Multi-resolution modeling of subassembly pin bundles for advanced fast reactor safety simulations

Comparisons of thermal-hydraulic performance and temperature distributions between two different modeling fidelities (sub-channel and RANS) are presented for the case of a 217-pin fuel assembly. While RANS-based methods are capable of resolving the details of sub-channel cross flows, the domain size to which they are applicable is somewhat limited. Sub-channel models, on the other hand, are capable of significantly larger solution domains in both space and time and can resolve the whole-core behaviors needed for transient analyses. Comparisons between the two methods show that while the sub-channel model is able to capture the overall temperature distribution in a pin bundle, it fails to resolve axial variations and edge channel effects caused by wire-wrap pin spacers. These observations emphasize the need for a multi-resolution approach. One where higher-fidelity models provide accurate, geometry-dependent parameters to lower-fidelity models that can capture much larger domains in both space and time. Conversely, lower-fidelity models can provide realistic, whole-plant boundary conditions to higher-fidelity models where the characterization of detailed, multi-physics phenomenon has critical importance.