Brittle Fracture and Vessel Thermal Stress

Question 1

The pressure stress on a reactor vessel wall is…

Answer 1

tensile across the entire wall.

Question 2

Brittle fracture is the fragmentation of metal resulting from the application of __________ stress at
relatively __________ temperatures.

Answer 2

tensile; low

Question 3

The conditions for brittle fracture of the reactor vessel are most closely approached at…

Answer 3

120°F, 400 psig

Question 4

Which one of the following comparisons results in a higher probability for brittle fracture of a reactor
vessel?

Answer 4

A high material strength in the reactor vessel rather than a high material ductility.

Question 5

Which one of the following reduces the probability of brittle fracture of the reactor vessel?

Answer 5

Small heatup and cooldown rates.

Question 6

Which one of the following comparisons results in a higher probability for brittle fracture of a reactor
vessel?

Answer 6

A tensile stress rather than a compressive stress.

Question 7

Which one of the following statements describes the relationship between brittle fracture and the
nil-ductility transition temperature?

Answer 7

Operation below the nil-ductility transition temperature will increase the probability of brittle
fracture.

Question 8

Which one of the following comparisons results in a higher probability for brittle fracture of a reactor
vessel?

Answer 8

Performing a 100°F/hr cooldown of the reactor rather than a 100°F/hr heatup.

Question 9

Which one of the following comparisons results in a higher probability for brittle fracture of a reactor
vessel?

Answer 9

Performing a 50°F/hr cooldown at 1,600 psia rather than a 50°F/hr cooldown at 1,200 psia.

Question 10

Brittle fracture of the reactor vessel wall is least likely to occur at…

Answer 10

400°F; 400 psig.

Question 11

Brittle fracture of the reactor vessel (RV) is most likely to occur during a reactor __________ when
RV temperature is __________ the nil-ductility transition temperature.

Answer 11

cooldown; below

Question 12

Which one of the following will normally prevent brittle fracture failure of a reactor vessel?

Answer 12

Operating above the nil-ductility transition temperature.

Question 13

Brittle fracture of the reactor vessel (RV) is least likely to occur during a __________ of the RV when
RV temperature is __________ the nil-ductility transition temperature.

Answer 13

heatup; above

Question 14

Brittle fracture of a low-carbon steel is more likely to occur when the temperature of the steel is
__________ the nil-ductility transition temperature; and will normally occur when the applied stress is
__________ the steel=s yield strength (or yield stress) at room temperature

Answer 14

less than; less than

Question 15

Which one of the following comparisons results in a higher probability for brittle fracture of a reactor
vessel?

Answer 15

A 50°F/hr cooldown rather than a 100°F/hr heatup.

Question 16

Which one of the following comparisons results in a lower probability for brittle fracture of a reactor
vessel?

Answer 16

A high gamma flux in the reactor rather than a high neutron flux.

Question 17

The nil-ductility transition temperature for a reactor vessel is the temperature…

Answer 17

below which the probability of brittle fracture significantly increases

Question 18

The nil-ductility transition temperature of the reactor vessel (RV) is the temperature…

Answer 18

below which the RV metal loses its ability to elastically deform as RV pressure increases.

Question 19

The nil-ductility transition temperature is the temperature above which…

Answer 19

a metal exhibits more ductile tendencies.

Question 20

The nil-ductility transition temperature is that temperature…

Answer 20

below which the probability of brittle fracture significantly increases.

Question 21

The likelihood of brittle fracture failure of the reactor vessel is reduced by…

Answer 21

reducing vessel pressure.

Question 22

Which one of the following reactor coolant system (RCS) conditions is least effective in preventing
brittle fracture of the reactor vessel?

Answer 22

Operating with a ramped RCS temperature as reactor power level increases.

Question 23

Why are reactor coolant system cooldown rate limitations established?

Answer 23

Prevent brittle fracture of the reactor vessel.

Question 24

The thermal stress experienced by the reactor vessel during a reactor coolant system heatup is…

Answer 24

compressive at the inner wall and tensile at the outer wall of the vessel.

Question 25

The total stress on the reactor vessel inner wall is greater during cooldown than heatup because...

