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Control Rods Flashcards

1
Q

A reactor is initially critical well below the point of adding heat (POAH) during a reactor startup.
Control rods are withdrawn for 20 seconds to establish a 0.5 DPM startup rate.
In response to the control rod withdrawal, reactor power will initially increase, and then…

A. continue increasing until the control rods are reinserted.

B. stabilize at a value slightly below the POAH.

C. stabilize at the POAH.

D. stabilize at a value slightly above the POAH.

A

stabilize at a value slightly above the POAH.

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2
Q

A reactor is initially critical below the point of adding heat during a reactor startup. If control
rods are manually inserted for 5 seconds, reactor power will decrease…

A. to a lower power level determined by subcritical multiplication.

B. temporarily, then return to the original power level due to subcritical multiplication.

C. temporarily, then return to the original power level due to a decrease in moderator temperature.

D. until inherent positive reactivity feedback causes the reactor to become critical at a lower
power level.

A

to a lower power level determined by subcritical multiplication.

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3
Q

A reactor is initially critical below the point of adding heat (POAH) during a reactor startup. If
control rods are manually withdrawn for 5 seconds, reactor power will initially increase and then…

A. stabilize at a critical power level below the POAH.

B. decrease and stabilize at the original value.

C. stabilize at a critical power level at the POAH.

D. decrease and stabilize below the original value.

A

stabilize at a critical power level at the POAH.

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4
Q

A reactor is operating at steady-state 50 percent power near the end of a fuel cycle when the
operator withdraws a group of control rods for 5 seconds. (Assume main turbine load remains
constant and the reactor does not trip.)
In response to the control rod withdrawal, actual reactor power will stabilize __________ the
initial power level and reactor coolant temperature will stabilize __________ the initial
temperature.

A. at; at

B. at; above

C. above; at

D. above; above

A

at; above

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5
Q

Initially, a reactor is operating at steady-state 50 percent power, when control rods are inserted a
short distance. Assume that main turbine-generator load remains constant and the reactor does
not trip.
In response to the control rod insertion, reactor power will initially decrease, and then…

A. stabilize in the source range.

B. stabilize at a lower value in the power range.

C. increase and stabilize above the original value.

D. increase and stabilize at the original value.

A

increase and stabilize at the original value.

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6
Q

A reactor is operating at steady-state 50 percent power near the end of a fuel cycle when the
operator inserts a group of control rods for 5 seconds. Assume that turbine load remains constant
and the reactor does not trip.
In response to the control rod insertion, reactor power will stabilize __________ the initial power
level and reactor coolant temperature will stabilize __________ the initial temperature.

A. at; at

B. at; below

C. below; at

D. below; below

A

at; below

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7
Q

A reactor has been shut down for three weeks with all control rods fully inserted. If a single
control rod is fully withdrawn from the core, neutron flux level will… (Assume the reactor
remains subcritical.)

A. increase and stabilize above the original level.

B. increase, then decrease and stabilize at the original level.

C. increase, then decrease and stabilize above the original level.

D. remain the same during and after the withdrawal.

A

increase and stabilize above the original level.

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8
Q

A reactor has been shut down for three weeks with all control rods fully inserted. If a center
control rod is fully withdrawn from the core, neutron flux level will… (Assume the reactor
remains subcritical.)

A. remain the same.

B. increase and stabilize at a new higher level.

C. increase temporarily then return to the original level.

D. increase exponentially until the operator reinserts the center control rod.

A

increase and stabilize at a new higher level.

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9
Q

Criticality has been achieved during a xenon-free reactor startup. The core neutron flux level is
low in the intermediate range with a stable 0.5 DPM startup rate (SUR). The operator begins
inserting control rods in an effort to stabilize the core neutron flux level near its current value.
The operator stops inserting control rods when the SUR indicates exactly 0.0 DPM.
Immediately after the operator stops inserting the control rods, the SUR will become __________;
and the core neutron flux level will __________.

A. positive; increase exponentially
.

B. positive; increase linearly

C. negative; decrease exponentially

D. negative; decrease linearly

A

positive; increase exponentially
.

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10
Q

The total amount of reactivity added by a control rod position change from a reference height to
any other rod height is called…

A. differential rod worth.

B. excess reactivity.

C. integral rod worth.

D. reference reactivity.

A

integral rod worth.

