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.

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

“…This is because the gas was injected into the bed through porous metallic plates, which have a relatively large flow resistance. A detailed description of the selfleveling experiments is presented in our previous studies [13,16].…”

“…In our previous studies [13,15], we introduced the inclination angle of the bed to quantitatively represent the overall shape change of the particle bed associated with leveling. Considering the characteristics of the leveling behavior observed in our experiments, the following empirical model was proposed for the time variation of the inclination angle:…”

“…The dimensionless terms u g /u t , μ l u t /σ l and (ρ p − ρ l )/ρ l on the right side of Equations (5) and (6) represent the intensity of gas flow, the properties of fluid-particle interactions and the buoyancy effect on particles, respectively. The above empirical model for bed inclination angle has been applied to several series of self-leveling experiments performed under various conditions of gas bubbling (depressurization boiling, conventional pool boiling, and gas injection) and its intensity, bed geometry (rectangular and cylindrical systems), and particle properties (size and density) [13,15]. In these comprehensive studies, it was successfully demonstrated that the proposed empirical model has wide applicability over various conditions for predicting the transient leveling development observed in experiments using spherical particles.…”

“…In the previous model for bed inclination angle, the specific time t 0 in Equation 4was the maximum time period, during which the change in the inclination angle was measured identically in all experimental cases. For example, it was set to 100, 180, and 300 s for the self-leveling experiments using depressurization boiling in a cylindrical system [13], gas injection in a cylindrical system [17], and gas injection in a rectangular system [13], respectively. Therefore, R(t 0 ) does not necessarily correspond to the inclination angle of the particle bed after self-leveling is fully developed.…”

“…To further clarify the mechanisms underlying selfleveling behavior, we conducted several series of experiments using simulant materials in collaboration with the Japan Atomic Energy Agency (JAEA) and Kyushu University in Japan [11][12][13][14][15][16][17]. In these experiments, depressurization boiling, conventional pool boiling and gas injection were used to simulate sodium boiling.…”