Additional neutron studies on the Oklo phenomenon

It was shown at the Libreville (Gabon) Symposium in 1975 that the conventional methods of reactor physics were applicable to the Oklo phenomenon. This type of studies was therefore continued and particular attention was paid to improving the calculation tools. The BINOCLE code, which makes detailed calculations of the neutron balance in a multiplying medium such as the Oklo ore at the time of the reactions, has been improved: a number of effects, in particular that of temperature, have been calculated in greater detail; the code has been adjusted more accurately and, with the addition of supplementary loops, is now more convenient to use. The CRITO code calculates the flux distributions and associated characteristics in a reactor formed by the juxtaposition of different media. Some examples of applications are given. The spectral indices and conversion coefficients are characteristics which are accessible to measurements which provide information on the amounts of water present at the time of the reactions. The interpretation of these characteristics is discussed. The paper also considers the question of reflectors, i.e. the role played by the lower grade ore surrounding the core of the reactors; more particularly, the rate of decrease in neutron flux in these media has been calculated as a function of their characteristics. An effort has been made to determine the distributions of the reaction rates measured in a number of cases: sector 2'P, boreholes SC 30, SC 30 bis and SC 36; the shape of the curves is reproduced extremely well and it is possible to discuss the role of certain parameters, such as the distribution of water or the amounts of neutron poison present at the time of the reactions. These are only a few examples; other applications have been described at this meeting (papers IAEA-TC-119/22 and 23). The studies are being continued. (author)