Fission Production Poisons

Question 1

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

A. breeder fuels.

B. burnable poisons.

C. fissionable fuels.

D. reactor poisons.

Answer 1

reactor poisons.

Question 2

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

A. have a longer half-life.

B. are stronger absorbers of thermal neutrons.

C. are produced in a larger percentage of fissions.

D. have a higher fission cross section for thermal neutrons.

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…

A. be produced in direct proportion to the fission rate in the core.

B. remain radioactive for thousands of years after the final reactor criticality.

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

D. migrate out of the fuel pellets and into the reactor coolant via pinhole defects in the clad.

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…

A. will be radioactive for thousands of years.

B. is produced in a relatively large percentage of thermal fissions.

C. has a relatively high probability of absorbing a fission neutron.

D. is formed as a gas and is contained within the fuel pellets and fuel rods.

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…

A. has a higher microscopic cross section for thermal neutron capture.

B. has a longer half-life.

C. is produced in a greater percentage of thermal fissions.

D. is formed as a gas and is contained in the fuel pellets.

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…

A. fission cross section.

B. absorption cross section.

C. elastic scatter cross section.

D. inelastic scatter cross section.

Answer 6

absorption cross section.

Question 7

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

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

B. Thermal neutrons interact with xenon-135 primarily through scattering reactions.

C. Xenon-135 is primarily a resonance absorber of epithermal neutrons.

D. Xenon-135 is produced from the radioactive decay of barium-135.
ANSWER: A.

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?

A. Uranium-235

B. Boron-10

C. Samarium-149

D. Xenon-135

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.

A. small; large

B. small; small

C. large; small

D. large; large

Answer 9

large; large

Question 10

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

A. increase, due to the decay of iodine-135.

B. decrease, because xenon-135 production from fission has stopped.

C. remain the same, because the decay of iodine-135 and xenon-135 balance each other out.

D. decrease initially, and then slowly increase due to the differences in the half-lives of iodine-135
and xenon-135.

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…

A. cesium-135.

B. iodine-135.

C. xenon-136.

D. iodine-136.

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.

A. 100

B. 30

C. 3

D. 0.3

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?

A. Primarily from fission, secondarily from iodine decay

B. Primarily from fission, secondarily from promethium decay

C. Primarily from iodine decay, secondarily from fission

D. Primarily from promethium decay, secondarily from fission

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?

A. Radioactive decay of I-135.

B. Radioactive decay of Cs-135.

C. Direct production from fission of U-235.

D. Direct production from fission of U-238.

Answer 14

Radioactive decay of I-135.

Question 15

One minute after a reactor trip from steady-state 100 percent reactor power, the greatest xenon-135
production rate will be from __________; and the greatest xenon-135 removal rate will be caused by
__________.

A. fission; xenon-135 decay

B. fission; neutron capture

C. iodine-135 decay; xenon-135 decay

D. iodine-135 decay; neutron capture

Answer 15

iodine-135 decay; xenon-135 decay

Question 16

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

A. beta decay.

B. gamma decay.

C. neutron capture.

D. gamma capture.

Answer 16

beta decay.

Question 17

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?

A. Production of xenon-135 directly from fission.

B. Production of xenon-135 from iodine-135 decay.

C. Loss of xenon-135 due to absorption of neutrons.

D. Loss of xenon-135 due to decay to cesium-135.

Answer 17

Loss of xenon-135 due to absorption of neutrons.

Question 18

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

A. 135^Xe β
→
−
135^
Cs

B. 135^Xe n
→ 134^Xe

C. 135^Xe α
→ 131^
Te

D. 135^Xe β
→
+
131^
I

Answer 18

135^Xe β
→
−
135^
Cs

Question 19

Xenon-135 undergoes radioactive decay to…

A. iodine-135.

B. cesium-135.

C. tellurium-135.

D. lanthanum-135.

Answer 19

cesium-135.

Question 20

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?

A. Primary is neutron capture; secondary is beta decay.

B. Primary is beta decay; secondary is neutron capture.

C. Beta decay and neutron capture contribute equally.

D. Not enough information is given to make a comparison.

Answer 20

Primary is neutron capture; secondary is beta decay.

Question 21

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?

A. Twice the 50 percent power concentration.

B. Less than twice the 50 percent power concentration.

C. More than twice the 50 percent power concentration.

D. Remains the same, because it is independent of power.

Answer 21

Less than twice the 50 percent power concentration.

Question 22

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?

