Entrainment and deposition modeling of liquid films with applications for BWR fuel rod dryout

While best estimate computer codes provide the licensing basis for nuclear power facilities, they also serve as analytical tools in overall plant and component design procedures. An ideal best estimate code would comprise of universally applicable mechanistic models for all its components. However, due to the limited understanding in these specific areas, many of the models and correlations used in these codes reflect high levels of empiricism. As a result, the use of such models is strictly limited to the range of parameters within which the experiments have been conducted. Disagreements between best estimate code predictions and experimental results are often explained by the mechanistic inadequacies of embedded models.; Significant mismatches between calculated and experimental critical power values are common observations in the analyses of Boiling Water Reactors (BWR). Based on experimental observations and calculations, these mismatches are attributed to the additional entrainment and deposition caused by spacer grids in BWR fuel assemblies. In COBRA-TF (Coolant Boiling in Rod Arrays-Two Fluid); a state of the art industrial best estimate code, these disagreements are hypothesized to occur due the absence of an appropriate spacer grid model.; In this thesis, development of a suitably detailed spacer grid model and integrating it to COBRA-TF is documented. The new spacer grid model is highly mechanistic so that the applicability of it is not seriously affected by geometric variations in different spacer grid designs.; COBRA-TF (original version) simulations performed on single tube tests and BWR rod bundles with spacer grids showed that single tube predictions were more accurate than those of the rod bundles. This observation is understood to arise from the non-availability of a suitable spacer grid model in COBRA-TF.; Air water entrainment experiments were conducted in a test section simulating two adjacent BWR sub channels to visualize the flow behavior at spacer grid locations. Portions of a prototypic 8 x 8 spacer grid and metal tubes simulating fuel rods were used for these experiments. Although the test section was unheated, the flow conditions were chosen to closely depict the normal operating conditions of a BWR core subchannel. Results confirmed that entrainment at spacer grid locations is significant. Subsequent COBRA-TF simulations mimicking enhanced entrainment and deposition at spacer grid locations showed marked effects on dryout prediction. This indicates that incorporation of a proper spacer grid model to COBRA-TF could in fact improve the overall code performance.; A method of problem analysis by decoupling the overall spacer grid hydrodynamic phenomenon into several smaller individual mechanisms is used in this work. This document includes the initial conceptualization of entrainment and deposition mechanisms, their mathematical formulation and results obtained after integrating them to COBRA-TF.