Year
2018
Abstract
The Spent Fuel Nondestructive Assay (NDA) project was established with the goal of strengthening the technical toolkit of safeguards inspectors and/or other interested parties. Several NDA techniques were initially considered and simulated. Eventually, the techniques were down-selected to only the most promising methods, including the Differential Die-Away Self-Interrogation (DDSI)-Passive Neutron Albedo Reactivity (PNAR) integrated instrument. DDSI-PNAR was shipped to the Swedish Central Interim Storage (Clab) facility in 2016, and field tests were performed in 2018. The purpose of the field trials was to perform neutron and gamma measurements for the SKB50 set of pressurized water reactor (PWR) and boiling water reactor (BWR) assemblies. The experiments were conducted at the Clab facility in Oskarshamn, Sweden over five different weeks, with approximately half of the time devoted to PWR assemblies and the other half devoted to BWR assemblies. A total of 50 assemblies were measured with a static, central measurement and with an axial scan. Additional measurements conducted include static measurements of different axial zones of BWR assemblies, four high voltage (HV) plateaus, and central and scan measurements of assemblies with a cadmium liner for PNAR analysis. The technical goals of the project, and therefore of the field trials, were to: (1) verify the initial enrichment, burnup, and cooling time of facility declaration, (2) detect the diversion or replacement of pins, (3) estimate fissile mass, (4) estimate decay heat, and (5) determine the reactivity of spent fuel assemblies.The DDSI measurements comprise a large amount of time-dependent neutron coincidence data from as many as 28 He tubes on two different sides of spent fuel assemblies, which can be used to characterize the fuel assemblies. The DDSI technique relies on the fact that spontaneous fission and (α,n) neutrons, primarily from 244Cm in spent power reactor fuel, thermalize in the water between pins and induce fissions in the fissile material in the assembly [1]. By measuring time-correlated neutrons from fission, we can characterize nuclear material [2]. The PNAR measurements work by calculating a ratio from counts in the detectors with and without a cadmium liner present. The ratio of counts has been shown to be proportional to multiplication in the assembly [3]. Both datasets and approaches are enhanced by the addition of the total gamma-ray counting data from an ion chamber (IC). The data will ultimately be analyzed with both DDSI and PNAR methodologies to determine multiplication, initial enrichment, burnup, cooling time, effective fissile mass, and heat. However, the preliminary analysis presented in this paper includes only determination of assembly multiplication, initial enrichment, and burnup.