A mixture model is developed for the simulation of a transient two-phase flow induced by the accidental depressurization of an enclosure containing initially high-pressure liquid. It is based on a three-equation system and a drift-flux model for describing the relative phase motion. The unsteady solution is obtained by means of a fully implicit scheme. An original treatment of the drift-flux term (based on the donor cell concept) in the energy equation is used. The comparison between the numerical results and those given by experiments for two accidental events is quite good. This fast-running approach (in terms of CPU time) allows real-time simulations, which are of primary importance for control system modeling and simulator design.