C.C. Keith and 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 (~10-10 a/o). 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), (α,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 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 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 (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 the 236U production
in the ore. 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 match
measured 236U results for the majority of samples.
The results indicate a correlation between the
observed 236U variations and ore deposit parameters
suggesting that236U can possibly be used to obtain
source information from uranium ore concentrates.
Associated Project(s):Investigation of Trace U-236 Content Variation in Geologic Uranium Deposits