Xenon-135 is considered a major fission product poison because it has a large:
A. fission cross section.
B. absorption cross section.
C. elastic scatter cross section.
D. inelastic scatter cross section.
ANSWER: B.
Comment: The question is technically incorrect because it is only a partial answer. A large absorption cross section is of no consequence if the yield is small.
QID: P59 (TOPIC: 192006 KNOWLEDGE: K1.03 [2.7/2.8])
Immediately after a reactor trip from sustained high power operation, xenon-135 concentration in the reactor will:
A. increase due to the decay of iodine already in the core.
B. decrease because xenon is produced directly from fission.
C. remain the same because the decay of iodine and xenon balance each other out.
D. decrease immediately, then slowly increase due to the differences in the half-lives of iodine and xenon.
ANSWER: A.
Comment: The question is technically incorrect because Choice A is an incomplete explanation. Iodine is always decaying. Concentration change depends on the difference between production and loss.
QID: P1359 (B458) (TOPIC: 192006 KNOWLEDGE: K1.03 [2.7/2.8])
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. 0.3
B. 3.0
C. 30
D. 100
ANSWER: A.
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: P2859 (B2760) (TOPIC: 192006 KNOWLEDGE: K1.05 [3.1/3.1])
Reactors A and B are operating at steady-state 100% power with equilibrium core Xe-135. The reactors are identical except that reactor A is operating near the end of core life and reactor B is operating near the beginning of core life. Which reactor is experiencing the most negative reactivity from equilibrium core Xe-135?
A. Reactor A due to the greater concentration of equilibrium core Xe-135
B. Reactor A due to the lower competition from the fuel for thermal neutrons
C. Reactor B due to the greater concentration of equilibrium core Xe-135
D. Reactor B due to the lower competition from the fuel for thermal neutrons
ANSWER: B.
Comment: The question is technically incorrect because of unsatisfactory wording. Xenon introduces reactivity change, not reactivity. The reactor does not experience negative reactivity from xenon. Reactivity is a physical property of the reactor. For a detailed discussion of reactivity and reactivity change see QID:P1246 in section Reactor Theory - Neutron Life Cycle. In addition, negative is negative.
QID: P259 (TOPIC: 192006 KNOWLEDGE: K1.06 [3.2/3.4])
A reactor has been operating at 50% power for one week when power is quickly ramped (over 4 hours) to 100%. How will the xenon-125 concentration in the core respond?
A. Decrease initially, then build to a new equilibrium concentration in 8 to 10 hours
B. Increase steadily to a new equilibrium concentration in 20 to 30 hours
C. Decrease initially, then build to a new equilibrium concentration in 40 to 50 hours
D. Increase steadily to a new equilibrium concentration in 70 to 80 hours
ANSWER: C.
Comment: The question is technically incorrect because "xenon-125" is not a major fission product poison.
QID: P1358 (B1361) (TOPIC: 192006 KNOWLEDGE: K1.07 [3.4/3.4])
A reactor has been operating at 75% power for two months. A manual reactor trip is required for a test. The trip will be followed immediately by a reactor startup with criticality scheduled to occur 12 hours after the trip. The greatest assurance that xenon reactivity will permit criticality during the startup will be attained if the reactor is operated at ____________ power for 48 hours prior to the trip and if criticality is rescheduled for ____________ hours after the trip.
A. 100%; 8
B. 100%; 16
C. 50%; 8
D. 50%; 16
ANSWER: D.
Comment: The question suffers the same defect as P2859. Xenon does not possess reactivity.
QID: P1660 (TOPIC: 192006 KNOWLEDGE: K1.07 [3.4/3.4])
Which one of the following combinations of core age (beginning of core life (BOL) or end of core life (EOL)) and long-term power history (20% or 100%) will require the greatest amount of positive reactivity addition to attain reactor criticality during peak core Xe-135 conditions after a reactor trip from equilibrium core Xe-135 conditions?
