Neutron Life Cycle

Question 1

Before a fission neutron could migrate out of a fuel pellet, the neutron was absorbed by the nucleus of
a uranium atom. The absorption occurred at a neutron energy of 2.1 MeV. If the neutron was
absorbed by a U-235 nucleus, the most likely outcome would be __________; if the neutron was
absorbed by a U-238 nucleus, the most likely outcome would be __________

Answer 1

fission; fission

Question 2

Initially, a reactor is subcritical with the effective multiplication factor (Keff) equal to 0.998. After a
brief withdrawal of control rods, Keff equals 1.002. The reactor is currently…

Answer 2

supercritical

Question 3

Which one of the following conditions describes a reactor that is exactly critical?

Answer 3

Keff = 1; ΔK/K = 0

Question 4

The ratio of the number of neutrons in one generation to the number of neutrons in the previous
generation is the…

Answer 4

effective multiplication factor.

Question 5

The effective multiplication factor (Keff) can be determined by dividing the number of neutrons in the
third generation by the number of neutrons in the __________ generation.

Answer 5

second

Question 6

The effective multiplication factor (Keff) describes the ratio of the number of fission neutrons at the
end of one generation to the number of fission neutrons at the __________ of the __________
generation.

Answer 6

end; previous

Question 7

A thermal neutron is about to interact with a U-238 nucleus in an operating reactor. Which one of the
following describes the most likely interaction and its effect on Keff?

Answer 7

The neutron will be scattered, thereby leaving Keff unchanged.

Question 8

A nuclear power plant is currently operating at steady-state 80 percent power near the end of its fuel
cycle. During the next 3 days of steady-state power operation, no operator action is taken.
How will Keff be affected during the 3-day period?

Answer 8

Keff will tend to decrease, but inherent reactivity feedback will maintain Keff at 1.0.

Question 9

A 1.5 MeV neutron is about to interact with a U-238 nucleus in an operating reactor. Which one of
the following describes the most likely interaction and its effect on Keff?

Answer 9

The neutron will be scattered, thereby leaving Keff unchanged

Question 10

During reactor refueling, burnable poisons are often installed in the core to help control Kexcess. Why
are more burnable poisons installed for the first fuel cycle than for subsequent fuel cycles?

Answer 10

More fission product poisons are present at the beginning of subsequent fuel cycles.

Question 11

Which one of the following defines K-excess?

Answer 11

Keff - 1

Question 12

The following are combinations of critical conditions that exist for the same reactor operating at the
point of adding heat at different times in core life. Which one of the following combinations indicates
the most amount of excess reactivity present in the core?

Answer 12

Control RCS Boron
Rod Position Concentration
50% inserted 1,000 ppm

Question 13

The following are combinations of critical conditions that exist for the same reactor operating at the
point of adding heat at different times in core life. Which one of the following combinations indicates
the least amount of excess reactivity present in the core?

Answer 13

Control RCS Boron
Rod Position Concentration
25% inserted 500 ppm

Question 14

Which one of the following is a reason for installing excess reactivity (Kexcess) in a reactor?

Answer 14

To compensate for the negative reactivity added by the power coefficient during a power increase.

Question 15

A reactor is operating at full power at the beginning of a fuel cycle. A neutron has just been absorbed
by a U-238 nucleus at a resonance energy of 6.7 electron volts.
Which one of the following describes the most likely reaction for the newly formed U-239 nucleus and
the effect of this reaction on Kexcess?

Answer 15

Decays over several days to Pu-239, which increases Kexcess.

Question 16

Which one of the following is a benefit of installing excess reactivity (Kexcess) in a reactor?

Answer 16

Ensures that the reactor can be made critical during a peak xenon condition after a reactor trip.

Question 17

Shutdown margin can be defined as the amount of reactivity…

Answer 17

by which the reactor is subcritical.

Question 18

The shutdown margin determination for an operating reactor assumes the complete withdrawal of…

Answer 18

a single control rod of high reactivity worth.

Question 19

With a reactor initially operating at steady-state 85 percent power with manual rod control, the
operator borates the reactor coolant system an additional 10 ppm. During the boration, the available
shutdown margin will…

Answer 19

increase and stabilize at a higher value.

