Fission Production Poisons

Question 1

Fission products that have large microscopic cross sections for capture of thermal neutrons are called…

Answer 1

reactor poisons.

Question 2

Fission product poisons can be differentiated from other fission products in that fission product
poisons…

Answer 2

are stronger absorbers of thermal neutrons.

Question 3

A fission product poison can be differentiated from all other fission products in that a fission product
poison will…

Answer 3

depress the power production in some core locations and cause peaking in others.

Question 4

A fission product poison can be differentiated from all other fission products in that a fission product
poison…

Answer 4

has a relatively high probability of absorbing a fission neutron.

Question 5

A fission product poison can be differentiated from all other fission products because a fission product
poison…

Answer 5

has a higher microscopic cross section for thermal neutron capture

Question 6

Xenon-135 is considered a major fission product poison because it has a large…

Answer 6

absorption cross section.

Question 7

Which one of the following is a characteristic of xenon-135?

Answer 7

Thermal neutron flux level affects both the production and removal of xenon-135.

Question 8

Which one of the following has the greatest microscopic cross section for absorption of a thermal
neutron?

Answer 8

Xenon-135

Question 9

Compared to other reactor poisons, the two characteristics that make xenon-135 a major reactor poison
are its relatively __________ thermal neutron absorption cross section and its relatively __________
variation in concentration for large reactor power changes.

Answer 9

large; large

Question 10

Immediately after a reactor trip from sustained high power operation, xenon-135 concentration in the
reactor will…

Answer 10

increase, due to the decay of iodine-135.

Question 11

Xenon-135 is produced in a reactor by two primary methods. One is directly from fission; the other is
from the decay of…

Answer 11

iodine-135.

Question 12

A reactor has been operating at full power for several weeks. Xenon-135 is being directly produced
as a fission product in approximately __________ percent of all fissions.

Answer 12

0.3

Question 13

Which one of the following describes the production mechanisms of xenon-135 in a reactor that is
operating at steady-state 100 percent power?

Answer 13

Primarily from iodine decay, secondarily from fission

Question 14

What is the major contributor to the production of xenon-135 in a reactor that has been operating at full
power for two weeks?

Answer 14

Radioactive decay of I-135.

Question 15

One hour after a reactor trip from sustained 100 percent power operation, the xenon-135 removal
process consists primarily of…

Answer 15

beta decay.

Question 16

Reactor power is increased from 50 percent to 60 percent in one hour. What is the most significant
contributor to the initial change in xenon-135 reactivity?

Answer 16

Loss of xenon-135 due to absorption of neutrons.

Question 17

In a shutdown reactor, which decay chain describes the primary means of removing xenon-135?

Answer 17

135Xe β → − 135Cs

Question 18

A nuclear power plant has been operating at 100 percent power for several months. Which one of the
following describes the relative contributions of beta decay and neutron capture to xenon-135 removal
from the reactor?

Answer 18

Primary is neutron capture; secondary is beta decay

Question 19

A reactor was operating at 50 percent power for one week when power was ramped to 100 percent.
Which one of the following describes the equilibrium xenon-135 concentration at 100 percent power?

Answer 19

Less than twice the 50 percent power concentration.

Question 20

A reactor was operating at 100 percent power for one week when power was decreased to 50 percent.
Which one of the following describes the equilibrium xenon-135 concentration at 50 percent power?

Answer 20

More than one-half the 100 percent power equilibrium concentration.

Question 21

A reactor has been operating at 25 percent power for 24 hours following a two-hour power reduction
from steady-state 100 percent power. Which one of the following describes the current status of the
xenon-135 concentration?

Answer 21

Decreasing toward equilibrium.

Question 22

Following a two-week shutdown, a reactor is taken critical and ramped to 100 percent power in 6
hours. How long will it take to achieve an equilibrium xenon-135 condition after the reactor reaches
100 percent power?

Answer 22

40 to 50 hours

Question 23

Which one of the following indicates that core xenon-135 concentration is in equilibrium?