Answer 25

both pressure stress and thermal stress are tensile at the inner wall during cooldown.

Question 26

The likelihood of brittle fracture failure of the reactor vessel is reduced by...

Answer 26

reducing reactor vessel pressure.

Question 27

Which one of the following will increase the compressive stress on the outside surface of the reactor vessel wall?

Answer 27

Reactor coolant system cooldown

Question 28

Which one of the following applies a compressive stress to the inner wall of the reactor vessel during a reactor coolant system heatup?

Answer 28

Thermal stress

Question 29

Which one of the following is the most limiting component for establishing reactor coolant system heatup/cooldown rate limits?

Answer 29

Reactor vessel

Question 30

Which one of the following stresses is compressive on the outer wall of the reactor vessel during a reactor coolant system cooldown?

Answer 30

Thermal stress

Question 31

Which one of the following will apply a compressive stress to the outside wall of the reactor vessel?

Answer 31

Performing a reactor coolant system cooldown.

Question 32

Reactor coolant system pressure-temperature limit curves are derived by using a conservative value for the reactor vessel nil-ductility transition temperature (NDTT). The conservative value used for the reactor vessel NDTT is __________ than the actual NDTT; the actual NDTT is verified periodically by __________.

Answer 32

higher; removing and testing irradiated specimens of reactor vessel material

Question 33

Which one of the following operating limitations is designed to prevent brittle fracture of the reactor vessel and/or the reactor coolant system (RCS)?

Answer 33

Maximum RCS pressure versus RCS temperature for a given RCS heatup rate

Question 34

A reactor is shutdown with the shutdown cooling system maintaining reactor coolant temperature at 240°F immediately following an uncontrolled rapid cooldown from 500°F. If reactor coolant temperature is held constant at 240°F, which one of the following describes the change in tensile stress on the inner wall of the reactor vessel (RV) over the next few hours?

Answer 34

Decreases, because the temperature gradient across the RV wall will decrease.

Question 35

Fast neutron irradiation of the reactor vessel results in __________ stresses within the vessel metal, thereby __________ the nil-ductility transition temperature.

Answer 35

increased; increasing

Question 36

Fast neutron irradiation adversely affects the reactor vessel primarily by causing...

Answer 36

metal embrittlement.

Question 37

Prolonged exposure of a reactor vessel to a fast neutron flux will cause the nil-ductility transition temperature to...

Answer 37

increase, due to changes in the material properties of the vessel wall

Question 38

The likelihood of reactor vessel brittle fracture is decreased by minimizing...

Answer 38

The amount of copper contained in the metal used for the reactor vessel.

Question 39

Which one of the following types of radiation most significantly reduces the ductility of a reactor vessel?

Answer 39

Fast neutrons

Question 40

After several years of operation, the maximum allowable stress to the reactor vessel is more limited by the inner wall than the outer wall because...

Answer 40

the inner wall experiences more neutron-induced embrittlement than the outer wall.

Question 41

Prolonged exposure to __________ will cause the nil-ductility transition temperature of the reactor vessel to __________.

Answer 41

neutron radiation; increase

Question 42

Two identical reactors have been in operation for the last 10 years. Reactor A has experienced 40 heatup/cooldown cycles with an average capacity factor of 50 percent. Reactor B has experienced 30 heatup/cooldown cycles with an average capacity factor of 60 percent. Which reactor will have the lower reactor vessel nil-ductility transition temperature, and why?

Answer 42

Reactor A, due to the lower average capacity factor.

Question 43

The two factors that have the greatest effect on the nil-ductility transition temperature of the reactor vessel over its life are...

Answer 43

fast neutron flux and vessel copper content.

Question 44

Two identical reactors have been in operation for the last 10 years. Reactor A has experienced 30 heatup/cooldown cycles with an average capacity factor of 60 percent. Reactor B has experienced 40 heatup/cooldown cycles with an average capacity factor of 50 percent. Which reactor will have the lower reactor vessel nil-ductility transition temperature, and why?

Answer 44

Reactor B, due to the lower average capacity factor.