Question 20

With a reactor initially operating at steady-state 75 percent power with manual rod control, the
operator dilutes the reactor coolant system boron concentration by 5 ppm. During the dilution, the
available shutdown margin will…

Answer 20

decrease and stabilize at a lower value.

Question 21

A nuclear power plant is operating with the following initial conditions:
• Reactor power is 50 percent.
• Rod control is in manual.
• Reactor coolant system (RCS) boron concentration is 600 ppm.
Disregarding the effects of fission product poisons, which one of the following will result in a decrease
in the available shutdown margin once the plant stabilizes?

Answer 21

Reactor power is increased to 55 percent with final RCS boron concentration at 580 ppm.

Question 22

Which one of the following changes will decrease the available shutdown margin in a reactor?
(Assume no operator actions.)

Answer 22

Depletion of burnable poisons during reactor operation.

Question 23

A reactor is operating at steady-state 100 percent power with manual rod control about three months
from the end of a fuel cycle. During the next two weeks of operation at 100 percent power, the
available shutdown margin will… (Assume no operator actions are taken.)

Answer 23

continuously increase.

Question 24

Reactivity is defined as the fractional change in…

Answer 24

the effective multiplication factor from criticality

Question 25

Which term is described by the following?

“The fractional change of the effective multiplication factor from criticality.”

Answer 25

Reactivity

Question 26

With Keff equal to 0.985, how much reactivity must be added to make the reactor critical? (Round
answer to the nearest 0.01 %ΔK/K.)

Answer 26

1.52 %ΔK/K

Question 27

With Keff equal to 0.987, how much reactivity must be added to make the reactor critical? (Round
answer to the nearest 0.01 %ΔK/K.)

Answer 27

1.32 %ΔK/K

Question 28

In a subcritical reactor, Keff was increased from 0.85 to 0.95 by rod withdrawal. Which one of the
following is the approximate amount of reactivity that was added to the core?

Answer 28

0.124 ΔK/K

Question 29

With Keff equal to 0.982, how much positive reactivity is required to make the reactor critical?
(Round answer to the nearest 0.01 %ΔK/K.)

Answer 29

1.83 %ΔK/K

Question 30

With Keff equal to 0.985, how much positive reactivity is required to make the reactor critical?
(Round answer to the nearest 0.01 %ΔK/K.)

Answer 30

1.52 %ΔK/K

Question 31

With Keff equal to 0.983, how much positive reactivity must be added to make the reactor critical?
(Round answer to the nearest 0.01 %ΔK/K.)

Answer 31

1.73 %ΔK/K

Question 32

Initially, a reactor was shutdown at a stable power level of 2.0 x 10-5 percent. After a small positive
reactivity addition, the current stable power level is 3.0 x 10-5 percent. If the initial Keff was 0.982,
what is the current Keff?

Answer 32

0.988

Question 33

A reactor near the end of a fuel cycle has been shut down from 100 percent power and cooled down to
140°F over three days. During the cooldown, boron concentration was increased by 100 ppm.
Given the following absolute values of reactivities added during the shutdown and cooldown, assign a
(+) or (!) as appropriate and choose the current value of core reactivity.
Control rods = ( ) 6.918 %ΔK/K
Xenon = ( ) 2.675 %ΔK/K
Power defect = ( ) 1.575 %ΔK/K
Boron = ( ) 1.040 %ΔK/K
Cooldown = ( ) 0.500 %ΔK/K

Answer 33

-3.208 %ΔK/K

Question 34

A reactor was operating at steady-state 100 percent power with all control rods fully withdrawn and
average reactor coolant temperature (Tave) at 588°F when a reactor trip occurred.
After the trip, Tave stabilized at the no-load temperature of 557°F and all control rods were verified to
be fully inserted.
Given the following information, select the current value of core reactivity. (Assume no operator
actions and disregard any reactivity effects of xenon.)
Power coefficient = -0.015 %ΔK/K/percent
Control rod worth = -6.918 %ΔK/K
Moderator temperature coefficient = -0.0012 %ΔK/K/°F

