Reactivity Coefficients

Question 1

The moderator temperature coefficient describes the change in reactivity per degree change in…

Answer 1

reactor coolant temperature

Question 2

Which one of the following isotopes is the most significant contributor to the resonance capture of
fission neutrons in a reactor at the beginning of a fuel cycle?

Answer 2

U-238

Question 3

Factors that affect the probability of resonance absorption of a neutron by a nucleus include…

Answer 3

kinetic energy of the nucleus, kinetic energy of the neutron, and excitation energy of the nucleus.

Question 4

Which one of the following isotopes is the most significant contributor to the resonance capture of
fission neutrons in a reactor at the end of a fuel cycle?

Answer 4

U-238

Question 5

Which one of the following has the smallest microscopic cross section for absorption of a thermal
neutron in an operating reactor?

Answer 5

Uranium-238

Question 6

Under which one of the following conditions is a reactor most likely to have a positive moderator
temperature coefficient?

Answer 6

Low reactor coolant temperature at the beginning of a fuel cycle.

Question 7

A reactor has operated at steady-state 100 percent power for the past 6 months. Compared to 6
months ago, the current moderator temperature coefficient is…

Answer 7

more negative, due to decreased reactor coolant boron concentration.

Question 8

Which one of the following contains the pair of nuclides that are the most significant contributors to
the total resonance capture in the core near the end of a fuel cycle?

Answer 8

U-238 and Pu-240

Question 9

Which one of the following conditions will cause the moderator temperature coefficient (MTC) to
become more negative? (Consider only the direct effect of the indicated change on MTC.)

Answer 9

The controlling bank of control rods is inserted 5 percent into the core.

Question 10

Which one of the following contains the nuclides responsible for most of the resonance capture of
fission neutrons in a reactor at the beginning of the sixth fuel cycle? (Assume that each refueling
process replaces one-third of the fuel.)

Answer 10

U-238 and Pu-240

Question 11

Which one of the following describes a situation where an increase in moderator temperature can add
positive reactivity?

Answer 11

At low moderator temperatures, an increase in moderator temperature can reduce neutron capture
by the moderator sufficiently to add positive reactivity.

Question 12

As the reactor coolant boron concentration increases, the moderator temperature coefficient becomes
less negative. This is because a 1°F increase in reactor coolant temperature at higher boron
concentrations results in a larger increase in the…

Answer 12

thermal utilization factor.

Question 13

In which one of the following conditions is the moderator temperature coefficient most negative?

Answer 13

End of a fuel cycle (EOC), high reactor coolant temperature

Question 14

During a nuclear power plant heatup near the end of a fuel cycle, the moderator temperature
coefficient becomes increasingly more negative. This is because…

Answer 14

a greater density change per °F occurs at higher reactor coolant temperatures.

Question 15

The moderator temperature coefficient will be least negative at a __________ reactor coolant
temperature and a __________ reactor coolant boron concentration.

Answer 15

low; high

Question 16

A reactor is operating at full power following a refueling outage. Compared to the current moderator
temperature coefficient (MTC), the MTC just prior to the refueling was...

Answer 16

more negative at all coolant temperatures.

Question 17

During a reactor coolant system cooldown, positive reactivity is added to the core if the moderator
temperature coefficient is negative. This is partially due to…

Answer 17

an increasing resonance escape probability.

Question 18

As the core ages, the moderator temperature coefficient becomes more negative. This is primarily
due to…

Answer 18

decreasing reactor coolant boron concentration.

Question 19

The moderator temperature coefficient will be most negative at a __________ reactor coolant
temperature and a __________ reactor coolant boron concentration.

Answer 19

high; low

Question 20

Which one of the following describes the initial reactivity effect of a moderator temperature decrease
in an undermoderated reactor?

Answer 20

Positive reactivity will be added because fewer neutrons will be absorbed at resonance energies
while slowing down.

