Reactivity Coefficient Flashcards
The moderator temperature coefficient describes the change in reactivity per degree change in…
A. fuel temperature.
B. fuel cladding temperature.
C. reactor vessel temperature.
D. reactor coolant temperature.
reactor coolant temperature.
Which one of the following isotopes is the most significant contributor to the resonance capture of
fission neutrons in a reactor at the beginning of a fuel cycle?
A. U-238
B. U-233
C. Pu-240
D. Pu-239
U-238
Factors that affect the probability of resonance absorption of a neutron by a nucleus include…
A. excitation energy of the neutron, kinetic energy of the nucleus, and kinetic energy of the neutron.
B. kinetic energy of the neutron, excitation energy of the nucleus, and excitation energy of the
neutron.
C. excitation energy of the nucleus, excitation energy of the neutron, and kinetic energy of the
nucleus.
D. kinetic energy of the nucleus, kinetic energy of the neutron, and excitation energy of the nucleus.
kinetic energy of the nucleus, kinetic energy of the neutron, and excitation energy of the nucleus.
Which one of the following isotopes is the most significant contributor to the resonance capture of
fission neutrons in a reactor at the end of a fuel cycle?
A. U-235
B. U-238
C. Pu-239
D. Pu-240
U-238
Which one of the following has the smallest microscopic cross section for absorption of a thermal
neutron in an operating reactor?
A. Uranium-235
B. Uranium-238
C. Samarium-149
D. Xenon-135
Uranium-238
Under which one of the following conditions is a reactor most likely to have a positive moderator
temperature coefficient?
A. High reactor coolant temperature at the beginning of a fuel cycle.
B. High reactor coolant temperature at the end of a fuel cycle.
C. Low reactor coolant temperature at the beginning of a fuel cycle.
D. Low reactor coolant temperature at the end of a fuel cycle.
Low reactor coolant temperature at the beginning of a fuel cycle.
A reactor has operated at steady-state 100 percent power for the past 6 months. Compared to 6
months ago, the current moderator temperature coefficient is…
A. more negative, due to control rod withdrawal.
B. less negative, due to control rod insertion.
C. more negative, due to a smaller reactor coolant boron concentration.
D. less negative, due to a greater reactor coolant boron concentration.
more negative, due to a smaller reactor coolant boron concentration.
Which one of the following contains the pair of nuclides that are the most significant contributors to
the total resonance capture in the core near the end of a fuel cycle?
A. U-238 and Pu-239
B. U-238 and Pu-240
C. Pu-239 and U-235
D. Pu-239 and Pu-240
U-238 and Pu-240
Which one of the following conditions will cause the moderator temperature coefficient (MTC) to
become more negative? (Consider only the direct effect of the indicated change on MTC.)
A. The controlling bank of control rods is inserted 5 percent into the core.
B. Fuel temperature decreases from 1500°F to 1200°F.
C. Reactor coolant boron concentration increases by 20 ppm.
D. Moderator temperature decreases from 500°F to 450°F.
The controlling bank of control rods is inserted 5 percent into the core.
Which one of the following contains the nuclides responsible for most of the resonance capture of
fission neutrons in a reactor at the beginning of the sixth fuel cycle? (Assume that each refueling
process replaces one-third of the fuel.)
A. U-235 and Pu-239
B. U-235 and U-238
C. U-238 and Pu-239
D. U-238 and Pu-240
U-238 and Pu-240
Which one of the following contains two isotopes that add significant negative reactivity when fuel
temperature increases near the end of a fuel cycle?
A. U-235 and Pu-239
B. U-235 and Pu-240
C. U-238 and Pu-239
D. U-238 and Pu-240
U-238 and Pu-240
Which one of the following describes a situation where an increase in moderator temperature can add
positive reactivity?
A. At low moderator temperatures, an increase in moderator temperature can reduce neutron leakage
from the core sufficiently to add positive reactivity.
B. At low moderator temperatures, an increase in moderator temperature can reduce neutron capture
by the moderator sufficiently to add positive reactivity.
C. At high moderator temperatures, an increase in moderator temperature can reduce neutron leakage
from the core sufficiently to add positive reactivity.
D. At high moderator temperatures, an increase in moderator temperature can reduce neutron capture
by the moderator sufficiently to add positive reactivity.
At low moderator temperatures, an increase in moderator temperature can reduce neutron capture
by the moderator sufficiently to add positive reactivity.
As the reactor coolant boron concentration increases, the moderator temperature coefficient becomes
less negative. This is because a 1°F increase in reactor coolant temperature at higher boron
concentrations results in a larger increase in the…
A. fast fission factor.
B. thermal utilization factor.
C. total nonleakage probability.
D. resonance escape probability.
thermal utilization factor.
In which one of the following conditions is the moderator temperature coefficient most negative?
A. Beginning of a fuel cycle (BOC), high reactor coolant temperature
B. BOC, low reactor coolant temperature
C. End of a fuel cycle (EOC), high reactor coolant temperature
D. EOC, low reactor coolant temperature
End of a fuel cycle (EOC), high reactor coolant temperature
During a nuclear power plant heatup near the end of a fuel cycle, the moderator temperature
coefficient becomes increasingly more negative. This is because…
A. as moderator density decreases, more thermal neutrons are absorbed by the moderator than by the
fuel.