Answer 34

-5.418 %ΔK/K

Question 35

A reactor is operating at steady-state 90 percent power with all control rods fully withdrawn and
average reactor coolant temperature (Tave) at 580°F. A reactor trip occurs, after which Tave stabilizes
at the no-load temperature of 550°F and all control rods are verified to be fully inserted.
Given the following information, calculate the current value of core reactivity. (Assume no operator
actions and disregard any reactivity effects of changes in xenon-135.)
Power coefficient = -0.01 %ΔK/K/percent
Control rod worth = -6.918 %ΔK/K
Moderator temperature coefficient = -0.01 %ΔK/K/°F

Answer 35

-6.018 %ΔK/K

Question 36

A reactor was operating at steady-state 100 percent power near the end of a fuel cycle when a reactor
trip occurred. Immediately after the trip, shutdown margin was determined to be -5.883 %ΔK/K.
Over the next 72 hours, the reactor coolant system was cooled down and reactor coolant boron
concentration was increased. The reactivities affected by the change in plant conditions are as
follows:
Reactivity Change (+) or (!)
Xenon = ( ) 2.675 %ΔK/K
Moderator temperature = ( ) 0.5 %ΔK/K
Boron = ( ) 1.04 %ΔK/K
What is the value of core reactivity 72 hours after the trip?

Answer 36

-3.748 %ΔK/K

Question 37

A reactor near the end of a fuel cycle has been shut down from 100 percent power and cooled down to
140°F over three days. During the cooldown, reactor coolant boron concentration was increased by
100 ppm.
Given the following absolute values of reactivities added during the shutdown and cooldown, assign a
(+) or (!) as appropriate and choose the current value of core reactivity.
Xenon = ( ) 2.5 %ΔK/K
Moderator temperature = ( ) 0.5 %ΔK/K
Power defect = ( ) 1.5 %ΔK/K
Control rods = ( ) 7.0 %ΔK/K
Boron = ( ) 1.0 %ΔK/K

Answer 37

-3.5 %ΔK/K

Question 38

A reactor near the end of a fuel cycle has been shut down from 100 percent power and cooled down to
140°F over three days. During the cooldown, reactor coolant boron concentration was increased by
100 ppm.
Given the following absolute values of reactivities added during the shutdown and cooldown, assign a
(+) or (!) as appropriate and choose the current value of core reactivity.
Moderator temperature = ( ) 0.50 %ΔK/K
Control rods = ( ) 6.50 %ΔK/K
Boron = ( ) 1.50 %ΔK/K
Power defect = ( ) 1.75 %ΔK/K
Xenon = ( ) 2.75 %ΔK/K

Answer 38

-3.0 %ΔK/K

Question 39

A reactor near the middle of a fuel cycle has been shut down from 100 percent power and cooled down
to 340°F over three days. During the cooldown, reactor coolant boron concentration was increased
by 200 ppm.
Given the following absolute values of reactivities added during the shutdown and cooldown, assign a
(+) or (!) as appropriate and choose the current value of core reactivity.
Xenon = ( ) 3.0 %ΔK/K
Boron = ( ) 3.5 %ΔK/K
Power defect = ( ) 4.0 %ΔK/K
Control rods = ( ) 7.0 %ΔK/K
Moderator temperature = ( ) 2.0 %ΔK/K

Answer 39

-1.5 %ΔK/K

Question 40

A reactor near the middle of a fuel cycle was operating at 100 percent power for two months when a
reactor trip occurred. During the 14 hours since the trip, the reactor has been cooled to 340°F and
reactor coolant boron concentration has been increased by 200 ppm.
Given the following absolute values of reactivities added during the shutdown and cooldown, assign a
(+) or (!) as appropriate and choose the current value of core reactivity.
Xenon = ( ) 2.0 %ΔK/K
Boron = ( ) 2.5 %ΔK/K
Power defect = ( ) 4.0 %ΔK/K
Control rods = ( ) 7.0 %ΔK/K
Moderator temperature = ( ) 2.0 %ΔK/K

Answer 40

5.5 %ΔK/K

Question 41

A reactor near the middle of a fuel cycle was initially operating at steady-state 100 percent power
when it was shut down and cooled down to 200°F over a three-day period. During the cooldown,
reactor coolant boron concentration was increased by 80 ppm.
Given the following absolute values of reactivities added during the shutdown and cooldown, assign a
(+) or (!) as appropriate and choose the current value of core reactivity.
Control rods = ( ) 6.75 %ΔK/K
Xenon = ( ) 2.50 %ΔK/K
Power defect = ( ) 2.00 %ΔK/K
Boron = ( ) 1.25 %ΔK/K
Moderator temperature = ( ) 0.50 %ΔK/K

Answer 41

3.0 %ΔK/K

Question 42

Which one of the following plant parameter changes will increase the shutdown margin for a
shutdown reactor near the end of a fuel cycle?

