Axial and Azimuthal Gamma Scanning of Nuclear Fuel Implications for Spent Fuel Characterization

A project to research the application of non-destructive assay (NDA) to spent fuel assemblies is underway among a team comprised of the European Commission, DG Energy, Directorate Nuclear Safeguards; the Swedish Nuclear Fuel and Waste Management Company, Uppsala University, the University of Michigan, Los Alamos National Laboratory, Lawrence Livermore National Laboratory, and Oak Ridge National Laboratory that collaboratively are advancing some of the goals of the Next Generation Safeguards Initiatives Spent Fuel (NGSI-SF) Project. The NGSISF team is working to achieve the following technical goals more easily and efficiently using nondestructive assay measurements of spent fuel assemblies in order to improve both international safeguards and repository safety: (1) verify the initial enrichment, burnup, and cooling time of facility declaration, (2) detect the diversion or replacement of pins, (3) estimate the plutonium mass, (4) estimate the decay heat, and (5) determine the reactivity of spent fuel assemblies. The measured neutron, gamma-ray, and heat signatures from spent fuel assemblies, as well as simulations, will be combined in advancement of the technical goals. This current study focuses primarily on the application of time-stamped list mode data acquisition applied in the context of a fixed collimator that allowed a thin axial portion of the fuel to be observed as the fuel assembly moved vertically past the collimator. Measurements were performed at the Central Interim Storage Facility for Spent Nuclear Fuel (which is abbreviated using the Swedish acronym: Clab) in Sweden, in 2013 and 2014. In total, fifty spent nuclear fuel assemblies were measured in detail, twenty-five boiling water reactors and twentyfive pressurized water reactor assemblies.