B. the change in the thermal utilization factor dominates the change in the resonance escape
probability.
C. a greater density change per °F occurs at higher reactor coolant temperatures.
D. the core transitions from an undermoderated condition to an overmoderated condition.
a greater density change per °F occurs at higher reactor coolant temperatures.
The moderator temperature coefficient will be least negative at a __________ reactor coolant
temperature and a __________ reactor coolant boron concentration.
A. high; high
B. high; low
C. low; high
D. low; low
low; high
A reactor is operating at full power following a refueling outage. Compared to the current moderator
temperature coefficient (MTC), the MTC just prior to the refueling was…
A. less negative at all coolant temperatures.
B. more negative at all coolant temperatures.
C. less negative below approximately 350°F coolant temperature and more negative above
approximately 350°F coolant temperature.
D. more negative below approximately 350°F coolant temperature and less negative above
approximately 350°F coolant temperature.
more negative at all coolant temperatures.
During a reactor coolant system cooldown, positive reactivity is added to the core if the moderator
temperature coefficient is negative. This is partially due to…
A. a decreasing thermal utilization factor.
B. an increasing thermal utilization factor.
C. a decreasing resonance escape probability.
D. an increasing resonance escape probability.
an increasing resonance escape probability.
As the core ages, the moderator temperature coefficient becomes more negative. This is primarily
due to…
A. fission product poison buildup in the fuel.
B. decreasing fuel centerline temperature.
C. decreasing control rod worth.
D. decreasing reactor coolant boron concentration.
decreasing reactor coolant boron concentration.
The moderator temperature coefficient will be most negative at a __________ reactor coolant
temperature and a __________ reactor coolant boron concentration.
A. low; low
B. high; low
C. low; high
D. high; high
high; low
Which one of the following describes the initial reactivity effect of a moderator temperature decrease
in an undermoderated reactor?
A. Negative reactivity will be added because more neutrons will be absorbed at resonance energies
while slowing down.
B. Negative reactivity will be added because more neutrons will be captured by the moderator.
C. Positive reactivity will be added because fewer neutrons will be absorbed at resonance energies
while slowing down.
D. Positive reactivity will be added because fewer neutrons will be captured by the moderator.
Positive reactivity will be added because fewer neutrons will be absorbed at resonance energies
while slowing down.
Which one of the following describes why the moderator temperature coefficient is more negative
near the end of a fuel cycle (EOC) compared to the beginning of a fuel cycle (BOC)?
A. Increased nucleate boiling near the EOC amplifies the negative reactivity added by a 1°F
moderator temperature increase.
B. Increased control rod insertion near the EOC amplifies the negative reactivity added by a 1°F
moderator temperature increase.
C. Decreased fuel temperature near the EOC results in reduced resonance neutron capture for a 1°F
increase in moderator temperature.
D. Decreased coolant boron concentration near the EOC results in fewer boron atoms leaving the core
for a 1°F moderator temperature increase.
Decreased coolant boron concentration near the EOC results in fewer boron atoms leaving the core
for a 1°F moderator temperature increase.
Which one of the following describes the initial reactivity effect of a moderator temperature decrease
in an overmoderated reactor?
A. Positive reactivity will be added because fewer neutrons will be captured by the moderator while
slowing down.
B. Positive reactivity will be added because fewer neutrons will be absorbed at resonance energies
while slowing down.
C. Negative reactivity will be added because more neutrons will be captured by the moderator while
slowing down.
D. Negative reactivity will be added because more neutrons will be absorbed at resonance energies
while slowing down.
Negative reactivity will be added because more neutrons will be captured by the moderator while
slowing down.
A reactor is operating at 100 percent power following a refueling outage. Compared to the moderator
temperature coefficient (MTC) just prior to the refueling, the current MTC is…
A. less negative at all coolant temperatures.
B. more negative at all coolant temperatures.
C. less negative below approximately 350°F coolant temperature and more negative above
approximately 350°F coolant temperature.
D. more negative below approximately 350°F coolant temperature and less negative above
approximately 350°F coolant temperature.
less negative at all coolant temperatures.
Which one of the following describes the initial reactivity effect of a moderator temperature increase
in an overmoderated reactor?
A. Negative reactivity will be added because more neutrons will be absorbed at resonance energies
while slowing down.
B. Negative reactivity will be added because more neutrons will be captured by the moderator while
slowing down.
C. Positive reactivity will be added because fewer neutrons will be absorbed at resonance energies
while slowing down.
D. Positive reactivity will be added because fewer neutrons will be captured by the moderator while
slowing down.
Positive reactivity will be added because fewer neutrons will be captured by the moderator while
slowing down.
How does the addition of boric acid to the reactor coolant affect the moderator temperature coefficient
(MTC) in an undermoderated reactor?
A. The initially negative MTC becomes more negative.
B. The initially negative MTC becomes less negative.
C. The initially positive MTC becomes more positive.
D. The initially positive MTC becomes less positive.
The initially negative MTC becomes less negative.
