In a core disruptive accident of a fast breeder reactor, the post accident heat removal is crucial to achieve in-vessel core retention. Therefore, a series of experiments on bubble behavior in a particle bed was performed to clarify three-phase flow dynamics in debris bed, which is essential in heat-removal capability, under coolant boiling conditions. Although in the past several experiments have been carried out in the gas-liquid-solid system to investigate the bubble dynamics, most of them were under lower solid holdup (? 0.5), where the solid-phase influence may be not so important as much as the liquid phase. While for this study, the solid holdup is much higher (> 0.55) where the particle-bubble interaction may be dominated. The current experiment was conducted in a 2D tank with the dimensions of 300 mm height, 200 mm width and 10 mm gap thickness. Water was used as liquid phase, while bubbles were generated by injecting nitrogen gas from the bottom of the tank. Various experimental parameters were taken, including different particle bed height (from 30 mm to 200 mm), various particle diameter (from 0.4 mm to 6 mm), different particle type (acrylic, glass, alumina and zirconia beads), and different nitrogen gas flow rate(around 1.75 ml/min and 2.7 ml/min). By using digital image analysis method, three kinds of bubble rise behavior were observed under current experimental conditions and confirmed by the quantitative detailed analysis of bubble rise properties including bubble departure frequency and bubble departure size. This experiment is expected in the future to provide appropriate quantitative data for the analysis and verification of SIMMER-III, an advanced fast reactor safety analysis code. © 2010 by ASME.