Higher resolution radial reflector modeling capabilities in MPACT

To provide high fidelity multiphysics simulations of nuclear reactors, the Consortium for Advanced Simulation of Light Water Reactors (CASL) is developing the Virtual Environment for Reactor Applications (VERA). MPACT, which is the primary deterministic neutron transport solver, employs the 2D/1D method to solve 3D problems, leveraging 2D method of characteristics (MOC) for radial transport and ID-NEM-P3 for axial transport. To this point, full- and quarter-core MPACT cases have used lower fidelity radial reflector models consisting of an explicit baffle representation, an assembly-width of moderator along the core periphery, and pin-wise representations of pad and vessel components. Building off this previous work, higher resolution capabilities have been added by incorporating a Cartesian subgrid to pins in the reflector, allowing for a more faithful representation of the cylindrical structural components. Two test problems are assessed in this work. The first is a 2D quarter core model of Watts Bar Unit 1 with a representative cycle of depletion. Comparisons are shown to the previous lower fidelity model, where it is shown that the use of higher resolution capabilities do not result in substantially different pin power or eigenvalue results. The second case shown is a 2D model representative of the NuScale small modular reactor (SMR) reflector configuration. This design has interesting characteristics, such as coolant channels in the reflector, and assessments are made highlighting the importance of their detailed modeling.