Simulation Study for Detection of Pin Diversion with the Differential Die-away Instrument Using Fresh Nuclear Fuel

2015 J.D. Williams Student Paper Winner The Differential Die-away (DDA) technique is currently being investigated at Los Alamos National Laboratory to better understand the development and deployment challenges for its use as a non-destructive, active interrogation method for assay of nuclear fuel assemblies. Inspired by the successful application for nuclear waste assay, this application of the DDA technique is based on the active interrogation by a pulsed DT neutron generator of a fuel assembly submerged in water. Induced fission neutrons from the fuel assembly are detected by surrounding 3He detectors, which provide information on the time of detection (list-mode data) of individual neutrons with respect to the time of the interrogating neutron pulse. The magnitude and dynamic evolution of the neutron population recorded by surrounding thermal neutron detectors reflects the fuel assembly characteristics and composition. In this paper, we report on results from a simulation study using MCNPX focusing on how random pin diversion scenarios (or partial defects) influence the dynamic evolution of the signal observed by individual 3He detectors. We use a combination of fresh low-enriched, natural, depleted uranium dioxide (UO2) fuel rods, and stainless steel rods to create perturbations in PWR-like fuel pin matrices mimicking removal or substitution of ten (10) pins to quantitatively determine the impact on the DDA signal, including signal magnitude, die-away time, and, importantly, individual detector sensitivity to diversion position in the fuel assembly. Analysis of the trends in the results calculated for the fresh fuel diversion cases provides insight into observable effects caused by similar types of diversion scenarios in spent fuel. This supports the conclusion that the DDA technique can provide a wealth of information about the assayed item and is suitable for spent fuel measurements.