A. The same as the 100 percent power equilibrium concentration.

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

C. One-half the 100 percent power equilibrium concentration.

D. Less than one-half the 100 percent power equilibrium concentration.

Answer 22

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

Question 23

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?

A. At equilibrium.

B. Decreasing toward an upturn.

C. Decreasing toward equilibrium.

D. Increasing toward a peak.

Answer 23

Decreasing toward equilibrium.

Question 24

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?

A. 70 to 80 hours

B. 40 to 50 hours

C. 8 to 10 hours

D. 1 to 2 hours

Answer 24

40 to 50 hours

Question 25

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

A. Xenon-135 production and removal rates are momentarily equal five hours after a power increase.

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

C. Xenon-135 is being produced equally by fission and I-135 decay.

D. A reactor is currently operating at 100 percent power.

Answer 25

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

Question 26

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?

A. Reactor A (EOC), due to the smaller 100 percent power thermal neutron flux.

B. Reactor A (EOC), due to the larger 100 percent power thermal neutron flux.

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

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

Answer 26

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

Question 27

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?

A. Reactor A (EOC), due to a greater equilibrium concentration of xenon-135.

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

C. Reactor B (BOC), due to a greater thermal neutron flux in the core.

D. Reactor B (BOC), due to a smaller accumulation of fission product poisons.

Answer 27

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

Question 28

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?

A. Decrease initially, and then build to a new equilibrium concentration in 8 to 10 hours.

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

C. Increase steadily to a new equilibrium concentration in 20 to 30 hours.

D. Increase steadily to a new equilibrium concentration in 70 to 80 hours.

Answer 28

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

Question 29

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?

A. Increasing

B. Decreasing

C. At equilibrium

D. Oscillating

Answer 29

Decreasing

Question 30

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?

A. 20 to 30 hours

B. 40 to 50 hours

C. 70 to 80 hours

D. Unable to determine without knowledge of previous power history

Answer 30

40 to 50 hours

Question 31

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.

A. the same as

B. about 80 percent of

C. about 50 percent of

D. less than 25 percent of

Answer 31

about 50 percent of

Question 32

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?

A. Increasing toward a peak.

B. Decreasing toward an upturn.

C. Increasing toward equilibrium.

D. Decreasing toward equilibrium.

Answer 32

Decreasing toward equilibrium.

Question 33

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?

A. 10 to 20 hours

B. 40 to 50 hours

C. 70 to 80 hours

D. Cannot determine without knowledge of previous power history

Answer 33

40 to 50 hours

Question 34

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 __________.

A. decrease; 4 to 6 hours

B. increase; 4 to 6 hours

C. decrease; 8 to 11 hours

D. increase; 8 to 11 hours

Answer 34

increase; 8 to 11 hours

Question 35

Initially, a reactor is operating at 50 percent power with equilibrium xenon-135. Then power is
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?

A. Greater than 588°F and decreasing slowly

B. Greater than 588°F and increasing slowly

C. Less than 588°F and decreasing slowly

D. Less than 588°F and increasing slowly

Answer 35

Greater than 588°F and decreasing slowly

Question 36

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?

A. Withdraw rods slowly during the entire period.

B. Withdraw rods slowly at first, and then insert rods slowly.

C. Insert rods slowly during the entire period.

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

Answer 36

Withdraw rods slowly at first, and then insert rods slowly.

Question 37

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?

A. Withdraw rods slowly during the entire period.

B. Withdraw rods slowly at first, and then insert rods slowly.

C. Insert rods slowly during the entire period.

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

Answer 37

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

Question 38

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

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

B. They can adversely affect core power distribution, and they can prevent reactor criticality during a
reactor startup.

C. They can cause excessively short reactor periods during power operation, and they can require
operation below full rated power.

D. They can cause excessively short reactor periods during power operation, and they can prevent
reactor criticality during a reactor startup.

Answer 38

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

Question 39

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?

A. Withdraw rods slowly during the entire period.

B. Withdraw rods slowly at first, and then insert rods slowly.

C. Insert rods slowly during the entire period.

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

Answer 39

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

Question 40

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?

A. Less than 572°F and increasing slowly.

B. Less than 572°F and decreasing slowly.

C. Greater than 572°F and increasing slowly.

D. Greater than 572°F and decreasing slowly.

Answer 40

Less than 572°F and increasing slowly.

Question 41

Initially, a reactor is operating at 80 percent power with equilibrium xenon-135. Then power is
increased to 100 percent over a 2-hour period. At the end of the power increase, the average reactor
coolant temperature is 585°F. Rod control is 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 reactor power reaches 100 percent?

