Reactor Kinetics and Neutron Sources

Question 1

Which one of the following is a characteristic of subcritical multiplication?

A. The subcritical neutron level is directly proportional to the neutron source strength.
B. Doubling the indicated count rate by reactivity additions will reduce the margin to criticality by
approximately one quarter.
C. For equal reactivity additions, it takes less time for the new equilibrium source range count rate to
be reached as Keff approaches unity.
D. An incremental withdrawal of any given control rod will produce an equivalent equilibrium count
rate increase, whether Keff is 0.88 or 0.92.

Answer 1

The subcritical neutron level is directly proportional to the neutron source strength.

Question 2

A nuclear power plant has been operating at 100 percent power for 2 months when a reactor trip
occurs. Two months after the reactor trip, with all control rods still fully inserted, a stable count rate
of 20 cps is indicated on the source range nuclear instruments.
The majority of the source range count rate is being caused by the interaction of __________ with the
detector.

A. intrinsic source neutrons
B. fission gammas from previous power operation
C. fission neutrons from subcritical multiplication
D. delayed fission neutrons from previous power operation

Answer 2

fission neutrons from subcritical multiplication

Question 3

The total neutron flux in a shutdown reactor is constant at 5.0 x 103 n/cm2-sec. If non-fission neutron
sources are supplying a constant flux of 1.0 x 102 n/cm2-sec, what is Keff?

A. 0.98
B. 0.96
C. 0.94
D. Cannot be determined without additional information.

Answer 3

0.98

Question 4

Reactor power was increased from 1.0 x 10-9 percent to 1.0 x 10-6 percent in 6 minutes. The average
startup rate was __________ decades per minute.

A. 0.5
B. 1.3
C. 2.0
D. 5.2

Answer 4

0.5

Question 5

Reactor power increases from 1.0 x 10-8 percent to 5.0 x 10-7 percent in two minutes. What was the
average startup rate during the power increase?

A. 0.95 DPM
B. 0.90 DPM
C. 0.85 DPM
D. 0.82 DPM

Answer 5

0.85 DPM

Question 6

During a reactor startup, reactor power increases from 1.0 x 10-8 percent to 2.0 x 10-8 percent in two
minutes. What was the average reactor period during the power increase?

A. 173 seconds
B. 235 seconds
C. 300 seconds
D. 399 seconds

Answer 6

173 seconds

Question 7

During a reactor startup, reactor power increases from 3.0 x 10-6 percent to 5.0 x 10-6 percent in two
minutes. What was the average reactor period during the power increase?

A. 357 seconds
B. 235 seconds
C. 155 seconds
D. 61 seconds

Answer 7

235 seconds

Question 8

A small amount of positive reactivity is added to a reactor that is critical in the source range. The
amount of reactivity added is much less than the effective delayed neutron fraction.
Which one of the following will have the most significant effect on the magnitude of the stable reactor
period achieved for this reactivity addition while the reactor is in the source range?

A. Prompt neutron lifetime

B. Fuel temperature coefficient

C. Moderator temperature coefficient

D. Effective delayed neutron precursor decay constant

Answer 8

Effective delayed neutron precursor decay constant

Question 9

A nuclear power plant is operating at steady-state 50 percent power in the middle of a fuel cycle.
Which one of the following will initially produce a positive startup rate?

A. Main turbine runback.

B. Unintentional boration.

C. Increase in main turbine load.

D. Closure of a letdown isolation valve.

Answer 9

Increase in main turbine load.

Question 10

The magnitude of the stable startup rate achieved for a given positive reactivity addition to a critical
reactor is dependent on the __________ and __________.

A. prompt neutron lifetime; axial neutron flux distribution

B. prompt neutron lifetime; effective delayed neutron fraction

C. effective delayed neutron precursor decay constant; effective delayed neutron fraction

D. effective delayed neutron precursor decay constant; axial neutron flux distribution

Answer 10

effective delayed neutron precursor decay constant; effective delayed neutron fraction

Question 11

A reactor is critical at 1.0 x 10-8 percent power during a reactor startup. β�eff for this reactor is 0.0072.
Which one of the following is the approximate amount of positive reactivity that must be added to the
core by control rod withdrawal to attain a stable startup rate of 1.0 DPM?

