Gap conductance modeling II: Optimized model for UO₂-Zircaloy interfaces

The model conventionally used to calculate heat transfer across the fuel-cladding gap in light water nuclear reactors is a modified version of the Ross-Stoute model. The model was modified to include gap distance in the formulation, which introduced additional uncertainties because the model parameters were not adjusted after the modification. In this study, this conventional model is optimized for uranium dioxide-Zircaloy interfaces using experimental data at high pressure for single- and multi-component gases. First, a calibration is performed for single-component gases. Second, the calibration is extended to multi-component gases, which allows for a demonstration of sources of uncertainty in the model. Third, a general form of the gap conductance model is optimized by combining both data sets. Difficulties arise due to: (i) inaccurate estimation of contact characteristics (e.g., number of solid contacts, deformation mechanism of surface irregularities, contact shapes) that are different for each experimental setup; (ii) the non-physical ratio of temperature jump distance to the gap distance for postulated model function form; (iii) an insufficient description of the appropriate heat transfer regime; and (iv) the pressure dependence of thermal conductivity for inert gases aside from helium. Lastly, a general model is optimized by setting the temperature jump distance at the wall to zero, which reduces possible uncertainties. This final analysis results in a more accurate prediction of the available experimental data. The Associated parameter uncertainty of the model is estimated by performing uncertainty propagation. Overall, the optimized model results in a larger gap conductance with significantly reduced error.