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

Abstract

During the material relocation phase of core disruptive accidents in sodium-cooled fast reactors, the rapid quenching and fragmentation of molten materials discharged from the reactor core into the lower plenum region can lead to the formation of debris beds. Coolant boiling may lead to leveling of the mound-shaped beds, which changes both the beds' coolability with decay heat in the fuel and the neutronic characteristics. In this study, a series of experiments using simulant materials were performed to develop an experimental database of self-leveling processes of particle beds in a cylindrical system. To simulate the coolant boiling in the beds in the experiments, a gas injection method was used to percolate nitrogen gas uniformly through the base of a bed with a conical-shaped mound. Time variations in bed height during the self-leveling process were measured for different particle sizes, densities and sphericities, and gas injection velocities. Using a dimensional analysis approach, a new model was proposed. This model correlates the experimental data on transient bed height with an empirical equation using a characteristic time for self-leveling development and an equilibrium bed height. The proposed model reasonably predicts the self-leveling development of particle beds.

Debris Bed Self-Leveling Mechanism and Characteristics for Core Disruptive Accident of Sodium-Cooled Fast Reactor: Review of Experimental and Modeling Investigations

“…→To verify observations from depressurization to traditional boilling condition [33,[38][39][40]42,43,46] Provide evidences, verifications and validations Model establishment [14,40] Predictive Modeling Investigations Model establishment [32][33][34]37,38] Model establishment [42,43,46] Model verification progression, were identified. In addition, several bottomheated experiments carried out at Indira Gandhi Centre for Atomic Research (IGCAR) further validated the occurrence of self-leveling under low heat fluxes (lower than the dry-out heat flux) and the imperfect horizontally level-off for largedensity particles [30].…”

“…en, through the introduction of a correlation factor estimating the particle-shape effect, the applicable range of this model was further expanded for non-spherical particles [38], so that the extensibility of this model for covering more complicated and more realistic conditions was revealed. While noticing the fact that the particle mound/bed height is also an important factor for the ability of cooling assessment of debris bed, the establishment of another predictive model focusing on the variation of the particle mound height was attempted for single-size particle (including spherical and non-spherical) cases to attain more useful understandings on the self-leveling development [46]. However, since it was further confirmed that the mixedsize particle characteristics could play a remarkable role in the progression of the self-leveling process, a predictive model for the variation of the particle bed height was subsequently proposed for being applicable in single-size and mixed-size particle cases [42,43].…”

“…Variation. Considering that the particle mound and bed heights were also the key influencing factors for the ability of cooling of debris beds, aimed at obtaining more valuable predictions for evaluating the debris bed self-leveling behavior, another model, which is applicable to predict the mound-height variation, was established in accordance with the 3D gas-injection experiments using single-size particles with shapes of the sphere and non-sphere [46]. While further noticing from the experiments that the heights of the lower part of particle beds (i.e., the volume of particle bed) could also play a significant role in the self-leveling development, a model for estimating the transient bed height was also developed on the basis of the gas-injection experiments for single and mixed particles [43].…”

“…While further noticing from the experiments that the heights of the lower part of particle beds (i.e., the volume of particle bed) could also play a significant role in the self-leveling development, a model for estimating the transient bed height was also developed on the basis of the gas-injection experiments for single and mixed particles [43]. e model for describing the time variation of the mound height H m (t) was developed as [46] H…”

“…where H m,level is the completely leveled-off mound height (i.e., the height of mound at t � ∞), and τ stands for the characteristic time constant, which was dependent on the rate of self-leveling progression. Suggesting that the fluid and particle properties and the gas velocity should noticeably influence the values of H m,level and τ, the following correlations were determined for H m,level and τ [46]:…”