Development and validation of the premixing-phase model in the three-dimensional fuel–coolant interaction code SEINA-3D

Abstract

Premixing during a fuel–coolant interaction determines the three-dimensional melt–water–vapor state available to a subsequent steam explosion. This study develops and validates the premixing-phase model of the three-dimensional fuel–coolant interaction code SEINA-3D. The model advances five coupled Eulerian material fields—liquid coolant, vapor, connected melt jet, melt droplets, and melt fragments—in one pressure-based framework. Mixed finite-volume/finite-difference discretization, explicit/implicit advancement, and improved SIMPLEPC pressure coupling form the numerical core; VOF–PLIC interface geometry and continuum-surface-force capillarity resolve the connected interfaces; and interphase exchange, breakup, and phase-change closures couple the fields. The Ransom faucet problem, SPHERIC Test 2, and a three-dimensional rising-bubble benchmark verify the transport, free-surface, and capillary treatments, while FARO L-24 validates the complete model under prototypical corium–water conditions. On the finest SPHERIC grid, the median NRMSE is 0.159 over the four water-level records and 0.139 over the eight pressure records, and the first P1 impact amplitude is reproduced within 10.2 %. The rising-bubble calculation preserves the gas volume and reproduces the published centroid, velocity, sphericity, and directional-size endpoints to within 5 %. In the 15-point FARO pressure comparison, the steam-only calculation has an RMSE of about 0.18 MPa and reproduces the approximately 2.2 MPa peak near 1.6 s; adding the reported L-24 hydrogen source raises the late-time pressure by about 0.48 MPa and quantifies the coupled sensitivity of the steam, noncondensable-gas, and headspace closures. The calculation also yields the material inventories, energy partition, void, azimuthal asymmetry, and a debris-size distribution consistent with the post-test recovery. These results establish the premixing capability of SEINA-3D and provide three-dimensional mixture states for steam-explosion load analysis.