In pebble bed reactor, pebbles flow very slowly in the stagnant region, which is defined according to the burn-up level of fuel pebbles. It is not allowed to exist in real reactor, since the stay time of fuel pebbles in these regions goes beyond the burn-up level, which increases the risk of leakage of radiation. This research shows that the stagnant region is related to the geometric parameters of the core and the physical properties of pebbles. Experimental setup has been designed to observe the phenomenon of stagnant region, and analysis based on a phenomenological method has been carried out. The phenomenological method is an approach to study the dense pebble flow by means of investigating the interface features of different areas composed of differently colored pebbles. In addition, additional simulations by the DEM model are in good agreement with the experimental results, which successfully verify the availability of the discrete element method. On the basis of these researches, several key parameters have been investigated through DEM simulations, including height of the experimental setup, friction coefficient between pebbles and base cone angle. It is proved that, the stagnant region existing in the pebble bed can be eliminated by improving the design of pebble bed and the physical properties of fuel pebbles. All of these are very helpful to guide the design of pebble-bed reactor.

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