Studies on the self-leveling behavior of debris bed are crucial in the assessment of core-disruptive accident (CDA) that could occur in sodium-cooled fast reactors (SFR). To clarify the mechanisms of this behavior, several series of experiments were elaborately designed and performed in recent years under the collaboration between Japan Atomic Energy Agency (JAEA) and Kyushu University (Japan). This paper presents the recent knowledge obtained from the newly developed large-scale experiments using gas-injection to simulate coolant boiling. Compared to previous investigations, it can cover a much wider range of gas velocities (presently up to a flow rate of around 300L/min). Based on the experimental data obtained, influence of various experimental parameters, including gas flow rate (∼ 300 L/min), water depth (180 mm and 400mm), bed volume (5L, 7L), particle size (2 ∼ 6 mm), particle density (beads of alumina, zirconia and stainless steel) along with particle shape (spherical and irregularly-shaped) on the leveling was checked and compared. In addition, the status of developing empirical model to predict the self-leveling over current setup was also presented. This work, which gives a large palette of favorable data for a better understanding and an improved estimation of CDAs in SFRs, is expected to benefit future analyses and verifications of computer models developed in advanced fast reactor safety analysis codes.

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