Will we have a second nuclear era? ~ Alvin M. Weinberg
Nuclear energy was first used to generate electricity in 1951 at the EBR-I liquid metal-cooled fast breeder reactor in Idaho, USA. However, it had existed on earth before this time. Notwithstanding the secret wartime CP-1 experiments under the University of Chicago’s Stagg Field stands in 1942, the earliest known nuclear reactors were actually operating around two billion years ago.
In what is now Gabon, Africa, back when eukaryotes were learning to metabolise oxygen, a fortuitous combination of highly concentrated uranium ore in porous sandstone – geologically separated from most other heavy metals – and significant groundwater volumes gave rise to natural nuclear fission. The uranium was a little different to today’s ore, as the proportion of the fissile isotope, U-235, had not naturally decayed for as long, and in fact made up at least 3% of the total – much like the low-enriched fuel in today’s reactors. Also like today’s reactors, the super-speedy, naturally emitted neutrons in the ore were slowed down or moderated by the water in the sandstone such that they could react with the fissile uranium in the thermal spectrum. The result was a collection of 16 natural nuclear reactors which sustained fission for hundreds of thousands of years.
Scientists have determined that this was a stable, self-regulating process. As heat was generated and warmed the rocks to a few hundred degrees, the ground water boiled. This gave rise to a void effect where neutrons were not moderated by enough liquid water to cause fission. (Today’s reactors use the same negative feedback as just one of many ways to automatically stabilise their reaction rates.) The ancient reactors would cool down for a few hours until enough liquid had collected to moderate neutrons again, restarting the reaction. The average power output was 100 kilowatts.
It is interesting that the right geological conditions could naturally – and predictably – give rise to stable, subterranean nuclear fission, whereas the extremely high isotopic purity and density of fissile material required by atom bombs could never have arisen spontaneously in the Earth’s crust. Indeed, it was careful investigation in 1972 by French technicians of the isotopic composition of ore from the Oklo mine in Gabon which revealed that the equivalent of 200 kg of U-235 was missing. Fortunately, rather than having been diverted to atomic weapons, it was actually still present, just in the form of fission products. In fact, along with the decay products of naturally generated plutonium, these elements had migrated barely a few metres from their original positions over billions of years.
This last fact is relevant when considering long term disposal of today’s radioactive waste, which will be immobilised and sealed in inert containers designed specifically for the purpose – not left raw within the ground. Repositories will be intentionally sited in the driest, most geologically stable places available, rather than randomly as nature would have it. And, ultimately, reactor designs which improve phenomenally upon the original ore-water-and-sandstone model will use and recycle their fuel far more efficiently, producing prodigious amounts of clean energy and by-products which need only hundreds – not millions – of years to lose their radioactivity.
There may have been many more natural reactors than just those we know of at Oklo and adjacent Okelobondo, their fossilised legacies sealed deep underground. Even with instruments sensitive enough to detect radioactivity thousands of times below any sort of potentially harmful level, they remain hidden. Either way, we know enough now that we needn’t worry. Now the challenge is to make this efficient and natural way of boiling water work for us, and more crucially for the wider natural world.