A Hybrid Numerical Framework for the TRR Ventilation Stack: CFD-Based Sampling Height Determination in Accordance with ISO 2889 and RELAP5 Hydraulic Benchmarking

This study presents a numerical framework for evaluating and qualifying radioactive effluent sampling systems innuclear facilities, focusing on the Tehran Research Reactor (TRR) ventilation stack under both normal andemergency conditions. Computational Fluid Dynamics (CFD) simulations using ANSYS Fluent and the ReynoldsStress Model (RSM) are employed to analyze flow behavior, species mixing, and aerosol transport to rigorouslyassess compliance with ISO 2889. The CFD results indicate that the optimal representative sampling heights are6.5 m for normal ventilation and 2.5 m for emergency conditions, ensuring improved flow uniformity. Analysis ofthe coefficient of variation (COV) for velocity, tracer gas, and aerosol particles, along with cyclonic flow angles andconcentration distributions, confirm satisfactory agreement with ISO 2889 criteria at these locations. In addition,a novel methodology is developed to couple 3D CFD with the 1D RELAP5 system code. Rather than using RELAP5to assess local mixing or sampling representativeness, high-fidelity CFD results are utilized to derive accurate formloss coefficients (K-factors) for the complex geometries of the ventilation system. These derived coefficients arethen integrated into the RELAP5 model to evaluate the large-scale bulk hydraulic behavior of the stack. Thisframework provides a practical approach for nuclear safety analysts who need to model ventilation systems usingcomputationally efficient 1D codes without running computationally expensive 3D simulations.