A 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 of improper terminology, namely the use of "adding negative reactivity", rather than introduction of a negative change in reactivity. For a detailed discussion of reactivity and reactivity change see QID:B2048 in section Reactor Theory - Neutron Life Cycle.
QID: B252 (TOPIC: 292004 KNOWLEDGE: K1.01 [3.2/3.2])
The moderator temperature coefficient describes a change in _______________ resulting from a change in _______________.
A. reactivity; moderator temperature
B. Keff; moderator temperature
C. moderator temperature; reactivity
D. moderator temperature; Keff
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: B353 (P350) (TOPIC: 292004 KNOWLEDGE: K1.02 [2.5/2.6])
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: B651 (P751) (TOPIC: 292004 KNOWLEDGE: K1.02 [2.5/2.6])
A reactor is currently at end-of-life in its fuel cycle, and it will be refueled next month. In comparison to the current moderator temperature coefficient (MTC), the MTC after refueling will be:
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: This question suffers the same defect as B353. Negative is negative. There is no correct choice.
QID: B752 (N/A) (TOPIC: 292004 KNOWLEDGE: K1.02 [2.5/2.6])
A 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. more negative below approximately 350 F coolant temperature and less negative above approximately 350 F coolant temperature.
B. less negative below approximately 350 F coolant temperature and more negative above approximately 350 F coolant temperature.
C. more negative at all coolant temperatures.
D. less negative at all coolant temperatures.
ANSWER: D.
Comment: This question suffers the same defect as B353. Negative is negative. There is no correct choice.
QID: B852 (TOPIC: 292004 KNOWLEDGE: K1.02 [2.5/2.6])
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. Control rods are inserted from 50% rod density to 75% rod density.
B. Fuel temperature decreases from 1500 F to 1200 F.
C. Recirculation flow increases by 10%.
D. Moderator temperature decreases from 500 F to 450 F.
ANSWER: A.
Comment: This question suffers the same defect as B353. Negative is negative. There is no correct choice.
QID: B1152 (TOPIC: 292004 KNOWLEDGE: K1.02 [2.5/2.6])
Which one of the following describes the change in the moderator temperature coefficient (MTC) of reactivity over core life? (Assume 100% power for all cases.)
A. Control rod withdrawal results in increased thermal neutron utilization, which results in a less negative MTC at end of fuel cycle (EOC).
B. Fission product poison buildup results in decreased thermal neutron utilization, which results in a more negative MTC at EOC.
C. Burnup of U-235 results in decreased thermal neutron utilization, which results in a more negative MTC at EOC.
D. Decreased voiding in the core results in increased thermal neutron utilization, which results in a less negative MTC at EOC.
ANSWER: A.
Comment: This question suffers the same defect as B353. Negative is negative. There is no correct choice.
QID: B1253 (TOPIC: 292004 KNOWLEDGE: K1.02 [2.5/2.6])
The moderator temperature coefficient of reactivity is ____________ negative at end of core life because, over core life, the utilization of thermal neutrons ____________.
A. more; decreases
B. less; decreases
C. more; increases
D. less; increases
ANSWER: D.
Comment: This question suffers the same defect as B353. Negative is negative. There is no correct choice.
QID: B1752 (P1752) (TOPIC: 292004 KNOWLEDGE: K1.02 [2.5/2.6])
Which one of the following describes the net reactivity effect of a decrease in moderator temperature in an undermoderated reactor core?
A. Negative reactivity will be added because more thermal neutrons will be captured by the moderator.
B. Negative reactivity will be added because more neutron leakage will occur.
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: This question suffers the same defect as B51. Reactivity is not added.
QID: B2052 (N/A) (TOPIC: 292004 KNOWLEDGE: K1.02 [2.5/2.6])
A reactor is shut down with the reactor vessel head removed for refueling. The core is covered by 23 feet of water with a temperature of 100 F. Which one of the following can both increase and decrease Keff depending on core burnout?