Answer 42

The reactor coolant system is allowed to heat up 30°F

Question 43

A nuclear power plant is operating at steady-state 70 percent power with manual rod control. Which
one of the following events will increase the available shutdown margin? (Assume that no
unspecified operator actions occur and the reactor does not trip.)

Answer 43

Power is decreased to 50 percent using boration.

Question 44

A reactor is shutdown with the reactor vessel head removed for refueling. The core is covered by 23
feet of refueling water at 100°F with a boron concentration of 2,000 ppm.

Answer 44

Refueling water temperature is increased to 105°F

Question 45

A reactor is operating at steady-state 80 percent power when the operator adds 10 gallons of
concentrated boric acid to the reactor coolant system (RCS). Over the next several minutes, the
operator adjusts control rod position as necessary to maintain a constant RCS average temperature.
When the plant stabilizes, the available shutdown margin will be __________; and axial power
distribution will have shifted toward the __________ of the core.

Answer 45

greater; top

Question 46

A nuclear power plant malfunction requires a rapid reactor power decrease from 100 percent to 90
percent. The crew performs the downpower transient using control rod insertion when necessary.
Reactor coolant boron concentration is not changed.
If the available shutdown margin at 100 percent power was 3.5 %ΔK/K, which one of the following
describes the available shutdown margin at the lower power level? (Ignore any changes in core
fission product reactivity.)

Answer 46

Equal to 3.5 %ΔK/K regardless of the reactivity effects of control rod insertion and power defect

Question 47

A reactor is shutdown with the reactor vessel head removed for refueling. The core is covered by 23
feet of refueling water at 105°F with a boron concentration of 2,200 ppm.
Which one of the following will increase core Keff?

Answer 47

A spent fuel assembly is replaced with a new fuel assembly.

Question 48

Reactors A and B are identical except that reactor A is operating at steady-state 80 percent power
while reactor B is operating at steady-state 100 percent power. Initial control rod positions are the
same for each reactor.
How will the shutdown margins (SDM) compare for the two reactors following a reactor trip?
(Assume no post-trip operator actions are taken that would affect SDM.)

Answer 48

Reactor A will have the greater SDM

Question 49

Initially, a reactor is operating at steady-state 50 percent power. A plant test requires a 4°F decrease
in reactor coolant system (RCS) average temperature (T-avg). The operator accomplishes this
temperature decrease by adjusting RCS boron concentration. No other operator actions are taken.
If the initial available shutdown margin was 3.0 %ΔK/K, which one of the following describes the
available shutdown margin at the lower RCS T-avg with the reactor still at steady-state 50 percent
power?

Answer 49

More than 3.0 %ΔK/K, because RCS boron concentration is higher.

Question 50

A reactor is initially operating at steady-state 60 percent power near the end of a fuel cycle when a
fully withdrawn control rod suddenly inserts completely into the core. No operator action is taken
and the plant control systems stabilize the reactor at a power level in the power range.
Compared to the initial available shutdown margin (SDM), the current available SDM is __________;
and compared to the initial core Keff, the current core Keff is __________.

Answer 50

the same; the same

Question 51

A nuclear power plant has just completed a refueling outage. Based on the expected core loading,
reactor engineers have predicted a control rod configuration at which the reactor will become critical
during the initial reactor startup following the refueling outage. However, the burnable poisons
scheduled to be loaded were inadvertently omitted.
Which one of the following describes the effect of the burnable poison omission on achieving reactor
criticality during the initial reactor startup following the refueling outage?

Answer 51

The reactor will become critical before the predicted critical control rod configuration is
achieved.

Question 52

A reactor is shutdown with the reactor vessel head removed for refueling. The core is covered by 23
feet of refueling water at 100°F with a boron concentration of 2,000 ppm.
Which one of the following will decrease core Keff?

Answer 52

An unrodded spent fuel assembly is removed from the core.

