Predicting Critical Heat Flux in Narrow Rectangular Channels: A CFD Approach for Subcooled Flow

The PALLAS-reactor is an advanced nuclear reactor designed for producing medical isotopes and for carrying out research on (medical) nuclear technology. RELAP5 is the primary tool for modeling the accident scenarios relevant to the licensing of this reactor. The Groeneveld Look-Up Tables (LUT) are the default model of RELAP5 for estimating Critical Heat Flux (CHF). Literature has shown that the LUT over-predict CHF inside narrow, rectangular channels such as the cooling channels of the core of the PALLAS-reactor. Empirical correlations such as that of Sudo-Kaminaga or its modified version by Kim et al. (SK-Kim) are better suited for these geometries. However, these correlations are typically implemented in one-dimensional (1D) System Thermal-Hydraulics codes whereby the quantities of interest are averaged-out over the control domains. Computational Fluid Dynamics (CFD) could capture three-dimensional effects that could help reducing unnecessary conservatism embedded in some 1D approaches (e.g. Sudo-Kaminaga or SK-Kim correlations). Therefore, CFD is to be employed to support the Deterministic Safety Analyses of the PALLAS-reactor if there are scenarios in which the SK-Kim correlation predicts CHF. Boiling flows inside round or annular vertical channels have been extensively modeled with CFD. However, attempts at modeling CHF in narrow, rectangular geometries are still scarce. Therefore, this work aims at developing and validating a CFD approach for predicting CHF in rectangular channels. Four combinations of turbulence interaction and turbulent dispersion models are investigated; the results are compared against experiments and the SK-Kim correlation; sensitivity analyses are conducted on the turbulence models as well as the mesh. Certain combinations of turbulence interaction and turbulent dispersion models promote the transport of steam towards the center of the channel, thus increasing CHF, while other cause the steam to remain confined at the wall, thereby lowering the CHF. This study provides several CFD options to estimate CHF depending on the required level of conservatism. Estimating CHF in a conservative manner is crucial for licensing the PALLAS-reactor and other reactors of similar design, thereby securing the production of medical isotopes in Europe and the rest of the world.