In 1972, technicians at the Tricastin nuclear plant at Pierrelatte in France made an alarming discovery. Routine tests appeared to show that a batch of uranium had been tampered with.
A test tube containing processed uranium ore known as yellow cake. Credit: Weirdmeister – Wikimedia.
Natural uranium consists of two isotopes. It is 99.28% uranium-238 and 0.72% uranium-235. But the French technicians found that some of the uranium-235 in their batch was missing. It was just 0.6% uranium-235 rather than 0.72%.
Had some of the precious and potentially deadly isotope been extracted elsewhere? Was this a sign of a clandestine nuclear weapons programme? An explanation was required and an investigation was launched by the French authorities.
Nuclear Reactors
Around a fifth of the electricity in the United States is generated by nuclear reactors. The fraction in the UK is similar, while in France three quarters is nuclear in origin. Currently, 440 civilian reactors generate electricity in forty-five countries.
Nuclear reactors produce energy in a chain reaction, which operates as follows. A uranium-235 nucleus fissions after absorbing an uncharged nuclear particle known as a neutron. This is because when the neutron binds to the uranium-235 nucleus sufficient energy is liberated to split the nucleus into two smaller fragments. Crucially, two or three neutrons are also released, along with a substantial amount of energy, and these neutrons may then go on to induce the fission of further uranium-235 nuclei. Each fission releases more neutrons and these neutrons may cause further fissions. In a nuclear reactor this process is controlled so that on average just one neutron from each fission event leads to a subsequent fission so the chain reaction continues at a steady rate.
A uranium-235 nucleus absorbs a neutron, which causes it to fission and release further neutrons.
Two ingredients are required to sustain a chain reaction—nuclear fuel and a moderator. The fuel in most nuclear reactors is enriched uranium which contains an artificially enhanced proportion of the readily fissionable isotope uranium-235. Typically, the fuel is enriched to between 3% and 5% uranium-235.
The moderator is necessary to slow down the neutrons, which greatly increases the chance that they will be captured by a uranium-235 nucleus. A good moderator is one that contains nuclei with a low mass, as this reduces the number of collisions needed to slow down the neutrons. (When a neutron hits a heavy nucleus it behaves like a rubber ball hitting a wall—it bounces off without losing much energy.) Another important feature of a good moderator is that its nuclei do not capture too many neutrons, which would dampen down the chain reaction. The most suitable materials are water, heavy water and ultra-pure graphite. The choice between these options is one of the most important factors in the design of a reactor.
Fermi’s first nuclear reactor known as Chicago Pile 1. It used natural uranium with blocks of graphite acting as the moderator, as can be seen in this drawing.
The nuclear reactor was developed in the United States during the Second World War, originally to manufacture plutonium for nuclear weapons. Enrico Fermi constructed the prototype in a disused squash court at the University of Chicago and it went critical for the first time on 2 December 1942. But was this really the world’s first nuclear reactor?
The Oklo Uranium Mine
The investigations by the French authorities in 1972 led to a uranium mine in the Central African country of Gabon, then a French overseas territory. The reason for the missing uranium-235 turned out to be quite incredible. Fermi’s Chicago nuclear reactor had been foreshadowed at least 1.7 billion years earlier by a natural nuclear reactor in a uranium deposit at Oklo near Franceville. Detailed analysis of the unusual nuclear isotopes found in the deposit has enabled physicists to construct a picture of what happened there long long ago.
Gabon, Central Africa.
In an exceptionally rich seam of uranium ore the groundwater acted as a moderator and sustained a nuclear chain reaction. The ore now contains isotopes of neodymium, ruthenium and other elements whose presence can only be explained by assuming these nuclei were produced by the fission of uranium-235. This also explains why the proportion of uranium-235 nuclei in uranium extracted from the ore is lower than elsewhere on Earth. In some areas its concentration is as low as 0.44% due to the fission of much of the isotope in the ancient natural reactor.
Through the analysis of xenon isotopes within the rock, it is estimated that the reactor would go critical for about half an hour generating around 100 kilowatts of power and boiling away the groundwater that was acting as a moderator for the nuclear chain reaction. With the loss of moderator the reactor would then shut down for about two and a half hours until it had cooled sufficiently for liquid water to seep back in and restart the reactor. This three-hour cycle is believed to have continued for hundreds of thousands of years.
Sixteen natural nuclear reactors were found in the Oklo area, as well as a seventeenth discovered at Bangombé, thirty kilometres to the southeast. This remarkable phenomenon is not known anywhere else in the world. It has been celebrated on stamps issued by the Republic of Gabon.
Could It Happen Today?
Although there may be other ancient natural nuclear reactors awaiting discovery, such a reactor is impossible today. The uranium-235 content of natural uranium is no longer sufficient to sustain a chain reaction with water acting as the moderator. In the distant past, uranium-235 formed a much greater component of uranium and this is what made the natural reactors possible. When the Oklo reactors went critical 1.7 billion years ago uranium-235 composed about 3.1% of the uranium in the ore, a similar concentration to the enriched uranium used in today’s commercial reactors. Uranium-235 undergoes alpha decay with a half-life of 704 million years, so it decays faster than uranium-238, which has a half-life of 4.5 billion years. As time passes the proportion of uranium-235 gradually decreases, so the era of natural fission reactors is long gone.
Oklo fossil reactor 15. The yellow mineral is uranium oxide (yellow cake). Credit: Robert D. Loss (Curtin University, Perth.)
Unfortunately the natural reactor deposits in Oklo no longer exist as they have now been completely mined out. French scientists are campaigning to preserve the last remaining natural reactor site at Bangombé, so that further research can be carried out in the future.
Further Information
There is further information about natural nuclear reactors in this article: Nature’s Nuclear Reactors: The 2-Billion-Year-Old Natural Fission Reactors in Gabon, Western Africa.
