text: Analyses carried out at Pierrelatte and Cadarache showed that magnesium uranates (or yellow cakes) from Gabon had a variable but constant U depletion. On July 7, 1972, researchers at Cadarache discovered an anomaly in uranium ore from Oklo in Gabon. Its U content was much lower than usual. Isotopic analyses revealed the origin of the U depletion: the depleted uranium came from Oklo ore in Gabon, mined by COMUF. A systematic analysis campaign was then carried out in the Cadarache and Pierrelatte laboratories (uranium content measurements, isotopic content measurements). On Oklo samples, Cadarache analysts noted a U depletion for magnesium uranate from the Mounana plant (U = 0.625%) and an even greater depletion for a magnesium uranate (Oklo M) (U = 0.440%): Oklo 310 and 311 ores have uranium contents of 12% and 46% respectively, and U contents of 0.592% and 0.625%. 
  In this context, J.F. Dozol took the initiative of analyzing magnesium uranate and ore samples from Oklo on the AEI MS 702 Spark Source Mass Spectrometer (SSMS).
  The advantage of the SSMS is its ability to produce substantial quantities of ions from all the elements present in the electrodes. The electrodes, between which a spark is generated, have to be conductive (to achieve this, Oklo samples were mixed with high-purity silver). All the isotopes in the sample, from lithium to uranium, are plotted on a photo plate. On examining the plate (see below), J.F. Dozol noted in particular the very high uranium content of Oklo 311 ore:
  - elements present in significant quantities around masses 85-105 and 130-150, corresponding to the two bumps of U fission yields. (The mass distribution of fission products follows a "camel's hump" curve, with two maxima),
  - the last lanthanides (holmium to lutetium) are not detected (beyond mass166). In nature, all 14 lanthanides are found; in nuclear fuel, having undergone fission reactions, the isotopes of the last lanthanides are not detected.
  The next step is isotopic analysis of certain elements on a thermal ionization mass spectrometer, after chemical separation of neodymium and samarium. From the first analyses of Oklo "M" uranate and "Oklo 311" ore, it is clear that neodymium and samarium have an isotopic composition much closer to that found in irradiated fuel than to that of the natural element. The detection of Nd and Sm isotopes not produced by fission indicates that these elements are also present in the natural state, from which their contribution can be subtracted.
  These results were passed on to neutron scientist Jean Claude Nimal (CEA Saclay), who estimated the neutron flux received by the analyzed sample on the basis of its U deficit. This made it possible to estimate the neutron capture by the isotopes Nd and Nd, leading to the additional formation of Nd and Nd respectively. This excess must be subtracted to obtain fission yields for uranium 235. As can be seen from the table below, the fission yields (M) agree with the results corrected (C) for the presence of natural neodymium and neutron capture.