A digital computer approach for predicting the dynamic response of surge tanks is presented. The applications of different models are presented for analyzing the primary pressure transients of CANDU reactors. Conservation equations for deformable control volume have been employed to describe the flow inside both of the closed distinct regions (phases). In this model, the upper region can be either in the superheated state or two-phase saturated state. The lower region can be in the subcooled state or two-phase saturated state. Energy and mass transfer processes occurring inside the surge tanks have been investigated and determined under various operating conditions. These processes are spray condensation, wall condensation, vapor flashing, heat transfer at interface, and heat transfer from heaters. Numerical results showed that this model favorably predicted the pressurizer pressure when compared with those calculated by adiabatic and equilibrium models employed in the SOPHT code and with data obtained from the Gentilly-2 site and Bruce NGS-A.