This paper describes the development, analysis, testing of a RELAP5-3D system thermal-hydraulics model for a 50-MWel-class pressurized water reactor–small modular reactor (PWR–SMR), similar to that by NuScale Power. This study focuses on a series of sensitivity tests to investigate the impacts of model changes. Parameters considered in the sensitivity study included the surge line junction resistance (SLJR), steam generator (SG) heat transfer area (SGHTA), SG primary flow area (SGPFA), SG secondary pressure (SGSP), and SG secondary flowrate (SGSF). Results for the reference and sensitivity simulations are compared with available design data. The flow in the primary circuit of the PWR–SMR is driven by natural circulation and can be sensitive to changes in hydraulic resistance and pressure drop in system components. Initial analysis results demonstrated significant flow oscillations. As a result of sensitivity studies, it was found that the surge line junction resistance needed to be increased by increasing the form loss coefficient from 4.9 to 30.0, to reduce mass flow oscillation amplitude to less than ±2%. Modifications to the steam generator heat transfer area, primary flow area, or secondary pressure have very little impact in reducing flow oscillations. However, it was found that the steam generator secondary flowrate will affect primary circuit flow oscillations, and when the SGSF was artificially increased from 68 kg/s (design data) to 91 kg/s (a 36% increase), the oscillations were eliminated, along with better matching with design data for core flowrate and inlet/outlet temperatures. Further improvements to the model for the steam generator will be required.