The scaling up of a hydrothermal oxidation (HTO) facility depends on the development of simulation tools. In this way, a new approach to simulate the reactor thermal behavior during a nonstationary regime such as the start, stop, or emergency stop of the unit is developed. A numerical procedure of nonstationary temperature profile simulation in the reactor is described in order to predict the reactor thermal behavior. This new model is validated from experimental data obtained with a quasi-adiabatic reactor, which allows one to follow the temperature profile evolution along the reactor during the HTO process. Performances of the numerical procedure are tested with the simulation of three kinds of real nonstationary conditions: facility start, pump defect, and waste concentration dilution. The simulation gives the evolution of temperature profiles of a reactive mixture versus time and models the external wall temperature of the reactor tube versus time. These three simulations show that the HTO reactor thermal operation can be considered as stable and sure. Furthermore, they put in a prominent position that the risks of reactor damage are limited.