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11
Q

Integral control rod worth can be described as the change in __________ for a __________ change
in rod position.

A. reactor power; total

B. reactivity; unit

C. reactor power; unit

D. reactivity; total

A

reactivity; total

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12
Q

A control rod is positioned in a reactor with the following neutron flux parameters:
Core average thermal neutron flux = 1 x 10^12 neutrons/cm^2
-sec
Control rod tip thermal neutron flux = 5 x 10^12 neutrons/cm^2
-sec
If the control rod is slightly withdrawn such that the tip of the control rod is located in a thermal
neutron flux of 1 x 10^13 neutrons/cm^2
-sec, the differential control rod worth will increase by a
factor of __________. (Assume the core average thermal neutron flux is constant.)

A. 0.5

B. 1.4

C. 2.0

D. 4.0

A

4.0

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13
Q

Integral rod worth is the…

A. change in reactivity per unit change in control rod position.

B. rod worth associated with the most reactive control rod.

C. change in worth of a control rod per unit change in reactor power.

D. reactivity added by moving a control rod from one position to another position.

A

reactivity added by moving a control rod from one position to another position.

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14
Q

Reactor power was ramped from 80 percent power to 100 percent power over 4 hours. The 80
percent conditions were as follows:
Reactor coolant system (RCS) boron concentration = 600 ppm
Control rod position = 110 inches
RCS average temperature = 575°F
The 100 percent conditions are as follows:
RCS boron concentration = 580 ppm
Control rod position = 130 inches
RCS average temperature = 580°F
Given the following reactivity coefficient/worth values, and ignoring fission product poison
reactivity changes, what was the average differential control rod worth during the power change?
Power coefficient = -0.03 %ΔK/K/percent
Moderator temperature coefficient = -0.02 %ΔK/K/°F
Differential boron worth = -0.01 %ΔK/K/ppm

A. -0.02 %ΔK/K/inch

B. -0.025 %ΔK/K/inch

C. -0.04 %ΔK/K/inch

D. -0.05 %ΔK/K/inch

A

-0.02 %ΔK/K/inch

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15
Q

A control rod is positioned in a reactor with the following neutron flux parameters:
Core average thermal neutron flux = 1.0 x 10^12 n/cm^2
-sec
Control rod tip thermal neutron flux = 5.0 x 10^12 n/cm^2
-sec
If the control rod is slightly inserted such that the control rod tip is located in a thermal neutron
flux of 1.0 x 10^13 n/cm^2
-sec, the differential control rod worth will increase by a factor of
__________. (Assume the core average thermal neutron flux is constant.)

A. 2

B. 4

C. 10

D. 100

A

4

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16
Q

A control rod is positioned in a reactor with the following neutron flux parameters:
Core average thermal neutron flux = 1.0 x 10^12 n/cm^12
-sec
Control rod tip thermal neutron flux = 4.0 x 10^12 n/cm^2
-sec
If the control rod is slightly inserted such that the control rod tip is located in a thermal neutron flux
of 1.2 x 10^13 n/cm^2
-sec, the differential control rod worth will increase by a factor of __________.
(Assume the core average thermal neutron flux is constant.)

A. 1/3

B. 3

C. 9

D. 27

A

9

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17
Q

A reactor is initially operating at steady state 70 percent power with the following conditions:
Reactor coolant system (RCS) boron concentration = 600 ppm
Control rod position = 110 inches
RCS average temperature = 575°F
Reactor power is increased to 100 percent. The 100 percent reactor power conditions are as
follows:
RCS boron concentration = 590 ppm
Control rod position = 130 inches
RCS average temperature = 580°F
Given the following reactivity coefficient/worth values, and ignoring fission product poison
reactivity changes, what was the average differential control rod worth during the power change?
Power coefficient = -0.03 %ΔK/K/percent
Moderator temperature coefficient = -0.02 %ΔK/K/°F
Differential boron worth = -0.01 %ΔK/K/ppm

A. -0.02 %ΔK/K/inch

B. -0.025 %ΔK/K/inch

C. -0.04 %ΔK/K/inch

D. -0.05 %ΔK/K/inch

A

-0.04 %ΔK/K/inch

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18
Q

A control rod is positioned in a reactor with the following neutron flux parameters:
Core average thermal neutron flux = 1.0 x 10^12 n/cm^2
-sec
Control rod tip thermal neutron flux = 4.0 x 10^12 n/cm^2
-sec
If the control rod is slightly inserted such that the control rod tip is located in a thermal neutron flux
of 1.6 x 10^13 n/cm^2
-sec, the differential control rod worth will increase by a factor of __________.
(Assume the core average thermal neutron flux is constant.)

