Assessment of a Two-Layer Material Point Method for Three-Dimensional Water-Granular Sloshing

Molten-pool sloshing is a critical safety concern during Core Disruptive Accidents (CDAs) in Sodium-cooled Fast Reactors (SFRs), as the inward collapse of the liquid fuel may lead to energetic neutronic recriticality. The presence of solid particles further complicates the flow physics through interphase drag, particle wall friction, and granular yielding. This study assesses a two-layer Material Point Method (MPM) for three-dimensional water-granular sloshing using pure-water comparisons and four centered water-acrylic-particle configurations from the KfK 5090 experimental campaign. The two phases use superposed momentum grids with implicit drag coupling, APIC transfer for water, and ASFLIP transfer for the granular phase. Pure-water calculations examine regional pressure sensitivity and free-surface evolution in centered and offset geometries. Closure parameters are selected using the layered saturated case D1PI-1 and are subsequently held fixed for the fully mixed case D1PI-2 and the dry-annulus cases D1P-1 and D1P-2. In D1PI-1, the five reported standardized residuals have magnitudes no greater than 1.10 under the adopted normalization, although the ensemble-mean wall and center heights are overpredicted and the center rebound exhibits appreciable realization variability. The other configurations expose substantial discrepancies in wall run-up, supported water arrival, and, for the thicker dry annulus, center rebound. Coupled grid-sensitivity studies for D1PI-1 and D1P-1 show comparatively stable peak timing but appreciable sensitivity of rebound height, and the 4 mm grid is retained as an engineering baseline. These results document calibration performance and limited cross-configuration transferability, and motivate further investigation of transient momentum exchange, segregation, and wetting before application to severe-accident conditions.