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Thermal Stress  Summary
Figure 2   Heatup Stress Profile

Material Science Volume 2 of 2
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PRESSURIZED THERMAL SHOCK DOE-HDBK-1017/2-93 Thermal Shock PRESSURIZED THERMAL SHOCK Personnel need to be aware how pressure combined with thermal stress can cause failure  of  plant  materials.    This  chapter  addresses  thermal  shock  (stress)  with pressure excursions. EO  1.6 DEFINE  the  term  pressurized  thermal  shock. EO  1.7 STATE how the pressure in a closed system effects the severity of  thermal  shock. EO  1.8 LIST the four plant transients that have the greatest potential for  causing  thermal  shock. EO  1.9 STATE  the  three  locations  in  a  reactor  system  that  are  of primary  concern  for  thermal  shock. Definition One  safety  issue  that  is  a  long-term  problem  brought  on  by  the  aging  of  nuclear  facilities  is pressurized thermal shock (PTS).  PTS is the shock experienced by a thick-walled vessel due to the combined stresses from a rapid temperature and/or pressure change.  Nonuniform temperature distribution and subsequent differential expansion and contraction are the causes of the stresses involved.  As the facilities get older in terms of full power operating years, the neutron radiation causes   a   change   in   the   ductility   of   the   vessel   material,   making   it   more   susceptible   to embrittlement.   Thus, if an older reactor vessel is cooled rapidly at high pressure, the potential for failure by cracking increases greatly. Evaluating  Effects  of  PTS 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  with  the  changes involved in a rapid cooldown of the reactor system, which causes thermal shock to the reactor vessel.   These changes are called transients.   Pressure in the reactor system raises the severity of the thermal shock due to the addition of stress from pressure.  Transients, which combine high system  pressure and a severe thermal  shock, are potentially more dangerous  due to the added effect of the tensile stresses on the inside of the reactor vessel wall.   In addition, the material toughness of the reactor vessel is reduced as the temperature rapidly decreases. MS-03 Page 6 Rev. 0







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