For certain types of chemical reactor whose catalyst decays rapidly but reversibly, continuous operation with a lower pressure drop is possible by use of a moving-bed radial flow reactor. In this type of reactor there is an upper limit on the gas flowrate imposed by the mechanical phenomenon of ’pinning’, or the formation of a cavity. This paper presents an analysis of the distribution of compressive stress in the particle bed with the gas stream flowing outward radially. A theoretical model is developed using the theories of particulate media mechanics to predict the dimensionless pressure drop for the initiation of the cavity. The theoretical results agree very well with experimental data. The shape of the interface between the cavity and the particle bed has also been measured.