Evaluation of modeling options for in-pellet power distribution and gap gas conductance for accurate fuel temperature predictions

Having the ability to predict fuel temperatures for efficient multi-physics steady state, depletion, and transient calculations with reasonable accuracy without the added burden of prohibitively expensive computation costs has been a major driving force in the nuclear industry. There are several parameters that have an immense impact on fuel surface and centerline temperatures. Sensitivity studies were performed to investigate the impact of gap gas conductance and internal pin power distribution on the fuel temperature predictions. As a result, areas of improvement in the CTF fuel performance model were identified by separating different effects, and analyzing the sensitivity of results to each model improvement. The performed studies demonstrated the importance of modeling internal pellet power distribution for accurate prediction of fuel centerline temperature. Furthermore, a new gap gas conductance modeling option that leverages the fuel performance code FRAPCON was implemented in the fuel rod model of CTF. Gap gas conductance data was pre-computed as a function of linear heat rate and fuel exposure, and was integrated into CTF as part of the new model. Using FRAPCON as a reference solution, the new FRAPCON-informed gap conductance model of CTF was found to calculate results within 2 degrees Kelvin of FRAPCON predictions with respect to fuel surface temperature. This study indicated the feasibility of developing an efficient framework for informing the low fidelity fuel rod models of thermal-hydraulic codes, in this case CTF, with more accurate pre-computed values by leveraging high fidelity fuel performance codes such as FRAPCON. CTF was able to utilize this tabulated data provided by the FRAPCON fuel performance code as well as to include the above mentioned improvements in each axial node of a given rod to provide a full three-dimensional representation.