A TURBULENCE MODEL SENSITIVITY ANALYSIS OF THERMAL-HYDRAULIC PROPERTIES ON THE PRE-CONCEPTUAL NIST NEUTRON SOURCE DESIGN

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The current reactor at the NIST, the National Bureau of Standards Reactor (NBSR) was operational in 1967, serving as a premiere user-facility to the international neutron scattering research community. The NBSR’s age has contributed to difficulties like longer outage times and increased maintenance costs, prompting an investigation into a replacement design. The NIST Neutron Source (NNS) is a proposed replacement design to replace the current NBSR. This paper investigates the thermalhydraulic behavior of the NNS’ compact core preliminary design using computational fluid dynamics (CFD) analysis. While developing the CFD model, any flow irregularities may significantly affect thermal-hydraulic characteristics such as the core’s pressure or velocity profiles. Therefore, a turbulence model must be carefully selected to balance computational costs and model uncertainties. This paper details a sensitivity analysis with multiple turbulence models to evaluate the resulting hydraulic behavior of the coolant flowing from the inlet plenum to the core. Various Reynolds Averaged Navier stokes (RANS) turbulence models in ANSYS Fluent® such as k-ε, k-ω, k-ω SST, realizable k-ε and Spallart-Allmaras are compared. The resulting velocity and pressure profiles for each turbulence model are compared for fit. Discussions of the mesh, assumptions, and boundary conditions are also provided in the text, demonstrating the limitations and methodologies of the study.