Question 53

Reactors A and B are identical except that reactor A is operating near the beginning of a fuel cycle
(BOC) and reactor B is operating near the end of a fuel cycle (EOC). Both reactors are operating at
100 percent power with all control rods fully withdrawn.
If the total reactivity worth of the control rods is the same for both reactors, which reactor will have the
smaller Keff five minutes after a reactor trip, and why?

Answer 53

Reactor A, because the power coefficient is less negative near the BOC

Question 54

A reactor is shutdown with the reactor vessel head removed for refueling. The core is covered by 23
feet of refueling water at 105°F with a boron concentration of 2,000 ppm.
Which one of the following will decrease Keff?

Answer 54

Refueling water temperature decreases by 5°F.

Question 55

Reactors A and B are identical except that reactor A is operating near the beginning of a fuel cycle
(BOC) and reactor B is operating near the end of a fuel cycle (EOC). Both reactors are operating at
100 percent power with all control rods fully withdrawn.
If the total reactivity worth of the control rods is the same for both reactors, which reactor will have the
greater Keff five minutes after a reactor trip, and why?

Answer 55

Reactor B, because the power coefficient adds more positive reactivity after a trip near the EOC.

Question 56

Initially, a nuclear power plant was operating at steady-state 70 percent power near the middle of a fuel
cycle when a control rod of moderate reactivity worth dropped into the core. The reactor did not trip.
Consider the following two possible operator responses:
Response 1: An operator adjusts the reactor coolant system (RCS) boron concentration to restore the
initial RCS temperatures.
Response 2: An operator partially withdraws some of the remaining control rods to restore the initial
RCS temperatures.
In a comparison between the two responses, which response, if any, will result in the greater available
shutdown margin (SDM) when the plant is stabilized at 70 percent power, and why?

Answer 56

Response 2, because a greater (than response 1) amount of negative reactivity will be added by the
control rods upon a reactor trip.

Question 57

A reactor is shutdown with the reactor vessel head removed for refueling. The core is covered by 23
feet of refueling water at 120°F with a boron concentration of 2,000 ppm. Source range
instrumentation indicates 100 cps.

Answer 57

Indication will decrease because of the cooperative effects of decreased Keff and decreased neutron
leakage from the core.

Question 58

A nuclear power plant was initially operating at equilibrium 100 percent power just prior to a refueling
outage. The plant was shut down, refueled, restarted, and is currently operating at equilibrium 100
percent power. Assume the 100 percent power fission rate did not change.
Which one of the following describes the current plant status as compared to the conditions just prior
to the refueling?

Answer 58

The reactor’s available shutdown margin is greater

Question 59

A nuclear reactor is shut down with the reactor vessel head removed for refueling. The core is
covered by 23 feet of refueling water at 100EF with a boron concentration of 2,000 ppm. Source
range count rate indicates 100 cps.
How will the source range count rate be affected if refueling water temperature increases to 120EF?

Answer 59

The count rate will increase, because the positive effect of increased core neutron leakage adds to
the positive effect of a greater Keff.

Question 60

Initially, a nuclear power plant is operating at steady-state 70 percent power near the middle of a fuel
cycle when a control rod drops into the core. The reactor does not trip. Consider the following two
possible operator responses:
Response 1: An operator adjusts the reactor coolant system (RCS) boron concentration to restore the
initial RCS temperatures.
Response 2: An operator partially withdraws some of the remaining control rods to restore the initial
RCS temperatures.
In a comparison between the two responses, which response, if any, will result in the smaller available
shutdown margin (SDM) when the plant is stabilized at 70 percent power, and why?

Answer 60

Response 1, because a smaller (than response 2) amount of negative reactivity will be added by the
control rods upon a reactor trip.

Question 61

A reactor is currently operating at steady-state 100 percent power near the beginning of a fuel cycle
(BOC). When the same reactor is operating at steady-state 100 percent power near the end of a fuel
cycle (EOC), how will the BOC and EOC shutdown margins compare? Assume the control rods are
fully withdrawn, and the total reactivity worths of the control rods are the same at BOC and EOC.

Answer 61

The EOC shutdown margin will be less negative because the power defect will add more positive
reactivity immediately after a reactor trip near the EOC.