A. 2

B. 4

C. 8

D. 16

A

16

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19
Q

Which one of the following expresses the relationship between differential rod worth (DRW) and
integral rod worth (IRW)?

A. DRW is the area under the IRW curve at a given rod position.

B. DRW is the slope of the IRW curve at a given rod position.

C. DRW is the IRW at a given rod position.

D. DRW is the square root of the IRW at a given rod position.

A

DRW is the slope of the IRW curve at a given rod position.

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20
Q

Which one of the following parameters typically has the greatest influence on the shape of a
differential rod worth curve?

A. Core radial neutron flux distribution

B. Core axial neutron flux distribution

C. Core xenon distribution

D. Burnable poison distribution

A

Core axial neutron flux distribution

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21
Q

During normal full power operation, the differential control rod worth is less negative at the top
and bottom of the core compared to the center regions due to the effects of…

A. reactor coolant boron concentration.

B. neutron flux distribution.

C. xenon concentration.

D. fuel temperature distribution.

A

neutron flux distribution

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22
Q

Which one of the following expresses the relationship between differential rod worth (DRW) and
integral rod worth (IRW)?

A. IRW is the slope of the DRW curve.

B. IRW is the inverse of the DRW curve.

C. IRW is the sum of the DRWs between the initial and final control rod positions.

D. IRW is the sum of the DRWs of all control rods at a specific control rod position.

A

IRW is the sum of the DRWs between the initial and final control rod positions.

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23
Q

As moderator temperature increases, the differential rod worth becomes more negative because…

A. moderator density decreases, which causes more neutron leakage out of the core.

B. the moderator temperature coefficient decreases, which causes decreased competition for
neutrons.

C. fuel temperature also increases, which decreases the rate of neutron absorption in the fuel.

D. moderator density decreases, which increases the neutron migration length.

A

moderator density decreases, which increases the neutron migration length.

24
Q

Differential rod worth will become most negative if reactor coolant temperature is __________
and reactor coolant boron concentration is __________.

A. increased; decreased

B. decreased; decreased

C. increased; increased

D. decreased; increased

A

increased; decreased

25
Q

A nuclear power plant is operating at 50 percent power with one group of control rods partially
inserted into the core. If the moderator temperature decreases by 5°F, the group differential
control rod worth will become…

A. more negative, due to better moderation of neutrons.

B. less negative, due to shorter neutron migration lengths.

C. more negative, due to increased moderator absorption of neutrons.

D. less negative, due to increased resonance absorption of neutrons.

A

less negative, due to shorter neutron migration lengths.

26
Q

As moderator temperature increases, the differential rod worth becomes…

A. more negative due to longer neutron diffusion lengths.

B. more negative due to decreased resonance absorption of neutrons.

C. less negative due to reduced moderation of neutrons.

D. less negative due to decreased moderator absorption of neutrons

A

more negative due to longer neutron diffusion lengths.

27
Q

A reactor is operating at 60 percent power near the end of a fuel cycle with the controlling group of
control rods inserted 5 percent into the core. Which one of the following will cause the group
differential rod worth to become less negative? (Consider only the direct effect of the indicated
change.)

A. Burnable poison rods become increasingly depleted.

B. Core Xe-135 concentration decreases toward an equilibrium value.

C. Reactor coolant temperature is allowed to decrease from 575°F to 570°F.

D. The group of control rods is inserted an additional 0.5 percent.

A

Reactor coolant temperature is allowed to decrease from 575°F to 570°F.

28
Q

A reactor startup is in progress from a cold shutdown condition. During the reactor coolant
heatup phase of the startup, the differential control rod worth will become __________ negative;
and during the complete withdrawal of the initial bank of control rods, the differential control rod
worth will become __________.

A. more; more negative initially and then less negative

B. more; less negative initially and then more negative

C. less; more negative during the entire withdrawal

D. less; less negative during the entire withdrawal

A

more; more negative initially and then less negative

29
Q

Which one of the following will cause the differential rod worth for a group of control rods to
become less negative? (Consider only the direct effect of the initiated change.)

