Recent knowledge from an experimental investigation on self-leveling behavior of debris bed

Cheng, S.; Yamano, Hidemasa   ; Suzuki, Toru; Tobita, Yoshiharu; Nakamura, Yuya*; Takeda, Shohei*; Nishi, Shimpei*; Zhang, B.*; Matsumoto, Tatsuya*; Morita, Koji*

To confirm the mechanisms of self-leveling behavior, several series of experiments were elaborately designed and performed in recent years under the constructive collaboration between Japan Atomic Energy Agency and Kyushu University. This paper summarizes the recent knowledge obtained from the newly developed large-scale experiments using gas-injection to simulate coolant boiling. Compared to previous investigations, it can cover a much wider range of gas velocities (presently up to a flow rate of around 300 L/min). The experiments were conducted in a cylindrical tank, in which water, nitrogen gas and different kinds of solid particles, simulate the coolant, vapor and fuel debris, respectively. Based on the quantitative data obtained, influence of various experimental parameters, including gas flow rate, water depth, particle size as well as particle density on the leveling was checked and compared. Moreover, with the help of dimensional analysis technique, a set of empirical correlations to predict the self-leveling development depending on particle size, particle density and gas injection velocity was proposed and validated over current conditions.