Fission fragments or daughters that have a substantial neutron absorption cross section and are not fissionable are called:
A. fissile materials.
B. fission product poisons.
C. fissionable nuclides.
D. burnable poisons.
ANSWER: B.
Comment: The question is technically incorrect because it implies that some fission products are fissionable. There are no fission products that are fissionable by reactor neutrons.
QID: B256 (P2658) (TOPIC: 292006 KNOWLEDGE: K1.02 [3.1/3.1])
Compared to other poisons in the core, the two characteristics that cause Xe-135 to be a major reactor poison are its relatively _________ absorption cross section and its relatively _________ variation in concentration for large reactor power changes.
A. small; large
B. small; small
C. large; small
D. large; large
ANSWER: D.
Comment: The answer is technically incorrect because a "relatively large variation" can occur in a concentration that is always small.
QID: B458 (P1359) (TOPIC: 292006 KNOWLEDGE: K1.03 [2.9/2.9])
A reactor has been operating at full power for several weeks. Xenon-135 is being produced as a fission product in approximately __________ of all fissions.
A. 100%
B. 30%
C. 3%
D. 0.3%
ANSWER: D.
Comment: The question is technically incorrect because it is ambiguous. Xenon-135 is being produced from nearly 6% of all fissions and directly from 0.3% of fissions.
QID: B128 (TOPIC: 292006 KNOWLEDGE: K1.04 [2.9/2.9])
Which one of the following describes the change in core xenon-135 concentration immediately following a power increase from equilibrium conditions?
A. Xenon concentration will initially decrease due to the Doppler coefficient increase, thus increasing the thermal utilization.
B. Xenon concentration will initially decrease due to the increased absorbtion of thermal neutrons by xenon-135.
C. Xenon concentration will initially increase due to the large increase in production from fission.
D. Xenon concentration will initially increase due to the decrease in thermal utilization as core flow increases.
ANSWER: B.
Comment: The question is technically incorrect because of the initial conditions are poorly defined (also see B1759 where equilibrium apparently refers to equilibrium power, not equilibrium xenon). What is "a power increase from equilibrium conditions" supposed to mean ... equilibrium power ... equilibrium Xenon ... or equilibrium everything? Equilibrium can be applied to power as in subcritical multiplication and to Xenon as for steady-state Xenon. Choice B is an incomplete explanation. Concentration change always depends on the difference between production and loss.
QID: B1759 (TOPIC: 292006 KNOWLEDGE: K1.04 [2.9/2.9])
Which one of the following describes the change in core xenon-135 concentration immediately following a 10% power increase from equilibrium 70% power over a two-hour period?
A. Xe-135 concentration will initially decrease due to the increased rate of decay of Xe-135 to Cs-135.
B. Xe-135 concentration will initially decrease due to the increased absorption of thermal neutrons by xenon-135.
C. Xe-135 concentration will initially increase due to the increased I-135 production rate directly from fission.
D. Xe-135 concentration will initially increase due to the increased production rate directly from fission.
ANSWER: B.
Comment: The question suffers the same defect as B128. Initial conditions are poorly defined and xenon behavior depends on both production and loss.
QID: B2259 (TOPIC: 292006 KNOWLEDGE: K1.04 [2.9/2.9])
Which one of the following describes the change in core xenon-135 concentration immediately following a power increase from 50% power equilibrium conditions?
A. Initially decreases due to the increased rate of xenon-135 radioactive decay.
B. Initially decreases due to the increased absorption of thermal neutrons by xenon-135.
C. Initially increases due to the increased xenon-135 production from fission.
D. Initially increases due to the increased iodine-135 production from fission.
ANSWER: B.
Comment: This question suffers the same defect as B128.
QID: B660 (TOPIC: 292006 KNOWLEDGE: K1.06 [2.7/2.7])
A reactor has been operating at 75% power for one week when power is decreased to 50% over a 1 hour period. Which one of the following statements explains how xenon concentration will initially change?
A. Decreases, because the xenon production rate from fission has decreased
B. Increases, because of the reduced rate of xenon burnout
C. Decreases, because the rate of xenon decay exceeds the rate of production from fission
D. Increases, because the concentration of iodine-135 increases
ANSWER: B.
Comment: This question is technically incorrect because xenon behavior depends on the difference between production and loss. Choice B is only a partial answer.
QID: B1561 (P1561) (TOPIC: 292006 KNOWLEDGE: K1.07 [3.2/3.2])
Which one of the following reactor startup conditions requires the least amount of control rod withdrawal to attain reactor criticality during peak core xenon-135 conditions after a reactor scram from equilibrium core xenon-135 conditions? (Assume equilibrium core xenon-135 reactivities at 20% power and 100% power do not change over core life.)
