Experimental Evaluation of Debris Bed Characteristics in Particulate Debris Sedimentation Behaviour

Abstract

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, ZrO2 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.

A new empirical model for self-leveling behavior of cylindrical particle beds

“…In addition, the definition of the initial inclination angle used to normalize the transient angle is ambiguous, although the values used in the analysis vary within a rather limited range (around 18 • -20 • ). According to our study on particle bed formation behavior [3], however, the initial inclination angle or bed height should depend heavily on particle settling conditions as well as particle properties.…”

“…A i , B i , C i Empirical constants (i = 1 and 2 in Equations (5) and (6), respectively) a i ∼ f i Empirical constants (i = 1 and 2 in Equations (8) and (9), respectively) A p Actual surface area of the particle (m 2 ) A S Surface area of a sphere having the same volume as the particle (m 2 ) Ar Gas-phase Archimedes number (= ρ g (ρ p − ρ l )gd 3 p /μ…”

“…According to our study on debris settling and bed formation behavior [3], the debris bed is typically formed in a conical shape with a convex mound or in a semi-conical shape with a concave mound. However, coolant boiling caused by decay heat in the fuel debris could ultimately lead to leveling-off of the debris bed.…”

Numerical Simulation of the Solid Particle Sedimentation and Bed Formation Behaviors Using a Hybrid Method

“…Experimental investigations with simulant materials composed of homogeneous solid particles were conducted recently [12,13] and, in connection with this study, an exploration was performed in our previous studies for mixtures of particles with different properties [14,15]. These experimental analyses discovered the nature of the shape characteristics of the particle bed and its dependence on some important factors, such as particle density and diameter as well as the diameter and length of the nozzle, which is used to discharge solid particles into the water pool [12][13][14][15]. As the experimental process consumes a high cost and long duration, the evaluation of such sedimentation behaviors is performed knowing the limited scope of reproducibility.…”

“…The objective of the present numerical study is to demonstrate the practicality of the hybrid method to particle sedimentation and bed formation behaviors of mixed particles as well as homogeneous ones. In the present study, three-dimensional simulations were performed and compared with results obtained in a series of particle sedimentation experiments in which gravity-driven solid particles [12,15] are discharged into a quiescent cylindrical water pool. Although the experiments using simulant materials did not reproduce the particle sedimentation and bed formation behaviors that are expected to occur under reactor accident conditions, the hybrid method will be validated for fundamental characteristics of the behaviors, which were measured in the experiments performed under controlled conditions.…”