Thernal ShockDOE-HDBK-1017/2-93PRESSURIZED THERMAL SHOCKStresses arising from coolant system pressureFigure 2 Heatup Stress Profileexerted against the inside vessel wall (whereneutron fluence is greatest) are always tensile innature. Stresses arising from temperaturegradients across the vessel wall can either betensile or compressive. The type of stress is afunction of the wall thickness and reverses fromheatup to cooldown. During system heatup, thevessel outer wall temperature lags the inner walltemperature. The stresses produced by thistemperature gradient and by system pressure willproduce the profile shown in Figure 2.During heatup, it can be seen that while thepressure stresses are always tensile, at the 1/4thickness (1/4 T), the temperature stresses arecompressive. Thus, the stresses at the 1/4 Tlocation tend to cancel during system heatup. Atthe 3/4 T location, however, the stresses fromboth 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 ofFigure 3 Cooldown Stress ProfileFigure 3 is obtained. During cooldown, the outerwall lags the temperature drop of the inner walland is at a higher temperature. It can be seenthat during cooldown, the stresses at the 3/4 Tlocation are tensile due to system pressure andcompressive due to the temperature gradient.Thus during cooldown, the stresses at the 3/4 Tlocation tend to cancel. At the 1/4 T location,however, the pressure and temperature stressesare both tensile and reinforce each other. Thus,the 1/4 T location is limiting during systemcooldown.Plant temperature transients that have the greatestpotential for causing thermal shock includeexcessive plant heatup and cooldown, plantscrams, plant pressure excursions outside ofnormal pressure bands, and loss of coolantaccidents (LOCAs). In pressurized water reactors (PWRs), the two transients that can cause themost severe thermal shock to the reactor pressure vessel are the LOCA with subsequent injectionof emergency core cooling system (ECCS) water and a severe increase in the primary-to-secondary heat transfer.Rev. 0Page 7MS-03
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