Answer 23

A reactor has been operated at 80 percent power for five days

Question 24

Reactors A and B are operating at steady-state 100 percent power with equilibrium xenon-135. The
reactors are identical except that reactor A is operating near the end of a fuel cycle (EOC) and reactor
B is operating near the beginning of a fuel cycle (BOC).
Which reactor has the greater concentration of xenon-135, and why?

Answer 24

Reactor B (BOC), due to the smaller 100 percent power thermal neutron flux.

Question 25

Reactors A and B are operating at steady-state 100 percent power with equilibrium xenon-135. The
reactors are identical except that reactor A is operating near the end of a fuel cycle (EOC) and reactor
B is operating near the beginning of a fuel cycle (BOC).
Which reactor is experiencing the most negative reactivity from equilibrium xenon-135?

Answer 25

Reactor A (EOC), due to lower competition from the fuel for thermal neutrons.

Question 26

A reactor has been operating at 50 percent power for one week when power is ramped to 100 percent
over a four-hour period. How will the xenon-135 concentration respond after power reaches 100
percent?

Answer 26

Decrease initially, and then build to a new equilibrium concentration in 40 to 50 hours.

Question 27

A reactor has been operating at a 50 percent power for 15 hours following a one-hour power reduction
from 100 percent. Which one of the following describes the current xenon-135 concentration?

Answer 27

Decreasing

Question 28

A reactor was operating for 42 weeks at a steady-state power level below 100 percent when a reactor
trip occurred. The reactor was returned to critical after 12 hours and then ramped to 60 percent power
in 6 hours.
How much time at steady-state 60 percent power will be required to reach an equilibrium xenon-135
concentration?

Answer 28

40 to 50 hours

Question 29

A reactor has been operating at 100 percent power for one week when power is ramped in 4 hours to 25
percent power. The new equilibrium xenon-135 concentration will be __________ the initial 100
percent equilibrium concentration.

Answer 29

about 50 percent of

Question 30

A reactor has been operating at a constant 50 percent power level for 15 hours following a one-hour
power reduction from steady-state 100 percent power. Which one of the following describes the
current xenon-135 concentration?

Answer 30

Decreasing toward equilibrium.

Question 31

A reactor was operating for 24 weeks at a steady-state power level below 100 percent when a reactor
trip occurred. The reactor was returned to critical after 12 hours, and then ramped to 80 percent
power in 6 hours.
Approximately how much time at steady-state 80 percent power will be required to reach an
equilibrium xenon-135 concentration?

Answer 31

40 to 50 hours

Question 32

A reactor was operating at 100 percent power for two weeks when power was decreased to 10 percent
in one hour. Immediately following the power decrease, xenon-135 concentration will __________
for a period of __________.

Answer 32

increase; 8 to 11 hours

Question 33

A reactor is initially operating at 50 percent of rated power with equilibrium xenon-135. Power is
then increased to 100 percent over a one-hour period and average reactor coolant temperature is
adjusted to 588°F using manual rod control. Rod control is left in Manual and no subsequent operator
actions are taken.
Considering only the reactivity effects of xenon-135 changes, which one of the following describes the
average reactor coolant temperature 8 hours after the power change is completed?

Answer 33

Greater than 588°F and decreasing slowly

Question 34

A reactor had been operating at 100 percent power for two weeks when power was reduced to 50
percent over a one-hour period. To maintain reactor power stable during the next 24 hours, which
one of the following incremental control rod manipulations will be required?

Answer 34

Withdraw rods slowly at first, and then insert rods slowly

Question 35

A reactor had been operating at 50 percent power for two weeks when power was increased to 100
percent over a three-hour period. To maintain reactor power stable during the next 24 hours, which
one of the following incremental control rod manipulations will be required?

Answer 35

Insert rods slowly at first, and then withdraw rods slowly.

Question 36

Which one of the following explains why xenon-135 oscillations are a concern in a reactor?

Answer 36

They can adversely affect core power distribution, and they can require operation below full rated
power.

Question 37

A reactor had been operating at 70 percent power for two weeks when power was increased to 100
percent over a two-hour period. To offset xenon-135 reactivity changes during the next 12 hours,
which one of the following incremental control rod manipulations will be required?

