We tend to think that humans are the only possible source of complex machinery on Earth. Leaving aside the exquisite complexity of biologically evolved organisms, it does seem to be true that the Earth creates less complexity than its human inhabitants. Yet, in the 1970s, nuclear excavators discovered a form of natural technology that not only humbled nuclear scientists with its simplicity, but which actually predated their achievements by several billion years. The startling discovery has supported decades of research, but its depths are still producing lessons for US regulators.
The objects in question are called the Oklo reactors, naturally occurring nuclear reactors named for the West African region of Gabon in which they reside. They’ve been dead for a very long time, probably over 1.5 billion years, but the evidence of their prior action is unmistakable. Sometime a bit less than 2 billion years ago, and lasting for about 300,000 years, the Oklo reactors held a series of stable nuclear fission reactions.
Upon their discovery, the central question about these reactors was simply: How could they possibly work? One early hypothesis was that the fundamental physical constants that restrict nuclear reactions today may have been different 2 billion years ago. Analysis of the wastes at Oklo, running all the way up to this very month, suggest that the physical constants have indeed been constant all along. That means the reactor would have needed a fissionable isotope in the same concentrations we require today.
Protection of nuclear enrichment technology is one of the defining international issues of our time. Many political theorists say that nuclear enrichment in Iran is the most likely cause of any future World War. If we have such trouble making raw uranium usable in stable fission reactions, how could an inanimate planet possibly do it?
Well, it didn’t. Back when these Oklo reactors first fired themselves up, Earth was barely half as old as it is today. That means that less time had passed since its initial formation from bits of galactic dust and rock. As a result, fewer radioactive half-lives had played out, and unstable isotopes were found in much higher concentrations. The most useful uranium isotope for nuclear power is uranium-235, which today accounts for just 0.7202% of any given natural sample of uranium. When the solar system first formed, that number would have been more like 17%, falling steadily until it reached the modern day value.
And 2 billion years ago? Scientists estimate the Oklo reactors would have had samples with roughly 3.6% uranium-235 — that’s close to the enrichment threshold of modern nuclear reactors. However, just packing the right material into a closed space does not a power plant make.
A stable nuclear fission reaction occurs because neutrons, one of the two large components of atomic nuclei, are knocked off of their original atoms so they can hit still more atoms, beginning the process again. For this to occur properly, you need enough “fissionable” isotope that the chain won’t fizzle out — if the neutrons thrown out by one fissionable atom don’t happen to impact any other fissionable atoms, then the reaction ends. Past about 3.2% concentration, probability says a reaction will continue all on its own.
But that only speaks to the relationship between atoms undergoing the fission process, and scientists were still stumped as to how that process got started in the first place. Neutrons released by the fission process are high-energy particles that tend to zip through even enriched uranium without impacting, or interacting at all. It turns out, the Oklo reactors got around this problem in much the same way that nuclear engineers did: water.