A. Less than 585°F, and decreasing slowly.

B. Less than 585°F, and increasing slowly.

C. Greater than 585°F, and decreasing slowly.

D. Greater than 585°F, and increasing slowly.

Answer 41

Less than 585°F, and decreasing slowly.

Question 42

Initially, a reactor is operating at 100 percent power with equilibrium xenon-135. Then power is
decreased to 40 percent over a 2-hour period. At the end of the power decrease, the average reactor
coolant temperature is 562°F. Rod control is 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 2 hours after reactor power reaches 40 percent?

A. Greater than 562°F, and decreasing slowly.

B. Greater than 562°F, and increasing slowly.

C. Less than 562°F, and decreasing slowly.

D. Less than 562°F, and increasing slowly.

Answer 42

Less than 562°F, and decreasing slowly.

Question 43

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 _____.

A. B; B

B. B; A

C. A; B

D. A; A

Answer 43

A; B

Question 45

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.

A. 100 percent; 8

B. 100 percent; 16

C. 50 percent; 8

D. 50 percent; 16

Answer 45

50 percent; 16

Question 46

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.

A. 20 percent; beginning

B. 20 percent; end

C. 100 percent; beginning

D. 100 percent; end

Answer 46

20 percent; beginning

Question 47

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.

A. 20 percent; beginning

B. 20 percent; end

C. 100 percent; beginning

D. 100 percent; end

Answer 47

100 percent; end

Question 48

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.

A. 60 percent; 18

B. 60 percent; 30

C. 100 percent; 18

D. 100 percent; 30

Answer 48

60 percent; 30

Question 49

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.)

A. The count rate will remain about the same.

B. The count rate will decrease for the entire period.

C. The count rate will initially decrease and then increase.

D. The count rate will initially increase and then decrease.

Answer 49

The count rate will initially decrease and then increase.

Question 50

A reactor trip occurred 16 hours ago following several months of operation at 100 percent power.
Reactor coolant temperature is being maintained at 557°F. The source range count rate is 400 cps,
and the source neutron production rate is constant. Assume that no operator action is taken during the
next 24 hours.
During the next 24 hours, the source range count rate will…

A. increase for the entire period.

B. decrease for the entire period.

C. initially increase, and then decrease for the rest of the period.

D. initially decrease, and then increase for the rest of the period.

Answer 50

increase for the entire period.

Question 51

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…

A. peaking.

B. override.

C. burnup.

D. oscillations.

Answer 51

oscillations.

Question 52

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

A. converging

B. diverging

C. diffusing

D. equalizing

Answer 52

converging

Question 53

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?

A. Steam generator level transients

B. Iodine-135 spiking

C. Xenon-135 oscillations

D. Inadvertent boron dilution

Answer 53

Xenon-135 oscillations

Question 54

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…

A. shift toward the top of the core.

B. shift toward the bottom of the core.

C. peak at the top and the bottom of the core.

D. remain symmetrical and peaked at the core midplane.

Answer 54

shift toward the top of the core.

Question 55

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.
What is the effect on power distribution in the core during the first 4 hours following the power
reduction?

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

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

C. There is no relative change in power distribution in the core.

D. It is impossible to determine without additional information.

Answer 55

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

Question 56

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

A. top and bottom

B. adjacent quadrants

C. center and periphery

D. opposite quadrants

Answer 56

top and bottom

Question 57

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.
During the power increase, axial power distribution will…

A. shift toward the top of the core.

B. shift toward the bottom of the core.

C. remain evenly distributed above and below the core midplane.

D. peak at the top and the bottom of the core.

Answer 57

shift toward the bottom of the core.

Question 58

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?

A. Feedwater variations

B. Dropped center control rod

C. Xenon-135 oscillations

D. Samarium-149 oscillations

Answer 58

Xenon-135 oscillations

Question 59

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

A. 40 to 50

B. 24 to 28

C. 12 to 14

D. 6 to 7

Answer 59

12 to 14

Question 60

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.
What is the effect on power distribution in the core during the first 4 hours following the power
increase?

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

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

C. There is no relative change in power distribution in the core.

D. It is impossible to determine without additional information.

Answer 60

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

Question 61

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 __________.

A. higher; upward

B. higher; downward

C. lower; upward

D. lower; downward

Answer 61

higher; downward

Question 62

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…

A. uranium-235 burnup.

B. xenon-135 buildup.

C. fuel temperature increase.

D. moderator temperature increase.

Answer 62

xenon-135 buildup.

