Experimental study and empirical model development for self-leveling behavior of debris bed using gas-injection

Abstract

Studies on the self-leveling behavior of debris bed are crucial in the assessment of core disruptive accident (CDA) that could occur in sodium-cooled fast reactors (SFR). To clarify the mechanisms underlying this behavior, several series of experiments were elaborately designed and conducted within a variety of conditions in recent years, under the close collaboration between Japan Atomic Energy Agency (JAEA) and Kyushu University (Japan). The current contribution, including knowledge from both experimental analyses and empirical model development, is focused on a recently developed comparatively larger-scale experimental facility using gas-injection to simulate the coolant boiling. Compared to the previous investigations, this facility can achieve a much wider range of gas velocities (presently up to a flow rate of around 300 L/min). Based on the experimental observation and quantitative data obtained, influence of various experimental parameters, including gas flow rate (~ 300 L/min), water depth (180 mm and 400 mm), bed volume (3 ~ 7 L), particle size (1 ~ 6 mm), particle density (beads of alumina, zirconia and stainless steel) along with particle shape (spherical and non-spherical) on the leveling is checked and compared. As for the empirical model development, aside from a base model which is restricted to predictions of spherical particles, the status of potential considerations on how to cover more realistic conditions (esp. debris beds formed with non-spherical particles), is also presented and discussed. This work, which gives a large palette of favorable data for a better understanding and an improved estimation of CDAs in SFRs, is expected to benefit future analyses and verifications of computer models developed in advanced fast reactor safety analysis codes.

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].…”

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

“…According to this method, particle sphericity can be determined by the well-known Ergun correlation [21]. The pressure drop measurement experiment is explained in detail in our recent papers [16,17].…”

“…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.…”

“…The initial water level in the tank was adjusted to 180 mm. Under this condition, water convection flow in the pool is comparatively less dominant than in self-leveling development [17]. The outer quadrate tank was also filled with water to improve visual observation inside the cylindrical tank.…”