Answer 37

Insert rods slowly at first, and then withdraw rods slowly

Question 38

A reactor is initially operating at 100 percent power with equilibrium xenon-135. Power is decreased
to 50 percent over a one-hour period and average reactor coolant temperature is adjusted to 572°F
using manual rod control. Rod control is left in Manual and no subsequent operator actions are taken.
Considering only the reactivity effects of xenon-135 changes, which one of the following describes the
average reactor coolant temperature 10 hours after the power change is completed?

Answer 38

Less than 572°F and increasing slowly.

Question 39

A reactor is initially operating at 80 percent power with equilibrium xenon-135. Power is increased
to 100 percent over a two-hour period and average reactor coolant temperature is adjusted to 585°F
using manual rod control. Rod control is left in Manual and no subsequent operator actions are taken.
Considering only the reactivity effects of xenon-135 changes, which one of the following describes the
average reactor coolant temperature 24 hours after the power change is completed?

Answer 39

Less than 585°F and decreasing slowly.

Question 40

A reactor is initially operating at 100 percent power with equilibrium xenon-135. Power is decreased
to 40 percent over a two-hour period and average reactor coolant temperature is adjusted to 562°F
using manual rod control. Rod control is left in Manual and no subsequent operator actions are taken.
Considering only the reactivity effects of xenon-135 changes, which one of the following describes the
status of the average reactor coolant temperature two hours after the power change is completed?

Answer 40

Less than 562°F and decreasing slowly.

Question 41

Two identical reactors have been operating at a constant power level for one week. Reactor A is at 50
percent power and reactor B is at 100 percent power. If both reactors trip at the same time, xenon-135
negative reactivity will peak first in reactor _____; and the highest xenon-135 reactivity peak will
occur in reactor _____.

Answer 41

A; B

Question 42

Two identical reactors have been operating at a constant power level for one week. Reactor A is at
100 percent power and reactor B is at 50 percent power. If both reactors trip at the same time,
xenon-135 concentration will peak first in reactor _____; and the highest peak xenon-135
concentration will occur in reactor _____.

Answer 42

B; A

Question 43

A reactor has been operating at 75 percent power for two months. A manual reactor trip is required
for a test. The trip will be followed immediately by a reactor startup with criticality scheduled to
occur 12 hours after the trip.
The greatest assurance that fission product poison reactivity will permit criticality during the startup
will exist if the reactor is operated at __________ power for 48 hours prior to the trip; and if criticality
is rescheduled for __________ hours after the trip.

Answer 43

50 percent; 16

Question 44

The amount of negative reactivity associated with peak xenon-135 is smallest after a reactor trip from
equilibrium __________ reactor power at the __________ of a fuel cycle.

Answer 44

20 percent; beginning

Question 45

The amount of negative reactivity associated with peak xenon-135 is greatest after a reactor trip from
equilibrium __________ reactor power at the __________ of a fuel cycle.

Answer 45

100 percent; end

Question 46

A reactor has been operating at 80 percent power for two months. A manual reactor trip is required
for a test. The trip will be followed by a reactor startup with criticality scheduled to occur 24 hours
after the trip.
The greatest assurance that xenon-135 reactivity will permit criticality during the reactor startup will
exist if the reactor is operated at __________ power for 48 hours prior to the trip; and if criticality is
rescheduled for __________ hours after the trip.

Answer 46

60 percent; 30

Question 47

A reactor trip occurred one hour ago following several months of operation at 100 percent power.
Reactor coolant temperature is being maintained at 550°F and the source range count rate is currently
400 cps. If no additional operator action is taken, how will the source range count rate respond during
the next 24 hours? (Assume a constant source neutron flux.)

Answer 47

The count rate will initially decrease and then increase.

Question 48

Slow changes in axial power distribution in a reactor that has operated at a steady-state power level for
a long time can be caused by xenon-135…

Answer 48

oscillations

Question 49

Xenon-135 oscillations that tend to dampen themselves over time are __________ oscillations.

Answer 49

converging

Question 50

Which one of the following occurrences can cause reactor power production to fluctuate between the
top and bottom of the core when steam demand is constant?

