In the concept of BWR-PB with a relatively low power level, the core region is filled with a large number of coated particles, which are directly cooled by boiling light water. It's significant to understand the two-phase flow characteristics in such a complicated pebble-bed structure. A visualization experiment was carried out to investigate the flow phenomena of steam-water two-phase flow in a pseudo-three-dimensional pebble bed using a high-speed video camera. The pebble bed in the experiment was constructed of hundreds of glass beads and a specially designed stainless heating plate, which was used to simulate the heat-generation of solid particles. Based on our observation, five typical flow regimes were identified to distinguish different phase distribution characteristics: bubbly flow, bubbly-slug flow, slug flow, slug-annular flow and pure annular flow. System pressure, mass flow rate and inlet subcooling were considered as the key influence factors for flow regime transition in the experiment. A flow pattern map for low pressure and low inlet subcooling condition was obtained from the experimental data.
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