Question 45

Which one of the following is the major contributor to embrittlement of a reactor vessel?

Answer 45

High-energy neutron radiation

Question 46

Which one of the following describes the effect of fast neutron irradiation on a reactor vessel?

Answer 46

Increased nil-ductility transition temperature

Question 47

Two identical reactors have been in operation for the last 10 years. Reactor A has experienced 30 heatup/cooldown cycles and has an average capacity factor of 60 percent. Reactor B has experienced 40 heatup/cooldown cycles and has an average capacity factor of 50 percent. Which reactor will have the higher reactor vessel nil-ductility transition temperature, and why?

Answer 47

Reactor A, due to the higher average capacity factor.

Question 48

Two identical reactors have been in operation for the last 10 years. Reactor A has experienced 40 heatup/cooldown cycles and has an average capacity factor of 50 percent. Reactor B has experienced 30 heatup/cooldown cycles and has an average capacity factor of 60 percent. Which reactor will have the higher reactor vessel nil-ductility transition temperature?

Answer 48

Reactor B, due to the higher average capacity factor.

Question 49

Two identical reactors are currently shut down for refueling. Reactor A has an average lifetime capacity factor of 60 percent and has been operating for 15 years. Reactor B has an average lifetime capacity factor of 75 percent and has been operating for 12 years. Which reactor, if any, will have the lower reactor vessel nil-ductility transition temperature, and why?

Answer 49

Both reactors will have approximately the same nil-ductility transition temperature because each reactor has produced approximately the same number of fissions.

Question 50

Two identical reactors are currently shut down for refueling. Reactor A has achieved an average lifetime capacity factor of 60 percent while operating for 15 years. Reactor B has achieved an average lifetime capacity factor of 60 percent while operating for 12 years. Which reactor, if any, will have the lower reactor vessel nil-ductility transition temperature, and why?

Answer 50

Reactor B, because it has produced less total fissions.

Question 51

Two identical reactors have been in operation for the last 10 years. Reactor A has experienced 30 heatup/cooldown cycles and has an average capacity factor of 60 percent. Reactor B has experienced 20 heatup/cooldown cycles and has an average capacity factor of 80 percent. Which reactor will have the higher reactor vessel nil-ductility transition temperature, and why?

Answer 51

Reactor B, due to the higher average capacity factor.

Question 52

A reactor is shut down for refueling following 18 months of operation at an average power level of 85 percent. During the shutdown, a reactor vessel metal specimen was removed from the reactor vessel for testing. The testing determined that the nil-ductility transition (NDT) temperature of the specimen decreased from 44°F to 42°F since the previous refueling shutdown. Which one of the following conclusions is warranted?

Answer 52

The test results are questionable because the specimen NDT temperature would not decrease during the described 18-month period of operation

Question 53

A reactor is shut down for refueling following 18 months of operation at an average power level of 85 percent. During the shutdown, a reactor vessel metal specimen was removed from the reactor vessel for testing. The testing determined that the nil-ductility transition (NDT) temperature of the specimen increased from 42°F to 44°F since the previous refueling shutdown. Which one of the following conclusions is warranted?

Answer 53

The test results are credible and the reactor vessel is more susceptible to brittle fracture now than after the previous refueling shutdown.

Question 54

A reactor is shut down for refueling following 18 months of operation at an average power level of 85 percent. During the shutdown, a reactor vessel metal specimen is removed from the reactor vessel for testing. The testing indicates that the nil-ductility transition (NDT) temperature of the specimen has decreased from 44°F to 32°F since the previous refueling shutdown. Which one of the following conclusions is warranted?

Answer 54

The test results are questionable because the actual specimen NDT temperature would not decrease during the described 18-month period of operation

Question 55

Two identical reactors are currently shut down for refueling. Reactor A has an average lifetime capacity factor of 90 percent and has been operating for 10 years. Reactor B has an average lifetime capacity factor of 80 percent and has been operating for 15 years. Which reactor will have the higher reactor vessel nil-ductility transition temperature, and why?

Answer 55

Reactor B, because it has produced significantly more fissions.