A. 0.2 %ΔK/K

B. 0.5 %ΔK/K

C. 1.0 %ΔK/K

D. 2.0 %ΔK/K

Answer 11

0.2 %ΔK/K

Question 12

A reactor is being started for the first time following a refueling outage. Reactor Engineering has
determined that during the upcoming fuel cycle, β�eff will range from a maximum of 0.007 to a
minimum of 0.005.
Once the reactor becomes critical, control rods are withdrawn to increase reactivity by 0.1 %ΔK/K.
Assuming no other reactivity additions, what will the stable reactor period be for this reactor until the
point of adding heat is reached?

A. 20 seconds

B. 40 seconds

C. 60 seconds

D. 80 seconds

Answer 12

60 seconds

Question 13

Reactors A and B are identical except that the reactors are operating at different times in core life.
The reactor A effective delayed neutron fraction is 0.007, and the reactor B effective delayed neutron
fraction is 0.005. Both reactors are currently subcritical with neutron flux level stable in the source
range.
Given:
Reactor A Keff = 0.999
Reactor B Keff = 0.998
If positive 0.003 ΔK/K is suddenly added to each reactor, how will the resulting stable startup rates
(SUR) compare? (Consider only the reactor response while power is below the point of adding heat.)

A. Reactor A stable SUR will be greater.

B. Reactor B stable SUR will be smaller.

C. Reactors A and B will have the same stable SUR because both reactors will remain subcritical.

D. Reactors A and B will have the same stable SUR because both reactors received the same amount
of positive reactivity.

Answer 13

Reactor A stable SUR will be greater.

Question 14

Given the following stable initial conditions for a reactor:
Power level = 1.0 x 10^-8 percent
Keff
= 0.999
Core β�eff = 0.006
What will the stable reactor period be following an addition of positive 0.15 %ΔK/K reactivity to the
reactor? (Assume the stable reactor period occurs before the reactor reaches the point of adding
heat.)

A. 30 seconds

B. 50 seconds

C. 80 seconds

D. 110 seconds

Answer 14

110 seconds

Question 15

Given the following stable initial conditions for a reactor:
Power level = 1.0 x 10^-8 percent
Keff
= 0.999
Core β�
A
E
A
eff = 0.006
What will the stable startup rate be following an addition of positive 0.2 %ΔK/K reactivity to the
reactor? (Assume the stable startup rate occurs before the reactor reaches the point of adding heat.)

A. 0.24 DPM

B. 0.33 DPM

C. 0.52 DPM

D. 1.30 DPM

Answer 15

0.52 DPM

Question 16

A nuclear power plant has just completed a refueling outage and a reactor startup is in progress.
Reactor engineers have determined that during the upcoming fuel cycle,
A
E
A
E
β
�
eff will range from a
minimum of 0.0052 to a maximum of 0.0064.
After the reactor becomes critical, control rods are withdrawn further to increase reactivity by an
additional 0.1 %ΔK/K. Assuming no other reactivity changes occur, what will the approximate
stable startup rate be for this reactor until the point of adding heat is reached?

A. 1.0 DPM

B. 0.6 DPM

C. 0.5 DPM

D. 0.3 DPM

Answer 16

0.5 DPM

Question 17

During a fuel cycle, plutonium isotopes are produced with delayed neutron fractions that are
__________ than the delayed neutron fractions for uranium isotopes, thereby causing reactor power
transients to be __________ near the end of a fuel cycle.

A. larger; slower

B. larger; faster

C. smaller; slower

D. smaller; faster

Answer 17

smaller; faster

Question 18

Following a reactor trip, when does the startup rate initially stabilize at –1/3 DPM?

