A new next generation safety analysis code, COMPASS, is designed based on the moving particle semi-implicit (MPS) method to provide local information for various key phenomena in core disruptive accidents of sodium-cooled fast reactors. In FY2006, improvement of basic fluid dynamics models for the COMPASS code was carried out and verified with fundamental verification calculations. In order to improve the numerical stability of MPS simulations, a fully implicit pressure solution algorithm was introduced instead of the two-stage MAC algorithm originally applied by MPS. With a newly developed free surface model, numerical difficulty caused by poor pressure solutions is overcome by involving free surface particles in the pressure Poisson equation. An improved algorithm was also proposed for surface tension calculation with the continuous surface force (CSF) model applied to the moving particle method. This algorithm provides higher numerical precision with the CSF model by interpolation between moving particles and background mesh. Application of the fully Lagrangian MPS method to solid-fluid mixture flow problems is straightforward. In FY2006, applicability of the MPS method to interactions between fluid and multi-solid bodies was investigated in comparison with dam-break experiments with solid balls. It was found that a modified pressure solution algorithm makes simulation with the passively moving solid model stable numerically. Though characteristic behavior of solids was successfully reproduced by the present numerical simulations, the comparisons with the experimental results showed that interactions between solids and solid-wall should be modeled for more precise simulations. Therefore, the discrete element method will be considered in the next stage.

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