Experimental evaluation of debris bed characteristics in particulate debris sedimentation behaviour

After a core-disruptive accident (CDA) in sodium-cooled fast reactor, degraded core material can form debris beds on core-support structure or in the lower inlet plenum of the reactor vessel. This paper reports an experimental evaluation on debris bed formation characteristic in CDA. Investigation of debris bed characteristic during debris sedimentation on core catcher plate is important from recriticality concern and also from cooling considerations to ensure the safety of the reactor main vessel in CDA. In the present study, to evaluate this characteristic, a series of experiments was performed by gravity driven discharge of solid particles as simulant debris from a nozzle into a quiescent water pool in isothermal condition at room temperature. The discharged solid particles with a maximum amount of 10 L finally accumulate on the debris tray, forming a bed with a convex or concave mound depending on the experimental parameters. The nozzle diameter, nozzle height, debris density, debris diameter and debris volume are taken as the experimental parameters. Currently, three types of spherical particles, namely Al₂O₃, ZrO ₂ and stainless steel (SS) with diameter of 2, 4, or 6 mm are employed to study the effect of key experimental parameter on debris bed mound shape. In addition, 2 mm non-spherical particles of SS were also utilized to investigate the effect of debris shape on altering mound profile. In experimental investigation with different debris volume, both developing and fully developed mound shapes were observed based on the effect of debris size, density and nozzle diameter. In this study, the investigated particle velocity of main stream settling particles was found increasing with nozzle diameter, which caused a decrement of mound height with an increment of mound dimple area. In nozzle height effect, shrinking of concavity on mound shape was observed with decreasing manner of impact velocity while height is reducing. From the visualization results of the experimental investigations, transformation of bed shape from convex to concave was observed with increasing repose angle incase of 4 mm Al₂O₃ particle. In general, transformation of bed shape was observed by increasing either nozzle diameter or particle density for all particle type. The present results could be useful to validate numerical models and simulation codes of particulate debris sedimentation.