A. When decay gamma heating starts adding negative reactivity.

B. When the long-lived delayed neutron precursors have decayed away.

C. When the installed neutron source contribution to the total neutron flux becomes significant.

D. When the short-lived delayed neutron precursors have decayed away.

Answer 18

When the short-lived delayed neutron precursors have decayed away.

Question 19

Delayed neutrons contribute more to reactor stability than prompt neutrons because they __________
the average neutron generation time and are born at a __________ kinetic energy.

A. increase; lower

B. increase; higher

C. decrease; lower

D. decrease; higher

Answer 19

increase; lower

Question 20

Which one of the following statements describes the effect of changes in the delayed neutron fraction
from the beginning of a fuel cycle (BOC) to the end of a fuel cycle (EOC)?

A. A given reactivity addition to a shutdown reactor at EOC yields a larger change in shutdown
margin (SDM) than at BOC.

B. A given reactivity addition to a shutdown reactor at EOC yields a smaller change in SDM than at
BOC.

C. A given reactivity addition to an operating reactor at EOC results in a higher startup rate (SUR)
than at BOC.

D. A given reactivity addition to an operating reactor at EOC results in a lower SUR than at BOC.

Answer 20

A given reactivity addition to an operating reactor at EOC results in a higher startup rate (SUR)
than at BOC.

Question 21

Delayed neutrons are important for reactor control because…

A. they are produced with a higher average kinetic energy than prompt neutrons.

B. they prevent the moderator temperature coefficient from becoming positive.

C. they are the largest fraction of the neutrons produced from fission.

D. they greatly extend the average lifetime of each neutron generation.

Answer 21

they greatly extend the average lifetime of each neutron generation.

Question 22

Two reactors are identical except that reactor A is near the end of a fuel cycle and reactor B is near the
beginning of a fuel cycle. Both reactors are operating at 100 percent power when a reactor trip occurs
at the same time on each reactor.
If no operator action is taken and the reactor systems for both reactors respond identically to the trip,
reactor A will attain a negative __________ second stable period; and reactor B will attain a negative
__________ second stable period.

A. 80; 56

B. 80; 80

C. 56; 56

D. 56; 80

Answer 22

80; 80

Question 23

Two reactors are identical except that reactor A is near the end of a fuel cycle and reactor B is near the
beginning of a fuel cycle. Both reactors are critical at 1.0 x 10-5 percent power.
If the same amount of positive reactivity is added to each reactor at the same time, the point of adding
heat will be reached first by reactor __________ because it has a __________ effective delayed
neutron fraction.

A. A; smaller

B. A; larger

C. B; smaller

D. B; larger

Answer 23

A; smaller

Question 24

Two reactors are identical except that reactor A is near the end of core life and reactor B is near the
beginning of core life. Both reactors are operating at 100 percent power when a reactor trip occurs at
the same time on each reactor. The trips insert equal amounts of negative reactivity, and no operator
actions are taken.
For the conditions above, a power level of 1.0 x 10-5 percent will be reached first by reactor
__________ because it has the __________ effective delayed neutron fraction.

A. A; larger

B. B; larger

C. A; smaller

D. B; smaller

Answer 24

A; smaller

Question 25

Which one of the following is the reason that delayed neutrons are so effective at controlling the rate of
reactor power changes?

A. Delayed neutrons make up a large fraction of the fission neutrons compared to prompt neutrons.

B. Delayed neutrons have a long mean generation time compared to prompt neutrons.

C. Delayed neutrons produce a large amount of fast fission compared to prompt neutrons.

D. Delayed neutrons are born with high kinetic energy compared to prompt neutrons.

Answer 25

Delayed neutrons have a long mean generation time compared to prompt neutrons.

Question 26

Which one of the following distributions of fission percentages occurring in a reactor will result in the
largest effective delayed neutron fraction?

Answer 26

90%
7%
3%

Question 27

Which one of the following distributions of fission percentages occurring in a reactor will result in the
smallest effective delayed neutron fraction?

Answer 27

60%
6%
34%

Question 28

Two reactors are identical except that reactor A is near the beginning of core life and reactor B is near
the end of core life. Both reactors are critical at 10-5 percent power.
If the same amount of positive reactivity is added to each reactor at the same time, the point of adding
heat will be reached first by reactor __________ because it has a __________ effective delayed
neutron fraction.

