Year
2006
Abstract
The nondestructive assay (NDA) of special nuclear material using the neutron-based method of multiplicity analysis has proven useful for accurate and rapid verification of many nuclear material types. Plutonium metal items are one material type that does not yield accurate results, as biases of up to 20% have been observed. The multiplicity analysis of plutonium metal items has shown the need for a multiplication-dependent assay correction factor to achieve accurate assay results. Empirically derived correction factors have successfully been used to obtain accurate assay values for metal items. Recently a new multiplicity-based analysis method, called the “weighted point model,” has been developed, which allows for the assay of metal items without the need for a multiplication-based correction factor. The weighted-point model requires a set of Monte Carlo calculated weighting factors. A set of generic weighting factors was determined with an idealized detector; the validity of using these weighting factors with a real multiplicity counter is unknown. A study was conducted to see if the weighted-point model using the generic weighting factors would result in accurate assay values for various neutron multiplicity counters. Monte Carlo calculations were undertaken to determine the multiplication correction factor for several different multiplicity counters. These multiplication correction factors were compared to the empirical correction factors and also to the correction factors based on the idealized detector. For multiplication values up to M=2, agreement was found between the calculated and empirical multiplication correction factors, which validates the use of the weighted-point model with the generic weighting factors for all multiplicity counters. Use of the weighted-point model will allow a facility to save time and effort by being able to assay plutonium metal items without having to calculate counter-specific weighting factors or the empirical multiplication correction factors.