Answer 50

Xenon-135 oscillations

Question 51

A reactor has been operating at 100 percent power for several weeks with a symmetrical axial power
distribution peaked at the core midplane. Reactor power is reduced to 50 percent using boration to
control reactor coolant temperature while maintaining control rods fully withdrawn.
During the power reduction, the axial power distribution will…

Answer 51

shift toward the top of the core.

Question 52

A reactor was initially operating at 100 percent power at the beginning of core life with equilibrium
xenon-135. Then, reactor power was reduced to 50 percent over a two-hour period.
The following information is given:
Prior to After
Power Change Power Change
Reactor power: 100 percent 50 percent
Reactor coolant
boron concentration: 740 ppm 820 ppm
Control rod position: Fully withdrawn Fully withdrawn
Control rod position: Fully withdrawn Fully withdrawn
What is the effect on power distribution in the core during the first 4 hours following the power
reduction?

Answer 52

Power production in the top of the core increases relative to the bottom of the core.

Question 53

When a reactor experiences xenon-135 oscillations, the most significant shifts in power generation
occur between the __________ of the core.

Answer 53

top and bottom

Question 54

A reactor has been operating at 80 percent power for several weeks with power production equally
distributed axially above and below the core midplane. Reactor power is increased to 100 percent
using boron dilution to control reactor coolant temperature while maintaining control rods fully
withdrawn.

Answer 54

shift toward the bottom of the core

Question 55

Which one of the following will cause reactor power production to fluctuate slowly between the top
and bottom of the core with steady-state steam demand?

Answer 55

Xenon-135 oscillations

Question 56

Xenon-135 oscillations take about __________ hours to get from maximum xenon-135 negative
reactivity to minimum xenon-135 negative reactivity

Answer 56

12 to 14

Question 57

A reactor was initially operating at 80 percent power near the beginning of a fuel cycle with
equilibrium xenon-135. Then, reactor power was increased to 100 percent over a 2 hour period.
The following information is provided:
Prior to After
Power Change Power Change
Reactor power: 100 percent 50 percent
Reactor coolant
boron concentration: 740 ppm 820 ppm
Control rod position: Fully Withdrawn Fully Withdrawn
What is the effect on power distribution in the core during the first 4 hours following the power
increase?

Answer 57

Power production in the top of the core decreases relative to the bottom of the core.

Question 58

A reactor has been operating at 100 percent power for one month following a refueling outage with
axial neutron flux distribution peaked in the bottom half of the core. An inadvertent reactor trip
occurs. The reactor is restarted, with criticality occurring 6 hours after the trip. Reactor power is
increased to 60 percent over the next 4 hours and then stabilized.
During the one-hour period immediately after power level is stabilized at 60 percent, the core axial
neutron flux peak will be located __________ in the core than the pre-scram peak location; and the
core axial neutron flux peak will be moving __________.

Answer 58

higher; downward

Question 59

A nuclear power plant is being returned to operation following a refueling outage. Fuel
preconditioning procedures require reactor power to be increased from 10 percent to 100 percent
gradually over a one-week period.
During this slow power increase, most of the positive reactivity added by the operator is required to
overcome the negative reactivity from…

Answer 59

xenon-135 buildup

Question 60

A reactor has been shut down for 7 days to perform maintenance. A reactor startup is performed, and
power level is increased to 50 percent over a 5 hour period.
When power reaches 50 percent, the magnitude of xenon-135 negative reactivity will be…

Answer 60

increasing toward an equilibrium value

Question 61

A reactor has been shut down for 5 days to perform maintenance. A reactor startup is performed, and
power is ramped to 75 percent over a 16-hour period.
When power reaches 75 percent, the concentration of xenon-135 will be…

Answer 61

increasing toward an equilibrium value.

Question 62

A reactor was shut down for 7 days to perform maintenance. Then, a reactor startup was performed,
and reactor power level was increased from 1 percent to 50 percent over a 2 hour period. Reactor
power is currently stable at 50 percent.
Ten hours after reactor power reaches 50 percent, the xenon-135 concentration will be…

Answer 62

increasing toward an equilibrium value.

