A model is proposed describing the corium pool behavior with a material composition presenting a miscibility gap. The model is described in the first part of this paper, and the state of its validation is developed in the second part, against SIMECO experiments. Qualitatively the model predicts the experimental behavior (domain of existence of two layers, phase separation in the boundary layers, and power split). Applicability to the reactor situation is discussed. It is also concluded that the time delay to obtain physicochemical equilibrium between liquid phases is of the same order of magnitude as the time delay necessary to obtain thermal-hydraulic steady state (established heat flux distribution).