So, let me tell you about the Oklo Effect. One of the results of working on the Atomic Energy [sic – should be Weights] Commission was that I became aware of some beautiful work that the French Atomic Energy Commission was doing. They were… of course, the French have a big nuclear generating programme most of their electric power comes from nuclear energy. And they get the uranium for this from some mines in Gabon in West Africa in a place near Oklo, so it has now become the Oklo Phenomenon – that is what I want to talk about. And, very careful analytical work had showed that the ratio of isotopes in various elements that they were studying as impurities, did not follow a normal isotope pattern, but followed a double-humped curve which was very characteristic of fission product elements. They also found that the concentration of uranium 235, which is the important one, was quite different from what it was in normal minerals. Instead of being, I think 0.67% it was substantially higher than that. And, to cut a long story short, again, with very excellent detective work and so forth, they came to the conclusion that the reason why there was this anomaly was that there had been, in prehistoric times, a nuclear reactor there. And what had happened, apparently was this. The half-life, the rate at which uranium 235 disintegrates naturally is about six or seven times as fast as uranium 238. So if you extrapolate back in time you would find that uranium would have a much higher concentration of uranium 235.

If you go back 1 billion years when some of the rocks were being formed, or even 2 billion years, perhaps, the ratio of 235 was as high as 25%. Now you will have heard of nuclear enrichment programmes, the enrichment of 235 to make the reactors work efficiently, will raise it from the one in nature, up to 3% or 4% quite normally – 25% meant that it was more susceptible to fission. But you need a moderator. You will have heard of water-cooled moderators, and we talked about heavy water in Harwell. It turns out that in this region of Oklo, there is a high concentration of uranium ores, billions of years ago; the concentration of uranium also had a concentration of 235. And then with the course of time, the rain fell, there were floods, lakes formed, the beds were covered with water which is a natural moderator, a nuclear reaction started. A nuclear reaction went on for thousands of years but as it went on it, of course, gave the fission products, which we’re digging up now, but it also evaporated the water because it heated the water and that evaporated. The water level sank. As the water levels sank it was no longer moderated so the nuclear reactor stopped. And the experiments that they did in France on the distribution of minerals and so forth, showed that there were probably several cycles lasting hundreds of thousands of years, of this natural nuclear reactor.

Now, there’s an important message there. Firstly, that the techniques of modern chemistry are such that you can get incredible and unexpected data from these things. But also, that it is possible for the environment to grow over, study, mask nuclear explosions – not that this is an argument to have them, but it does mean that in the distant past there have been nuclear reactors which, as a phenomenon we only started finding out about in the middle of the 20^th century.