A. During long-term full power operation, fuel temperature decreases as the fuel pellets come into
contact with the fuel clad.

B. The reactor coolant system is cooled from 170°F to 120°F in preparation for refueling.

C. Core xenon-135 builds up in the lower half of the core.

D. During the fuel cycle, the quantity of burnable poisons decreases.

A

The reactor coolant system is cooled from 170°F to 120°F in preparation for refueling.

30
Q

The main reason for designing and operating a reactor with a flattened neutron flux distribution is
to…

A. provide even burnup of control rods.

B. reduce neutron leakage from the core.

C. achieve a higher average power density.

D. provide more accurate nuclear power indication.

A

achieve a higher average power density.

31
Q

Which one of the following is a reason for neutron flux shaping in a reactor core?

A. To minimize local power peaking by more evenly distributing the core thermal neutron flux.

B. To reduce thermal neutron leakage by decreasing the neutron flux at the periphery of the
reactor core.

C. To reduce the size and number of control rods needed to shut down the reactor during a reactor
trip.

D. To increase differential control rod worth by peaking the thermal neutron flux at the top of the
reactor core.

A

To minimize local power peaking by more evenly distributing the core thermal neutron flux.

32
Q

Which one of the following includes two reasons for control rod bank/group overlap?

A. Provides a more uniform differential rod worth, and minimizes axial neutron flux peaking.

B. Provides a more uniform differential rod worth, and allows dampening of xenon-induced
neutron flux oscillations.

C. Ensures that all rods remain within the allowable tolerance between their individual position
indicators and their group counters, and ensures rod insertion limits are not exceeded.

D. Ensures that all rods remain within their allowable tolerance between individual position
indicators and their group counters, and provides a more uniform axial flux distribution.

A

Provides a more uniform differential rod worth, and minimizes axial neutron flux peaking.

33
Q

Which one of the following includes two reasons for control rod bank/group overlap?

A. Provide a more uniform axial power distribution and provide a more uniform differential rod
worth.

B. Provide a more uniform differential rod worth and provide a more uniform radial power
distribution.

C. Provide a more uniform radial power distribution and maintain individual and group rod
position indicators within allowable tolerances.

D. Maintain individual and group rod position indicators within allowable tolerances and provide
a more uniform axial power distribution.

A

Provide a more uniform axial power distribution and provide a more uniform differential rod
worth.

34
Q

One purpose of using control rod bank/group overlap is to…

A. ensure adequate shutdown margin.

B. provide a more uniform differential rod worth.

C. allow dampening of xenon-induced neutron flux oscillations.

D. ensure control rod insertion limits are not exceeded.

A

provide a more uniform differential rod worth.

35
Q

A reactor has been operating at 100 percent power for several weeks near the middle of a fuel cycle
with all control rods fully withdrawn. Which one of the following describes why most of the
power is being produced in the lower half of the reactor core?

A. Xenon-135 concentration is lower in the lower half of the core.

B. The moderator to fuel ratio is lower in the lower half of the core.

C. The fuel loading in the lower half of the core contains a higher uranium-235 enrichment.

D. The moderator temperature coefficient of reactivity is adding less negative reactivity in the
lower half of the core.

A

The moderator temperature coefficient of reactivity is adding less negative reactivity in the
lower half of the core.

36
Q

A reactor is operating at steady-state 75 percent power in the middle of a fuel cycle. Which one of
the following actions will cause the greatest shift in reactor power distribution toward the top of
the core? (Assume control rods remain fully withdrawn.)

A. Decrease reactor power by 25 percent.

B. Decrease reactor coolant boron concentration by 10 ppm.

C. Decrease average reactor coolant temperature by 5°F.

D. Decrease reactor coolant system operating pressure by 15 psia.

A

Decrease reactor power by 25 percent.

37
Q

A reactor has been operating at 100 percent power for three weeks shortly after a refueling outage.
All control rods are fully withdrawn. Which one of the following describes why most of the
power is being produced in the lower half of the core?

