The Natural Nuclear Reactor: Lessons for the Future

Matthew Morris looks to the past in solving the problem of high-level radioactive waste.

Around two billion years ago, something strange started happening deep underground at Oklo in Gabon; a process that was not discovered by humans until 1938: nuclear fission. Nuclear fission is the reaction used in nuclear power plants today and provides around 10% of the world’s energy. Modern nuclear reactors are extremely expensive and are complex feats of engineering, making what occurred at Oklo all the more fascinating.

The natural reactor at Oklo was first discovered by French scientists in 1972 when testing uranium ore for mining. They found the enrichment level (or concentration) of the uranium-235 isotope (235U), the crucial isotope which undergoes nuclear fission, was at 0.7171% as opposed to the global standard of 0.7202%. Although this may appear to be a miniscule amount, it represents 200kg of uranium lost; an amount equivalent to 6 nuclear bombs. The identification of missing uranium was extremely concerning to the scientists involved, as uranium falling into the wrong hands could be catastrophic. However, after a short investigation, the true reason for the depleted uranium was determined to be nuclear fission, and this natural phenomenon was reported to the world. It was found that this natural nuclear reactor was actually made up of 17 separate reactors: 16 at Oklo and one 30km south-east, at Bangombé. These reactors released a combined power output of 100kW, consistently, for up to one million years; enough to power around 1,000 lightbulbs.

For the natural reactors at Oklo to occur, many factors needed to be present at just the right levels. Oklo had a combination of sufficiently enriched 235U ore, free neutrons to initiate the chain reaction, and groundwater to act as a moderator to the reaction. This unique combination of factors is why the reactors at Oklo are the only known examples of naturally occurring nuclear fission throughout geological history. Unfortunately, all 16 reactors at Oklo have been mined for their uranium reserves. Thankfully, plans to mine the last remaining reactor at Bagombé were halted after uproar from the scientific community, although it is not clear if mining of the reactor has now occurred.

The most relevant lesson from the natural fission reactors is what it tells us about the capability of rocks to prevent the contamination of the biosphere with radioactive contaminants. High-level radioactive waste (HLW) is a major problem facing our generation as we currently have no permanent storage facility for this level of waste anywhere in the world. However, the reactors at Oklo have been invaluable in finding a solution for this waste. The radioactive by-products from the continued nuclear fission have remained isolated, deep underground for around two billion years, demonstrating the effectiveness of geological disposal of HLW. The UK has been working towards the construction of a geological disposal facility (GDF) for many years, as have many countries around the world. It is essential that this work continues and is not forgotten with a collection of temporary measures. It must be considered with urgency to prevent the problem from falling onto future generations to solve.

From Issue 20