Which one of the following statements describes subcritical multiplication during a reactor startup?
A. Subcritical multiplication is the process of using source neutrons to maintain an equilibrium neutron population when Keff is less than 1.
B. As Keff approaches unity, a smaller change in neutron level occurs for a given change in Keff.
C. The equilibrium subcritical neutron level is dependent on the source strength and the time between successive reactivity insertions.
D. As Keff approaches unity, less time is required to reach the equilibrium neutron level for a given change in Keff.
ANSWER: A.
Comment: The question is defective because subcritical multiplication is a process that occurs in the Sub-Critical region, whether a reactor startup is in progress or not. Choice "A" applies to equilibrium subcritical multiplication, whenever it exists. Subcritical multiplication also applies to transient conditions in the Sub-Critical region, where the non-fission source neutron emissions are not in balance with fission neutron losses. Choice "C" incorrectly refers to "reactivity" insertion.
QID: B176 (TOPIC: 292003 KNOWLEDGE: K1.01 [2.9/3.0])
A reactor is being taken critical by periodically withdrawing control rods in equal reactivity increments. Which one of the following statements describes reactor conditions as Keff approaches unity?
A. The neutron level change for successive rod increment pulls becomes smaller.
B. A longer period of time is required to reach the equilibrium neutron level after each rod withdrawal.
C. A rod withdrawal will result in the reactor becoming slightly supercritical due to a "prompt jump" and then return to a subcritical level.
D. If the rod withdrawal is stopped for several hours the neutron level will decrease to source level.
ANSWER: B.
Comment: The question is unsatisfactory because use of equal reactivity increments is not an operational procedure and may mislead the student into believing the operator has knowledge of the keff value. The question could just as well be posed in terms of a power doubling startup procedure.
QID: B1449 (P1348) (TOPIC: 292003 KNOWLEDGE: K1.01 [2.9/3.0])
A reactor is shut down by 1.8% delta-K/K. Positive reactivity is added which increases stable neutron count rate from 15 to 300 cps. What is the current value of Keff?
A. 0.982
B. 0.990
C. 0.995
D. 0.999
ANSWER: D.
Comment: The question is technically incorrect because it states that "positive reactivity is added", rather than a positive change in reactivity is introduced. For a detailed discussion of reactivity and reactivity change see QID:B2048 in section Reactor Theory - Neutron Life Cycle.
QID: B1849 (P1448) (TOPIC: 292003 KNOWLEDGE: K1.01 [2.9/3.0])
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 suffer the same defect as B1449; "positive reactivity is NOT added". In addition, all choices are incorrectly indicated to represent reactivity ... instead of reactivity change. There is no correct answer.
QID: B1949 (P448) (TOPIC: 292003 KNOWLEDGE: K1.01 [2.9/3.0])
A subcritical reactor has an initial source 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: This question suffers the same defect as B1849. "Reactivity" is not added to the core and the choices are indicated to represent reactivity. There is no correct answer.
QID: B2149 (P848) (TOPIC: 292003 KNOWLEDGE: K1.01 [2.9/3.0])
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 will be the final equilibrium source range count rate?
A. 150 cps
B. 200 cps
C. 300 cps
D. 400 cps
ANSWER: D.
Comment: This question suffers the same defect as B1849. "Reactivity" is not added to the core.
QID: B850 (TOPIC: 292003 KNOWLEDGE: K1.04 [2.5/2.5])
The average effective delayed neutron fraction (ßeff) can be defined as:
ANSWER: C.
Comment: The question is technically incorrect because the definition of ßeff is erroneous. In modifying beta to betaeff adjustment is made to the magnitude of beta to account for the fact that delayed neutrons are born at lower energy than prompt neutrons, which results in fewer fast fissions (a reduction in beta) and in reduced fast leakage (an increase in beta). This adjustment in beta allows all fission neutrons, prompt and delayed, to be treated mathematically as if born at the same energy ... and in no way changes the definition, or character, of beta from a yield term to one that quantifies the fraction of fissions caused by delayed neutrons. See NUKEFACT #1 - Beta is NOT the delayed neutron (population) fraction. There is no correct choice.
QID: B1050 (TOPIC: 292003 KNOWLEDGE: K1.04 [2.5/2.5])
Compared to the effective delayed neutron fraction (ßeff), the delayed neutron fraction ( ß ):
A. changes due to fuel depletion, whereas ßeff will remain constant over core life.
B. is based on a finite-sized reactor, whereas ßeff is based on an infinite-sized reactor.
C. describes the fraction of fission neutrons born delayed, whereas ßeff describes the fraction of fissions caused by delayed neutrons.