A. A spent fuel assembly is removed from the core.
B. Refueling water temperature decreases to 95 F.
C. A fresh neutron source is installed in the core.
D. Movable incore source range instrumentation is repositioned to increase source range count rate.
ANSWER: B.
Comment: The question is technically incorrect because of poor wording and terminology. The question should read, " Which one of the following can "either" increase "or" decrease ... ?" In addition, the effect depends on "core depletion."
QID: B2452 (P951) (TOPIC: 292004 KNOWLEDGE: K1.02 [2.5/2.6])
During a reactor vessel 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: This question suffers the same defect as B51. Reactivity is not added.
QID: B2652 (P2650) (TOPIC: 292004 KNOWLEDGE: K1.02 [2.5/2.6])
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: This question suffers the same defect as B51. Reactivity is not added.
QID: B2853 (TOPIC: 292004 KNOWLEDGE: K1.02 [2.5/2.6])
Which one of the following describes the change in the moderator temperature coefficient (MTC) of reactivity over core life? (Assume 100% power for all cases.)
A. MTC becomes less negative because as control rods are withdrawn from the core, the increase in the number of neutrons leaking from the core for a 1 F increase in moderator temperature is smaller.
B. MTC becomes less negative because as U-238 depletes, a 1 F increase in moderator temperature results in fewer neutrons escaping resonance capture.
C. MTC becomes more negative because as U-235 depletes, a 1 F increase in moderator temperature permits more neutrons to leak out of the core.
D. MTC becomes more negative because as fission product poisons build up, the increase in the number of neutrons being absorbed by fission product poisons for a 1 F increase in moderator temperature is larger.
ANSWER: A.
Comment: This question suffers the same defect as B353. Negative is negative. There is no correct choice.
QID: B2952 (P2950) (TOPIC: 292004 KNOWLEDGE: K1.02 [2.5/2.6])
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: This question suffers the same defect as B51. Reactivity is not added.
QID: B1553 (P1951) (TOPIC: 292004 KNOWLEDGE: K1.04 [2.6/2.7])
A reactor is operating at 70% power. Which one of the following will result in a less negative fuel temperature coefficient? (Consider only the direct effect of the change in each listed parameter.)
A. Increase in Pu-240 inventory in the core
B. Increase in moderator temperature
C. Increase in fuel temperature
D. Increase in void fraction
ANSWER: C.
Comment: This question suffers the same defect as B353. Negative is negative.
QID: B452 (P2251) TOPIC: 292004 KNOWLEDGE: K1.05 [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: This question suffers the same defect as B51. Reactivity is not associated with fuel temperature.
QID: B552 (P2451) (TOPIC: 292004 KNOWLEDGE: K1.05 [2.9/2.9])
Which one of the following describes how the magnitude of the Doppler coefficient of reactivity is affected over core life?
A. It becomes more negative due to the buildup of Pu-240.
B. It becomes less negative due to the buildup of fission products.
C. It becomes more negative initially due to gadolinium burnout, then less negative due to fuel depletion.
D. It remains essentially constant.
ANSWER: A.
Comment: This question suffers the same defect as B353. Negative is negative. There is no correct choice.
QID: B1353 (TOPIC: 292004 KNOWLEDGE: K1.05 [2.9/2.9])
Compared to beginning of core life, the Doppler coefficient of reactivity is ____________ negative at end of core life due to ____________. (Assume the same initial fuel temperature.)
A. less; depletion of U-238
B. more; burnout of gadolinium
C. less; buildup of fission products
D. more; buildup of Pu-240
ANSWER: D.
Comment: This question suffers the same defect as B353. Negative is negative. There is no correct choice.
QID: B2053 (P2052) (TOPIC: 292004 KNOWLEDGE: K1.05 [2.9/2.9])
Compared to operating at a low power level, the fuel temperature (Doppler) coefficient of reactivity at a high power level is ____________ negative due to ____________. (Assume the same core age.)
A. less; buildup of fission product poisons
B. more; improved pellet-to-clad heat transfer
C. less; higher fuel temperature
D. more; increased neutron flux
ANSWER: C.
