The problem of reactor vessel-wall cooldown due to high-pressure safety injection in a stagnated primary system is considered. The approach is based on the Regional Mixing Model, which has been previously documented and applied to various tests. This work is devoted to reactor predictions and experimental simulations made for use in the U.S. Nuclear Regulatory Commission’s Integrated Pressurized Thermal Shock study. A brief summary of the model and some refinements to it are also presented.