D. considers only the decay constant of the longest lived delayed neutron precursors, whereas ßeff considers the weighted average of all the decay constants.
ANSWER: C.
Comment: This question suffers the same defect as B850. ßeff is a yield term. There is no correct answer.
QID: B2751 (P2748) (TOPIC: 292003 KNOWLEDGE: K1.05 [3.7/3.7])
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 reactor period of 26 seconds?
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: This question suffers the same defect as B1849. "Reactivity" is not added to the core and the choices are indicated to represent reactivity. There is no correct answer.
QID: B250 (TOPIC: 292003 KNOWLEDGE: K1.06 [3.7/3.7])
Without delayed neutrons in the neutron cycle, when positive reactivity is added to a critical reactor, the reactor will:
A. experience a prompt jump in power level followed by a decrease to the initial power level.
B. experience a rapid but controllable power increase.
C. begin an uncontrollable rapid power increase.
D. not be able to attain criticality.
ANSWER: C.
Comment: This question suffers the same defect as B1849. "Reactivity" is not added to the core.
QID: B451 (P47) (TOPIC: 292003 KNOWLEDGE: K1.6 [3.7/3.7])
A small amount of 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 reactor period achieved for this reactivity addition?
A. Moderator temperature coefficient
B. Fuel temperature coefficient
C. Prompt neutron lifetime
D. Average effective decay constant
ANSWER: D.
Comment: The question suffers the same defect as B1849; 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: B1250 (P1548) (TOPIC: 292003 KNOWLEDGE: K1.06 [3.7/3.7])
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 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; larger
B. B; larger
C. A; smaller
D. B; smaller
ANSWER: C.
Comment: The question suffers the same defect as B1849; reactivity is not added.
QID: B1349 (P1248) (TOPIC: 292003 KNOWLEDGE: K1.06 [3.7/3.7])
Two reactors are identical 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, 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 seconds-1 for both reactors.)
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 BWR has a worth of -0.9700 delta-rho.
QID: B1649 (P1649) (TOPIC: 292003 KNOWLEDGE: K1.06 [3.7/3.7])
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 B1349, 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: B2450 (P348) (TOPIC: 292003 KNOWLEDGE: K1.06 [3.7/3.7])
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 reactivity addition to a shutdown reactor at EOL yields a larger change in shutdown margin (SDM) than at BOL.
B. A given reactivity addition to a shutdown reactor at EOL yields a smaller change in SDM than at BOL.
C. A given reactivity addition to an operating reactor at EOL results in a longer reactor period than at BOL.
D. A given reactivity addition to an operating reactor at EOL results in a shorter reactor period than at BOL.
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 B1349). The question also suffers the same defect as B1849; reactivity is not added.
QID: B2651 (P1149) (TOPIC: 292003 KNOWLEDGE: K1.06 [3.7/3.7])
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 lifetime of each neutron generation.
ANSWER: D.
Comment: The question is technically incorrect because of erroneous terminology. There is no "average lifetime" for a neutron generation. A neutron generation requires a "generation time", lg, to traverse a life cycle.
QID: B2850 (P2849) (TOPIC: 292003 KNOWLEDGE: K1.06 [3.7/3.7])
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: This question suffers the same defect as B1849; reactivity is not added.
QID: B551 (TOPIC: 292003 KNOWLEDGE: K1.07 [3.3/3.3])
A reactor is operating at 50% power with the following conditions:
Power defect = 0.03% delta-K/K
Shutdown margin = 0.05% delta-K/K
Effective delayed neutron fraction = 0.007
Effective prompt neutron fraction = 0.993
How much positive reactivity must be added to take this reactor "prompt critical"?
A. 0.03% delta-K/K
B. 0.05% delta-K/K
C. 0.7% delta-K/K
D. 0.993% delta-K/K
ANSWER: C.
Comment: The question is technically incorrect because the values given for the power defect and for the four choices are incorrectly indicated to represent reactivity, instead of (delta-rho). In addition, reactivity is not added and the operator never "takes" the reactor to prompt criticality. There is no correct choice.
QID: B664 (TOPIC: 292003 KNOWLEDGE: K1.07 [3.3/3.3])
A critical reactor will become prompt critical if the reactivity added is equal to the effective:
A. delayed neutron decay constant.
B. delayed neutron fraction.
C. prompt neutron decay constant.
D. prompt neutron fraction.
ANSWER: B.
Comment: The question is technically incorrect because of erroneous terminology. Reactivity is not added.
