Estimating nuclear material holdup: a challenging non destructive assay problem

Holdup is the residual amount of special nuclear material (SNM) remaining in a processing facility after the bulk materials have been cleaned out. In commercial uranium processing facilities, quantification of holdup is a major challenge because of the highly variable shapes and sizes of the deposits. Any method that attempts to generalize and calibrate deposit shapes in order to quantify holdup will be prone to high uncertainties. Uncertainties on the order of ±50% are typical in holdup results. In nuclear security and safeguards applications, a high uncertainty in the estimation of holdup can result in a large amount of material unaccounted for (MUF) thereby increasing the threats posed due to material theft, material diversion, and facility misuse. Nondestructive assay (NDA) methods are useful for monitoring and estimating nuclear material holdup. Gamma-ray based as well as neutron counting based NDA methods are employed. The paper will highlight some of the recent advances in detector technology and mathematical approaches that are showing promise towards improving the accuracy of quantification. For example, using gamma-ray imaging with germanium or cadmium zinc telluride (CZT) detectors for delineating the deposit shape and size, mathematical methods such as Monte Carlo and the In Situ Object Counting System (ISOCSTM) have the potential for reducing the uncertainty in holdup estimation. Limitations of using gamma-ray methods when the deposit thickness approaches "Infinite thickness" at the gamma ray energy of interest and the use of neutron NDA techniques to complement gamma-ray methods will be discussed. (author)