A. EOL and 20% power
B. EOL and 100% power
C. BOL and 20% power
D. BOL and 100% power
ANSWER: B.
Comment: The question suffers the same defect as P2859. "Reactivity" is not added.
QID: P1160 (TOPIC: 192006 KNOWLEDGE: K1.08 [3.3/3.4])
Xenon-135 oscillations take about ____________ hours to get from maximum xenon-135 negative reactivity to minimum xenon-135 negative reactivity.
A. 40 to 50
B. 24 to 28
C. 12 to 14
D. 6 to 7
ANSWER: C.
Comment: The question suffers the same defect as P2859. Xenon does not possess reactivity.
QID: P353 (B355) (TOPIC: 192006 KNOWLEDGE: K1.09 [3.0/3.1])
A plant is being returned to operation following a refueling outage. Fuel preconditioning requires reactor power being 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 suffers the same defect as P2859. "Reactivity" is not added and xenon does not possess reactivity.
QID: P1263 (TOPIC: 192006 KNOWLEDGE: K1.09 [3.0/3.1])
A reactor has been shut down for seven days to perform maintenance. A reactor startup is performed and power is ramped to 50% over a 5 hour period. When power reaches 50%, the magnitude of core xenon negative reactivity will be:
A. increasing toward a peak.
B. increasing toward equilibrium.
C. decreasing toward equilibrium.
D. decreasing toward a valley.
ANSWER: B.
Comment: The question suffers the same defect as P2859. Xenon does not possess reactivity. "Valley", as used in Choice D, is not conventional terminology and is an inappropriate description of a "minimum."
QID: P1661 (TOPIC: 192006 KNOWLEDGE: K1.09 [3.0/3.1])
A reactor has been shut down for 5 days to perform maintenance. A reactor startup is performed and power is ramped to 75% over a 16 hour period. When power reaches 75%, the magnitude of core xenon-135 negative reactivity will be:
A. decreasing toward a valley.
B. increasing toward a peak.
C. decreasing toward equilibrium.
D. increasing toward equilibrium.
ANSWER: D.
Comment: The question suffers the same defect as P2859. Xenon does not possess reactivity. "Valley", as used in Choice A, is not conventional terminology and is an inappropriate description of a "minimum."
QID: P1860 (B2259) (TOPIC: 192006 KNOWLEDGE: K1.11 [3.1/3.1])
Which one of the following describes the change in core xenon-135 concentration immediately following a power increase from 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: The question suffers the same defect as P59. Concentration change depends on the difference between production and loss.
QID: P360 (TOPIC: 192006 KNOWLEDGE: K1.12 [3.1/3.1])
Compare a reactor that has been operating at 50% power for several days when a reactor trip occurs, to a reactor that had been operating at full power prior to the trip. For the 50% power reactor, xenon would peak _____________ and the peak xenon reactivity would be ______________.
A. earlier; the same
B. at the same time; the same
C. earlier; less negative
D. at the same time; less negative
ANSWER: C.
Comment: The question suffers the same defect as P2859. Xenon does not possess reactivity. In addition, 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 reactivity change being smaller.
QID: P663 (TOPIC: 192006 KNOWLEDGE: K1.12 [3.1/3.1])
Following a reactor trip, negative reactivity from xenon initially increases due to:
A. xenon production from the decay of iodine-135.
B. xenon production from the spontaneous fission of uranium.
C. the reduction of xenon removal by decay.
D. the reduction of xenon removal by recombination.
ANSWER: A.
Comment: The question suffers the same defect as P2859. Xenon does not possess reactivity. In addition, the question suffers the same defect as P59. Concentration change, and reactivity change, depend on the difference between production and loss.
QID: P963 (TOPIC: 192006 KNOWLEDGE: K1.12 / K1.13 [3.1/3.1])
A reactor has been operating at full power for several days when it is shut down rapidly (within 2 hours) for maintenance. How will core xenon reactivity change?