Question 56

A reactor is shut down for refueling following 18 months of operation at an average power level of 85 percent. During the shutdown, a reactor vessel metal specimen was removed from the reactor vessel for testing. The tests determined that the nil-ductility transition (NDT) temperature of the specimen increased from 42°F to 72°F since the previous refueling shutdown. Which one of the following conclusions is warranted?

Answer 56

The test results are questionable because the specimen NDT temperature would increase by less than indicated during the described 18-month period of operation.

Question 57

A reactor is shut down for refueling. During the shutdown, a reactor vessel metal specimen was removed from the reactor vessel for testing. The specimen was last tested six years ago and then returned to its original location in the reactor vessel. During the subsequent six years, the reactor has completed several 18 month fuel cycles with an average power level of 85 percent. The tests determined that the nil-ductility transition (NDT) temperature of the specimen has remained unchanged at 44°F since it was last tested. Which one of the following conclusions is warranted?

Answer 57

The test results are questionable because the specimen NDT temperature should have increased since it was last tested

Question 58

Two identical reactors are currently shut down for refueling. Reactor A has achieved an average lifetime capacity factor of 60 percent while operating for 12 years. Reactor B has achieved an average lifetime capacity factor of 60 percent while operating for 15 years. Which reactor, if any, will have the lower reactor vessel nil-ductility transition temperature?

Answer 58

Reactor A, because it has produced less total fissions.

Question 59

Two identical reactors are currently shut down for refueling. Reactor A has an average lifetime capacity factor of 90 percent and has been operating for 24 years. Reactor B has an average lifetime capacity factor of 72 percent and has been operating for 30 years. Which reactor, if any, will have the lower reactor vessel nil-ductility transition temperature?

Answer 59

Both reactors will have approximately the same nil-ductility transition temperature because each reactor has produced approximately the same number of fissions.

Question 60

Which one of the following comparisons results in a higher probability for brittle fracture of a reactor vessel?

Answer 60

A high fast neutron flux in the reactor rather than a high gamma flux.

Question 61

Two identical reactors are currently shut down for refueling. Reactor A has an average lifetime capacity factor of 90 percent and has been operating for 16 years. Reactor B has an average lifetime capacity factor of 80 percent and has been operating for 18 years. Which reactor, if any, will have the lower reactor vessel nil-ductility transition temperature, and why?

Answer 61

Both reactors will have approximately the same nil-ductility transition temperature because each reactor has produced approximately the same number of fissions.

Question 62

Which one of the following comparisons results in a lower probability for brittle fracture of a reactor vessel?

Answer 62

A high gamma flux in the reactor rather than a high fast neutron flux.

Question 63

A nuclear power plant is shut down with the reactor coolant system at 1,200 psia and 350°F. Which one of the following would be most likely to cause a pressurized thermal shock to the reactor vessel?

Answer 63

A rapid cooldown followed by a rapid pressurization.

Question 64

Pressurized thermal shock is a condition that can occur following a rapid __________ of the reactor coolant system if system pressure is rapidly __________.

Answer 64

cooldown; increased

Question 65

Which one of the following reactor coolant system (RCS) events would be most likely to cause a pressurized thermal shock to the reactor vessel?

Answer 65

Continuous emergency coolant injection to the RCS during and after a complete and unisolable rupture of a steam generator steam outlet nozzle.

Question 66

During a severe reactor coolant system overcooling transient, a major concern is...

Answer 66

brittle fracture of the reactor vessel

Question 67

An uncontrolled cooldown is a brittle fracture concern because it creates a large __________ stress at the __________ wall of the reactor vessel.

Answer 67

tensile; inner

Question 68

During an uncontrolled cooldown of a reactor coolant system, the component most susceptible to brittle fracture is the...

Answer 68

reactor vessel.

Question 69

Which one of the following describes the thermal stress placed on the reactor vessel wall during a cooldown of the reactor coolant system?

Answer 69

Tensile at the inner wall, compressive at the outer wall.

Question 70

A nuclear power plant heatup is in progress using reactor coolant pumps. The thermal stress applied to the reactor vessel is...

Answer 70

compressive at the inner wall and tensile at the outer wall.