In June 1972, nuclear scientists at the Pierrelatte uranium enrichment plant in south-east France noticed a strange deficit in the amount of uranium-235 they were processing.

That’s a serious problem in a uranium enrichment plant where every gram of fissionable material has to be carefully accounted for.

The Oklo nuclear reactor is 1.8 billion years old. Photo source taringa.net)

The dilema was the ratio of uranium isotopes in their samples. Natural uranium contains three isotopes, always in the same ratios: uranium-238 (99.2744 per cent), uranium-235 (0.7202 per cent) and uranium-234 (0.0054 per cent).

The problem was with the uranium-235 of which there was only 0.600 per cent.

Physicists soon traced the anomaly to the supply of uranium ore from Gabon in West Africa, which contained far less uranium-235 than the ore from anywhere else on the planet. This caused some consternation among nuclear scientists.

So France’s top nuclear scientists began an investigation and, in the process, made one of the more remarkable discoveries in recent history.

This kind of depleted uranium is only found inside nuclear reactors, which burn uranium-235. That set off a hunt for a reactor that could have produced this stuff.

On 25 September 1972, they announced that the depleted uranium had come from Gabon in Africa, where nuclear scientists had discovered a 2 billion year-old nuclear reactor at the site of the Oklo uranium mines near a town called Franceville.

This was thought to be a ‘naturally occurring’ deposit of uranium where the concentration of uranium-235 had been high enough to trigger a self-sustaining nuclear reaction.

The only one of its kind anywhere on the planet. Just extraordinary!

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