Question 21

Which one of the following describes why the moderator temperature coefficient is more negative
near the end of a fuel cycle (EOC) compared to the beginning of a fuel cycle (BOC)?

Answer 21

Decreased coolant boron concentration near the EOC results in fewer boron atoms leaving the core
for a 1°F moderator temperature increase.

Question 22

Which one of the following describes the initial reactivity effect of a moderator temperature decrease
in an overmoderated reactor?

Answer 22

Negative reactivity will be added because more neutrons will be captured by the moderator while
slowing down.

Question 23

A reactor is operating at 100 percent power following a refueling outage. Compared to the moderator
temperature coefficient (MTC) just prior to the refueling, the current MTC is...

Answer 23

less negative at all coolant temperatures.

Question 24

Which one of the following describes the initial reactivity effect of a moderator temperature increase
in an overmoderated reactor?

Answer 24

Positive reactivity will be added because fewer neutrons will be captured by the moderator while
slowing down.

Question 25

How does the addition of boric acid to the reactor coolant affect the moderator temperature coefficient (MTC) in an undermoderated reactor?

Answer 25

The initially negative MTC becomes less negative.

Question 26

Compared to the moderator temperature coefficient (MTC) of reactivity near the beginning of a fuel cycle, the MTC near the end of a fuel cycle is: (Assume 100 percent power for all cases.)

Answer 26

more negative, because as reactor coolant boron concentration decreases, the thermal utilization of fission neutrons increases.

Question 27

Which one of the following describes the initial reactivity effect of a moderator temperature increase in an undermoderated reactor?

Answer 27

Negative reactivity will be added because more neutrons will be absorbed by U-238 at resonance energies while slowing down.

Question 28

When compared to the beginning of a fuel cycle, the moderator temperature coefficient at 100 percent power near the end of a fuel cycle is...

Answer 28

more negative, because fewer boron-10 nuclei are removed from the core for a given moderator temperature increase.

Question 29

How does increasing the reactor coolant boron concentration affect the moderator temperature coefficient (MTC) in an overmoderated reactor?

Answer 29

The initially positive MTC becomes more positive

Question 30

A reactor is shut down near the middle of a fuel cycle with the shutdown cooling system in service. The initial reactor coolant temperature is 160ºF. In this condition, the reactor is undermoderated. Then, a heatup and pressurization is performed to bring the reactor coolant system to normal operating temperature and pressure. The reactor remains subcritical. During the heatup, Keff will…

Answer 30

decrease continuously.

Question 31

Why does the fuel temperature coefficient becomes less negative at higher fuel temperatures?

Answer 31

The amount of Doppler broadening per degree change in fuel temperature diminishes

Question 32

Which one of the following will cause the Doppler power coefficient to become more negative?

Answer 32

Lower power level

Question 33

A reactor is operating continuously at steady-state 100 percent power. As core burnup increases, the fuel temperature coefficient becomes __________ negative because the average fuel temperature __________.

Answer 33

more; decreases

Question 34

Which one of the following pairs of nuclides is responsible for most of the negative reactivity associated with a fuel temperature increase near the end of a fuel cycle?

Answer 34

U-238 and Pu-240

Question 35

A nuclear power plant is operating at steady-state 70 percent power. Which one of the following will result in a less negative fuel temperature coefficient? (Consider only the direct effect of the change in each listed parameter.)

Answer 35

Increase in fuel temperature.

Question 36

Compared to operation at a low power level, the fuel temperature coefficient of reactivity at a high power level is __________ negative due to __________.

Answer 36

less; higher fuel temperature

Question 37

Refer to the curve of microscopic cross section for absorption versus neutron energy for a resonance peak in U-238 (see figure below). If fuel temperature increases, the area under the curve will __________; and negative reactivity will be added to the core because __________.

Answer 37

remain the same; neutrons of a wider range of energies will be absorbed by U-238

Question 38

Which one of the following describes how the magnitude of the fuel temperature coefficient of reactivity is affected as the core ages?

