NuScale SMR Core Verification via MCNPX2.7 and Assessment of Alternative Burnable Absorbers Beyond Natural Gadolinium

The NuScale small modular reactor (SMR) employs burnable absorbers (BAs) to control initial excess reactivity, making the choice of optimal BA materials a key factor in ensuring both safety and fuel utilization efficiency. This study focuses on verifying the NuScale reactor core model and evaluating alternative BAs using MCNPX 2.7. Model verification was performed by benchmarking the results against previously published studies, with agreement demonstrated across critical neutronic parameters, including criticality, power distribution, and fuel burnup.Following this verification, a comparative neutronic evaluation was conducted to examine the performance of alternative BA materials relative to natural gadolinium recommended in the final safety analysis report. Erbium and enriched gadolinium were employed, using the same concentrations as those used for natural gadolinium. Also, enriched gadolinium was utilized at adjusted concentrations, which were half that of natural gadolinium, to attain the same initial excess reactivity as natural gadolinium.The simulation results revealed that enriched gadolinia, at the same concentration of natural gadolinium, effectively suppressed initial excess reactivity at the beginning of cycle and maintained better performance across other neutronic metrics. This makes it a strong candidate for improving reactivity control during the burnup cycle while preserving reactivity coefficients and flux distribution in NuScale SMR cores.