Technical Cross-cutting Issues for the Next Generation Safeguards Initiatives Spent Fuel Nondestructive Assay Project

Ever since there has been spent fuel (SF), researchers have madenondestructive assay (NDA) measurements of that fuel to learnabout its content. In general these measurements have focused onthe simplest signatures (passive photon and total neutron emission)and the analysis has often focused on diversion detectionand on determining properties such as burnup (BU) and coolingtime (CT). Because of shortcomings in current analysis methods,inspectorates and policy makers are interested in improving thestate-of-the-art in SF NDA. For this reason the U.S. Departmentof Energy, through the Next Generation Safeguards Initiative(NGSI), targeted the determination of elemental Pu mass in SFas a technical goal. As part of this research effort, fourteen nondestructiveassay techniques were studied. This wide range of techniqueswas selected to allow flexibility for the various needs of thesafeguards inspectorates and to prepare for the likely integrationof one or more techniques having complementary features. In thecourse of researching this broad range of NDA techniques, severalcross-cutting issues were identified. This paper will describesome common issues and insights. In particular we will describethe following: (1) induced and non-induced fission-based techniques,(2) the role of neutron absorbers with emphasis on howthese absorbers vary in SF as a function of initial enrichment, BU,and CT, as well as how some NDA techniques are more or lesssensitive to neutron absorbers; and (3) the need to partition themeasured signal among different isotopic sources and why thispartitioning indicates which NDA techniques best integrate; (4)the importance of the “first generation” concept in the context ofboth diversion detection and in the context of determining Pumass because the first generation indicates both the spatial andisotopic origins of the detected signal; and (5) the unique roleplayed by 238U and why in most cases it primarily acts as an amplifierof the signal generated by 235U, 239Pu, and 241Pu.