Year
2014
Abstract
Uranium-236 is naturally present in uranium ores at very low concentrations (~10-10 atom percent). With the increasing precision of mass spectroscopy, specifically accelerator mass spectroscopy (AMS), variations in 236U 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 of 236U in ore deposits is through neutron capture on 235U through a naturally occurring neutron flux present in uranium ore bodies. The neutron flux is mainly the result of spontaneous fission (SF), (a,n) reactions, and cosmic radiation. Different mineral components, water concentrations, and uranium densities/mineralization throughout the world will therefore result in a variation of 236U in the ore bodies. Developing a model accounting for the primary mechanisms for production and loss of 236U could 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 predict 236U 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 (a,n) reactions were calculated using a computational code, SOURCES 4C. MCNPX2.7 was used, with the parallel source term, to model a simplified ore body and calculate the 236U production in the ore. Several parameters were needed for the MCNP model, including elemental composition, water content, and geometry of the ore body. The input parameters were not known exactly, and max/min bounds were estimated from various calculations and placed on the parameters. The MCNP predictions of 236U/238U atom ratio were compared to AMS measured values for several samples, and the Monte Carlo forward model predictions were shown to provide good agreement with the measured 236U results for the majority of samples. The results indicate a correlation between the observed 236U variations and ore deposit parameters. Since these variations do not seem to be a random occurrence, 236U can be possibly used to obtain source information from given uranium ore concentrates.