A. A; smaller

B. A; larger

C. B; smaller

D. B; larger

Answer 28

B; smaller

Question 29

A nuclear power plant is operating at steady-state 50 percent power when a control rod is ejected from
the core. Which one of the following distributions of fission percentages in the core would result in
the highest startup rate? (Assume the reactivity worth of the ejected control rod is the same for each
distribution.)

Answer 29

60%
8%
32%

Question 30

Two reactors are identical except that reactor A is near the end of core life and reactor B is near the
beginning of core life. Both reactors are operating at 100 percent power when a reactor trip occurs at
the same time on each reactor. No operator action is taken and the reactor systems for both reactors
respond identically to the trip.
Ten minutes after the trip, the greater thermal neutron flux will exist in reactor __________ because it
has a __________ effective delayed neutron fraction.

A. A; larger

B. B; larger

C. A; smaller

D. B; smaller

Answer 30

B; larger

Question 31

Two reactors are identical except that reactor A is near the beginning of core life and reactor B is near
the end of core life. Both reactors are operating at 100 percent power when a reactor trip occurs at the
same time on each reactor. No operator action is taken and the reactor systems for both reactors
respond identically to the trip.
Ten minutes after the trip, the greater thermal neutron flux will exist in reactor __________ because it
has a __________ effective delayed neutron fraction.

A. A; larger

B. B; larger

C. A; smaller

D. B; smaller

Answer 31

A; larger

Question 32

A step positive reactivity addition of 0.001 ΔK/K is made to a reactor with a stable neutron flux and an
initial Keff of 0.99. Consider the following two cases:
Case 1: The reactor is near the beginning of a fuel cycle.
Case 2: The reactor is near the end of a fuel cycle.
Assume the initial neutron flux is the same for each case.
Which one of the following correctly compares the prompt jump in neutron flux levels and the final
stable neutron flux levels for the two cases?

A. The prompt jump will be greater for case 1, but the final stable neutron flux level will be the same
for both cases.

B. The prompt jump will be greater for case 2, but the final stable neutron flux level will be the same
for both cases.

C. The prompt jump will be the same for both cases, but the final stable neutron flux level will be
greater for case 1.

D. The prompt jump will be the same for both cases, but the final stable neutron flux level will be
greater for case 2.

Answer 32

The prompt jump will be greater for case 2, but the final stable neutron flux level will be the same
for both cases.

Question 33

A reactor is critical in the source range during the initial reactor startup immediately following a
refueling outage. The effective delayed neutron fraction is 0.0062. The operator adds positive
reactivity to establish a stable 0.5 DPM startup rate.
If the reactor had been near the end of a fuel cycle with an effective delayed neutron fraction of 0.005,
what would the approximate stable startup rate be after the addition of the same amount of positive
reactivity?

A. 0.55 DPM

B. 0.65 DPM

C. 0.75 DPM

D. 0.85 DPM

Answer 33

0.65 DPM

Question 34

The following data is given for the fuel in an operating reactor:

What is the delayed neutron fraction for this reactor?

A. 0.0052

B. 0.0054

C. 0.0062

D. 0.0068

Answer 34

0.0062

Question 34

The following data is given for the fuel in an operating reactor:

What is the delayed neutron fraction for this reactor?

A. 0.0052

B. 0.0058

C. 0.0072

D. 0.0078

Answer 34

0.0058

Question 35

Which characteristic of delayed neutrons is primarily responsible for enhancing the stability of a
reactor following a reactivity change?

A. They are born at a lower average energy than prompt neutrons.

B. They are more likely to experience resonance absorption than prompt neutrons.

C. They comprise a smaller fraction of the total neutron flux than prompt neutrons.

D. They require more time to be produced following a fission event than prompt neutrons.

Answer 35

They require more time to be produced following a fission event than prompt neutrons.

