Moderator temperature coefficient is defined as the change in core reactivity per degree change in:
A. coolant temperature.
B. reactor vessel temperature.
C. cladding temperature.
D. fuel temperature.
ANSWER: A.
Comment: The question is technically incorrect because it provides an imprecise definition, The moderator coefficient is very specifically defined as being the change in reactivity associated with "a one oF increase in moderator temperature".
QID: P250 (B51) (TOPIC: 192004 KNOWLEDGE: K1.01 [3.1/3.2])
The reactor is critical below the point of adding heat during a normal reactor startup at end of core life. Select the reactivity coefficient that will add the most negative reactivity if reactor coolant temperature increases by 1 F.
A. Void coefficient
B. Pressure coefficient
C. Fuel temperature coefficient
D. Moderator temperature coefficient
ANSWER: D.
Comment: The question is technically incorrect because it states that "negative reactivity will be added", rather than a negative change in reactivity is introduced. For a detailed discussion of reactivity and reactivity change see QID:P1246 in section Reactor Theory - Neutron Life Cycle.
QID: P350 (B353) (TOPIC: 192004 KNOWLEDGE: K1.02 [3.0/3.2])
Which one of the following will directly result in a less negative fuel temperature coefficient? (Consider only the effect of the change in the listed parameters.)
A. Increase in fuel burnup
B. Decrease in fuel temperature
C. Increase in void fraction
D. Decrease in moderator temperature
ANSWER: D.
Comment: The question is technically incorrect because there are no gradations of negativity. Negative is negative and positive is positive. The question refers to the magnitude of the negative fuel temperature coefficient becoming "smaller."
QID: P1150 TOPIC: (192004 KNOWLEDGE: K1.03 [2.9/3.1])
The reactor has operated at steady-state 100% power for the past 6 months. Compared to 6 months ago, 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 decreased reactor coolant system (RCS) boron concentration.
D. less negative due to increased RCS boron concentration.
ANSWER: C.
Comment: The question suffers the same defect as P350. Negative is negative.
QID: P2150 (TOPIC: 192004 KNOWLEDGE: K1.03/K1.06 [2.9/3.1])
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% 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.
ANSWER: A.
Comment: The question suffers the same defect as P350. Negative is negative.
QID: P50 (TOPIC: 192004 KNOWLEDGE: K1.06 [3.1/3.1])
Why does increasing reactor coolant boron concentration cause the moderator temperature coefficient to become less negative?
A. Reactor coolant temperature increases result in a larger increase in the thermal utilization factor.
B. Reactor coolant temperature increases result in an increase in the resonance escape probability.
C. Reactor coolant temperature increases result in an increase in the total nonleakage probability.
D. The change in resonance escape probability dominates the change in the thermal utilization factor.
ANSWER: A.
Comment: The question suffers the same defect as P350. Negative is negative.
QID: P123 (TOPIC: 192004 KNOWLEDGE: K1.06 [3.1/3.1])
In which of the following conditions is the moderator temperature coefficient most negative?
A. Beginning of core life (BOL), high temperature
B. BOL, low temperature
C. End of core life (EOL), high temperature
D. EOL, low temperature
ANSWER: C.
Comment: The question suffers the same defect as P350. Negative is negative.
QID: P252 (TOPIC: 192004 KNOWLEDGE: K1.06 [3.1/3.1])
During a plant heat-up at end of core life, 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 degree F occurs at higher reactor coolant temperatures.
D. the core transitions from an undermoderated condition to an overmoderated condition.
ANSWER: C.
Comment: The question suffers the same defect as P350. Negative is negative.
QID: P450 (TOPIC: 192004 KNOWLEDGE: K1.06 [3.1/3.1])
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
ANSWER: C.
Comment: The question suffers the same defect as P350. Negative is negative.
QID: P751 (B651) (TOPIC: 192004 KNOWLEDGE: K1.06 [3.1/3.1])
The reactor is operating at full power following a refueling outage. In comparison 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.
