As well known, the very slow pebble flow (VSPF) exists in the pebble bed high temperature gas cooled reactor, which is regarded as the 4th generation of nuclear power plant, one of the most promising reactors in the 21th century. Unfortunately, we still know very little on the fundamentals of the VSPF, a specific type of granular flows.

In general, a granular flow is a collection of a large number of discrete but closely packed solid particles. A granular flow may behave as either an elastic solid or a fluid, depending on the local stress conditions. A rapid granular flow behaves much more like a fluid rather than a solid collection, and sometimes can even be analogically analyzed by the kinetic theory. However, the very slow granular flow (VSGF) is showing its elastic solid behavior at any instantaneous time and fluid-like behavior in a long time. It can shear like fluid, dissipate energy by collision, and also it can support loads and form finite piling slopes like solid. It is more complicated than the rapid flow and cannot be easily analyzed solely by the fluid or kinetic theories.

We regard the VSPF as a special VSGF. In the pebble bed under gravity, the pebbles flow out of the bed very slowly, almost one-by-one. Thus, the flow-out process is an intermittent or discrete process in time. We can regard each flow-out process as an “original disturbance” (OD), and the transfer process of OD throughout the bed as a “subsequent disturbance” (SD). The SD is decaying in time as the pebble collision is dissipative. Thus, the VSPF can be specifically defined as a special case in which the OD and SD processes are not overlapped. In other words, the effect of SD has already been dissipated almost completely before the next OD occurs.

In this study, we just show some examples for the interesting phenomenon of VSPF both experimentally and numerically. Based on the OD and SD theory, we show the typical processes and basic characteristics of OD and SD to validate their existences and justify the definitions of VSPF by the OD and SD.

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