QID: B950 (TOPIC: 292003 KNOWLEDGE: K1.07 [3.3/3.3])
A reactor is operating at 75% power with the following conditions:
Total control rod worth = -0.0753 delta-K/K
Shutdown margin = 0.0042 delta-K/K
Effective delayed neutron fraction = 0.0058
Effective prompt neutron fraction = 0.9942
How much positive reactivity must be added to make the reactor "prompt critical"?
A. 0.0042 delta-K/K
B. 0.0058 delta-K/K
C. 0.0753 delta-K/K
D. 0.9942 delta-K/K
ANSWER: B.
Comment: The question is technically incorrect because the values given for total control rod worth and the four choices are incorrectly indicated to represent reactivity, instead of (delta-rho). In addition, reactivity is not added. There is no correct choice
QID: B1150 (P1948) (TOPIC: 292003) KNOWLEDGE: K1.07 [3.3/3.3])
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 is technically incorrect because of erroneous terminology. Reactivity is not added.
QID: B2051 (TOPIC: 292003 KNOWLEDGE: K1.07 [3.3/3.3])
A reactor is exactly critical at the point of adding heat with a xenon-free core. Reactor vessel temperature is 175 F. The operator then inserts control rods until a negative 100 second period is attained and then stops control rod motion.
When rod motion is stopped, reactor period will immediately _________ until power approaches the equilibrium subcritical multiplication source range level and then approach ________.
A. stabilize at negative 100 seconds; infinity.
B. stabilize at negative 100 seconds; zero.
C. lengthen and then stabilize; infinity.
D. lengthen and then stabilize; zero.
ANSWER: C.
Comment: The question is defective because of poor terminology. At equilibrium subcritical multiplication the reactor period is "infinite seconds".
QID: B2951 (P2949) (TOPIC: 292003 KNOWLEDGE: K1.07 [3.3/3.3])
A reactor is operating at 75% power with the following conditions:
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 is technically incorrect because the values given for the power defect and the four choices are indicated to represent reactivity, instead of (delta-rho). In addition, reactivity is not added. There is no correct choice.
QID: B49 (TOPIC: 292003 KNOWLEDGE: K1.08 [2.7/2.8])
After initial criticality, the reactor period is stabilized. The source range channels are repositioned so that the count rate is 100 cps. Sufficient positive reactivity is added to establish a 120-second period. How much time will it take for the count rate to increase to 10,000 cps with no additional operator action?
A. 1.2 minutes
B. 4 minutes
C. 9.21 minutes
D. 15.82 minutes
ANSWER: C.
Comment: The question is technical incorrect because the answer does not account for the power increase due to the introduction of a positive reactivity change. For a stable period of 120-seconds, the reactivity would be slightly less than +0.0005. Even if this small reactivity change were input as a step, the minimum change in power level, by prompt jump, would be to 108.5 cps and the time to 10,000 counts would then be reduced to 542.8 seconds. In addition, the terminology is poor: "after initial criticality, the reactor period is stabilized" ... what does this mean? A stabilized reactor period does not have to be at steady state. Positive reactivity is not added. There is no correct choice.
QID: B127 (TOPIC: 292003 KNOWLEDGE: K1.08 [2.7/2.8])
A reactor is operating at a power level of 120 watts. A control rod is inserted, which results in a stable negative 80-second period. Which one of the following is closest to the reactor power level 2 minutes after rod insertion? (Assume the period stabilized immediately after rod insertion.)
A. 27 watts
B. 32 watts
C. 49 watts
D. 54 watts
ANSWER: A.
Comment: The question is technically incorrect because it does not provide sufficient information for evaluation. In particular, the power reduction during the introduction of a negative reactivity change is unknown and impossible to determine. In order to attain a -80 second period, negative reactivity must be at least -0.0050 ... such that the associated power reduction, due to reactivity change alone, will be substantial. For a beta of 0.0064, the prompt drop in power for introduction of a -0.0050 delta-rho from criticality would be from 120 watts to 70 watts ... even before the power decay on a stable negative period begins.
QID: B50 (OPIC: 292003 KNOWLEDGE: K1.09 [2.5/2.6])
During a reactor startup, the reactor is critical at 3000 counts per second. A control rod is notched out, resulting in a doubling time of 85 seconds. How much time is required for the reactor to reach 888,000 cps?
A. 341 seconds
B. 483 seconds
C. 697 seconds
D. 965 seconds
ANSWER: C.
Comment: This question suffers the same defect as B49. The power increase during control rod movement is not accounted for. There is no correct choice.