A. The fuel loading in the lower half of the core contains a higher U-235 enrichment.

B. Reactor coolant boron is adding more negative reactivity in the upper half of the core.

C. There is a greater concentration of Xe-135 in the upper half of the core.

D. The moderator temperature coefficient of reactivity is adding more negative reactivity in the
upper half of the core.

A

The moderator temperature coefficient of reactivity is adding more negative reactivity in the
upper half of the core.

38
Q

If core quadrant power distribution (sometimes called quadrant power tilt or azimuthal tilt) is
maintained within design limits, which one of the following conditions is most likely?

A. Axial power distribution is within design limits.

B. Radial power distribution is within design limits.

C. Nuclear instrumentation is indicating within design accuracy.

D. Departure from nucleate boiling ratio is within design limits.

A

Radial power distribution is within design limits.

39
Q

A reactor was restarted following a refueling outage and is currently at the point of adding heat.
Which one of the following describes the change in axial power distribution as reactor power is
increased to 5 percent by control rod withdrawal?

A. Shifts toward the bottom of the core.

B. Shifts toward the top of the core.

C. Shifts from the center of the core toward the top and bottom of the core.

D. Shifts from the top and bottom of the core toward the center of the core.

A

Shifts toward the top of the core.

40
Q

By maintaining the radial and axial core power distributions within their prescribed limits, the
operator is assured that __________ will remain within acceptable limits.

A. power density (kW/foot) and departure from nucleate boiling ratio (DNBR)

B. DNBR and shutdown margin

C. core delta-T and power density (kW/foot)

D. shutdown margin and core delta-T

A

power density (kW/foot) and departure from nucleate boiling ratio (DNBR)

41
Q

Consider a reactor core with four quadrants: A, B, C, and D. The reactor is operating at
steady-state 90 percent power when a fully withdrawn control rod in quadrant C drops to the
bottom of the core. Assume that no operator actions are taken and reactor power stabilizes at 88
percent.
How are the maximum upper and lower core power tilt values (sometimes called quadrant power
tilt ratio or azimuthal power tilt) affected by the dropped rod?

A. Upper core value decreases while lower core value increases.

B. Upper core value increases while lower core value decreases.

C. Both upper and lower core values decrease.

D. Both upper and lower core values increase.

A

Both upper and lower core values increase.

42
Q

A reactor is operating at steady-state 100 percent power when a single control rod fully inserts
from the fully withdrawn position. After the initial transient, the operator returns the reactor to
100 percent power with the control rod still fully inserted.
Compared to the initial core axial neutron flux shape, the current core axial neutron flux shape will
have a…

A. minor distortion, because the fully inserted control rod has nearly zero reactivity worth.

B. minor distortion, because the fully inserted control rod is an axially uniform poison.

C. major distortion, because the upper and lower core halves are tightly coupled in the vicinity of
the control rod.

D. major distortion, because the power production will be drastically reduced in the vicinity of the
control rod.

A

minor distortion, because the fully inserted control rod is an axially uniform poison.

43
Q

After a control rod is fully inserted (from the fully withdrawn position), the effect on the axial flux
shape is minimal. This is because…

A. the differential rod worth is constant along the length of the control rod.

B. the fully inserted control rod is an axially uniform poison.

C. a control rod only has reactivity worth if it is moving.

D. a variable poison distribution exists throughout the length of the control rod.

A

the fully inserted control rod is an axially uniform poison.

44
Q

The control rod insertion limits generally rise as reactor power increases because…

A. the power defect becomes more negative as power increases.

B. the control rod worth becomes more negative as power increases.

C. the fuel temperature coefficient becomes more negative as power increases.

D. the equilibrium xenon-135 reactivity becomes more negative as power increases.

A

the power defect becomes more negative as power increases.

45
Q

Control rod insertion limits are established for power operation because excessive rod insertion
will…

A. adversely affect core power distribution.

B. generate excessive liquid waste due to dilution.

C. cause reduced control rod lifetime.

D. cause unacceptable fast and thermal neutron leakage.

A

adversely affect core power distribution.

46
Q

Control rod insertion limits ensure that control rods will be more withdrawn as reactor power
__________ to compensate for the change in __________.

A. increases; xenon reactivity

B. decreases; xenon reactivity

C. increases; power defect

D. decreases; power defect

A

increases; power defect

47
Q

Why are control rod insertion limits established for power operation?

