Neutron Life Cycle Flashcards
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 __________.
A. fission; fission
B. fission; capture
C. capture; fission
D. capture; capture
fission; fission
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…
A. prompt critical.
B. supercritical.
C. exactly critical.
D. subcritical.
supercritical.
Which one of the following conditions describes a reactor that is exactly critical?
A. Keff = 0; ΔK/K = 0
B. Keff = 0; ΔK/K = 1
C. Keff = 1; ΔK/K = 0
D. Keff = 1; ΔK/K = 1
Keff = 1; ΔK/K = 0
The ratio of the number of neutrons in one generation to the number of neutrons in the previous
generation is the…
A. effective multiplication factor.
B. fast fission factor.
C. nonleakage factor.
D. reproduction factor
effective multiplication factor.
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.
A. first
B. second
C. third
D. fourth
second
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.
A. beginning; next
B. beginning; previous
C. end; next
D. end; previous
end; previous
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?
A. The neutron will be scattered, thereby leaving Keff unchanged.
B. The neutron will be absorbed and the nucleus will fission, thereby decreasing Keff.
C. The neutron will be absorbed and the nucleus will fission, thereby increasing Keff.
D. The neutron will be absorbed and the nucleus will decay to Pu-239, thereby increasing Keff.
The neutron will be scattered, thereby leaving Keff unchanged.
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?
A. Keff will gradually increase during the entire period.
B. Keff will gradually decrease during the entire period.
C. Keff will tend to increase, but inherent reactivity feedback will maintain Keff at 1.0.
D. Keff will tend to decrease, but inherent reactivity feedback will maintain Keff at 1.0.
Keff will tend to decrease, but inherent reactivity feedback will maintain Keff at 1.0.
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?
A. The neutron will be scattered, thereby leaving Keff unchanged.
B. The neutron will be absorbed and the nucleus will fission, thereby decreasing Keff.
C. The neutron will be absorbed and the nucleus will fission, thereby increasing Keff.
D. The neutron will be absorbed and the nucleus will decay to Pu-239, thereby increasing Keff
The neutron will be scattered, thereby leaving Keff unchanged.
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?
A. Control rod worth is lower at the beginning of subsequent fuel cycles.
B. More fuel reactivity is present at the beginning of subsequent fuel cycles.
C. More fission product poisons are present at the beginning of subsequent fuel cycles.
D. Reactor coolant boron concentration is higher at the beginning of subsequent fuel cycles.
More fission product poisons are present at the beginning of subsequent fuel cycles.
Which one of the following defines K-excess?
A. Keff - 1
B. Keff + 1
C. (Keff - 1)/Keff
D. (1-Keff)/Keff
Keff - 1
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?
50% inserted
1,000 ppm
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?
25% inserted
500 ppm
Which one of the following is a reason for installing excess reactivity (Kexcess) in a reactor?
A. To compensate for the conversion of U-238 to Pu-239 during a fuel cycle.
B. To compensate for burnout of Xe-135 and Sm-149 during a power increase.
C. To ensure the fuel temperature coefficient remains negative during a fuel cycle.
D. To compensate for the negative reactivity added by the power coefficient during a power
increase.
To compensate for the negative reactivity added by the power coefficient during a power
increase.
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?
A. Decays over several days to Pu-239, which increases Kexcess.
B. Decays over several days to Pu-240, which increases Kexcess.
C. Immediately undergoes fast fission, which decreases Kexcess.
D. Immediately undergoes thermal fission, which decreases Kexcess.
Decays over several days to Pu-239, which increases Kexcess.
Which one of the following is a benefit of installing excess reactivity (Kexcess) in a reactor?
A. Ensures that sufficient control rod negative reactivity is available to shut down the reactor.
B. Ensures that the reactor can be made critical during a peak xenon condition after a reactor trip.
C. Ensures that positive reactivity additions result in controllable reactor power responses.
D. Ensures that the U-235 fuel enrichment is the same at the beginning and the end of a fuel cycle.
Ensures that the reactor can be made critical during a peak xenon condition after a reactor trip.
Shutdown margin can be defined as the amount of reactivity…
A. inserted by burnable poisons at beginning of a fuel cycle.
B. added by boron in the reactor coolant system.
C. by which the reactor is subcritical.
D. that would be inserted by shutdown bank rods.
by which the reactor is subcritical.
The shutdown margin determination for an operating reactor assumes the complete withdrawal of…
A. a single control rod of high reactivity worth.
B. a symmetrical pair of control rods of high reactivity worth.
C. a single control rod of average reactivity worth.
D. a symmetrical pair of control rods of average reactivity worth.
a single control rod of high reactivity worth.
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…
A. decrease and stabilize at a lower value.
B. initially decrease, then increase to the original value as coolant temperature changes.
C. increase and stabilize at a higher value.
D. initially increase, then decrease to the original value as coolant temperature changes
increase and stabilize at a higher value.
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…
A. increase and stabilize at a higher value.
B. increase, then decrease to the original value as coolant temperature changes.
C. decrease and stabilize at a lower value.
D. decrease, then increase to the original value as coolant temperature changes.
decrease and stabilize at a lower value.
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?
A. Reactor power is reduced to 45 percent with final RCS boron concentration at 620 ppm.
B. Reactor power is increased to 55 percent with final RCS boron concentration at 580 ppm.
C. Control rods are withdrawn 3 inches with no change in steady-state reactor power or RCS boron
concentration.
D. Control rods are inserted 3 inches with no change in steady-state reactor power or RCS boron
concentration.
Reactor power is increased to 55 percent with final RCS boron concentration at 580 ppm.
Which one of the following changes will decrease the available shutdown margin in a reactor?
(Assume no operator actions.)
A. Depletion of fuel during reactor operation.
B. Depletion of burnable poisons during reactor operation.
C. Buildup of samarium-149 following a reactor power transient.
D. Buildup of xenon-135 following a reactor power transient.
Depletion of burnable poisons during reactor operation.
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.)
A. continuously increase.
B. continuously decrease.
C. initially increase, and then decrease.
D. initially decrease, and then increase
continuously increase.
Reactivity is defined as the fractional change in…
A. reactor power per second.
B. neutron population per second.
C. reactor period from criticality.
D. the effective multiplication factor from criticality
the effective multiplication factor from criticality