Question 63

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…

A. increasing toward a peak value.

B. increasing toward an equilibrium value.

C. decreasing toward an equilibrium value.

D. decreasing toward an upturn.

Answer 63

increasing toward an equilibrium value.

Question 64

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…

A. decreasing toward an upturn.

B. increasing toward a peak value.

C. decreasing toward an equilibrium value.

D. increasing toward an equilibrium value.

Answer 64

increasing toward an equilibrium value.

Question 65

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

A. increasing toward a downturn.

B. increasing toward an equilibrium value.

C. decreasing toward an equilibrium value.

D. decreasing toward an upturn.

Answer 65

increasing toward an equilibrium value.

Question 66

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 __________.

A. sooner; the same

B. sooner; smaller

C. later; the same

D. later; smaller

Answer 66

sooner; the same

Question 67

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?

A. Increasing toward an equilibrium value.

B. Burning out faster than it is being produced.

C. Increasing toward a peak value.

D. At equilibrium.

Answer 67

Burning out faster than it is being produced.

Question 68

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.

A. high; beginning

B. low; beginning

C. high; end

D. low; end

Answer 68

high; end

Question 69

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 __________.

A. sooner; the same

B. sooner; smaller

C. later; the same

D. later; smaller

Answer 69

later; the same

Question 70

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…

A. fuel burnup.

B. xenon-135 buildup.

C. fuel temperature increase.

D. moderator temperature increase.

Answer 70

fuel temperature increase.

Question 71

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.

A. 8 to 10

B. 20 to 25

C. 40 to 50

D. 70 to 80

Answer 71

40 to 50

Question 72

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?

A. There will be no change, because iodine-135 concentration is constant.

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

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

D. Xenon-135 concentration will steadily decrease to a new equilibrium value.

Answer 72

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

Question 73

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

A. Decreases, due to the increased rate of xenon-135 radioactive decay.

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

C. Increases, due to the increased xenon-135 production rate from fission.

D. Initially increases, due to the increased iodine-135 production rate from fission.

Answer 73

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

Question 74

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?

A. Increasing toward a peak.

B. Decreasing toward an upturn.

C. Increasing toward equilibrium.

D. Decreasing toward equilibrium.

Answer 74

Decreasing toward equilibrium.

Question 75

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.

A. 8 to 10

B. 20 to 25

C. 40 to 50

D. 70 to 80

Answer 75

70 to 80

Question 76

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?

A. Increasing toward a peak.

B. Increasing toward equilibrium.

C. Decreasing toward an upturn.

D. Decreasing toward equilibrium.

Answer 76

Increasing toward a peak.

Question 77

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?

A. 575°F and stable.

B. Less than 575°F and increasing.

C. Less than 575°F and decreasing.

D. Less than 575°F and stable.

Answer 77

Less than 575°F and decreasing.

Question 78

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?

A. Increasing toward a peak.

B. Decreasing toward an upturn.

C. Decreasing toward equilibrium.

D. At equilibrium.

Answer 78

Decreasing toward equilibrium.

Question 79

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 __________.

A. earlier; the same

B. at the same time; the same

C. earlier; less negative

D. at the same time; less negative

Answer 79

earlier; less negative

Question 80

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

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

B. xenon-135 production from the spontaneous fission of uranium-235.

C. the reduction of xenon-135 removal by decay.

D. the reduction of xenon-135 removal by recombination.

Answer 80

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

Question 81

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

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

B. the same as the concentration at the time of the trip and increasing.

C. 50 percent lower than the concentration at the time of the trip and decreasing.

D. 50 percent higher than the concentration at the time of the trip and increasing.

Answer 81

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

Question 82

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?

A. Peak in 2 to 4 hours and then decay to near zero in about 1 day.

B. Peak in 2 to 4 hours and then decay to near zero in 3 to 4 days.

C. Peak in 6 to 10 hours and then decay to near zero in about 1 day.

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

Answer 82

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

Question 83

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?

A. At least twice the concentration at the time of the trip and decreasing.

B. Less than one-half the concentration at the time of the trip and decreasing.

C. At or approaching a peak concentration.

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

Answer 83

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

Question 84

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.

A. less; positive

B. less; negative

C. greater; positive

D. greater; negative

Answer 84

greater; negative

Question 85

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

A. Decreases, due to xenon-135 removal by decay.

B. Decreases, due to the reduction in xenon-135 production directly from fission.

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

D. Increases, due to xenon-135 production from the spontaneous fission of uranium.

Answer 85

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

Question 86

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 __________.

A. increase; decreasing

B. increase; increasing

C. decrease; decreasing

D. decrease; increasing

Answer 86

increase; decreasing

Question 87

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?

