A subcritical reactor has an initial source/startup range count rate of 150 cps with a shutdown reactivity of -2.0% delta-K/K. How much positive reactivity must be added to establish a stable count rate of 300 cps?
A. 0.5% delta-K/K
B. 1.0% delta-K/K
C. 1.5% delta-K/K
D. 2.0% delta-K/K
ANSWER: B.
Comment: The question is technically incorrect because it states that "positive reactivity must be added", rather than a positive change in reactivity is introduced. For a detailed discussion of reactivity and reactivity change see QID:P1246 in section Reactor Theory - Neutron Life Cycle. In addition, all choices are incorrectly indicated to represent reactivity ... instead of reactivity change. There is no correct answer.
QID: P848 (B2149) (TOPIC: 192003 KNOWLEDGE: K1.01 [2.7/2.8])
A subcritical reactor has an initial Keff of 0.8 at a source range count rate of 100 cps. Positive reactivity is added until Keff equals 0.95. What is the final equilibrium source range count rate?
A. 150 cps
B. 200 cps
C. 300 cps
D. 400 cps
ANSWER: D.
Comment: The question suffers the same defect as P448. Reactivity is not added.
QID: P1348 (B1449) (TOPIC: 192003 KNOWLEDGE: K1.01 [2.7/2.8])
A reactor is shutdown by 1.8% delta-K/K. Positive reactivity is added which increases stable neutron count rate from 15 to 300 cps. Assuming the reactor is still subcritical, what is the current value of Keff?
A. 0.982
B. 0.990
C. 0.995
D. 0.999
ANSWER: D.
Comment: The question suffers the same defect as P448. Reactivity is not added.
QID: P1448 (B1840) (TOPIC: 192003 KNOWLEDGE: K1.01 [2.7/2.8])
A subcritical reactor has an initial source/startup range count rate of 150 cps with a shutdown reactivity of -2.0% delta-K/K. Approximately how much positive reactivity must be added to establish a stable count rate of 600 cps?
A. 0.5% delta-K/K
B. 1.0% delta-K/K
C. 1.5% delta-K/K
D. 2.0% delta-K/K
ANSWER: C.
Comment: The question suffers the same defect as P448. Reactivity is not added. In addition the four choices are incorrectly indicated to represent reactivity. There is no correct answer.
QID: P1748 (TOPIC: 192003 KNOWLEDGE: K1.01 [2.7/2.8])
A subcritical reactor has an initial source/startup range count rate of 60 cps with a shutdown reactivity of -2.0% delta-K/K. How much positive reactivity must be added to establish a stable count rate of 300 cps?
A. 0.4% delta-K/K
B. 0.6% delta-K/K
C. 1.4% delta-K/K
D. 1.6% delta-K/K
ANSWER: D.
Comment: The question suffers the same defect as P448. Reactivity is not added. In addition the four choices are incorrectly indicated to represent reactivity. There is no correct answer.
QID: P47 (B451) (TOPIC: 192003 KNOWLEDGE: K1.06 [3.2/3.3])
A given amount of positive reactivity is added to a critical reactor in the source (startup) range. The amount added is less than the average effective delayed neutron fraction. Which one of the following will have a significant effect on the magnitude of the stable startup rate achieved for this addition?
A. Prompt neutron lifetime
B. Fuel temperature coefficient
C. Average effective decay constant
D. Moderator temperature coefficient
ANSWER: C.
Comment: The question suffers the same defect as P448; reactivity is not added. In addition, this question is technically incorrect because there are three correct answers. Both the moderator and fuel temperature coefficients will eventually have a significant effect on the magnitude of the stable rate.
QID: P248 (TOPIC: 192003 KNOWLEDGE: K1.06 [3.2/3.3])
The magnitude of the stable startup rate achieved for a given positive reactivity addition to a critical reactor is dependent on the ________________ and ________________.
A. prompt neutron lifetime; axial flux distribution
B. prompt neutron lifetime; average delayed neutron fraction
C. average effective decay constant; average delayed neutron fraction
D. average effective decay constant; axial flux distribution
ANSWER: C.
Comment: The question suffers the same defect as P448. Reactivity is not added.
QID: P2748 (B2751) (TOPIC: 192003 KNOWLEDGE: K1.06 [3.2/3.3])
A reactor is exactly critical at 10-8% power during a reactor startup. ß for this reactor is 0.0072. Which one of the following is the approximate amount of positive reactivity that must be added to the core by control rod withdrawal to initiate a reactor power increase toward the point of adding heat with a stable startup rate of 1 dpm?
