Material Science

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Material Science
2010-09-14 10:09:05
Chapter5 Thermal Shock

Chapter5 Thermal Shock
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  1. Experienced by a thick-walled vessel due to the combined stresses from a rapid temperature and/or pressure change. Non-uniform temperature distribution and subsequent differential expansion and contraction are the causes of the stresses involved
    Pressurized thermal shock
  2. Identify the three components that will set limits on the heatup and cooldown rates
    • Flanges
    • Reactor Vessel Head
    • Reactor Vessel
  3. Identify the basis for determining heatup and cooldown rate limits
    • Failure Life - impact on the life of the plant prior to fatigue becoming a likely failure made
    • Limiting thermal stress - created when the large components of the reactor vessel are heated and cooled
  4. Given a minimum pressure and temperature (MPT) graph, determine the limiting temperature or pressure.
    Minimum Bolt up 70oF
  5. Given a minimum pressure and temperature graph determine the minimum temperature allowed to tension the vessel head and state basis
    • 80oF
    • Most limiting temperature based on most limiting component
  6. State the action typically taken upon discovering the heatup or cooldown rate has been exceeded.
    No immediate hazard, only requires an assessment of the impact on the future fatigue life of the plant
  7. State the reason for using soaktimes
    So that heating can be carefully controlled. In this manner thermal stresses are minimized
  8. State when soak times become significant
    When a limiting component is at room temperature or below and very close to its RTNDT temperature limitations
  9. Identify two stresses that are the result of thermal shock to plant materials
    • Tensile stress
    • Compressive stress
  10. State the two causes of thermal stress
    • Non-uniform heating of a uniform metal
    • Uniform heating of a non-uniform material
  11. Describe why thermal shock is a major concern in reactor system when rapidly heating or cooling a thick walled vessel
    Concern due to the magnitude of the stresses involved with rapid heating or cooling
  12. List three operational limits that are specifically intended to reduce the severity of the thermal shock
    • Heatup and Cooldown rate limits
    • Temperature limits for placing systems into operation
    • Specific temperatures for specific pressures for system operations
  13. STATE how the pressure in a closed system affects the severity of thermal shock
    Pressure raises the severity due to additive effect of thermal and pressure tensile stresses
  14. List the four plant transients that have the greatest potential for causing thermal shock
    • Excessive heatup and cooldown
    • Plant Scrams
    • Plant pressure excursions outside of normal pressure bands
    • Loss of coolant accident (LOCA)
  15. State the three locations of primary concern for thermal shock in a reaction system
    • Reactor Vessel
    • Reactor Vessel Head
    • Reactor Vessel Flange
    • Pressurizer
    • CVCS
    • Pressurizer Spray Line

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