Development of a Mean Bubble Size Approach to Improve Pool Scrubbing Swarm Region Modeling

Abstract

Development of a Mean Bubble Size Approach to Improve Pool Scrubbing Swarm Region Modeling Erol Biçer Department of Energy System Engineering Graduate School of Engineering Seoul National University Pool scrubbing is a critical safety mechanism for mitigating severe accidents in conventional and advanced nuclear reactors, such as Small Modular Reactors (SMRs). The process involves complex interactions between hydrodynamic, thermal, and chemical processes as gas bubbles containing fission products pass through a water pool, effectively trapping radioactive substances and preventing their escape. Despite its importance, current pool scrubbing codes have significant limitations due to simplifying empirical assumptions, ultimately leading to underestimation of the Decontamination Factor (DF) - a figure-of-merit for measuring the retention capacity of the pool scrubbing process. One major limitation is the inadequate representation of bubble dynamics in pool scrubbing codes. The typical approach involves using a constant bubble diameter in the swarm rise region, even though the DF exponentially depends on the surface area-to- volume ratio of the bubbles. Furthermore, parameters such as the effective residence time of aerosols and net deposition velocities are based on factors (such as aspect ratio and rise velocity) that are influenced by the size of individual oblate bubbles. Therefore, using a constant bubble size for all conditions during the swarm rise region undermines the usability of pool scrubbing codes. Recent findings from The Integration of Pool Scrubbing Research to Enhance Source-term Calculations (IPRESCA) expert group highlighted the importance of improving pool scrubbing modeling by incorporating advanced correlations into existing models of bubble hydrodynamics with the help of Computational Fluid Dynamics (CFD) to enhance pool scrubbing estimations. Acknowledging the research gaps, this study aims to enhance the safety and efficiency of advanced nuclear reactor containment systems by improving the modeling of the pool scrubbing swarm rise region through a mean bubble size approach. The research addressed limitations and unresolved issues by developing a framework to simulate gas-liquid flow under pool scrubbing conditions. This framework captures the three-dimensional effects and enables the generation of additional data to complement the existing limited range of experiments for mean bubble size correlation. Tailored solutions for bubble dynamics were integrated into the two- fluid model, including the Interfacial Area Transport Equation (IATE) model to calculate bubble sizes, bubble-induced turbulence modeling, and closure models to reduce the heavy computational load faced by previous studies. Considering the diameter decrease along the flow direction, a correlation for bubble size was developed and validated using the available experimental data and existing correlations. This correlation is implemented into the pool scrubbing code SPARC- 90, resulting in a more mechanistic bubble size model that can also be adopted in other pool scrubbing codes. The multidimensional two-fluid framework incorporated turbulence and phase interaction modeling, accurately representing bubble sizes under pool scrubbing conditions. Predicting the flow variables and bubble diameters accurately enabled the research to extend the scarce experimental data for a broader range. As a result, the newly proposed correlation for bubble size showed good predictive accuracy, with an absolute average error of approximately 5.07%. The correlation is validated against various experimental datasets and compared with other existing correlations, demonstrating its applicability across various conditions and, finally, implementing the new bubble size model into SPARC-90 improved DF predictions. Validation from FNC, KAD, and LACE-ESPANA experiments resulted in average DF increases of 12.06%, 28.63%, and 44.83% correspondingly, showcasing an overall average improvement of 35.89% and a 30% reduction in the Underestimation Factor (UF) across all pool scrubbing experiments. In conclusion, this research has addressed the limitations of existing pool scrubbing models by developing a new bubble size model for the swarm region after modeling a multidimensional two-fluid gas-liquid flow and constructing a bubble size correlation. Keywords Pool Scrubbing, Swarm Rise Region, Decontamination Factor, Bubble Dynamics, Turbulence Modeling, SPARC-90 Code, IATE, CFD, Mean Bubble Diameter Correlation. Student Number: 2017-33400