[en] A model which predicts the one-dimensional time-dependent coolant behavior within an internally heated or superheated particle bed is briefly described. The dryout and the quench processes are analyzed in detail, with specific predictions for such quantities as the time for the bed to quench or dry out and the pressure build-up after step changes in power. The importance and implications of using the correct form of the relative permeabilities are discussed. Transient processes predicted to occur during an LWR accident are described