Development of a dynamic-mesh porosity transport model for multi-dimensional fuel performance codes

The porosity redistribution within nuclear fuel pellets exposed to high power ratings plays a critical role in the thermo-mechanical behavior of fast reactor fuel. Traditional fuel performance codes predict porosity migration through advection-dominated transport equations often assuming a fixed geometry, and limiting their accuracy in asymmetric conditions. A novel dynamic-mesh porosity migration model has been developed to address these limitations. For verification and demonstration purposes, the model has been implemented in OFFBEAT, a multidimensional OpenFOAM-based fuel performance code. The solver dynamically adjusts the fuel pellet geometry to model the evolution of the central hole caused by pore migration. Mesh quality is preserved throughout the simulation by means dynamic-mesh algorithms involving the resolution of a mesh-motion equation to diffuse the displacement imposed at the mesh boundaries to all the domain points. The methodology incorporates modifications to the traditional porosity transport equation, correcting the advective fluxes in the governing equations to account for mesh points movement. A simple mechanistic model to determine the hole expansion velocity as a function of the local porosity, pore velocity and inner fuel radius is proposed. The model's parameters are calibrated using open literature experimental data, demonstrating the solver capability to predict central void diameters within acceptable discrepancy. The dynamic-mesh solver shows good accuracy in predicting off-centered hole formations and aligns well with post-irradiation examination data. This new approach preserves the foundational principles of existing porosity migration models while offering enhanced flexibility and accuracy in asymmetric heat transfer scenarios.