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Reactivity Coefficients Flashcards

1
Q

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

A

reactor coolant temperature

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2
Q

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?

A

U-238

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3
Q

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

A

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

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4
Q

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?

A

U-238

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5
Q

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

A

Uranium-238

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6
Q

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

A

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

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7
Q

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…

A

more negative, due to decreased reactor coolant boron concentration.

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8
Q

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?

A

U-238 and Pu-240

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9
Q

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.)

A

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

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10
Q

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.)

A

U-238 and Pu-240

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11
Q

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

A

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

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12
Q

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…

A

thermal utilization factor.

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13
Q

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

A

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

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14
Q

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

A

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

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15
Q

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

A

low; high

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16
Q 
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...
A

more negative at all coolant temperatures.

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17
Q

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…

A

an increasing resonance escape probability.

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18
Q

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

A

decreasing reactor coolant boron concentration.

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19
Q

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

A

high; low

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20
Q

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

A

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

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21
Q

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)?

A

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

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22
Q

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

A

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

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23
Q 
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...
A

less negative at all coolant temperatures.

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24
Q

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

A

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

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25
Q

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

A

The initially negative MTC becomes less negative.

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26
Q

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.)

A

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

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27
Q

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

A

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

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28
Q

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…

A

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

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29
Q

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

A

The initially positive MTC becomes more positive

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30
Q

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…

A

decrease continuously.

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31
Q

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

A

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

32
Q

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

A

Lower power level

33
Q

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
__________.

A

more; decreases

34
Q

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?

A

U-238 and Pu-240

35
Q

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.)

A

Increase in fuel temperature.

36
Q

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

A

less; higher fuel temperature

37
Q

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 __________.

A

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

38
Q

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

A

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

39
Q

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.)

A

end; more Pu-240 is in the core

40
Q

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 __________.

A

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

41
Q

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 __________.

A

decrease; remain the same

42
Q

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 __________.

A

taller and more narrow; decrease

43
Q

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 __________.

A

increase; remain the same

44
Q

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 __________.

A

increase; decrease

45
Q

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 __________.

A

decrease; increase

46
Q

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.)

A

30 percent to 40 percent

47
Q

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 ________.

A

shorter and broader; increase

48
Q

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

A

less; smaller

49
Q

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

A

Moderator void fraction, fuel temperature, moderator temperature

50
Q

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.)

A

The change in fuel temperature.

51
Q

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?

A

Moderator void fraction, fuel temperature, and moderator temperature

52
Q

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…

A

increased fuel temperature.

53
Q

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…

A

increased fuel temperature.

54
Q

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

A

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

55
Q

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 __________.

A

higher; smaller

56
Q

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

A

less; fewer

57
Q

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

A

burnable poisons deplete.

58
Q

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?

A

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.

59
Q

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.

A

the same

60
Q

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.

A

10 times

61
Q

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

A

unit; total

62
Q

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).

A

606 ppm

63
Q

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.)

A

491 ppm

64
Q

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.)

A

450 ppm

65
Q

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

A

509 ppm

66
Q

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.)

A

425 ppm

67
Q

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.)

A

575 ppm

68
Q

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.)

A

620 ppm

69
Q

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.)

A

590 ppm

70
Q

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.)

A

560 ppm

71
Q

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

A

30 percent to 60 percent

72
Q

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

A

2 percent to 5 percent

73
Q

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

A

60 percent to 100 percent

74
Q

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?

A

25 percent to 65 percent

75
Q

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

A

10 percent to 15 percent

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