Numerical simulation on LMR molten-core centralized sloshing behaviors with single/multi-phase smoothed particle hydrodynamics based on novel density formulation

The Smoothed Particle Hydrodynamics is one of the best-known mesh-free particle method based on Lagrangian framework that can easily handle various types of physics because of its simplicity in expressing and solving governing equations. Due to its Lagrangian nature, it is capable of simulating free surface liquid motion and highly deformable geometry without any surface treatment. It is also effective for multi-fluid or multi-phase flow and phase change in which fluids of various densities coexist in calculation domain. In terms of severe accident of nuclear power plant, this particle-based method can be a good additional alternative to the mesh-based Eulerian methods for certain phenomena containing free surface flow, multi-phase flow or non-linear deformation such as tsunami, molten corium behaviors, etc. The sloshing motion of fluid is a typical free surface flow which occurs often also in the nuclear engineering field. Specifically in transient phase of core disruptive accident of liquid metal reactor, a neutronically active multiphase pool can be formed which is composed of solid fuel, molten fuel, refrozen fuel, fission gas, fuel vapor, solid steel particles, and so on. In this complex configuration, abrupt pressure build-up due to the local vapor generation or local power excursion can initiate so-called centralized sloshing which has a potential for energetic re-criticalities of fuel, depending on initial conditions of molten pool and flow disturbances. In this study, the 3D single/two phase centralized sloshing behavior has been simulated using GPU parallelized in-house SPH code. The original SPH density and continuity equations are re-formulated in terms of normalized densities in order to handle two-phase flow with high density ratio. Using this approach, maximum sloshing height and arrival time in various conditions are calculated with single/two phase SPH solver, and compared with the benchmark experiments. The results of SPH simulations show good agreement with the benchmark experiments both in qualitative and quantitative manners, especially for the high resolution simulation. Also, it has been identified that the two-phase SPH simulation best predicts the sloshing height since it can prevent the local numerical errors from particle deficiency in sloshing peak and gas trapping motions.