Answer 38

It becomes more negative, due to the buildup of Pu-240.

Question 39

In a comparison of the fuel temperature coefficient at the beginning and end of a fuel cycle, the fuel temperature coefficient is more negative at the __________ of a fuel cycle because __________. (Assume the same initial fuel temperature throughout the fuel cycle.)

Answer 39

end; more Pu-240 is in the core

Question 40

Refer to the curve of microscopic cross section for absorption versus neutron energy for a 6.7 electron volt (eV) resonance peak in U-238 for a reactor operating at 50 percent power (see figure below). If fuel temperature decreases by 50°F, the area under the curve will __________; and positive reactivity will be added to the core because __________.

Answer 40

remain the same; fewer neutrons will be absorbed by U-238 overall

Question 41

Refer to the curve of microscopic cross section for absorption versus neutron energy for a resonance peak in U-238 in a reactor operating at 80 percent power (see figure below). If reactor power is increased to 100 percent, the height of the curve will __________; and the area under the curve will __________.

Answer 41

decrease; remain the same

Question 42

Refer to the drawing of a curve showing the neutron absorption characteristics of a typical U-238 nucleus at a resonance neutron energy (see figure below). The associated reactor is currently operating at steady-state 80 percent power. During a subsequent reactor power decrease to 70 percent, the curve will become __________; and the percentage of the core neutron population lost to resonance capture by U-238 will __________.

Answer 42

taller and more narrow; decrease

Question 43

Refer to the curve of microscopic cross section for absorption versus neutron energy for a resonance peak in U-238 in a reactor operating at 80 percent power (see figure below). If reactor power is decreased to 60 percent, the height of the curve will __________; and the area under the curve will __________.

Answer 43

increase; remain the same

Question 44

If the average temperature of a fuel pellet decreases by 50°F, the microscopic cross-section for absorption of neutrons at a resonance energy of U-238 will __________; and the microscopic cross-sections for absorption of neutrons at energies that are slightly higher or lower than a U-238 resonance energy will __________.

Answer 44

increase; decrease

Question 45

If the average temperature of a fuel pellet increases by 50°F, the microscopic cross-section for absorption of neutrons at a resonance energy of U-238 will __________; and the microscopic cross-sections for absorption of neutrons at energies that are slightly higher or lower than a U-238 resonance energy will __________.

Answer 45

decrease; increase

Question 46

Which one of the following 10 percent reactor power level changes produces the largest amount of negative reactivity from the fuel temperature coefficient? (Assume that each power level change produces the same increase/decrease in fuel temperature.)

Answer 46

30 percent to 40 percent

Question 47

Refer to the drawing of a curve showing the neutron absorption cross-section for U-238 at a resonance energy (see figure below). The reactor associated with the curve is operating at 80 percent power. If reactor power is increased to 90 percent over the next few hours, the curve will become ________; and the percentage of the core neutron population lost to resonance capture by U-238 will ________.

Answer 47

shorter and broader; increase

Question 48

A reactor has an initial effective fuel temperature of 800EF. If the effective fuel temperature increases to 1,000EF, the fuel temperature coefficient will become __________ negative; because at higher effective fuel temperatures, a 1EF increase in effective fuel temperature produces a __________ change in Doppler broadening

Answer 48

less; smaller

Question 49

Which one of the following groups contain parameters that, if varied, will each have a direct effect on the power coefficient?

Answer 49

Moderator void fraction, fuel temperature, moderator temperature

Question 50

Which one of the following is responsible for the largest positive reactivity addition immediately following a reactor trip from 100 percent power at the beginning of a fuel cycle? (Assume reactor coolant system parameters stabilize at their normal post-trip values.)

Answer 50

The change in fuel temperature.

Question 51

A nuclear power plant is initially operating at steady-state 50 percent power. Which one of the following contains only parameters that, if varied, will each directly change the magnitude of the power defect?

