Three-Dimensional Numerical Simulation of the Release and Migration Behavior of Fission Gas Bubbles in Advanced Reactor Pool Under Serious Accident

Abstract

Three-dimensional numerical study on the release and migration of bubbles in liquid metal pool is carried out for severe accidents in advanced reactors. The fission gas bubble migration will affect the release and distribution of radioactive materials in the molten pool, and then affect the evaluation of the source term in the reactor. The simulations are conducted based on the Volume of Fluid (VOF) method for both water and molten Lead Bismuth Eutectic alloy (LBE). The bubbles are initialized with different diameters or generated by gas injection. The methodology is firstly validated by the simulations of single bubbles rising in water. The simulation results, such as the rising velocity and aspect ratio, of the bubbles with different equivalent diameters agree well with the experiments. Then the model is applied to predict the release and migration behavior of bubbles in molten LBE. It is found that the terminal rising velocity and aspect ratio of single bubbles in LBE have similar profiles to those in water. The rising velocity increases rapidly at first, and then fluctuates within a certain range. The bubbles in molten LBE have similar behavior to typical bubble migration in liquid. At the typical gas injection speed of 1 m/s, the bubble coalescence is observed in molten LBE. The distribution of the terminal rising velocity and equivalent diameter has a bifurcation which is influenced by the bubble wake. The results of the simulations with molten LBE are comparable to theoretical results. This work provides better understanding of the fission gas bubble migration behavior under severe accidents in advanced reactors and will contribute to the safety analysis as well as the design optimization for lead-cooled fast reactors.