Technical Cross-cutting Issues for the Next Generation Safeguards Initiative's Spent Fuel Nondestructive Assay Project

摘要

Ever since there has been spent fuel (SF), researchers have made nondestructive assay (NDA) measurements of that fuel to learn about its content. In general these measurements have focused on the simplest signatures (passive photon and total neutron emission) and the analysis has often focused on diversion detection and on determining properties such as burnup (BU) and cooling time (CT). Because of shortcomings in current analysis methods, inspectorates and policy makers are interested in improving the state-of-the-art in SF NDA. For this reason the U.S. Department of Energy, through the Next Generation Safeguards Initiative (NGSI), targeted the determination of elemental Pu mass in SF as a technical goal. As part of this research effort, fourteen nondestructive assay techniques were studied. This wide range of techniques was selected to allow flexibility for the various needs of the safeguards inspectorates and to prepare for the likely integration of one or more techniques having complementary features. In the course of researching this broad range of NDA techniques, several cross-cutting issues were identified. This paper will describe some common issues and insights. In particular we will describe the following: (1) induced and non-induced fission-based techniques, (2) the role of neutron absorbers with emphasis on how these absorbers vary in SF as a function of initial enrichment, BU, and CT, as well as how some NDA techniques are more or less sensitive to neutron absorbers; and (3) the need to partition the measured signal among different isotopic sources and why this partitioning indicates which NDA techniques best integrate; (4) the importance of the "first generation" concept in the context of both diversion detection and in the context of determining Pu mass because the first generation indicates both the spatial and isotopic origins of the detected signal; and (5) the unique role played by ~(238)U and why in most cases it primarily acts as an amplifier of the signal generated by ~(235)U, ~(239)Pu, and ~(241)Pu.