SPENT FUEL CHARACTERIZATION USING THE DIFFERENTIAL DIE-AWAY SELF-INTERROGATION TECHNIQUE

This paper summarizes the development of the Differential Die-Away Self-Interrogation Technique (DDSI) as a non-destructive assay (NDA) method to measure the fissile mass present in 17x17 pressurized water reactor (PWR) spent fuel assemblies. The Monte Carlo N-Particle eXtended transport code, MCNPX [1], was used to create a library of 64 spent fuel assembly models [2] for simulations purposes within the modeled DDSI geometry, in order to study the detector response to assemblies of varying burnup (BU), initial enrichment (IE), and cooling time (CT). The comprehensive range of these parameters was intended to reflect the range of possible spent fuel assemblies encountered at various facilities worldwide, and the resulting isotopic complexity within those samples. The purpose of modeling is to relate the DDSI detector response to the given fissile mass. Based on this quantitative relation, fissile mass in spent fuel can then be measured experimentally. This paper outlines the DDSI detector configuration, and the determination of two viable ratios, the late gate doubles-to-singles (D/S) ratio and the late-to-early-gate doubles (L/E)D ratio, which both track with the fissile content of the assemblies. A 239Pu-effective fissile mass concept was used as the basis for quantifying fissile mass, and relating such masses to the ratios. Both ratios were heavily dependent on BU, CT and IE of the assemblies and exhibited similar trends with the salient structure depending on IE. Multiplication within the assembly was used as a key intermediate parameter to develop a predictive model for effective mass values based on the ratios obtained from simulation. Using this model, there was a maximum of 6% deviation across the library between the true effective mass and the predicted effective mass based on the D/S ratio. This work is part of a larger effort sponsored by the Next Generation Safeguards Initiative (NGSI) to develop an integrated instrument, comprised of individual NDA techniques with complementary features, that is fully capable of determining Pu mass in spent fuel assemblies [3].