Evaluating Effects of P T S:

Changes from one steady-state temperature or pressure to another are of interest for evaluating the effects of PTS on the reactor vessel integrity. This is especially true along with the changes included in a rapid cooldown of the reactor system that causes thermal shock to the reactor vessel. These changes are known as transients. Pressure in the reactor system increased the severity of the thermal shock because of the further of stress from pressure. Transients, who combine high system pressure and a severe thermal shock, are potentially more dangerous because of the added effect of the tensile stresses on the within of the reactor vessel wall.  Further, the material toughness of the reactor vessel is decreased as the temperature rapidly reduced.

Stresses arising from coolant system pressure exerted against the inner vessel wall (where the neutron fluence is greatest) are always tensile in nature. Stresses arising from temperature gradients across the vessel wall could either be compressive or tensile. The categories of stress are a function of the wall thickness and reverses from heatup to cooldown.  In During system heatup, the vessel outer wall temperature lags the inner wall temperature.