Modeling Approach and Predictions of Boron Mixing in Core Bypass of ESBWR

Computational fluid dynamics (CFD) is used to predict the mixing and transport of a Boric acid solution in the core bypass region of the General Electric (GEH) Economic Simplified Boiling Water Reactor (ESBWR) during an anticipated transient without scram (ATWS) scenario. Transient boundary conditions are synthesized from a set of system code predictions. Full-scale CFD predictions are completed using a representative geometry. Symmetry is used to reduce the domain to 1/8th of the core bypass. Forty-five million finite volume cells are used to define the computational domain inside the core barrel which is made up of the small passages between the fuel channels and an outer peripheral open space. The size of the model posed special challenges in the FLUENT CFD model development and execution. The solution required a large number of iterations with reduced solver under-relaxation factors to ensure stable convergence. Steady-state predictions were completed to quantify the sensitivity of the results to variations in selected boundary conditions. These results show good transport of the Boron into the inner regions of the core bypass with no significant sensitivity of the result to changes in the distribution of flow from the top surface. The results also showed no significant impact to changing the specific gravity of the incoming jets from 1.05 to 1.0. Only a significant change in the leakage rate from the bypass and/or a change in the mass flow rate of injected Boron solution were able to impact the distribution and quantity of the Boron in the core. These results are in line with expectations. The predictions indicate efficient penetration of the Boron into the interior regions of the core bypass during an ATWS.