Modeling nuclear fuel assemblies through porous zones in a Small Modular Reactor: fluid dinamic considerations

This work aims to qualify the use of porous zones for representing fuel assemblies of a proposed SMR reactor in numerical models in other to reduce the computational demand required to study these structures. It employs computational fluid dynamics (CFD) methods to calculate the conservation equations of mass, momentum, and energy within a control volume. Initially, a detailed geometry of the fuel assembly was created and used for isothermal simulations. Based on the results of pressure drop and velocity, equations were used to calculate the coefficients of porosity and pressure drop of the system. These were then utilized to configure a second geometry, consisting of hexahedros divided into thirteen sub-regions according to their cross-sectional area, each having different porosities and pressure drop coefficents. Finally, the results of the two simulations were compared to verify their convergence to allow the use of the porous geometry. The outcomes suggests that, for models with a control volume significantly larger than a single fuel assembly, such as a complete nuclear reactor vessel, the use of porous zones is advantageous, as the variations in average velocity and pressure drop along the length of the structure are small, with the maximum axial velocity variation of -10.99%. However, if the objective is to conduct a more detailed analysis of the entire assembly, this strategy is not recommended, since some specific aspects of fluid behavior are not well capturated, such as radial velocity differences.