Question 36

For an operating reactor, the effective delayed neutron fraction may differ from the delayed neutron
fraction because, compared to prompt neutrons, delayed neutrons…

A. are less likely to leak out of the reactor core, and are less likely to cause fast fission.

B. are less likely to cause fast fission, and require more time to complete a neutron generation.

C. require more time to complete a neutron generation, and spend less time in the resonance
absorption energy region.

D. spend less time in the resonance absorption energy region, and are less likely to leak out of the
reactor core.

Answer 36

are less likely to leak out of the reactor core, and are less likely to cause fast fission.

Question 37

Given the following data for a reactor:
* The average delayed neutron fraction is 0.0068.
* The effective delayed neutron fraction is 0.0065.
The above data indicates that this reactor is operating near the __________ of a fuel cycle; and a
typical delayed neutron is __________ likely than a typical prompt neutron to cause another fission in
this reactor.

A. beginning; less

B. beginning; more

C. end; less

D. end; more

Answer 37

beginning; less

Question 38

Initially, a reactor is critical at a stable power level well below the point of adding heat (POAH). When
considering the following two cases, assume the reactor remains below the POAH.
Case 1: A step addition of positive 1.0 x 10^-4 ΔK/K.
Case 2: A step addition of negative 1.0 x 10^-4 ΔK/K.
The time required for reactor power to change by a factor of 10 will be greater for case _____, because
delayed neutrons are more effective at slowing reactor power changes when reactor power is
__________.

A. 1; increasing

B. 1; decreasing

C. 2; increasing

D. 2; decreasing

Answer 38

2; decreasing

Question 39

Two identical reactors, A and B, are critical at 1.0 x 10-8 percent power near the beginning of a fuel
cycle. Simultaneously, positive 0.001 ΔK/K is added to reactor A, and negative 0.001 ΔK/K is added
to reactor B. One minute later, which reactor, if any, will have the shorter period and why?

A. Reactor A, because delayed neutrons are less effective at slowing down power changes when the
fission rate is increasing.

B. Reactor B, because delayed neutrons are less effective at slowing down power changes when the
fission rate is decreasing.

C. The periods in both reactors will be the same because their effective delayed neutron fractions are
the same.

D. The periods in both reactors will be the same because the absolute values of the reactivity additions
are the same.

Answer 39

Reactor A, because delayed neutrons are less effective at slowing down power changes when the
fission rate is increasing.

Question 40

The following data is given for the fuel in an operating reactor just prior to a refueling shutdown.

During the refueling, one-third of the fuel assemblies were offloaded and replaced with new fuel
assemblies consisting of uranium having an average U-235 enrichment of 3.5 percent by weight.
Which one of the following describes how the above data will change as a result of completing the
refueling outage?

A. The delayed neutron fraction for U-235 will decrease.

B. The delayed neutron fraction for Pu-239 will decrease.

C. The fraction of the total fission rate attributed to U-235 will increase.

D. The fraction of the total fission rate attributed to Pu-239 will increase

Answer 40

The fraction of the total fission rate attributed to U-235 will increase.

Question 41

Which one of the following is the major cause for the change in the delayed neutron fraction from the
beginning to the end of a fuel cycle?

A. Burnup of the burnable poisons.

B. Changes in the fuel composition.

C. Buildup of fission product poisons.

D. Shift in the core axial power distribution

Answer 41

Changes in the fuel composition.

Question 42

Given the following data for the fuel in an operating reactor:

What is the delayed neutron fraction for this reactor?

A. 0.0044

B. 0.0055

C. 0.0063

D. 0.0071

Answer 42

0.0055

Question 43

A nuclear reactor is operating at steady-state 100 percent power in the middle of a fuel cycle. Which
one of the following changes would cause the core effective delayed neutron fraction to increase?

A. The fast nonleakage factor increases.

B. The fast nonleakage factor decreases.

C. The thermal utilization factor increases.

D. The thermal utilization factor decreases.

Answer 43

The fast nonleakage factor decreases.

