Simulation of a Slow Loss-of-Flow Accident in an Open-Pool Research Reactor Using Computational Fluid Dynamics

This study presents a three-dimensional numerical methodology to analyze a slow loss-of-flow accident in the IEA-R1 research reactor, located in São Paulo, Brazil. Numerical simulations were conducted using ANSYS-CFX®, and cladding and fluid temperatures were compared with experimental data available from the International Atomic Energy Agency. Seven turbulence models were individually tested; however, most (e.g. zero equation, eddy viscosity transport equation, Speziale, Sarkar, and Gatski (SSG) reynolds stress model, shear stress transport, and detached eddy simulation) either failed to converge or were computationally prohibitive due to spatial and temporal resolution requirements. Only the standard k–ω model yielded satisfactory performance, offering stable convergence and good agreement with the experimental data in both the forced and natural convection regimes. This performance is attributed to the model’s formulation based on the specific dissipation rate of turbulent kinetic energy and its suitability for resolving the viscous sublayer with a fine mesh (y+≈ 1 and Comax≤ 1).The results indicated flow reversal at 39.3°C at 88.1 s, compared with the experimental value of 38.6 ± 0.5°C at 88 s, and a maximum cladding temperature of 49.1°C at 111 s, in good agreement with the experimental data (46.0 ± 0.5°C at 114 s). The overall deviations were within 7% for temperatures, which falls inside the expected ±20% benchmark range of several system codes (RELAP, MERSAT, CATHARE), confirming the reliability of ANSYS-CFX for complex transient scenarios. Despite a conservative tendency in the temperature prediction, the results demonstrate that ANSYS-CFX is a valuable complement, not a substitute, to system codes, particularly for analyzing localized phenomena under specific timeframes that require high spatial and temporal resolution.