A. Peak in 2 to 4 hours and then decay to near zero in about 1 day.
B. Peak in 2 to 4 hours and then decay to near zero in 3 to 4 days.
C. Peak in 6 to 10 hours and then decay to near zero in about 1 day.
D. Peak in 6 to 10 hours and then decay to near zero in 3 to 4 days.
ANSWER: D.
Comment: The question suffers the same defect as P2859. Xenon does not possess reactivity.
QID: P2262 (B2461) (TOPIC: 192006 KNOWLEDGE: K1.12 [3.1/3.1])
Fourteen (14) hours after a reactor trip 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: The question suffers the same defect as P2859. "Reactivity" is not added and xenon does not possess reactivity.
QID: P2363 (TOPIC: 192006 KNOWLEDGE: K1.12 [3.1/3.1])
Following a reactor trip, negative reactivity from xenon-135 initially:
A. increases due to xenon production from the decay of iodine-135.
B. increases due to xenon production from the spontaneous fission of uranium.
C. decreases due to xenon removal by decay.
D. decreases due to the reduction in xenon production directly from fission.
ANSWER: A.
Comment: The question suffers the same defect as P2859. Xenon does not possess reactivity. In addition, the question suffers the same defect as P59. Concentration change, and reactivity change, depend on the difference between production and loss.
QID: P1760 (TOPIC: 192006 KNOWLEDGE: K1.13 [2.9/3.0])
A plant was shut down following three months of operation at full power. The shutdown occurred over a 3 hour period with a constant rate of power decrease. Which one of the following describes the reactivity added by core xenon during the shutdown?
A. Xenon buildup added negative reactivity.
B. Xenon buildup added positive reactivity.
C. Xenon burnout added negative reactivity.
D. Xenon burnout added positive reactivity.
ANSWER: A.
Comment: The question suffers the same defect as P2859. "Reactivity" is not added and xenon does not possess reactivity.
QID: P262 (TOPIC: 192006 KNOWLEDGE: K1.14 [3.2/3.3])
Four hours after a reactor trip from equilibrium full power operation, a reactor is taken critical and power is immediately stabilized for critical data. To maintain a constant reactor power, the operator must add __________ reactivity because xenon concentration is __________.
A. positive; increasing
B. positive; decreasing
C. negative; increasing
D. negative; decreasing
ANSWER: A.
Comment: The question suffers the same defect as P2859. "Reactivity? is not added.
QID: P361 (TOPIC: 192006 KNOWLEDGE: K1.14 [3.2/3.3])
A plant has been operating at 100% power for two months when a reactor trip occurs. Six hours after the trip, 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 is building in.
B. negative reactivity because xenon is building in.
C. positive reactivity because xenon is decaying away.
D. negative reactivity because xenon is decaying away.
ANSWER: A.
Comment: The question suffers the same defect as P2859. "Reactivity" is not added.
QID: P561 (B562) (TOPIC: 192006 KNOWLEDGE: K1.14 [3.2/3.3])
Following a seven-day shutdown, a reactor startup is performed and the plant is taken to 40% power over a 6-hour period. After stabilizing at 40% 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: The question suffers the same defect as P2859. "Reactivity" is not added.
QID: P1462 (B1461) (TOPIC: 192006 KNOWLEDGE: K1.14 [3.2/3.3])
A reactor has been operating at 100% power for two weeks. Power is then decreased over a 1- hour period to 10%. Assuming manual rod control, which one of the following operator actions is required to maintain a constant reactor coolant temperature at 10% power during the next 24 hours?
A. Add negative reactivity during the entire period
B. Add positive reactivity during the entire period
C. Add positive reactivity, then negative reactivity
D. Add negative reactivity, then positive reactivity
ANSWER: C.
Comment: The question suffers the same defect as P2859. "Reactivity" is not added.