A. Scram from 20% power at beginning of core life (BOL)
B. Scram from 20% power at end of core life (EOL)
C. Scram from 100% power at BOL
D. Scram from 100% power at EOL
ANSWER: A.
Comment: The question is technically incorrect because Xenon-135 does not possess reactivity. The xenon reactivity effect on the core, delta-rho, may be assumed to remain the same over core life.
QID: B135 (TOPIC 292006 KNOWLEDGE: K1.08 [2.8/3.2])
When comparing control rod worth (CRW) during a reactor startup from 100% peak xenon-135 and a reactor startup from xenon-free conditions:
A. center CRW will be higher during the peak xenon startup than during the xenon-free startup.
B. peripheral CRW will be higher during the peak xenon startup than during the xenon-free startup.
C. center and peripheral CRWs will be the same regardless of core xenon conditions.
D. it is impossible to determine how xenon will affect the worth of center and peripheral control rods.
ANSWER: B.
Comment: The question is technically incorrect because of undefined terminology. Control rod worth (CRW) has no meaning. Control rods are valued either in differential rod worth or integral rod worth.
QID: B2454 (TOPIC: 292006 KNOWLEDGE: K1.08 [2.8/3.2])
Sustained operation at 100% power requires periodic withdrawal of control rods to compensate for:
A. buildup of fission product poisons and decreasing control rod worth.
B. fuel depletion and buildup of fission product poisons.
C. decreasing control rod worth and burnable poison burnout.
D. burnable poison burnout and fuel depletion.
ANSWER: B.
Comment: The question is technically incorrect. Early in a fuel cycle control rods may be periodically inserted to compensate for rapid burnable poison depletion.
QID: B355 (P353) (TOPIC: 292006 KNOWLEDGE: K1.09 [2.5/2.5])
A plant is being returned to operation following a refueling outage. Fuel preconditioning requires reactor power to be increased from 10% to full power gradually over a one week period. During this slow power increase, most of the positive reactivity added by the operator is required to overcome the negative reactivity from:
A. fuel burnup.
B. xenon buildup.
C. fuel temperature increase.
D. moderator temperature increase.
ANSWER: B.
Comment: The question is technically incorrect because xenon does not possess reactivity and because the operator does not add reactivity.
QID: B562 (P561) (TOPIC: 292006 KNOWLEDGE: K1.09 [2.5/2.5])
Following a seven day shutdown, a reactor startup is performed and a plant is taken to 100% power over a 16-hour period. After reaching 100% power, what type of reactivity will the operator need to add to compensate for xenon changes over the next 24 hours?
A. Negative only
B. Negative, then positive
C. Positive, then negative
D. Positive only
ANSWER: D.
Comment: This question suffers the same defect as B355. The operator does not add reactivity.
QID: B1461 (P1462) (TOPIC: 292006 KNOWLEDGE: K1.10 [2.9/2.9])
A reactor has been operating at 100% power for two months when a reactor scram occurs. Four hours later, the reactor is critical and stable at 10% power. Which one of the following operator actions is required to maintain reactor power at 10% over the next 24 hours?
A. Add positive reactivity during the entire period
B. Add negative reactivity during the entire period
C. Add positive reactivity, then negative reactivity
D. Add negative reactivity, then positive reactivity
ANSWER: C.
Comment: This question suffers the same defect as B355. The operator does not add reactivity.
QID: B1362 (TOPIC: 292006 KNOWLEDGE: K1.11 [2.6/2.7])
A reactor has been operating at 100% power for two weeks when power is reduced to 50%. During the next 2 hours, what must the operator do to compensate for a change in core Xe-135?
A. The operator must add positive reactivity because Xe-135 is decaying.
B. The operator must add negative reactivity because Xe-135 is decaying.
C. The operator must add positive reactivity because Xe-135 is building in.
D. The operator must add negative reactivity because Xe-135 is building in.
ANSWER: C.
Comment: This question suffers the same defect as B355. The operator does not add reactivity.
QID: B2461 (P2262) (TOPIC: 292006 KNOWLEDGE: K1.12 [2.8/2.3])
Fourteen (14) hours after a reactor scram from 100% power, equilibrium xenon conditions, the amount of core xenon will be:
A. lower than 100% equilibrium xenon, and will have added a net positive reactivity since the trip.
B. lower than 100% equilibrium xenon, and will have added a net negative reactivity since the trip.
C. higher than 100% equilibrium xenon, and will have added a net positive reactivity since the trip.
D. higher than 100% equilibrium xenon, and will have added a net negative reactivity since the trip.
ANSWER: D.