Comment: This question suffers the same defect as B353. Negative is negative. There is no correct choice.
QID: B2453 (P2352) (TOPIC: 292004 KNOWLEDGE: K1.05 [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: This question suffers the same defect as B51. Reactivity is not added.
QID: B2553 (P2651) (TOPIC: 292004 KNOWLEDGE: K1.05 [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: This question suffers the same defect as B353. Negative is negative.
QID: B2753 (P2751) (TOPIC: 292004 KNOWLEDGE: K1.05 [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: This question suffers the same defect as B51. Reactivity is not added.
QID: B125 (TOPIC: 292004 KNOWLEDGE: K1.10 [3.2/3.2])
Which one of the following will cause the void coefficient to become less negative? (Consider only the indicated changes.)
A. Core void fraction increases.
B. Fuel temperature decreases.
C. Gadolinium burns out.
D. Control rods are partially inserted.
ANSWER: B.
Comment: This question suffers the same defect as B353. Negative is negative.
QID: B354 (TOPIC: 292004 KNOWLEDGE: K1.10 [3.2/3.2])
Which one of the following is the primary reason the void coefficient becomes less negative with core burnup toward the end of core life?
A. The thermal neutron flux increases.
B. The thermal diffusion length decreases.
C. The fuel centerline temperature increases.
D. The control rod density decreases.
ANSWER: D.
Comment: This question suffers the same defect as B353. Negative is negative.
QID: B2153 (TOPIC: 292004 KNOWLEDGE: K1.10 [3.2/3.2])
Which one of the following describes why most reactor power is produced in the lower half of a core (versus the upper half) that has been operating at 100% power for several weeks at the beginning of a fuel cycle?
A. Xenon concentration is higher in the upper half of the core.
B. The moderator-to-fuel ratio is higher in the upper half of the core.
C. The void coefficient is adding more negative reactivity in the upper half of the core.
D. Control rods are adding more negative reactivity in the upper half of the core.
ANSWER: C.
Comment: This question suffers the same defect as B51. Reactivity is not added.
QID: B953 (TOPIC: 292004 KNOWLEDGE: K1.11 [2.5/2.6])
Which one of the following describes how and why the void coefficient changes as void fraction increases during a control rod withdrawal at power?
A. Becomes more negative due to a greater fractional loss of moderator for a 1% void increase at higher void fractions
B. Becomes more negative due to the reduction in the fast fission contribution to the neutron population
C. Becomes less negative due to a greater fraction of neutrons lost to leakage from the core
D. Becomes less negative due to the increased absorption of neutrons by U-238
ANSWER: A.
Comment: This question suffers the same defect as B353. Negative is negative. There is no correct choice.
QID: B272 (TOPIC: 292004 KNOWLEDGE: K1.14 [3.3/3.3])
During a normal power increase from 20% to 100%, the smallest negative reactivity addition will be caused by the change in:
A. void content.
B. fuel temperature.
C. xenon concentration.
D. moderator temperature.
ANSWER: D.
Comment: This question suffers the same defect as B51. Reactivity is not added.
QID: B1653 (TOPIC: 292004 KNOWLEDGE: K1.14 [3.3/3.3])
Which one of the following lists the moderator temperature coefficient (MTC), fuel temperature coefficient (FTC), and void coefficient (VC) in typical order of magnitude from most negative to least negative at 50% power at the end of core life?
A. FTC, VC, MTC
B. FTC, MTC, VC
C. VC, MTC, FTC
D. VC, FTC, MTC
ANSWER: C.
Comment: This question suffers the same defect as B353. Negative is negative.
QID: B2353 (TOPIC: 292004 KNOWLEDGE: K1.14 [3.3/3.3])
During a normal power decrease from 100% to 20%, the smallest positive reactivity addition will be caused by the change in:
A. void percentage.
B. fuel temperature.
C. xenon concentration.
D. moderator temperature.
ANSWER: D.
Comment: This question suffers the same defect as B51. Reactivity is not added.