A nonequilibrium three-region model is developed for the accurate prediction of the pressure in the pressurizer under both transient and accident conditions. The mathematical model derived from the general conservation equations includes all of the important thermal-hydraulics processes occurring in the pressurizer: bulk flashing and condensation, wall condensation, and interfacial heat and mass transfer, etc. The Stanton number for the interfacial heat transfer coefficient is obtained by fitting the experimental results in terms of the surge rate. The bubble rising and rain-out models are developed to describe bulk flashing and condensation, respectively. To obtain the wall condensation rate, a one-dimensional heat conduction equation is solved by the pivoting method. The mathematical model is numerically solved by the back substitution and successive iteration method for fast convergence and stability. For verification, several numerical tests are done on a mild transient in the Shippingport nuclear power plant, an experimental test done at the Massachusetts Institute of Technology, and the Three Mile Island accident. It is proved that predicted results are in better agreement with experimental tests than those by previous models. Sensitivity analysis is done to see the effect of each model on the behavior of the pressurizer. Discrepancy between results predicted with the three- and the tworegion models becomes apparent in an outsurge after insurge transient. Although the interfacial heat transfer of the pressurizer can be neglected in the case of the high water level, it becomes one of the most dominant processes in the low level. The wall condensation rate becomes important with an increase in pressure due to an insurge transient.