Answer 51

Moderator void fraction, fuel temperature, and moderator temperature

Question 52

A reactor is initially critical at the point of adding heat during a xenon-free reactor startup near the beginning of a fuel cycle. Reactor power is ramped to 50 percent over a 4 hour period. During the power increase, most of the positive reactivity added by the operator is necessary to overcome the negative reactivity associated with the...

Answer 52

increased fuel temperature.

Question 53

A nuclear power plant has been operating at steady-state 50 percent power for one month following a refueling outage. Then, reactor power is ramped to 100 percent over a 2-hour period. During the power increase, most of the positive reactivity added by the operator is necessary to overcome the negative reactivity associated with the...

Answer 53

increased fuel temperature.

Question 54

As reactor coolant boron concentration decreases, the differential boron worth (ΔK/K/ppm) becomes...

Answer 54

more negative, due to a smaller number of boron molecules in the core.

Question 55

With higher concentrations of boron in the reactor coolant, the core neutron flux distribution shifts to __________ energies where the absorption cross section of boron is __________.

Answer 55

higher; smaller

Question 56

Differential boron worth (ΔK/K/ppm) will become __________ negative as moderator temperature increases because, at higher moderator temperatures, a 1 ppm increase in reactor coolant boron concentration will add __________ boron atoms to the core

Answer 56

less; fewer

Question 57

Differential boron worth (ΔK/K/ppm) becomes more negative as...

Answer 57

burnable poisons deplete.

Question 58

The following are the initial conditions for a nuclear power plant: • Reactor power is 50 percent. • Average reactor coolant temperature is 570°F. • Reactor coolant boron concentration is 400 ppm. After a power increase, the current plant conditions are as follows: • Reactor power is 80 percent. • Average reactor coolant temperature is 582°F. • Reactor coolant boron concentration is 400 ppm. Which one of the following describes the current differential boron worth (DBW) in ΔK/K/ppm compared to the initial DBW?

Answer 58

The current DBW is less negative because a 1 ppm increase in reactor coolant boron concentration will add fewer boron-10 atoms to the core.

Question 59

The amount of boric acid required to increase the reactor coolant boron concentration by 50 ppm at 1,200 ppm is approximately __________ as the amount of boric acid required to increase the reactor coolant boron concentration by 50 ppm at 100 ppm.

Answer 59

the same

Question 60

The amount of pure water required to decrease the reactor coolant boron concentration by 20 ppm at 100 ppm is approximately __________ the amount of pure water required to decrease the reactor coolant boron concentration by 20 ppm at 1,000 ppm.

Answer 60

10 times

Question 61

A reactivity coefficient measures a/an __________ change in reactivity, while a reactivity defect measures a __________ change in reactivity

Answer 61

unit; total

Question 62

Given the following initial parameters: Reactor coolant boron concentration = 600 ppm Moderator temperature coefficient = -0.015 %ΔK/K/°F Differential boron worth = -0.010 %ΔK/K/ppm Which one of the following is the final reactor coolant boron concentration required to decrease average reactor coolant temperature by 4°F. (Assume no change in control rod position or reactor/turbine power).

Answer 62

606 ppm

Question 63

Given the following initial parameters: Reactor coolant boron concentration = 500 ppm Moderator temperature coefficient = -0.012 %ΔK/K/°F Differential boron worth = -0.008 %ΔK/K/ppm Which one of the following is the final reactor coolant boron concentration required to increase average coolant temperature by 6°F. (Assume no change in control rod position or reactor/turbine power.)

Answer 63

491 ppm

Question 64

Given the following initial parameters: Power coefficient = -0.016 %ΔK/K/percent Differential boron worth = -0.010 %ΔK/K/ppm Control rod worth = -0.030 %ΔK/K/inch Reactor coolant boron concentration = 500 ppm Which one of the following is the final reactor coolant boron concentration required to support increasing reactor power from 30 percent to 80 percent by boration/dilution with 10 inches of outward control rod motion. (Ignore any change in fission product poison reactivity.)