A. 0.2% delta-K/K
B. 0.5% delta-K/K
C. 1.0% delta-K/K
D. 2.0% delta-K/K
ANSWER: A.
Comment: The question suffers the same defect as P448. Reactivity is not added. In addition the four choices are incorrectly indicated to represent reactivity. There is no correct answer.
QID: P48 (B1950) (TOPIC: 192003 KNOWLEDGE: K1.07 [3.0/3.0])
Over core life, plutonium isotopes are produced with delayed neutron fractions that are ______________ than uranium delayed neutron fractions, thereby causing reactor power transients to be ______________ near the end of core life.
A. larger; slower
B. larger; faster
C. smaller; slower
D. smaller; faster
ANSWER: D.
Comment: The question is technically incorrect because for large negative reactivities the stable period is -80 seconds at both BOL and EOL (see P1248).
QID: P348 (B2450) (TOPIC: 192003 KNOWLEDGE: K1.07 [3.0/3.0])
Which one of the following statements describes the effect of changes in the delayed neutron fraction from beginning of core life (BOL) to end of core life (EOL)?
A. A given set of plant parameters at EOL yields a greater shutdown margin (SDM) than at BOL.
B. A given set of plant parameters at EOL yields a smaller SDM than at BOL.
C. A given reactivity addition at EOL results in a higher startup rate (SUR) than it would at BOL.
D. A given reactivity addition at EOL results in a lower SUR than it would at BOL.
ANSWER: C.
Comment: The question suffers the same defect as P448. Reactivity is not added. In addition, the correct choice suffers the same defect as P48, it is not true for large negative reactivity.
QID: P1149 (B2651) (TOPIC: 192003 KNOWLEDGE: K1.07 [3.0/3.0])
Delayed neutrons are important for reactor control because:
A. they are produced with higher average kinetic energy than prompt neutrons.
B. they prevent the moderator temperature coefficient from becoming positive.
C. they are the largest fraction of the neutrons produced from fission.
D. they greatly extend the average neutron generation lifetime.
ANSWER: D.
Comment: This question suffers the same defect as P1945 (Neutrons). There is no such thing as an "average neutron generation lifetime."
QID: P1248 (B1349) (TOPIC: 192003 KNOWLEDGE: K1.07 [3.0/3.0])
Two reactors are identical in every way except that reactor A is at end of core life and reactor B is at the beginning of core life. Both reactors are operating at 100% power when a reactor trip occurs at the same time on each reactor. If the reactor systems for each reactor respond identically to the trip and no operator action is taken, reactor A will attain a negative ________ second stable period and reactor B will attain a negative ________second stable period. (Assume control rod worth equals -0.9700 delta-K/K and lambdaeff equals 0.0124 sec-1.)
A. 80; 56
B. 80; 80
C. 56; 56
D. 56; 80
ANSWER: B.
Comment: The question is technically incorrect because control rod worth is indicated to represent reactivity. In fact, it represents a change in reactivity which is delta-rho. The value of control rod worth has obviously been misstated as a value for keff after scram. No control rod in a PWR has a worth of -0.9700 delta-rho.
QID: P1548 (B1250) (TOPIC: 192003 KNOWLEDGE: K1.07 [3.0/3.0])
Two reactors are identical in every way except that reactor A is at the end of core life and reactor B is at the beginning of core life. Both reactors are critical at 10-5% power. If the same amount of positive reactivity is added to each reactor at the same time, the point of adding heat will be reached first by reactor ______ because it has a ___________ delayed neutron fraction.
A. A; smaller
B. A; larger
C. B; smaller
D. B; larger
ANSWER: A.
Comment: The question suffers the same defect as P448. Reactivity is not added.
QID: P1649 (B1649) (TOPIC: 192003 KNOWLEDGE: K1.07 [3.0/3.0])
Two reactors are identical in every way except that reactor A is at the end of core life and reactor B is at the beginning of core life. Both reactors are operating at 100% power when a reactor trip occurs at the same time on each reactor. If the reactor systems for each reactor respond identically to the trip and no operator action is taken, a power level of 10-5% will be reached first by reactor _____ because it has a ____________ delayed neutron fraction.
A. A; larger
B. B; larger
C. A; smaller
D. B; smaller
ANSWER: C.
