Fission under our feet

A mass spectrometer is a complex mixture of wires, electrodes, metal piping and whirring machinery. Inside, chemical compounds are ionised, accelerated and deflected by powerful electric and magnetic fields, separating according to their mass. Banks of the devices line the nuclear fuel processing site at Pierrelatte in Southern France. They are among the most important tools at the disposal of the engineers and scientists that analyse the raw nuclear fuel arriving from mines across the world.

The scientists at Pierrelatte were expecting a routine mass spectrometry analysis of a uranium ore sample received from Gabon, a former French colony, in May 1972. Analysis of uranium ore is rarely exciting, as the proportions of the three different forms of uranium, known as isotopes, is remarkably stable. Uranium-238 (U-238) is by far the most common, with a natural abundance of 99.2743%.

Unfortunately, U-238 is not able to sustain the nuclear chain reactions required to generate electricity in power plants. U-235 is a far more valuable isotope for this purpose, but only makes up seven out of every 1000 uranium nuclei dug from the ground.

Uranium ore is always analysed in processing sites like the one at Pierrelatte to check that the proportions of U-235 are as expected. This is to ensure that none of the U-235 has already been used up in another nuclear plant or been illegally stolen for use in nuclear weapons.

This is why the scientists were shocked to find that the sample they received from Oklo in Gabon contained just 0.7171% U-235. The difference between this sample and the expected value was just 0.0031% – it might not seem like much, but it was enough to make everyone in the plant sit up and take notice, and with good reason. In total, about 500 kg of U-235 was missing, enough to make half-a-dozen nuclear weapons as powerful as the one that levelled Hiroshima in 1945.

So where had the uranium gone? As it turned out, the answer was right under their feet.

Two billion years ago, the world’s first nuclear reactors were created. Of course, humans were not responsible – these reactors were entirely natural in their origin. Oklo was one of seventeen sites that maintained stable natural nuclear chain reactions, producing a continuous, but modest, amount of energy. The concept of a natural nuclear reactor had been hypothesised in 1956 by Paul Kuroda, who highlighted that under certain conditions, which mimic those created in man-made nuclear plants, deposits of uranium could spontaneously erupt in self-sustaining nuclear reactions.

Modern nuclear power plants are driven by nuclear fission reactions, in which uranium, or other nuclear fuel such as plutonium, breaks down into smaller elements releasing nuclear energy and fast-moving neutrons. If these neutrons are slowed down by a moderator, typically water or graphite, they can then slam into another uranium atom in a series of fission chain reactions. The nuclear energy produced by these reactions is used to boil water and produce steam, which turns turbines and produces electricity.

The chain reactions are controlled by water, which slows neutrons down enough to break uranium atoms apart and acts as a coolant, and by control rods, which are made of elements (such as silver, iridium and cadmium) that absorb neutrons and therefore shut down the fission reactions entirely.

Natural nuclear reactors must have the same features as a modern nuclear plant:

1. The natural uranium must have few impurities and the local geology (thickness and geometry) must allow for spontaneous fission to take place.
   
2. The natural uranium must contain a high amount of U-235 (uranium ores go through an enrichment process before being used as fuel, which increases the proportion of U-235 from the initial 0.7202% to roughly 3%)
   
3. There must be a moderator, which can slow down neutrons to allow fission to occur.
   
4. There must not be a significant amount of neutron absorbing elements, such as those from which control rods are made, which would inhibit any self-sustaining fission.

All these conditions were met at Oklo and the sixteen other sites in Gabon’s Franceville Basin. Remarkably, it is thought that the natural reactors operated for hundreds of thousands of years, switching on and off every 30 minutes, as the moderating water boiled away and slowly returned through the surrounding porous sandstone. Were it not for the mass spectrometers at Pierrelatte, they might never have been discovered.

But why two billion years ago? And why did the reactors run for only a short amount of time?

The relative proportions of the isotopes of uranium have changed significantly since the Earth was formed. This is because they have different half lives (the time it takes for half of a given mass of a radioactive substance to decay into other compounds).

U-234 is the shortest lived, with a half life of 245,000 years – this is why so little remains today. U-238 takes the longest to decay, with a half life of nearly 4.5 billion years, while U-235 – the isotope of interest in terms of nuclear fission – has a half life of 703 million years. All this means that the proportion of U-235 in relation to U-238 has changed significantly since the Earth formed. U-235 has been estimated to have made up as much as 34% of all uranium around 4 billion years ago.

The question remains, then, if a high relative concentration of U-235 is required to sustain nuclear fission why did the site at Oklo wait nearly two billion years from this peak U-235 abundance, by which time the abundance of U-235 had fallen to roughly 4%?

Surprisingly for a story that has so far taken place underground, the answer lies in the air. Specifically, with oxygen. Uranium is only present in trace quantities in rocks on Earth. For large deposits to form, uranium must be dissolved in water and concentrated in areas where it precipitates back out of these solutions – much like how salt that has been dissolved in boiling water condenses back at the bottom of the pan when the water cools. However, without the presence of oxygen, uranium forms stable compounds that are not easily dissolved in water. Uranium must be in its oxidised form to easily form soluble complexes that allow it to be picked up and deposited by hydrothermal circulation.

Around the same time as the establishment of the Oklo fission reactor, the Earth was going through a period known as “The Great Oxidation Event” (GOE), in which the abundance of oxygen in the atmosphere rose from c.1% to c.15%. It is thought that the emergence of cyanobacteria, which generate energy through photosynthesis, releasing oxygen in the process, were responsible. Initially, this oxygen was captured in minerals that quickly became oxidised – indeed, bands of oxidised compounds found in ancient sediment layers were some of the key pieces of evidence for the GOE.  W these minerals could not take on any more oxygen, it began to accumulate in the atmosphere. The increased abundance of atmospheric oxygen favoured the formation of soluble uranium compounds, allowing it to be deposited in large veins.

Without the perfect balance of U-235 abundance and atmospheric oxygen concentration, the natural nuclear fission sites at Oklo and the surrounding area could never have roared into life.

There remains a fight to save the Oklo site from the fate of the other sites in the Franceville Basin. Aggressive mining has destroyed evidence of the other natural reactors, so Oklo now represents the sole opportunity to study these remarkable snapshots in Earth’s history. The Oklo natural nuclear reactors operated so long ago, but were so stable and existed without contamination of their surroundings. There may be much to learn about our own nuclear power plants from the natural sites that came before.