Irradiation experiments are a critical aspect of the nuclear fuels and materials qualification process. Determining the flow conditions of these experiments is necessary for understanding response of nuclear components during transient and normal operation. Quality flow data is lacking for cartridge-type irradiation experiments with annular flow natural circulation designs. Data were obtained for informing this type of design using computational fluid dynamics (CFD) and experimentation. The CFD simulation was verified and validated and the CFD code was used to perform a sensitivity study. The verification and validation of the CFD modeling proves the code’s ability to quantify the thermal hydraulics of internally heated concentric tube thermosyphons, which has not been shown in literature. The CFD code was then used to determine modeling and geometrical sensitivities on the solution of transient and pseudo-steady state operation of the internally heated concentric tube thermosyphon.