Comment: This question is defective because it allows a correct choice to be made for the wrong reason. Similar questions for a supercritcal reactor are based on a shorter positive period at End-of-Life, due to a smaller delayed neutron fraction (beta). If that is the intended reason for this question, then it contradicts question P1248, where both reactor A and B reach a negative stable rate of -80 seconds. In this case, the prompt drop from trip is greater at EOL and the subsequent stable periods for power decay in the two reactors are the same, namely - 80 seconds. Thus, reactor A does not reach the power level sooner because of a shorter negative period but because of a larger prompt drop. The fact that the delayed neutron fraction is shorter does not specifically identify the reason for the behavior.
QID: P2849 (B2850) (TOPIC: 192003 KNOWLEDGE: K1.07 [3.0/3.0])
Two reactors are identical in every way except that reactor A is at the beginning of core life and reactor B is at the end of core life. Both reactors are critical at 10-5% power. If the same amount of positive reactivity is added to each reactor at the same time, the point of adding heat will be reached first by reactor ______ because it has a ___________ delayed neutron fraction.
A. A; smaller
B. A; larger
C. B; smaller
D. B; larger
ANSWER: C.
Comment: The question suffers the same defect as P448. Reactivity is not added.
QID: P748 (B664) (TOPIC: 192003 KNOWLEDGE: K1.08 [2.8/2.9])
A critical reactor will become prompt critical when reactivity is added equal in magnitude to the:
A. shutdown margin.
B. average effective delayed neutron fraction.
C. average effective decay constant.
D. worth of the most reactive rod.
ANSWER: B.
Comment: The question suffers the same defect as P448. Reactivity is not added.
QID: P949 (TOPIC: 192003 KNOWLEDGE: K1.08 [2.8/2.9])
A reactor is operating at 75% power with the following conditions:
Power defect = -0.0157 delta-/K/K
Shutdown margin = 0.0241 delta-/K/K
Effective delayed neutron fraction = 0.0058
Effective prompt neutron fraction = 0.9942
How much positive reactivity must be added to take the reactor "prompt critical"?
A. 0.0157 delta-K/K
B. 0.0241 delta-K/K
C. 0.0058 delta-K/K
D. 0.9942 delta-K/K
ANSWER: C.
Comment: The question suffers the same defect as P448. Reactivity is not added. In addition the power defect and the four choices are incorrectly indicated as representing reactivity. There is no correct answer.
QID: P1449 (B1850) (TOPIC: 192003 KNOWLEDGE: K1.08 [2.8/2.9])
A reactor is exactly critical several decades below the point of adding heat (POAH) with a xenon-free core. The operator continuously withdraws control rods until a positive 60 second reactor period is reached and then stops control rod motion. When rod withdrawal is stopped, reactor period will immediately: (Neglect reactivity effects of fission products.)
A. stabilize at 60 seconds until power reaches the POAH.
B. lengthen, and then stabilize at a value greater than 60 seconds until power reaches the POAH.
C. stabilize, and then slowly and continuously lengthen until power reaches the POAH.
D. lengthen, and then continue to slowly lengthen until power reaches the POAH.
ANSWER: B.
Comment: The question is technically incorrect because of improper terminology. Criticality is an exact condition
QID: P1948 (B1150) (TOPIC: 192003 KNOWLEDGE: K1.08 [2.8/2.9])
Positive reactivity is continuously added to a critical reactor. Which one of the following values of Keff will first result in a prompt critical reactor?
A. 1.0001
B. 1.001
C. 1.01
D. 1.1
ANSWER: C.
Comment: The question suffers the same defect as P448. Reactivity is not added. In addition, the wording is poor. The reactor is only critical prior to the initiation of positive reactivity change.
QID: P2949 (B2951) (TOPIC: 192003 KNOWLEDGE: K1.08 [2.8/2.9])
A reactor is operating at equilibrium 75% power with the following conditions:
Total power defect = -0.0185 delta-K/K
Shutdown margin = 0.0227 delta-K/K
Effective delayed neutron fraction = 0.0061
Effective prompt neutron fraction = 0.9939
How much positive reactivity must be added to make the reactor "prompt critical"?
A. 0.0061 delta-K/K
B. 0.0185 delta-K/K
C. 0.0227 delta-K/K
D. 0.9939 delta-K/K
ANSWER: A.
Comment: The question suffers the same defect as P448. Reactivity is not added. In addtion the power defect and the four choices are incorrectly indicated as representing reactivity. There is no correct answer.