Question 63

A reactor startup is being performed 5 hours after a reactor trip from 100 percent power with
equilibrium xenon-135. The reactor is currently at 10 percent power, and is being returned to 100
percent power at 2.0 percent per minute instead of the normal rate of 0.5 percent per minute.
At the faster rate of power increase, the minimum amount of xenon-135 will occur __________ than
normal; and the amount of equilibrium xenon-135 at 100 percent power will be __________.

Answer 63

sooner; the same

Question 64

A reactor was operating at 100 percent power for 8 weeks when a reactor trip occurred. The reactor
was critical 6 hours later and power was increased to 100 percent over the next 6 hours.
What was the status of xenon-135 concentration when power reached 100 percent?

Answer 64

Burning out faster than it is being produced

Question 65

Xenon-135 poisoning in a reactor is most likely to prevent a reactor startup following a reactor
shutdown from __________ power at the __________ of core life.

Answer 65

high; end

Question 66

A reactor startup is in progress 5 hours after a reactor trip from 100 percent power with equilibrium
xenon-135. The reactor is currently at 10 percent power, and is being returned to 100 percent power
at 0.25 percent per minute instead of the normal rate of 0.5 percent per minute.
At the slower rate of power increase, the maximum amount of xenon-135 will occur __________ than
normal; and the amount of equilibrium xenon-135 at 100 percent power will be __________

Answer 66

later; the same

Question 67

A nuclear power plant was operating at 100 percent power for 3 months near the beginning of a fuel
cycle when a reactor trip occurred. Eighteen hours after the reactor trip, the reactor was critical at the
point of adding heat. Then, reactor power was increased to 100 percent over a three-hour period.
During the three-hour reactor power increase to 100 percent, most of the positive reactivity added by
the operator was required to overcome the negative reactivity from…

Answer 67

fuel temperature increase.

Question 68

A reactor was operating at 100 percent power for two weeks when power was quickly reduced to 50
percent. Core xenon-135 will reach a new equilibrium concentration in __________ hours

Answer 68

40 to 50

Question 69

A reactor that has been operating at 100 percent power for two weeks is reduced in power to 50
percent. What happens to the xenon-135 concentration in the core?

Answer 69

Xenon-135 concentration will initially build up, and then decrease to a new equilibrium value

Question 70

Which one of the following describes the initial change in xenon-135 concentration immediately
following a power increase from steady-state power operation?

Answer 70

Decreases, due to the increased rate of neutron absorption by xenon-135.

Question 71

A reactor has been operating at 50 percent power for 12 hours following a one-hour power reduction
from steady-state 100 percent power. Which one of the following describes the current xenon-135
concentration?

Answer 71

Decreasing toward equilibrium.

Question 72

A reactor that had been operating at 100 percent power for about two months was shut down over a
two-hour period. Following the shutdown, xenon-135 will reach a steady-state concentration in
__________ hours.

Answer 72

70 to 80

Question 73

A reactor has been operating at 30 percent power for three hours following a one-hour power reduction
from steady-state 100 percent power. Which one of the following describes the current xenon-135
concentration?

Answer 73

Increasing toward a peak.

Question 74

A nuclear power plant is initially operating at steady-state 100 percent power in the middle of a fuel
cycle. The operators decrease main generator load while adding boric acid to the reactor coolant
system over a period of 30 minutes. At the end of this time period, reactor power is 70 percent and
average reactor coolant temperature is 575°F. All control rods remain fully withdrawn and in manual
control.
Considering only the reactivity effects of xenon-135 changes, which one of the following describes the
status of the average reactor coolant temperature 60 minutes after the power change is completed?

Answer 74

Less than 575°F and decreasing.

Question 75

A reactor has been operating at 70 percent power for 20 hours following a one-hour power reduction
from steady-state 100 percent power. Which one of the following describes the current xenon-135
concentration?

Answer 75

Decreasing toward equilibrium.

Question 76

A reactor had operated at 100 percent power for several days when a reactor trip occurred. If the
reactor had operated at 50 percent power prior to the trip, the xenon-135 concentration would peak
__________; and the peak xenon-135 concentration would be __________.

Answer 76

earlier; less negative

Question 77

Following a reactor trip, negative reactivity from xenon-135 initially increases due to…

Answer 77

xenon-135 production from the decay of iodine-135.