A. Add positive reactivity during the entire period.

B. Add negative reactivity during the entire period.

C. Add positive reactivity at first, and then negative reactivity.

D. Add negative reactivity at first, and then positive reactivity.

Answer 87

Add positive reactivity at first, and then negative reactivity.

Question 88

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 88

A. Reactor A Reactor A

Question 89

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

A. independent of

B. directly proportional to

C. inversely proportional to

D. dependent on, but not directly proportional to

Answer 89

dependent on, but not directly proportional to

Question 90

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?

A. Xenon-135 buildup added negative reactivity.

B. Xenon-135 buildup added positive reactivity.

C. Xenon-135 burnout added negative reactivity.

D. Xenon-135 burnout added positive reactivity.

Answer 90

Xenon-135 buildup added negative reactivity.

Question 91

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 __________.

A. positive; increasing

B. positive; decreasing

C. negative; increasing

D. negative; decreasing

Answer 91

positive; increasing

Question 92

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…

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

B. negative reactivity, because the xenon-135 concentration is increasing.

C. positive reactivity, because the xenon-135 concentration is decreasing.

D. negative reactivity, because the xenon-135 concentration is decreasing.

Answer 92

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

Question 93

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?

A. Negative only

B. Negative, then positive

C. Positive only

D. Positive, then negative

Answer 93

Positive only

Question 94

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?

A. Add negative reactivity during the entire period.

B. Add positive reactivity during the entire period.

C. Add positive reactivity at first, and then negative reactivity

D. Add negative reactivity at first, and then positive reactivity

Answer 94

Add positive reactivity at first, and then negative reactivity

Question 95

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?

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

B. Power increases toward the point of adding heat, due to the decay of Sm-149.

C. Power decreases toward a stable shutdown neutron level, due to the buildup of Xe-135.

D. Power decreases toward a stable shutdown neutron level, due to the buildup of Sm-149.

Answer 95

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

Question 96

Initially, a reactor is 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…

A. inserted periodically for the duration of the 12 hours.

B. withdrawn periodically for the duration of the 12 hours.

C. inserted periodically for 4 to 6 hours, and then withdrawn periodically.

D. withdrawn periodically for 4 to 6 hours, and then inserted periodically.

Answer 96

withdrawn periodically for the duration of the 12 hours.

Question 97

Initially, a reactor is shut down with no xenon-135 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…

A. withdrawn periodically for the duration of the 12 hours.

B. inserted periodically for the duration of the 12 hours.

C. withdrawn periodically for 4 to 6 hours, and then inserted periodically.

D. inserted periodically for 4 to 6 hours, and then withdrawn periodically.

Answer 97

withdrawn periodically for the duration of the 12 hours.

Question 98

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.

A. insert; increase

B. insert; decrease

C. withdraw; increase

D. withdraw; decrease

Answer 98

insert; increase

Question 99

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.

A. insert; increase

B. insert; decrease

C. withdraw; increase

D. withdraw; decrease

Answer 99

insert; increase

Question 100

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?

A. Above the initial 75 percent power position and inserting slowly.

B. Above the initial 75 percent power position and withdrawing slowly.

C. Below the initial 75 percent power position and inserting slowly.

D. Below the initial 75 percent power position and withdrawing slowly.

Answer 100

Below the initial 75 percent power position and inserting slowly.

Question 101

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 __________.

A. positive; increasing

B. negative; increasing

C. positive; decreasing

D. negative; decreasing

Answer 101

negative; decreasing

Question 102

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?

A. Higher than 582°F and increasing slowly.

B. Higher than 582°F and decreasing slowly.

C. Lower than 582°F and increasing slowly.

D. Lower than 582°F and decreasing slowly.

Answer 102

Lower than 582°F and decreasing slowly.

Question 103

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?

A. Withdraw rods slowly during the entire period.

B. Withdraw rods slowly at first, and then insert rods slowly.

C. Insert rods slowly during the entire period.

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

Answer 103

Withdraw rods slowly during the entire period.

Question 104

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…

A. greater, because the higher fission rate at BOC produces xenon-135 at a faster rate.

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

C. smaller, because the lower fission rate at BOC produces xenon-135 at a slower rate.

D. smaller, because the higher thermal neutron flux at BOC removes xenon-135 at a faster rate.

Answer 104

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

Question 105

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

A. 0 percent to 10 percent

B. 30 percent to 40 percent

C. 60 percent to 70 percent

D. 90 percent to 100 percent

Answer 105

0 percent to 10 percent