Question 44

Given the following data for a reactor:
* The average delayed neutron fraction is 0.0052.
* The effective delayed neutron fraction is 0.0054.
The above data indicates that the reactor is operating near the __________ of a fuel cycle, and that a
typical delayed neutron is __________ likely than a typical prompt neutron to cause another fission in
this reactor.

A. beginning; less

B. beginning; more

C. end; less

D. end; more

Answer 44

end; more

Question 45

A reactor core has a delayed neutron importance factor of 1.02. If the average delayed neutron
fraction in the core is 0.0057, the effective delayed neutron fraction is…

A. equal to 0.0057.

B. less than 0.0057.

C. greater than 0.0057.

D. unpredictable without additional information.

Answer 45

greater than 0.0057.

Question 46

Which one of the following is the primary reason that delayed neutrons are more effective than prompt
neutrons at controlling the rate of reactor power changes?

A. Delayed neutrons have a longer mean generation time than prompt neutrons.

B. Delayed neutrons produce a larger amount of core fissions than prompt neutrons.

C. Delayed neutrons make up a larger fraction of fission neutrons than prompt neutrons.

D. Delayed neutrons are born with a lower average kinetic energy than prompt neutrons.

Answer 46

Delayed neutrons have a longer mean generation time than prompt neutrons.

Question 47

Two identical reactors, A and B, with identical fuel compositions, are initially critical at
1.0 x 10^-8 percent power. Then, suddenly and simultaneously, positive 0.001 ΔK/K is added to
reactor A while negative 0.001 ΔK/K is added to reactor B.
One minute later, which reactor will have the shorter period, and why? (Note: λeff is the effective
delayed neutron precursor decay constant.)

A. Reactor A, because the value of λeff shifts toward the value of the decay constant for the
shorter-lived delayed neutron precursors when reactivity is positive.

B. Reactor A, because the value of λeff shifts toward the value of the decay constant for the
longer-lived delayed neutron precursors when reactivity is positive.

C. Reactor B, because the value of λeff shifts toward the value of the decay constant for the
shorter-lived delayed neutron precursors when reactivity is negative.

D. Reactor B, because the value of λeff shifts toward the value of the decay constant for the
longer-lived delayed neutron precursors when reactivity is negative.

Answer 47

Reactor A, because the value of λeff shifts toward the value of the decay constant for the
shorter-lived delayed neutron precursors when reactivity is positive.

Question 48

A reactor is critical at a constant power level of 1.0 x 10-8 percent. Consider the following two cases:
Case 1: A step addition of positive 0.001 ΔK/K.
Case 2: A step addition of negative 0.001 ΔK/K.
Which case will produce the faster rate of power change one minute after the reactivity addition, and
why?

A. Case 1, because the effective delayed neutron fraction is smaller during a power increase.

B. Case 1, because the effective delayed neutron precursor decay constant is larger during a power
increase.

C. Case 2, because the effective delayed neutron fraction is smaller during a power decrease.

D. Case 2, because the effective delayed neutron precursor decay constant is larger during a power
decrease.

Answer 48

Case 1, because the effective delayed neutron precursor decay constant is larger during a power
increase.

Question 49

Which one of the following describes a condition in which a reactor is prompt critical?

A. A very long reactor period makes reactor control very sluggish and unresponsive.

B. Fissions are occurring so rapidly that the effective delayed neutron fraction approaches zero.

C. Any increase in reactor power requires a reactivity addition equal to the fraction of prompt
neutrons in the core.

D. The net positive reactivity in the core is greater than or equal to the magnitude of the effective
delayed neutron fraction.

Answer 49

The net positive reactivity in the core is greater than or equal to the magnitude of the effective
delayed neutron fraction.

Question 50

A critical reactor will become prompt critical when the reactivity is equal to the…

A. shutdown margin.

B. effective delayed neutron fraction.

C. effective decay constant.

D. worth of the most reactive rod.

Answer 50

effective delayed neutron fraction.