ANSWER: B.
Comment: The question suffers the same defect as P350. Negative is negative.
QID: P951 (B2452) (TOPIC: 192004 KNOWLEDGE: K1.06 [3.1/3.1])
During a reactor coolant system (RCS) cooldown, positive reactivity is added to the core (assuming a negative moderator temperature coefficient). This is partially due to:
A. a decrease in the thermal utilization factor.
B. an increase in the thermal utilization factor.
C. a decrease in the resonance escape probability.
D. an increase in the resonance escape probability.
ANSWER: D.
Comment: The question suffers the same defect as P250. Reactivity is not added.
QID: P1250 (TOPIC: 192004 KNOWLEDGE: K1.06 [3.1/3.1])
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 system boron concentration.
ANSWER: D.
Comment: The question suffers the same defect as P350. Negative is negative.
QID: P1450 (TOPIC: 192004 KNOWLEDGE: K1.06 [3.1/3.1])
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
ANSWER: B.
Comment: The question suffers the same defect as P350. Negative is negative.
QID: P1752 (B1752) (TOPIC: 192004 KNOWLEDGE: K1.06 [3.1/3.1])
Which one of the following describes the net reactivity effect of a moderator temperature decrease in an undermoderated reactor core?
A. Negative reactivity will be added because more neutron leakage will occur.
B. Negative reactivity will be added because more thermal neutrons will be captured by the moderator.
C. Positive reactivity will be added because less neutron leakage will occur.
D. Positive reactivity will be added because less thermal neutrons will be captured by the moderator.
ANSWER: C.
Comment: The question suffers the same defect as P250. Reactivity is not added.
QID: P1850 (TOPIC: 192004 KNOWLEDGE: K1.06 [3.1/3.1])
Which one of the following describes why the moderator temperature coefficient is more negative at the end of core life (EOL) compared to the beginning of core life (BOL)?
A. Increased nucleate boiling at the EOL amplifies the negative reactivity added by a 1 F moderator temperature increase.
B. Increased control rod insertion at the EOL amplifies the negative reactivity added by a 1 F moderator temperature increase.
C. Decreased fuel temperature at the EOL results in reduced resonant neutron capture for a 1 F increase in moderator temperature.
D. Decreased coolant boron concentration at the EOL results in fewer boron atoms leaving the core for a 1 F moderator temperature increase.
ANSWER: D.
Comment: The question suffers the same defect as P350. Negative is negative.
QID: P2650 (B2652) (TOPIC: 192004 KNOWLEDGE: K1.06 [3.1/3.1])
Which one of the following describes the net reactivity effect of a moderator temperature decrease in an overmoderated reactor core?
A. Negative reactivity will be added because more neutron leakage will occur.
B. Negative reactivity will be added because more neutrons will be captured by the moderator.
C. Positive reactivity will be added because less neutron leakage will occur.
D. Positive reactivity will be added because fewer neutrons will be captured by the moderator.
ANSWER: B.
Comment: The question suffers the same defect as P250. Reactivity is not added.
QID: P2750 (TOPIC: 192004 KNOWLEDGE: K1.06 [3.1/3.1])
The reactor is operating at full 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.
ANSWER: A.
Comment: The question suffers the same defect as P350. Negative is negative.
QID: P2950 (B2952) (TOPIC: 192004 KNOWLEDGE: K1.06 [3.1/3.1])
Which one of the following describes the net reactivity effect of a moderator temperature increase in an overmoderated reactor core?
A. Negative reactivity will be added because more neutron leakage will occur.
B. Negative reactivity will be added because more neutrons will be captured by the moderator.
C. Positive reactivity will be added because less neutron leakage will occur.
D. Positive reactivity will be added because fewer neutrons will be captured by the moderator.
ANSWER: D.
Comment: The question suffers the same defect as P250. Reactivity is not added.