Answer 64

450 ppm

Question 65

A nuclear power plant is operating at steady-state 100 percent power. Given the following initial parameters, select the final reactor coolant boron concentration required to decrease average coolant temperature by 6°F. (Assume no change in control rod position or reactor/turbine power.) Reactor coolant boron concentration = 500 ppm Moderator temperature coefficient = -0.012 %ΔK/K/°F Differential boron worth = -0.008 %ΔK/K/ppm

Answer 65

509 ppm

Question 66

Given the following initial parameters: Power coefficient = -0.020 %ΔK/K/percent Differential boron worth = -0.010 %ΔK/K/ppm Differential rod worth = -0.025 %ΔK/K/inch Reactor coolant boron concentration = 500 ppm Which one of the following is the final reactor coolant boron concentration required to support increasing reactor power from 30 percent to 80 percent by boration/dilution with 10 inches of outward control rod motion? (Ignore any change in fission product poison reactivity.)

Answer 66

425 ppm

Question 67

Given the following initial parameters: Power coefficient = -0.020 %ΔK/K/percent Differential boron worth = -0.010 %ΔK/K/ppm Differential rod worth = -0.025 %ΔK/K/inch Reactor coolant boron concentration = 500 ppm Which one of the following is the final reactor coolant boron concentration required to support decreasing reactor power from 80 percent to 30 percent by boration/dilution with 10 inches of inward control rod motion? (Ignore any change in fission product poison reactivity.)

Answer 67

575 ppm

Question 68

Given the following initial parameters: Power coefficient = -0.020 %ΔK/K/percent Differential boron worth = -0.010 %ΔK/K/ppm Differential rod worth = -0.025 %ΔK/K/inch Reactor coolant boron concentration = 600 ppm Which one of the following is the final reactor coolant boron concentration required to support increasing reactor power from 40 percent to 80 percent with 40 inches of outward control rod motion? (Ignore any change in fission product poison reactivity.)

Answer 68

620 ppm

Question 69

Given the following initial parameters: Power coefficient = -0.020 %ΔK/K/percent Differential boron worth = -0.010 %ΔK/K/ppm Differential rod worth = -0.025 %ΔK/K/inch Reactor coolant boron concentration = 500 ppm Which one of the following is the final reactor coolant boron concentration required to support decreasing reactor power from 100 percent to 30 percent by boration/dilution with 20 inches of inward control rod motion? (Ignore any change in fission product poison reactivity.)

Answer 69

590 ppm

Question 70

Given the following initial parameters: Power coefficient = -0.020 %ΔK/K/percent Differential boron worth = -0.010 %ΔK/K/ppm Differential rod worth = -0.020 %ΔK/K/inch Reactor coolant boron concentration = 600 ppm Which one of the following is the final reactor coolant boron concentration required to support increasing reactor power from 20 percent to 50 percent with 10 inches of control rod withdrawal? (Ignore any change in fission product poison reactivity.)

Answer 70

560 ppm

Question 71

Ignoring the effects of changes in fission product poisons, which one of the following power changes requires the greatest amount of positive reactivity addition?

Answer 71

30 percent to 60 percent

Question 72

Ignoring the effects of changes in fission product poisons, which one of the following power changes requires the smallest amount of positive reactivity addition?

Answer 72

2 percent to 5 percent

Question 73

Ignoring the effects of changes in fission product poisons, which one of the following power changes requires the greatest amount of positive reactivity addition?

Answer 73

60 percent to 100 percent

Question 74

Ignoring the effects of changes in fission product poisons, which one of the following reactor power changes requires the greatest amount of positive reactivity addition?

Answer 74

25 percent to 65 percent

Question 75

Ignoring the effects of changes in fission product poisons, which one of the following power changes requires the smallest amount of positive reactivity addition?

Answer 75

10 percent to 15 percent