Thermal Shock

Front 1

Identify the two stresses that are the result of thermal shock (stress) to plant materials.

Back 1

Tensile Stress – pull a material apart
Compressive Stress – push a material together

Front 2

State the two causes of thermal shock

Back 2

-Non uniform heating or cooling of a thick material

-Uniform heating of non uniform materials, expansion of one material constrains the expansion of a different material

Front 3

Thermal stresses are a major concern in reactor systems due to the magnitude of the stresses involved.
With rapid heating (or cooling) of a _____ such as the reactor pressure vessel, one part of the wall may try to expand (or contract) while the adjacent section, which has not yet been exposed to the temperature change, tries to restrain it.
Both sections end up under stress.

Back 3

thick-walled vessel

Front 4

List the three operational limits that are specifically intended to reduce the severity of thermal shock

Back 4

Heatup and Cooldown rates
Temperature limits for placing systems in service
Specific temperatures for specific pressures during system operation.

Front 5

Thick wall vessels combine stress from rapid temperature and/or pressure change
_____ is the shock experienced by a thick-walled vessel due to the combined stresses from a rapid temperature and/or pressure change

Back 5

Pressurized Thermal Shock

Front 6

____ from the Fission process causes changes in vessel wall material
Becomes more susceptible to embitterment
More likely to fracture or form cracks at lower temperature

Back 6

Neutron Radiation

Front 7

Pressure Effect: Remember that pressure always present as part of the tensile stress in the vessel wall material
Overall effect:______

Back 7

Pressure Stress (+ stress)
Thermal Stress (+ stress)
Embitterment (lower allowable stress before fracture)

Front 8

List the four plant transients that have the greatest potential for causing thermal shock.

Back 8

Rapid & frequent Heatup and Cool down cycles
SCRAMs (TRIPS)
Pressure excursions above normal pressure band
LOCA (Loss of Coolant Accidents) because of cool water injection into vessel exceeding cooldown rate.

Front 9

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 _____

Back 9

decreases

Front 10

State the three locations in a reactor system that are of primary concern for thermal shock

Back 10

Reactor Vessel Wall
Recirculation Loops
Feedwater Lines

Front 11

Metals that sustain plastic strain or deformation before fracture
Metal stretches before break (i.e. bending metal back and forth)

Better because it occurs over time, still intact until failure, and
Provides some warning (deformation)

Back 11

Ductile Fracture

Front 12

Metals that sustain little or no plastic strain or deformation before fast fracture at low stress load
Crack propagates rapidly from an initial flaw
Splitting along definite planes or Grain Boundaries (Cleavage)
Similar to snapping a bolt head off

Back 12

Brittle Fracture

Front 13

_____ temperature is the temperature below which a ductile material becomes brittle

Dependent on heat treatment nature and amounts of impurity elements

Back 13

Nil-ductility Transition

Front 14

Smaller grain size tends to make materials more _____

Back 14

ductile

Front 15

Smaller grain size has lower ____

Back 15

NDT temperatures

Front 16

DESCRIBE the two changes made to reactor pressure vessels to decrease NDT.

Back 16

Grain size controlled by heat treating during manufacturing(quenching)
Adding small amounts of nickel and manganese to low-carbon steel(increases toughness)

Front 17

LIST the three conditions necessary for brittle fracture to occur

Back 17

Flaw such as a small crack (all materials have flaws!)
Sufficient stress causing a small deformation at crack tip
Temperature < NDT

If these conditions ALL exist together, then Brittle Fracture is a major concern

Front 18

Flaw such as a small crack (all materials have flaws!)
Sufficient stress causing a small deformation at crack tip
Temperature < NDT

If these conditions ALL exist together, then ___ is a major concern

Back 18

Brittle Fracture

Front 19

STATE the three conditions that tend to mitigate crack initiation.

Back 19

Smaller Grain Size
Higher Operating Temperature
Lower Stress

Front 20

LIST the five factors that determine the fracture toughness of a material.

Back 20

Metal Composition
Metal Temperature
Deformation of the crystal structure
Grain size
Crystalline form

Front 21

_____ is to the right of curve
Special limits may also apply
Initial Critical
Low Power Operation
Power Operation

Back 21

Safe Operating Region

Front 22

IDENTIFY the three components that will set limits on the heatup and cooldown rates.

Back 22

Large Flanges
Reactor Vessel Head
Reactor Vessel

Front 23

____ are used so that heating or cooling thermal stresses can be carefully controlled
An example of a____ is to cool the reactor coolant to a specified temperature and to stay at that temperature for a specific time period
Allows temperature gradient to equalize (heat more evenly from cold to hot side) to reduce thermal stress

Back 23

Soak times

Front 24

Soak Time VERY SIGNIFICANT when vessel is very close to ____

Back 24

NDT temperature limits