QID: P51 (TOPIC: 192004 KNOWLEDGE: K1.07 [2.9/2.9])
Why does the fuel temperature (Doppler) coefficient becomes less negative at higher fuel temperatures?
A. As reactor power increases, the rate of increase in the fuel temperature diminishes.
B. Neutrons penetrate deeper into the fuel, resulting in an increase in the fast fission factor.
C. The amount of self-shielding increases, resulting in less neutron absorption by the inner fuel.
D. The amount of Doppler broadening per degree change in fuel temperature diminishes.
ANSWER: D.
Comment: The question suffers the same defect as P350. Negative is negative. In addition, the definition of the Doppler coefficient in Choice C is incorrectly indicated to be per degree "change" in fuel temperature.
QID: P651 (TOPIC: 192004 KNOWLEDGE: K1.07 [2.9/2.9])
Which one of the following will cause the Doppler power coefficient to become more negative?
A. Increased clad creep
B. Increased pellet swell
C. Lower power level
D. Higher coolant boron concentration
ANSWER: C.
Comment: The question suffers the same defect as P350. Negative is negative.
QID: P1052 (TOPIC: 192004 KNOWLEDGE: K1.07 [2.9/2.9])
As core age increases, for the same power level the Doppler power coefficient of reactivity becomes ______________ negative because fuel temperature ______________.
A. more; decreases
B. more; increases
C. less; decreases
D. less; increases
ANSWER: C.
Comment: The question suffers the same defect as P350. Negative is negative.
QID: P1851 (TOPIC: 192004 KNOWLEDGE: K1.07 [2.9/2.9])
Which one of the following pairs of isotopes is responsible for the negative reactivity associated with a fuel temperature increase near the end of core life?
A. U-235 and Pu-239
B. U-235 and Pu-240
C. U-238 and Pu-239
D. U-238 and Pu-240
ANSWER: D.
Comment: The question suffers the same defect as P250. Reactivity is not associated with a fuel temperature increase.
QID: P1951 (B1553) (TOPIC: 192004 KNOWLEDGE: K1.07 [2.9/2.9])
A reactor is operating at 70% power. Which one of the following will directly result in a less negative fuel temperature coefficient? (Consider only the effect of the change in each listed parameter.)
A. Increase in fuel temperature
B. Increase in moderator temperature
C. Increase in moderator voids
D. Increase in Pu-240 inventory in the core
ANSWER: A.
Comment: The question suffers the same defect as P350. Negative is negative.
QID: P2052 (B2053) (TOPIC: 192004 KNOWLEDGE: K1.07 [2.9/2.9])
Compared to operation at a low power level, the fuel temperature coefficient of reactivity at a high power level is ____________ negative due to ____________. (Assume the same core age.)
A. less; improved pellet-to-clad heat transfer
B. more; buildup of fission product poisons
C. less; higher fuel temperature
D. more; increased neutron flux
ANSWER: C.
Comment: The question suffers the same defect as P350. Negative is negative.
QID: P2352 (B2453) (TOPIC: 192004 KNOWLEDGE: K1.07 [2.9/2.9])
Refer to the drawing of microscopic cross section for absorption versus neutron energy for a resonance peak in U-238 (see figure below). 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
ANSWER: C.
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Comment: The question suffers the same defect as P250. Reactivity is not added.
QID: P2451 (B552) (TOPIC: 192004 KNOWLEDGE: K1.07 [2.9/2.9])
Which one of the following describes how the magnitude of the fuel temperature coefficient of reactivity is affected over core life?
A. It remains essentially constant over core life.
B. It becomes more negative due to the buildup of Pu-240.
C. It becomes less negative due to the decrease in RCS boron concentration.
D. It becomes more negative initially due to buildup of fissions product poisons, then less negative due to fuel depletion.
ANSWER: B.
Comment: The question suffers the same defect as P350. Negative is negative.
QID: P2651 (B2553) (TOPIC: 192004 KNOWLEDGE: K1.07 [2.9/2.9])
The fuel temperature (Doppler) coefficient of reactivity is more negative at the ____________ of a fuel cycle because ________________. (Assume the same initial fuel temperature throughout the fuel cycle.)
