C.C. Keith W.S. Charlton “Investigation of Trace U-236 Content Variation in Geologic Uranium Deposits”, 55th Annual Meeting of the Institute of Nuclear Materials Management, Atlanta, Georgia, 20-14 July 2014.
Uranium-236 is naturally present in uranium ores at very low concentrations (~10-10a/o). With the increasing precision of mass spectroscopy, specifically accelerator mass spectroscopy (AMS), variations in236U concentration have been able to be distinguished and are known to exist due to various physical, chemical, or even biological processes. One mechanism for production of236U in ore deposits is through neutron capture on235U through a naturally occurring neutron flux present in uranium ore bodies. The neutron flux is mainly the result of spontaneous fission (SF), (α,n) reactions, and cosmic radiation. Different mineral components, water concentrations, and uranium densities/ mineralization throughout the world will therefore result in a variation of236U in the ore bodies. Developing a model accounting for the primary mechanisms for production and loss of236U would be used to narrow the original source of uranium involved in a proliferation or safeguards-violation scenario. A model of the neutron source in a uranium ore body is needed to compute the neutron flux and predict236 U concentrations for different deposits types. To account for cosmic radiation, an analytical approach was used to calculate neutron production for evaporation, muon-capture, and photoneutrons. Spontaneous fission and (α,n) reactions were calculated using a computational code, SOURCES 4C. MCNPX2.7 was used, with the source term, to model a simplified ore body with estimated variations in input parameters and calculate the236U production in the ore. The MCNP predictions of236U/238U atom ratio were compared to AMS measured values for several samples, and the Monte Carlo forward model predictions were shown to match measured236U results for the majority of samples. The results indicate a correlation between the observed236U variations and ore deposit parameters suggesting that236U can possibly be used to obtain source information from uranium ore concentrates.