235 emit neutrons at speeds so high that most of the neutrons bounce off other atoms and fly away. But if there is sufficient quantities of the radioactive material together, the emitted neutrons bounce around inside the mass, some slowing down enough to be absorbed into another atom’s nucleus. This extra neutron causes the nucleus to become unstable and immediately split, which releases a large amount of energy.
If there is enough radioactive material in sufficient density that a lot of nuclei split very rapidly (critical mass), the reaction increases exponentially, and results in an explosion. Any less than that (subcritical mass), and it causes a sustained fission reaction, giving off energy as heat and radiation.
The Oklo deposits didn’t have enough uranium in a small enough space to cause either of these events, which puzzled scientists until the geography of the deposits was taken into account. The ore deposit was sandwiched between two sandstone layers, which allowed water to seep into the spaces between the ore deposits.
This water acted as a neutron moderator, causing the neutrons to slow down enough to cause hit and split uranium nuclei and cause nuclear fission. The resulting fission would generate heat enough to boil off the water, causing the reaction to slow and stop. Once the ore had cooled down enough, water began to seep back into the cracks and the process began again.
On average, it was calculated that this process too 3 hours; a 30 minute fission cycle, followed by a two and a half hour cooldown cycle where water would seep back into the ore, before the process started up again.
Some of the neutrons released during the fission of uranium 235 were captured by the more abundant uranium 238, which became uranium 239 and, after emitting two electrons, turned into plutonium 239. More than two tons of plutonium 239 were generated within the Oklo deposit. Although almost all this material, which has a 24,000-year half life, has since decayed away, some of the plutonium itself underwent fission, as evidenced by the presence of its characteristic fission products.
The abundance of those lighter elements allowed scientists to deduce that fission reactions must have gone on for hundreds of thousands of years. From the amount of uranium 235 consumed, they calculated the total energy released was in the order of 15,000 megawatt-years.
Over 5 tons of uranium 235 was fissioned over the life of the Oklo reactor which caused the initial discrepancy in the isotope ratios discovered in France and began the investigation into the mine.
So far, it has been the only site on the planet where natural, sustained nuclear fission has appeared to have occurred; but there may be evidence from gamma-ray results taken by the Mars Odyssey Orbiters gamma-ray spectrometer which shows that an abundance of radioactive uranium, thorium and potassium in one particular spot on Mars, may be caused by similar natural fission reactions taking place there billions of years ago, although better and more evidence would be needed before any final judgements could be made. So far it seems that only this planet has had its own natural nuclear reactor.