Question 78

Twenty-four hours after a reactor trip from 100 percent power with equilibrium xenon-135, the
xenon-135 concentration will be approximately…

Answer 78

the same as the concentration at the time of the trip and decreasing

Question 79

A reactor had been operating at 100 percent power for several days when it was shut down over a
two-hour period for maintenance. How will the xenon-135 concentration change after the shutdown?

Answer 79

Peak in 6 to 10 hours and then decay to near zero in 3 to 4 days.

Question 80

A reactor had operated at 100 percent power for three weeks when a reactor trip occurred. Which one
of the following describes the concentration of xenon-135 in the core 24 hours after the trip?

Answer 80

Approximately the same as the concentration at the time of the trip.

Question 81

Fourteen hours after a reactor trip from 100 percent power with equilibrium xenon-135, the
concentration of xenon-135 will be __________ than the 100 percent power equilibrium xenon-135
concentration; and xenon-135 will have added a net __________ reactivity since the trip.

Answer 81

greater; negative

Question 82

How does the amount of xenon-135 change immediately following a reactor trip from 100 percent
power with equilibrium xenon-135?

Answer 82

Increases, due to xenon-135 production from the decay of iodine-135.

Question 83

Given:
$ A reactor was operating at 100 percent power for six weeks when a reactor trip occurred.
$ A reactor startup was performed, and criticality was reached 16 hours after the trip.
$ Two hours later, the reactor is currently at 30 percent power with control rods in Manual.
If no operator actions are taken over the next hour, average reactor coolant temperature will
__________ because xenon-135 concentration is __________

Answer 83

increase; decreasing

Question 84

A reactor was operating at 100 percent power for 2 months when a reactor trip occurred. Four hours
later, the reactor is critical and stable at 10 percent power.
Which one of the following operator actions is required to maintain reactor coolant temperature stable
over the next 18 hours?

Answer 84

Add positive reactivity at first, and then negative reactivity

Question 85

Nuclear reactors A and B are identical and are operating near the middle of a fuel cycle. Reactor A is
operating at steady-state 100 percent power, while reactor B is operating at steady-state 50 percent
power. The integral control rod worth is the same for both reactors.
Which one of the following describes which reactor will have the greater Keff at three minutes and at
three days following a reactor trip? (Assume that all control rods fully insert and that no subsequent
operator actions affecting reactivity are taken.)

Answer 85

Three Three
Minutes Days
Reactor A Reactor A

Question 86

After a reactor shutdown from equilibrium xenon-135 conditions, the peak xenon-135 negative
reactivity is __________ the pre-shutdown power level.

Answer 86

dependent on, but not directly proportional to

Question 87

A reactor was shut down following three months of operation at full power. The shutdown occurred
over a three-hour period with a constant rate of power decrease.
Which one of the following describes the reactivity added by xenon-135 during the shutdown?

Answer 87

Xenon-135 buildup added negative reactivity

Question 88

Four hours after a reactor trip from 100 percent power operation with equilibrium xenon-135, a reactor
is taken critical and power is immediately stabilized for critical data. To maintain a constant reactor
power, the operator must add __________ reactivity because xenon-135 concentration is __________.

Answer 88

positive; increasing

Question 89

A nuclear power plant has been operating at 100 percent power for two months when a reactor trip
occurs. Shortly after the reactor trip, a reactor startup is commenced. Four hours after the trip,
reactor power is at 5 percent. To maintain reactor power at 5 percent over the next hour, the operator
must add…

Answer 89

positive reactivity, because the xenon-135 concentration is increasing.

Question 90

Following a 7 day shutdown, a reactor startup is performed and the reactor is taken to 100 percent
power over a 16-hour period. After reaching 100 percent power, what type of reactivity addition will
be needed to compensate for xenon-135 changes over the next 24 hours?

Answer 90

Positive only

Question 91

A reactor has been operating at 100 percent power for two weeks. Power is then decreased over a one
hour period to 10 percent.
Assuming manual rod control, which one of the following operator actions is required to maintain a
constant reactor coolant temperature at 10 percent power during the next 24 hours?

