Study on Modeling of Nuclear Fuel End Drop during a Hypothetical Accident

Single pin models of nuclear fuel have been used to model fuel assemblies on the assumption that all pins in an assembly deform in the same pattern. These single pin models are detailed models that use shell elements to represent the fuel cladding. It is increasingly significant to model the entire fuel assembly in order to capture separate deformation patterns for each individual nuclear fuel pin during a hypothetical accident. However, modeling each individual fuel pin of an entire fuel assembly using shell elements can result in models with a large number of elements that may require impractically large CPU times to run. Therefore, using beam elements to model nuclear fuel could be a computationally efficient option. Both single pin shell model and single pin beam model are developed using LS-DYNA to analyze two end drop scenarios for 15x15 PWR fuel: 19 ft drop (linear behavior) and 30 ft drop (non-linear behavior) to provide insights on the appropriateness of modeling fuel pin using beam elements. It is shown that the difference in the maximum/minimum total axial strains calculated with both models is about 3% and 14% for 19 ft end drop and 30 ft drop, respectively. An advantage of the single pin shell model is that it permits consideration of internal pressures and may capture local buckling. Therefore, use of beam models needs to take into account the possible significance of internal pressurization and local buckling for the problem to be analyzed.