A. end; more Pu-240 is in the core
B. end; more fission products are in the core
C. beginning; more U-238 is in the core
D. beginning; less fission products are in the core
ANSWER: A.
Comment: The question suffers the same defect as P350. Negative is negative.
QID: P2751 (B2753) (TOPIC: 192004 KNOWLEDGE: K1.07 [2.9/2.9])
Refer to the drawing 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% 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
ANSWER: C.
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Comment: The question suffers the same defect as P250. Reactivity is not added.
QID: P652 (TOPIC: 192004 KNOWLEDGE: K1.08 [3.1/3.1])
Which one of the following adds the most positive reactivity following a reactor trip/scram from full power at the beginning of core life? (Assume reactor coolant system parameters stabilize at their normal post-trip values.)
A. Void coefficient
B. Pressure coefficient
C. Fuel temperature coefficient
D. Moderator temperature coefficient
ANSWER: C.
Comment: The question suffers the same defect as P250. Reactivity is not added.
QID: P1353 (TOPIC: 192004 KNOWLEDGE: K1.08 [3.1/3.1])
A reactor has been taken critical following a four day shutdown at the beginning of core life. Reactor power is ramped to 50% over the next 8 hours. During the power increase, most of the positive reactivity added by the operator is necessary to overcome the negative reactivity associated with the:
A. buildup of core Xe-135.
B. increased fuel temperature.
C. burnout of burnable poisons.
D. increased reactor coolant temperature.
ANSWER: B.
Comment: The question suffers the same defect as P250. Reactivity is not added. In addition, reactivity is not associated with increased fuel temperature.
QID: P1551 (TOPIC: 192004 KNOWLEDGE: K1.08 [3.1/3.1])
A reactor has been operating at steady state 50% power for one month following a refueling outage. Reactor power is ramped to 100% over the next 2 hours. During the power increase, most of the positive reactivity added by the operator is necessary to overcome the negative reactivity associated with the:
A. increased reactor coolant temperature.
B. buildup of core Xe-135.
C. burnout of burnable poisons.
D. increased fuel temperature.
ANSWER: D.
Comment: The question suffers the same defect as P250. Reactivity is not added. In addition, reactivity is not associated with increased fuel temperature.
QID: P552 (TOPIC: 192004 KNOWLEDGE: K1.09 [2.8/2.9])
As reactor coolant boron concentration is reduced differential boron reactivity worth (delta-K/K per ppm) becomes:
A. less negative due to the increased number of water molecules in the core.
B. more negative due to the increased number of water molecules in the core.
C. less negative due to the decreased number of boron molecules in the core.
D. more negative due to the decreased number of boron molecules in the core.
ANSWER: D.
Comment: The question is technically incorrect because differential boron reactivity worth is indicated to represent "reactivity" per ppm. In addition, the question suffers the same defect as P350. Negative is negative.
QID: P1152 (TOPIC: 192004 KNOWLEDGE: K1.10 [2.9/2.9])
Differential boron reactivity worth will become _______ negative as moderator temperature increases because, at higher moderator temperatures, a 1 ppm increase in reactor coolant system boron concentration will add _______ boron atoms to the core.
A. more; fewer
B. more; more
C. less; fewer
D. less; more
ANSWER: C.
Comment: The question suffers the same defect as P350. Negative is negative.
QID: P1252 (TOPIC: 192004 KNOWLEDGE: K1.10 [2.9/2.9])
Differential boron worth (delta-K/K/ppm) becomes more negative as:
A. burnable poisons deplete.
B. boron concentration increases.
C. moderator temperature increases.
D. fission product poison concentration increases.
ANSWER: A.
Comment: The question is technically incorrect because differential boron worth is incorrectly indicated to represent reactivity per ppm. In addition, the question suffers the same defect as P350. Negative is negative.