Robert West University of Wisconsin Madison, 53706
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Natural Nuclear Reactors The Dklo phenomenon
On December 2.1942 in a smash court under the Universitv of Chicago stadium, the historic experiment of Enrico ~ e r n i i and his team succeeded and the first man-made, self-sustaining nuclear chain reaction took place. For 30 years it was assumed that this was the first nuclear reactor on the earth, but recently evidence has turned up showing that a t least three natural fission reactors functioned about 1.8 billion years ago, in Africa near Oklo in what is now the country of Gabon (1,Z). By fortunate coincidence the geological formations in which the nuclear chain reactions took place have heen almost undisturbed since the Precambrian Era, leaving these fossil nuclear reactors still in place. The Oklo phenomenon was discovered in June 1972, when an analvst in the French Atomic Enerev laboratories a t pierrelake obtained an unexpected result i i a routine analysis of uranium for i s o t o ~ i ccontent. The concentration of 2"wU in natural uranium'is normally 0.7202 f 0.0006%, and had been thought to be constant everywhere on earth. The sample analyzed a t Pierrelatte showed a 23%2Ucontent of 0.7171 i 0.001. At first it was thought that the deviation was due to an error in measurement hutcareful repetition showed that it was real. This deviation from the normal value is quite small, and it is to the credit of the analyst that it did not go unnoticed. Large amounts of natural uranium are processed to remove most of the 2"g2U by gaseous diffusion of UFe. The analysts assumed that the sample showing low 23592Ucontent had been contaminated with commercial depleted uranium, and searched for the source of contamination. They therefore examined UFc. ... t h e refined uranium salts. and eventuallv the concentrates shipped from various mines. Gradually it became clear that thev had discovered a natural uranium occurrence of anomalous isotopic composition rather than an instance of contamination. The depleted uranium all came from the Compaignie des Minerais d'Uranium de Franceville (COMUF). a comDanv operating two mines a t Oklo and Mounana in Gabon. 1t was soon found that nearly all the uranium shimed bv this com.. p.rttyduringrhe previouryearand n hnli,ntnr,unti~~y rco more than -IrO r c m , had .ess rhan the normal ''>.,,I 1 r
