Oklo - Part I

Oklo is a region in the west African equatorial country of Gabon.  What makes it notable to geologists is that it is the site of an unusual event which took place some two billion years ago.

Back in 1956, when Gabon was a French colony, prospectors from the Commissariat à l'énergie atomique discovered uranium deposits in the Oklo region near the city of Franceville in the southeastern part of the country.

France at the time was developing both reactors for nuclear energy as well being engaged in nuclear weapon research (they developed a nuclear bomb in 1960).

France mined uranium at Oklo for domestic nuclear reactors for 40 years until the deposits were exhausted and the mine closed.

In 1972, however, French physicist Francis Perrin noted an unusual thing when routinely testing the uranium from Oklo.

In order to understand what Perrin discovered, we have to first take a little digression on isotopes.

Let's start with atoms.  Everyone should know that atoms have a nucleus composed of protons and neutrons and the nucleus is orbited by electrons.  This is the simplified Bohr model of the atom (named for the famous Danish physicist Niels Bohr).  Protons have a positive charge, electrons have a negative charge, and neutrons are electrically neutral.  At right is a simple model of a carbon atom.

Now think of the periodic table of the elements.  Each entry on the table is an element.  Elements are atoms that differ from each other by the number of protons in their nucleus (this number is called the atomic number).  Hydrogen (H), for example, has an atomic number of 1 on the periodic table and has one proton in its nucleus.  Add another proton and you have helium (He).  Toss in 6 protons and you have carbon (C) as shown above.  Uranium (U) has 92 protons in the nucleus.

While the number of protons determines the element, atoms of the same element can have differing numbers of neutrons in the nucleus.  These different forms of the same element are called isotopes from the Greek roots isos (ἴσος "equal") and topos (τόπος "place") because isotopes of a single element occur in the same place on the periodic table.

Different elements have differing numbers of naturally-occurring isotopes (the theory behind all of this is complex and beyond the scope of this post).  Some isotopes are heavy in that they contain more neutrons in the nucleus than protons whereas others are light in that they contain fewer neutrons in the nucleus than protons.  In addition, some of these light or heavy isotopes are radioactive meaning they spontaneously decay into other elements over time (the rest of the isotopes are stable and don't decay).  The time it takes radioactive isotopes to decay is given by their half-life - the amount of time it takes for half of the isotopes in a sample to decay.

Two naturally-occurring isotopes are uranium are U-235 and U-238. The 235 and the 238 are the mass numbers which denotes the number of protons and neutrons in the nucleus of these isotopes.  Since all uranium has 92 protons (the element's atomic number), U-235 has 235-92 or  143 neutrons and U-238 has 238-92 or 146 neutrons.  Both of these isotopes are heavy and radioactive but they each have a different half-life.  The half-life of U-235 is 703.8 million years and the half-life of U-238 is 4.47 billion years.  Quite a difference.  Since they have different rates of decay, the ratio of U-235/U-238 will change over time (that ratio will decrease since U-235 decays more quickly).

In natural uranium deposits today, U-235 is about 0.72% of the total.  As we go back into geologic time, however, this ratio will increase.  Around 1.7 billion years ago, that ratio would have been around 3.1% (the other 97% being U-238.  Another important different between U-235 and U-238 is that U-235 is fissible.  This means that it will split apart when bombarded by free neutrons - a process called nuclear fission.

n + U-235 => Ba-141 + Kr-92 + 3 n

A free neutron (n) colliding with U-235 will split it into Ba-141 (barium) and Kr-92 (krypton) plus 3 free neutrons.  These 3 free neutrons can than fly off and split other U-235 isotopes in a self-sustaining chain reaction.  This process also releases energy hence its use in nuclear reactors (a LOT of energy - a chunk of U-235 the size of a grain of rice can produce as much energy as 3 tons of coal!).

Now back to the story of Oklo and the French physicist Francis Perrin.  What he found when testing samples from Oklo is that the ratio of U-235 wasn't around 0.72% as expected.  It was lower.  Further testing showed some samples were as low as 0.44% and there were similarly lower than expected ratios of other isotopes like neodymium (Nd) and ruthenium (Ru).  What caused these anomalously low ratios?

Well, it turns out that these ratios would be exactly what you would expect if these isotopes were in a nuclear reactor.  It also turns out that back in 1956 a chemistry professor at the University of Arkansas named Paul Kazuo Kuroda predicted that these types of nuclear fission reactions could occur naturally under favorable conditions.

Further research confirmed that nearly 2 billion years ago, nuclear fission reactions similar to what occurs today in the containment vessels of nuclear reactors, naturally developed in a mass of uranium ore at Oklo.

In my next post, I'll discuss a bit more about the geology of this unique site.