Compared to the moderator temperature coefficient (MTC) of reactivity near the beginning of a fuel
cycle, the MTC near the end of a fuel cycle is: (Assume 100 percent power for all cases.)
A. more negative, because as U-235 depletes, more fission neutrons are able to escape resonance
capture.
B. less negative, because as U-238 depletes, more fission neutrons are able to escape resonance
capture.
C. more negative, because as reactor coolant boron concentration decreases, the thermal utilization of
fission neutrons increases.
D. less negative, because as control rods are withdrawn from the core, the thermal utilization of
fission neutrons increases.
more negative, because as reactor coolant boron concentration decreases, the thermal utilization of
fission neutrons increases.
Which one of the following describes the initial reactivity effect of a moderator temperature increase
in an undermoderated reactor?
A. Negative reactivity will be added because more neutrons will be absorbed by U-238 at resonance
energies while slowing down.
B. Negative reactivity will be added because more neutrons will be captured by the moderator while
slowing down.
C. Positive reactivity will be added because fewer neutrons will be absorbed by U-238 at resonance
energies while slowing down.
D. Positive reactivity will be added because fewer neutrons will be captured by the moderator while
slowing down.
Negative reactivity will be added because more neutrons will be absorbed by U-238 at resonance
energies while slowing down.
When compared to the beginning of a fuel cycle, the moderator temperature coefficient at 100 percent
power near the end of a fuel cycle is…
A. more negative, because fewer boron-10 nuclei are removed from the core for a given moderator
temperature increase.
B. less negative, because more boron-10 nuclei are removed from the core for a given moderator
temperature increase.
C. more negative, because a smaller fraction of the neutron flux will leak out of the core following a
given moderator temperature increase.
D. less negative, because a larger fraction of the neutron flux will leak out of the core following a
given moderator temperature increase.
more negative, because fewer boron-10 nuclei are removed from the core for a given moderator
temperature increase.
How does increasing the reactor coolant boron concentration affect the moderator temperature
coefficient (MTC) in an overmoderated reactor?
A. The initially negative MTC becomes more negative.
B. The initially negative MTC becomes less negative.
C. The initially positive MTC becomes more positive.
D. The initially positive MTC becomes less positive.
The initially positive MTC becomes more positive.
If fuel temperature increases, the area under the curve will __________; and negative reactivity will be
added to the core because __________.
A. increase; neutrons of a wider range of energies will be absorbed by U-238
B. increase; more neutrons will be absorbed by U-238 at the resonance neutron energy
C. remain the same; neutrons of a wider range of energies will be absorbed by U-238
D. remain the same; more neutrons will be absorbed by U-238 at the resonance neutron energy
remain the same; neutrons of a wider range of energies will be absorbed by U-238
Refer to the curve of microscopic cross section for absorption versus neutron energy for a 6.7 electron
volt (eV) resonance peak in U-238 for a reactor operating at 50 percent power (see figure below).
If fuel temperature decreases by 50°F, the area under the curve will __________; and positive
reactivity will be added to the core because __________.
A. decrease; fewer neutrons will be absorbed by U-238 overall
B. decrease; fewer 6.7 eV neutrons will be absorbed by U-238 at the resonance energy
C. remain the same; fewer neutrons will be absorbed by U-238 overall
D. remain the same; fewer 6.7 eV neutrons will be absorbed by U-238 at the resonance energy
remain the same; fewer neutrons will be absorbed by U-238 overall
Refer to the curve of microscopic cross section for absorption versus neutron energy for a resonance
peak in U-238 in a reactor operating at 80 percent power (see figure below).
If reactor power is increased to 100 percent, the height of the curve will __________; and the area
under the curve will __________.
A. increase; increase
B. increase; remain the same
C. decrease; decrease
D. decrease; remain the same
decrease; remain the same
Refer to the drawing of a curve showing the neutron absorption characteristics of a typical U-238
nucleus at a resonance neutron energy (see figure below). The associated reactor is currently
operating at steady-state 80 percent power.
During a subsequent reactor power decrease to 70 percent, the curve will become __________; and the
percentage of the core neutron population lost to resonance capture by U-238 will __________.
A. shorter and broader; increase
B. shorter and broader; decrease
C. taller and more narrow; increase
D. taller and more narrow; decrease
taller and more narrow; decrease
Refer to the curve of microscopic cross section for absorption versus neutron energy for a resonance
peak in U-238 in a reactor operating at 80 percent power (see figure below).
If reactor power is decreased to 60 percent, the height of the curve will __________; and the area
under the curve will __________.
A. increase; increase
B. increase; remain the same
C. decrease; decrease
D. decrease; remain the same
increase; remain the same
Refer to the drawing of a curve showing the neutron absorption cross-section for U-238 at a resonance
energy (see figure below). The reactor associated with the curve is operating at 80 percent power.
If reactor power is increased to 90 percent over the next few hours, the curve will become ________;
and the percentage of the core neutron population lost to resonance capture by U-238 will ________.
A. shorter and broader; increase
B. shorter and broader; decrease
C. taller and more narrow; increase
D. taller and more narrow; decrease
shorter and broader; increase
