Quasi-static pebble flow, or so-called the very slow pebble flow, in a pebble bed reactor, with evident randomicity and dispersibility, is extremely complex. Improving the knowledge of the movement mechanism of quasi-static pebble flow can be beneficial to the safety of the pebble bed reactor. This study utilizes a phenomenological method and a discrete element method to investigate the interface features of two regions composed of differently colored pebbles. A pseudo-two dimensional experimental setup is established to facilitate the observation of movement of pebble. Then, the DEM simulation is carried out to analyze the further details of particle movement mechanism. To some extent, the two methods are closely related and mutually confirmed. In this study, some special phenomena are observed, such as the non-uniformity, mixing zone, stagnant zones, the propagation of voids, slow flow zone, etc. Moreover, some basic issues on the movement mechanism and characteristics of quasi-static pebble flow are discussed, e.g. the interpretation of force analysis inside the pebble packing, propagation and distribution of voids, formation of equilibrium arches, the effects of stagnant zone on the flow field, and so on. These characteristics of the quasi-static pebble flow are very different from the continuous flow, and the understanding of these characteristics is very helpful for the design and analysis of pebble bed reactors.

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