Experimental Studies and Empirical Models for the Transient Self-Leveling Behavior in Debris Bed

Abstract

Studies on the self-leveling behavior in debris beds are crucial in the assessment of core-disruptive accidents (CDAs) that could occur in sodium-cooled fast reactors (SFRs). To clarify this behavior, a series of experiments have been performed in which nitrogen gas has been percolated uniformly through a particle bed. In these experiments, solid particles and water contained in a rectangular tank simulate respectively fuel debris and coolant. Based on the data obtained, an empirical model was developed to describe the transient variation in the bed inclination angle during the self-leveling process. Good agreement has been obtained between calculated and experimental values. Verification of the model has been confirmed through detailed analysis of the effects of experimental parameters such as particle size, particle density, and gas flow rate. Its applicability to extended conditions was further discussed by performing modeling simulations and comparing results against experimental data obtained from a larger-scale experimental system that employed a conventional boiling method. With further improvements, the model will be tested under more realistic reactor conditions and is expected to benefit future analyses and simulations of CDAs in SFRs.

An investigation on debris bed self-leveling behavior with non-spherical particles

“…the discrete element method) [14][15][16][17]. However, due to the extremely complex and uncertain nature of the three-phase flow involved in the leveling phenomenon [2,9], empirical approach is still regarded as an attractive and indispensable option at present stage because of its distinct advantage in calculation efficiency. On one hand, with an effective empirical model, experimental database can be expanded (interpolated or extrapolated) with much lower cost.…”

“…However, it should be noticed that up until now, the developed empirical model (hereafter referred to as 'base model') is restricted to calculations of debris beds composed of spherical particles [9,[19][20][21]. Recognizing that in actual reactor accident conditions debris mounds formed with irregularly shaped particles are more commonly encountered; therefore there is a pressing need to check whether the base model is extendable to cover such complex situations.…”

“…On the other hand, the derivation and analyses of empirical approach do provide useful knowledge for computer model improvement and verifications. Focusing on these aspects, originally by applying dimensional analysis technique, a simple empirical approach was successfully advanced based on the experimental data from the quasi-two-dimensional (quasi-2D) leveling test (shown in Figure 3) [9,18]. Motivated by the potential of utilizing this approach at actual reactor accident conditions, recently its wide applicability has been further confirmed through several validation projects involving a great amount of experimental data from various conditions, including difference in bubbling mode (boiling or gas injection), bed geometry, and range of experimental parameters [9,[19][20][21].…”

“…Overall, as illustrated in Figure 3, those experiments can be generally divided into two categories, namely the macroscopic leveling experiments and microscopic flow-regime investigations. Due to the non-transparency of particle beds, the macroscopic leveling experiments were mainly performed to clarify the overall characteristics of leveling [2,3,[8][9][10], namely the role of experimental parameters (such as particle properties and bubbling rate) on its onset and evolution. In those experiments, to simulate the coolant boiling during CDAs, various experimental methods, including depressurization boiling, bottom-heated boiling as well as gas-injection, have been employed.…”

Numerical study on sedimentation behavior of solid particles used as simulant fuel debris

“…Email: zaman@nucl.kyushu-u.ac.jp imental data and no significant insights on debris sedimentation behavior that can reasonably demonstrate the debris bed shape. Noting the importance of the selfleveling characteristics on the heat removal capability of debris beds, our research group performed experimental investigations on self-leveling behavior initiating from a typical shape of debris bed with conical mounds [6,7]. However, to obtain a realistic debris bed shape for a particular scenario of debris sedimentation, the process of debris bed formation has to be clarified taking into account the sedimentation process of the debris as discussed in our earlier paper [8].…”