Fissile Material Measurements using the Differential Die-Away Self Interrogation Technique

Currently, there is substantial research effort focused on quantifying plutonium (Pu) mass in spent fuel using non-destructive assay (NDA) techniques [1, 2]. Of the techniques being investigated, Differential Die-Away Self-Interrogation (DDSI) is a recently proposed, neutron-based NDA technique capable of quantifying the total fissile content in an assembly. Unlike the conventional Differential Die-Away (DDA) technique, DDSI does not require an external neutron source for sample interrogation, but rather, uses the spontaneous fission neutrons originating from 244Cm within the spent fuel for self-interrogation. The essence of the technique lies in the time separation between the detection of spontaneous fission neutrons from 244Cm and the detection of induced fission neutrons at a later time. The DDSI detector design imposes this time separation by optimizing the die-away times (t) of the detector and sample interrogation regions to obtain an early and late neutron distribution respectively. The ratio of the count rates in the late gate to the early gate for singles, doubles, and triples is directly proportional to the fissile content present in the sample, which has already been demonstrated for simplified fuel cases using the Monte Carlo N- Particle eXtended (MCNPX) code [3, 4, 5]. The current work applies the DDSI concept to more complex samples, specifically spent Pressurized Water Reactor (PWR) assemblies with varying isotopics resulting from a range of initial enrichment, burnup, and cooling time. We assess the feasibility of using the late gate to early gate ratio as a reliable indicator of overall fissile mass for a range of assemblies by defining a 239Pu effective mass which indicates the mass of 239Pu that would yield the same DDSI signal as the combined mass of major fissile isotopes present in the sample. This work is important for assessing the individual capability of the DDSI instrument in quantifying fissile mass in an assembly in order to use this information for a possible integration with another NDA instrument for direct Pu mass determination.