A. To minimize the worth of a dropped control rod.

B. To maintain a negative moderator temperature coefficient.

C. To provide adequate shutdown margin after a reactor trip.

D. To ensure sufficient positive reactivity is available to compensate for the existing power
defect.

A

To provide adequate shutdown margin after a reactor trip.

48
Q

A reactor has been operating at 80 percent power for four weeks with the controlling rod
bank/group inserted 10 percent from the fully withdrawn position.
Which one of the following will be most affected by inserting the controlling bank/group an
additional 5 percent? (Assume steady-state reactor power does not change.)

A. Total xenon-135 reactivity

B. Radial power distribution

C. Quadrant (azimuthal) power distribution

D. Axial power distribution

A

Axial power distribution

49
Q

A reactor is operating at steady-state 75 percent power with all control rods fully withdrawn.
Assuming the reactor does not trip, which one of the following compares the effects of dropping
(full insertion) a center control rod to the effects of partially inserting (50 percent) the same control
rod?

A. A dropped rod causes a greater change in shutdown margin.

B. A dropped rod causes a smaller change in shutdown margin.

C. A dropped rod causes a greater change in axial power distribution.

D. A dropped rod causes a greater change in radial power distribution.

A

A dropped rod causes a greater change in radial power distribution.

50
Q

A reactor is operating at steady-state 75 percent power with all control rods fully withdrawn.
Assuming the reactor does not trip, which one of the following compares the effects of dropping
(full insertion) a center control rod to the effects of partially inserting (50 percent) the same control
rod?

A. A partially inserted rod causes a greater change in axial power distribution.

B. A partially inserted rod causes a greater change in radial power distribution.

C. A partially inserted rod causes a greater change in shutdown margin.

D. A partially inserted rod causes a smaller change in shutdown margin.

A

A partially inserted rod causes a greater change in axial power distribution.

51
Q

Assuming the reactor power does not trip, which one of the following compares the effects of
dropping (full insertion) a center control rod to the effects of partially inserting (50 percent) the
same control rod?

A. A dropped rod causes a smaller change in axial power distribution.

B. A dropped rod causes a smaller change in radial power distribution.

C. A dropped rod causes a smaller change in shutdown margin.

D. A dropped rod causes a greater change in shutdown margin.

A

A dropped rod causes a smaller change in axial power distribution.

52
Q

A reactor is operating at steady-state 85 percent power with all control rods fully withdrawn.
Assuming the reactor does not trip, which one of the following compares the effects of partially
inserting (50 percent) a center control rod to the effects of dropping (full insertion) the same
control rod?

A. A partially inserted rod causes a smaller change in axial power distribution.

B. A partially inserted rod causes a smaller change in radial power distribution.

C. A partially inserted rod causes a greater change in shutdown margin.

D. A partially inserted rod causes a smaller change in shutdown margin.

A

A partially inserted rod causes a smaller change in radial power distribution.

53
Q

A reactor is operating at steady-state 100 percent power at the beginning of a fuel cycle with all
control rods fully withdrawn. Assuming the reactor does not trip, which one of the following
compares the effects of dropping a control rod in the center of the core to dropping an identical
control rod at the periphery of the core?

A. Dropping a center control rod causes a greater change in shutdown margin.

B. Dropping a center control rod causes a smaller change in shutdown margin.

C. Dropping a center control rod causes a greater change in axial power distribution.

D. Dropping a center control rod causes a greater change in radial power distribution.

A

Dropping a center control rod causes a greater change in radial power distribution.

54
Q

A reactor has been operating at 80 percent power for four weeks with the controlling rod group
inserted 15 percent from the fully withdrawn position.
Which one of the following will be significantly affected by withdrawing the controlling rod group
an additional 5 percent? (Assume steady-state reactor power does not change.)

A. Total xenon-135 reactivity

B. Axial power distribution

C. Radial power distribution

D. Quadrant (azimuthal) power distribution

A

Axial power distribution

55
Q

A reactor is operating at steady-state 100 percent power with all control rods fully withdrawn when
one control rod at the core periphery falls completely into the core. Assuming no reactor trip and
no operator action, which one of the following will change significantly as a result of the dropped
rod?

A. Axial power distribution only.

B. Axial power distribution and shutdown margin.

C. Radial power distribution only.

D. Radial power distribution and shutdown margin.

A

Radial power distribution only.

TEST (22 decks)

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