Answer 91

Add positive reactivity at first, and then negative reactivity

Question 92

A reactor had been operating for two months at 100 percent power when a trip occurred. Fifteen
hours later, during a reactor startup, the reactor has achieved criticality and reactor power is currently
1.0 x 10-4 percent.
Which one of the following describes the response of reactor power over the next 2 hours without any
further operator actions?

Answer 92

Power increases toward the point of adding heat, due to the decay of Xe-135.

Question 93

A reactor is initially shut down with no xenon-135 in the core. Over the next 4 hours, the reactor is
made critical and power level is increased to10 percent. The shift supervisor has directed that power
level and reactor coolant temperature be maintained constant for 12 hours.
To accomplish this objective, control rods will have to be…

Answer 93

withdrawn periodically for the duration of the 12 hours.

Question 94

A reactor is initially shut down with no xenon in the core. Over the next 4 hours, the reactor is made
critical and power level is increased to 25 percent. The shift supervisor has directed that power level
and reactor coolant temperature be maintained constant for 12 hours.
To accomplish this objective, control rods will have to be…

Answer 94

withdrawn periodically for the duration of the 12 hours.

Question 95

Initially, a reactor was operating at steady-state 70 percent power. Then, reactor power was increased
to 100 percent over a 1 hour period. To keep reactor coolant system temperature stable during the
next 2 hours, the operator must gradually __________ the control rods or __________ the reactor
coolant boron concentration.

Answer 95

insert; increase

Question 96

A reactor is operating at 60 percent power immediately after a one-hour power increase from
steady-state 40 percent power. To keep reactor coolant temperature stable over the next two hours,
the operator must __________ control rods or __________ reactor coolant boron concentration.

Answer 96

insert; increase

Question 97

Initially, a nuclear power plant was operating at 100 percent power with equilibrium xenon-135.
Then, power was decreased to 75 percent over a one-hour period. The operator is currently adjusting
control rod position as necessary to maintain average reactor coolant temperature constant.
What will the control rod position and directional trend be 30 hours after power reached 75 percent?

Answer 97

Below the initial 75 percent power position and inserting slowly.

Question 98

A nuclear power plant had been operating at 100 percent power for two months when a reactor trip
occurred. Soon afterward, a reactor startup was performed. Twelve hours after the trip, the startup
has been paused with reactor power at 5 percent.
To maintain reactor power and reactor coolant temperatures stable over the next hour, the operator
must add __________ reactivity because the xenon-135 concentration will be __________.

Answer 98

negative; decreasing

Question 99

Initially, a nuclear power plant is operating at steady-state 100 percent reactor power in the middle of
a fuel cycle. Then, the operators decrease main generator load to 90 percent over a one-hour period
while adding boric acid to the reactor coolant system. After the required amount of boric acid is
added, reactor power is 90 percent and average reactor coolant temperature is 582°F. All control rods
remain fully withdrawn and in manual control.
If no other operator actions are taken, which one of the following describes the average reactor coolant
temperature after an additional hour?

Answer 99

Lower than 582°F and decreasing slowly.

Question 100

A reactor has been shut down for 7 days following 2 months of steady-state 100 percent power
operation. A reactor startup is then performed and the reactor is taken to 100 percent power over a
12-hour period. After 100 percent power is reached, what incremental control rod positioning will be
needed to compensate for xenon-135 changes over the next 24 hours?

Answer 100

Withdraw rods slowly during the entire period.

Question 101

A nuclear power plant was initially operating at steady-state 100 percent power at the end of a fuel
cycle (EOC) when the plant was shut down for refueling. After refueling, the reactor was restarted
and the plant is currently operating at steady-state 100 percent power at the beginning of a fuel cycle
(BOC). Assume the average energy released by each fission did not change.
Compared to the equilibrium xenon-135 concentration at 100 percent power just prior to the refueling,
the current equilibrium xenon-135 concentration is…

Answer 101

greater, because the lower thermal neutron flux at BOC removes xenon-135 at a slower rate.

Question 102

With xenon-135 initially at equilibrium, which one of the following power changes will produce the
greater change in equilibrium xenon-135 negative reactivity?

Answer 102

0 percent to 10 percent