[en] In recent years, a new 4. generation of nuclear reactors is under development. The most promising candidate to fulfil the requirement in terms of sustainability, safety, economic competitiveness, reduced nuclear waste production and proliferation resistance is the Sodium-cooled Fast Reactor concept. In this context, the ASTRID project is launched at CEA since 2009. The ASTRID project involves large R and D investment aiming to demonstrate on an industrial scale the relevance and performance of innovative technological breakthroughs. One of these breakthroughs is an innovative severe accident mitigation strategy developed for the ASTRID reactor CFV-V3 core design. The mitigation is achieved by implementing special design mitigation transfer tubes into the core and an in-vessel core catcher. The transfer tubes are envisaged to evacuate the mixture of reactive molten materials from the core region and thus decrease the probability and amplitude of further energetic power excursions. For the pre-conceptual phase of French Generation IV SFR program, it is necessary to have a robust demonstration of the performance of the mitigation devices in order to reduce uncertainties influencing the safety assessment. The theoretical demonstration is currently based on best-estimate calculations with code SIMMER. Previous reactor calculations for the ASTRID design on severe accident scenarios with mitigation strategy have demonstrated that the discharge of degraded core inventory via transfer tubes may be efficient. On the other hand, SIMMER simulations highlighted uncertainties linked to the degraded core motion, in particular solid debris components inside the transfer tubes. The consideration of solid particles is of high importance in CFV concept where the low energetic natural behavior of the core prevents massive fuel melting and enhances the production of fuel debris from pellet fragmentation. In this context, the thesis focuses on the relocation mechanism of the degraded core inventory via transfer tubes in the frame of the SIMMER code. The objective is to improve SIMMER modelling of particle-size solid debris dynamics, in order to predict the scenario of molten/degraded core discharge with a higher confidence and to contribute therefore to the safety evaluation of future generation SFRs. The thesis work constitutes the development and validation of a comprehensive modelling set for the dynamics of dense particulate flows in the SIMMER-V code. Firstly, the general context in which the thesis work has been defined is exposed. Secondly, we focus on the localized physical phenomena taking place during the discharge process via the transfer tube and its current state-of-the-art numerical modelling by SIMMER code. In contrast to the state-of-the-art approach, the main outcomes of the bibliographic review around solid particle dynamics are presented. Concluding on this, the developed models (inspired by granular flow theory correlations) and their numerical implementation into the SIMMER code environment are introduced. The impact of such models is demonstrated through comparison with the original SIMMER particle approach through simplified test cases for which analytical or empirical solutions exist. After, the performances of the new models are studied for an integral effect test together with a sensitivity study highlighting the influence of uncertain modelling parameters. Then we return to the initial motivation of the thesis work about the simulation of the whole reactor in case of a reference accidental scenario. Following the entire transient evolution, the mitigation performance applying the newly developed particle dynamics models is assessed. Lastly, the conclusions are summarized, and perspectives for future research on the continuation of this work are outlined in the seventh chapter. (author)