Comment: This question suffers the same defect as B1561. Xenon does not add reactivity.
QID: B1463 (TOPIC: 292006 KNOWLEDGE: K1.13 [2.6/2.6])
Which one of the following describes a reason for the direction of change in core xenon-135 reactivity immediately after a reactor shutdown?
A. The production rate of I-135 as a fission product significantly decreases.
B. The production rate of Xe-135 as a fission product significantly decreases.
C. The removal rate of I-135 by neutron absorption
significantly decreases.
D. The removal rate of Xe-135 by neutron absorption significantly decreases.
ANSWER: D.
Comment: This question suffers the same defects as B1759 and B1561. The necessary initial conditions are not provided, Choice D is an incomplete answer, and xenon does not possess reactivity. The wording used, "direction of change", is poor.
QID: B461 (TOPIC: 292006 KNOWLEDGE: K1.14 [3.1/3.2])
Four hours after a reactor trip from a long-term, steady-state, 100% power run, the reactor has been taken critical and is to be maintained at 1% to 2% power. Which one of the following operator actions is required?
A. Add positive reactivity because xenon is building in
B. Add negative reactivity because xenon is building in
C. Add negative reactivity because xenon is decaying away
D. Add positive reactivity because xenon is decaying away
ANSWER: A.
Comment: This question suffers the same defect as B355. The operator does not add reactivity.
QID: B964 (P2262) (TOPIC: 292006 KNOWLEDGE: K1.14 [3.1/3.2])
Sixteen hours after a reactor scram from 100% power, equilibrium xenon condition, the amount of core xenon will be:
A. lower than 100% equilibrium xenon, and will have added a net positive reactivity since the scram.
B. higher than 100% equilibrium xenon, and will have added a net positive reactivity since the scram.
C. lower than 100% equilibrium xenon, and will have added a net negative reactivity since the scram.
D. higher than 100% equilibrium xenon, and will have added a net negative reactivity since the scram.
ANSWER: D.
Comment: This question suffers the same defect as B1561. Xenon does not add reactivity.
QID: B1164 (TOPIC: 292006 KNOWLEDGE: K1.14 [3.1/3.2])
A reactor is operating at 100% power with equilibrium xenon conditions at the beginning of a fuel cycle when a reactor scram occurs. The reactor is taken critical 4 hours later. Which one of the following describes the effect of xenon on control rod worth when the reactor becomes critical?
A. Increasing xenon concentration at the periphery of the core will cause periphery rods to exhibit high-worth characteristics.
B. Peak thermal flux at the periphery of the core will cause periphery rods to exhibit high-worth characteristics.
C. Peak thermal flux at the center of the core will cause center rods to exhibit high-worth characteristics.
D. Decreasing xenon concentration at the center of the core will cause center control rods to exhibit high-worth characteristics.
ANSWER: B.
Comment: This question suffers the same defect as B135. Control rod worth is not a defined term.
QID: B1762 (TOPIC: 292006 KNOWLEDGE: K1.14 [3.1/3.2])
A reactor is operating at 100% power with equilibrium xenon conditions at the beginning of a fuel cycle when a reactor scram occurs. The reactor is taken critical 4 hours later. Which one of the following describes the effect of core xenon-135 on control rod worth when the reactor becomes critical?
A. High xenon-135 concentration at the periphery of the core will cause periphery rods to exhibit relatively high-worth characteristics.
B. High xenon-135 concentration at the periphery of the core will cause central rods to exhibit relatively high-worth characteristics.
C. High xenon-135 concentration at the center of the core will cause peripheral rods to exhibit relatively high-worth characteristics.
D. High xenon-135 concentration at the center of the core will cause central rods to exhibit relatively high-worth characteristics.
ANSWER: C.
Comment: This question suffers the same defect as B135. Control rod worth is not a defined term.
QID: B1862 (TOPIC: 292006 KNOWLEDGE: K1.14 [3.1/3.2])
A plant has been operating at 100% power for two months when a reactor scram occurs. Four hours after the scram, the reactor is taken critical and power is raised to 2%. To maintain power stable at 2%, the operator must add:
A. positive reactivity because xenon-135 is building in.
B. negative reactivity because xenon-135 is building in.
C. positive reactivity because xenon-135 is decaying away.
D. negative reactivity because xenon-135 is decaying away.
ANSWER: A.
Comment: This question suffers the same defect as B355. The operator does not add reactivity.