The purpose of the present study is to evaluate and improve the capability of the SIMMER code to demonstrate severe accident mitigation in Sodium-cooled Fast Reactors (SFR). Here the numerical representation of the mitigation strategy with controlled material transfer is carried out with SIMMER. By using the advanced multi-phase modelling option of the code, the particle-size solid debris are treated with an independent momentum field. This modelling choice is encouraged for the simulation of Unprotected Loss Of Flow (ULOF) type transients in French SFR core design where low enthalpy degraded fuel is expected to manifest in solid fragments. Applying this new particulate approach in SIMMER code requires the reevaluation of current SIMMER fluid dynamics modelling. In this context, this paper assesses the applicability of SIMMER momentum models to account for the peculiar behaviour of a purely particulate phase. We present the first improved model of interphase momentum transfer and a sensitivity study on solid particles-wall stress models. The assessment is performed through the investigation of simplified test cases that can be associated to reactor scenarios during severe accident mitigation. The test cases show that the new modifications in the SIMMER code bring significant improvements in predicting the discharge behaviour of degraded core mixture.

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