Figure 2 Heatup Stress Profile

Thernal Shock DOE-HDBK-1017/2-93 PRESSURIZED THERMAL SHOCK Stresses arising from coolant system pressure Figure 2 Heatup Stress Profile exerted against the inside vessel wall (where neutron fluence is greatest) are always tensile in nature. Stresses arising from temperature gradients across the vessel wall can either be tensile or compressive. The type of stress is a function of the wall thickness and reverses from heatup to cooldown. During system heatup, the vessel outer wall temperature lags the inner wall temperature. The stresses produced by this temperature gradient and by system pressure will produce the profile shown in Figure 2. During heatup, it can be seen that while the pressure stresses are always tensile, at the 1/4 thickness (1/4 T), the temperature stresses are compressive. Thus, the stresses at the 1/4 T location tend to cancel during system heatup. At the 3/4 T location, however, the stresses from both temperature and pressure are tensile and thus, reinforce each other during system heatup. For this reason the 3/4 T location is limiting during system heatup. During system cooldown, the stress profile of Figure 3 Cooldown Stress Profile Figure 3 is obtained. During cooldown, the outer wall lags the temperature drop of the inner wall and is at a higher temperature. It can be seen that during cooldown, the stresses at the 3/4 T location are tensile due to system pressure and compressive due to the temperature gradient. Thus during cooldown, the stresses at the 3/4 T location tend to cancel. At the 1/4 T location, however, the pressure and temperature stresses are both tensile and reinforce each other. Thus, the 1/4 T location is limiting during system cooldown. Plant temperature transients that have the greatest potential for causing thermal shock include excessive plant heatup and cooldown, plant scrams, plant pressure excursions outside of normal pressure bands, and loss of coolant accidents (LOCAs). In pressurized water reactors (PWRs), the two transients that can cause the most severe thermal shock to the reactor pressure vessel are the LOCA with subsequent injection of emergency core cooling system (ECCS) water and a severe increase in the primary-to- secondary heat transfer. Rev. 0 Page 7 MS-03