The purpose of the Next Generation Safeguards Initiative (NGSI)-Spent Fuel (SF) project is to strengthen the technical toolkit of safeguards inspectors and/or others. NGSI-spent fuel is working to achieve the following technical goals more easily and efficiently than in the past using nondestructive assay (NDA) measurements of spent fuel assemblies: (1) verify the initial enrichment, burnup, and cooling time of facility declaration, (2) detect the diversion or replacement of pins, (3) quantify the plutonium mass in spent fuel [which is also a function of the variables in (1)], (4) estimate the decay heat, and (5) estimate the reactivity. In the first phase of the NGSI initiative, libraries of virtual pressurized-water-reactor (PWR) spent fuel assemblies were developed as a function of various reactor conditions. Simulated passive gamma-ray spectra were analyzed, along with a range of other NDA techniques, to investigate a methodology to determine initial enrichment, burnup, and cooling time. In the context of passive gamma research, a set of equations that quantifies the 137Cs count rate, 134Cs/137Cs count-rate ratio, and 154Eu/137Cs count-rate ratio was constructed as a model to extract those parameters. In the second phase of the project, an experimental activity was undertaken. Since August 2013 a set of measurement campaigns has been conducted at the Central Interim Storage Facility for Spent Nuclear Fuel (Clab), in collaboration with Swedish Nuclear Fuel and Waste Management Company (SKB). One purpose of the measurement campaigns was to acquire passive gamma spectra with high-purity germanium and lanthanum bromide scintillation detectors from PWR and boiling-water-reactor spent fuel assemblies. The absolute 137Cs count rate and the 154Eu/137Cs, 134Cs/137Cs, 106Ru/137Cs, and 144Ce/137Cs ratios were extracted by the Fixed energy Response function Analysis with Multiple efficiencies (FRAM) code. The values have been used to construct corresponding model functions, which describe each measured quantity’s behavior over various combinations of burnup, cooling time, and initial enrichment, and were used to determine those same quantities in each measured spent fuel assembly. The results obtained in comparison with the operator declared values, as well as the methodology developed, will be discussed in detail.