The possibility for wide-spread implementation of small modular reactors (SMRs) in the coming years comes with associated challenges for international nuclear safeguards: more effective and efficient use of safeguards resources will be needed and safeguards methodologies for monitoring SMRs may need to be developed. Of particular interest are methods for unattended monitoring, which could complement on-site inspections at facilities. One possibility for remote monitoring is using neutron detectors to determine the time-evolution of the core fissile composition. Although ex-core neutron detectors are commonly used in nuclear power plants to monitor the short-term behaviour of the core, this technique would be based on long-term monitoring over one or more operating cycles. In this work, we investigate the possibilities for using ex-core neutron detectors to monitor the long-term composition of a 160 MWth NuScale integral pressurised water reactor (iPWR) core. This core was chosen as a first case study, given the wide spectrum of SMR concepts. The considered iPWR module is housed in a pool of borated water, which significantly affects the neutron flux outside the reactor. Depletion calculations in Serpent2 have been performed for a 2D model of the core to determine the evolution of the fuel composition over a two-year operating cycle. Using variance reduction techniques, the thermal and fast neutron fluxes were determined at various distances into the water outside the iPWR module. Changes in the ex-core neutron flux and its relationship to the flux in the core with fuel depletion were studied. It was found that the relationship between the ex-core neutron flux and the flux in the core changes during depletion. Different possible sources to these changes were studied to investigate the validity of the proposed safeguards monitoring technique for an iPWR module housed in water.
Year
2023
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Abstract