Question 51

A reactor is operating at 75 percent power with the following conditions:
Power defect
= -0.0157 Δ/K/K
Shutdown margin
= 0.0241 Δ/K/K
Effective delayed neutron fraction = 0.0058
Effective prompt neutron fraction = 0.9942
How much positive reactivity must be added to make the reactor prompt critical?

A. 0.0157 ΔK/K

B. 0.0241 ΔK/K

C. 0.0058 ΔK/K

D. 0.9942 ΔK/K

Answer 51

0.0058 ΔK/K

Question 52

A reactor with a xenon-free core is critical several decades below the point of adding heat (POAH).
The operator continuously withdraws control rods until a positive 0.5 DPM startup rate (SUR) is
reached and then stops control rod motion.
When rod motion is stopped, the SUR will immediately… (Ignore any reactivity effects from fission
product poisons.)

A. stabilize at 0.5 DPM until power reaches the POAH.

B. decrease, and then stabilize at a value less than 0.5 DPM until power reaches the POAH.

C. stabilize at 0.5 DPM, and then slowly and continuously decrease until power reaches the POAH.

D. decrease, and then continue to slowly decrease until power reaches the POAH

Answer 52

decrease, and then stabilize at a value less than 0.5 DPM until power reaches the POAH.

Question 53

Which one of the following is the smallest listed value of Keff that will result in a prompt critical
reactor?

A. 1.0001

B. 1.001

C. 1.01

D. 1.1

Answer 53

1.01

Question 54

A reactor initially has a stable positive 1.0 DPM startup rate with no control rod motion several
decades below the point of adding heat (POAH). Control rods are inserted until a positive 0.5 DPM
startup rate is attained and then stopped.
When rod insertion is stopped, startup rate will immediately…

A. stabilize at 0.5 DPM until power reaches the POAH.

B. increase, and then stabilize at a value greater than 0.5 DPM until power reaches the POAH.

C. continuously decrease until startup rate becomes zero when power reaches the POAH.

D. increase, and then slowly and continuously decrease until startup rate becomes zero when power
reaches the POAH.

Answer 54

increase, and then stabilize at a value greater than 0.5 DPM until power reaches the POAH.

Question 55

A reactor was stable at 80 percent power when the operator withdrew a control rod continuously for 2
seconds. Which one of the following affects the amount of Aprompt jump@ increase in reactor
power for the control rod withdrawal?

A. The total control rod worth

B. The differential control rod worth

C. The duration of control rod withdrawal

D. The magnitude of the fuel temperature coefficient

Answer 55

The differential control rod worth

Question 56

A reactor is operating at steady-state 75 percent power with the following conditions:
Power defect
= -0.0185 ΔK/K
Shutdown margin
= -0.0227 ΔK/K
Effective delayed neutron fraction = 0.0061
Effective prompt neutron fraction = 0.9939
How much positive reactivity must be added to make the reactor prompt critical?

A. 0.0061 ΔK/K

B. 0.0185 ΔK/K

C. 0.0227 ΔK/K

D. 0.9939 ΔK/K

Answer 56

0.0061 ΔK/K

Question 57

Refer to the partially labeled reactor response curve shown below for a reactor that was initially stable
in the source range. Both axes have linear scales. A small amount of positive reactivity was added
at time = 0 sec.
The response curve shows __________ versus time for a reactor that was initially __________.

A. startup rate; subcritical

B. startup rate; critical

C. reactor fission rate; subcritical

D. reactor fission rate; critical

Answer 57

reactor fission rate; subcritical

Question 58

Two reactors are critical at the same power level well below the point of adding heat. The reactors are
identical except that reactor A is near the beginning of a fuel cycle (BOC) and reactor B is near the end
of a fuel cycle (EOC).
If a step addition of positive 0.001 ΔK/K is added to each reactor, the size of the prompt jump in power
level observed in reactor B (EOC) will be __________ than in reactor A (BOC); and the stable startup
rate observed in reactor B (EOC) will be __________ than in reactor A (BOC). (Assume the power
level in each reactor remains below the point of adding heat.)

