Apparatus used at Oklo for determining the isotopic composition of the uranium

The ''Oklometer'', developed soon after the discovery of the Oklo fossil reactor, is used for measuring the degree of 235U enrichment of the uranium in the deposit at a laboratory near the mine without recourse to expensive or delicate techniques. The uranium to be measured must be in the form of a uranyl nitrate solution without chemical impurities or radioactive daughter products of uranium. This is achieved by grinding ore which contains about 5 g of uranium, treating the powder with a H2SO4-HNO3 mixture and then carrying out purification in an ion-exchange column. The solution is transferred to a cell with a scintillator. The area (s5) on the pulse spectrum of the 186-keV peak of the 235U is measured. A 60-keV gamma beam (241Am source) is then allowed to pass through the cell. The area of the transmitted peak is s1. While s5 depends both on the degree of enrichment and on the concentration of the solution, s1 depends only on the concentration. With s5 and s1, and a calibration curve plotted with a gradually diluted solution of natural uranium, it is easy to deduce the degree of enrichment of a sample. The stability of the apparatus is checked periodically, using a standard. Two ''Oklometers'' are in use - one at Mounana and the other at Saclay. The results agree to within +-1.5% with those obtained by mass spectrometry