A. smaller; smaller

B. smaller; larger

C. larger; smaller

D. larger; larger

Answer 58

larger; larger

Question 59

Refer to the partially labeled reactor response curve shown below for a reactor that was initially
subcritical in the source range and remained below the point of adding heat. A small amount of
positive reactivity was added at time = 0 sec.
The response curve shows __________ versus time for a reactor that is currently (at time = 60 sec)
__________.

A. startup rate; exactly critical

B. startup rate; supercritical

C. reactor fission rate; exactly critical

D. reactor fission rate; supercritical

Answer 59

reactor fission rate; supercritical

Question 60

A reactor is operating at equilibrium 75 percent power with the following conditions:
Total power defect
= -0.0176 ΔK/K
Shutdown margin
= -0.0234 ΔK/K
Effective delayed neutron fraction = 0.0067
Effective prompt neutron fraction = 0.9933
How much positive reactivity must be added to make the reactor prompt critical?

A. 0.0067 ΔK/K

B. 0.0176 ΔK/K

C. 0.0234 ΔK/K

D. 0.9933 ΔK/K

Answer 60

0.0067 ΔK/K

Question 61

Given the following information for a reactor:
Reactivity (ρ)
= 0.0060
Average delayed neutron fraction (β�) = 0.0058
Effective delayed neutron fraction (β�eff) = 0.0062
The reactor is __________, and the reactor fission rate is __________.

A. prompt critical; constant

B. prompt critical; increasing

C. not prompt critical; constant

D. not prompt critical; increasing

Answer 61

not prompt critical; increasing

Question 62

Which one of the following is a characteristic of a neutron source installed in a reactor?

A. Maintains the production of neutrons high enough to allow the reactor to achieve criticality.

B. Provides a means to allow reactivity changes to occur in a subcritical reactor.

C. Generates a sufficient neutron population to start the fission process and initiate subcritical
multiplication.

D. Provides a neutron level that is detectable on the source range nuclear instrumentation

Answer 62

Provides a neutron level that is detectable on the source range nuclear instrumentation

Question 63

Neutron sources are installed in a reactor for which one of the following reasons?

A. To decrease the amount of fuel load required for criticality.

B. To compensate for neutrons being absorbed by burnable poisons.

C. To augment the shutdown neutron flux to allow detection on nuclear instrumentation.

D. To provide sufficient neutron flux to achieve criticality during a reactor startup following a
long-term shutdown.

Answer 63

To augment the shutdown neutron flux to allow detection on nuclear instrumentation.

Question 64

Which one of the following neutron reactions yields the highest neutron production rate immediately
following a reactor trip from extended power operations during the tenth fuel cycle? (Ignore any
contribution from an installed neutron source.)

A. Alpha-neutron reactions

B. Beta-neutron reactions

C. Photo-neutron reactions

D. Spontaneous fission

Answer 64

Photo-neutron reactions

Question 65

Which one of the following neutron sources undergoes the most significant source strength reduction
during the hour immediately following a reactor trip from steady-state 100 percent power?

A. Spontaneous fission reactions

B. Photo-neutron reactions

C. Alpha-neutron reactions

D. Transuranic isotope decay

Answer 65

Photo-neutron reactions

Question 66

Which one of the following is the neutron source that produces the greatest neutron flux for the first
few days following a reactor trip from extended high power operations?

A. Spontaneous neutron emission from the control rods.

B. Photo-neutron reactions in the moderator.

C. Spontaneous fission in the fuel.

D. Alpha-neutron reactions in the fuel

Answer 66

Photo-neutron reactions in the moderator.

Question 67

Which one of the following describes the purpose of a neutron source that is installed in a reactor
during refueling for the third fuel cycle?

A. Ensures shutdown neutron level is large enough to be detected by nuclear instrumentation.

B. Provides additional excess reactivity to increase the length of the fuel cycle.

C. Amplifies the electrical noise fluctuations observed in source range instrumentation during
shutdown.

D. Supplies the only shutdown source of neutrons available to begin a reactor startup.

Answer 67

Ensures shutdown neutron level is large enough to be detected by nuclear instrumentation.