During a reactor startup, the first reactivity addition caused the count rate to increase from 20 to 40 cps. The second reactivity addition caused the count rate to increase from 40 to 160 cps. Which one of the following statements describes the reactivity additions?
A. The first reactivity addition was larger.
B. The second reactivity addition was larger.
C. The first and second reactivity additions were equal.
D. There is not enough data given to determine the relationship of reactivity values.
ANSWER: A.
Comment: The question is technically incorrect because it states that "reactivity is 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.
QID: P1665 (TOPIC: 192008 KNOWLEDGE: K1.01 [3.4/3.5])
During a reactor startup, the first reactivity addition caused the count rate to increase from 20 to 30 cps. The second reactivity addition caused the count rate to increase from 30 to 60 cps. Assume keff was 0.97 prior to the first reactivity addition. Which one of the following statements describes the magnitude of the reactivity additions?
A. The first reactivity addition was approximately 50% larger than the second.
B. The second reactivity addition was approximately 50% larger than the first.
C. The first and second reactivity additions were approximately the same.
D. There is not enough data given to determine the relationship of reactivity values.
ANSWER: C.
Comment: This question suffers the same defect as P565. Reactivity is not added. In addition, the question is technically incorrect because Choice C is incorrect. Choice C contradicts P565.
QID: P965 (TOPIC: 192008 KNOWLEDGE: K1.02 [2.8/3.0])
Which one of the following will be controlled by an operator to add positive reactivity when taking the reactor critical during a reactor startup?
A. Reactor coolant system (RCS) boron and control rods only
B. Control rods and moderator temperature only
C. Moderator temperature, RCS boron, and control rods
D. RCS flow, control rods, and moderator temperature
ANSWER: A.
Comment: This question suffers the same defect as P565. Reactivity is not added.
QID: P65 (B266) (TOPIC: 192008 KNOWLEDGE: K1.03 [3.9/4.0])
While withdrawing control rods during an approach to criticality, the stable count rate doubles. If the same amount of reactivity that caused the first doubling is added again, stable count rate will __________ and the reactor will be __________.
A. double; subcritical
B. more than double; subcritical
C. double; critical
D. more than double; critical
ANSWER: D.
Comment: This question suffers the same defect as P565. Reactivity is not added. In addition, a PWR startup does not introduce "equal" reactivity increments. The question does not pose an "operational" situation and can be misleading.
QID: P265 (TOPIC: 192008 KNOWLEDGE: K1.03 [3.9/4.0])
A reactor startup is in progress and the reactor is slightly subcritical. Assuming the reactor remains subcritical, a short control rod withdrawal will cause the reactor startup rate indication to increase rapidly in the positive direction, and then:
A. rapidly decrease and stabilize at a negative 1/3 DPM.
B. gradually decrease and stabilize at zero.
C. stabilize until the point of adding heat (POAH) is reached; then decrease to zero.
D. continue a rapid increase until the POAH is reached; then decrease to zero.
ANSWER: B.
Comment: The question is technically incorrect because units are not specified for reactor startup rate. The rate will stabilize at zero "dpm." It is always good practice to assign units, where applicable.
QID: P1065 (B1565) (TOPIC: 192008 KNOWLEDGE: K1.03 [3.9/4.0])
During a reactor startup, equal increments of positive reactivity are being sequentially added and the count rate is allowed to reach equilibrium after each addition. Which one of the following statements concerning the equilibrium count rate applies after each successive reactivity addition?
A. The time required to reach equilibrium count rate is the same.
B. The time required to reach equilibrium count rate is shorter.
C. The numerical change in equilibrium count rate increases.
D. The numerical change in equilibrium count rate is the same.
ANSWER: C.
Comment: This question suffers the same defect as P65. Reactivity is not added and the question does not pose an "operational" situation.
QID: P1166 (TOPIC: 192008 KNOWLEDGE: K1.03 [3.9/4.0])
Which one of the following describes the change in count rate resulting from a short control rod withdrawal with Keff at 0.99 as compared to an identical control rod withdrawal with Keff at 0.95? (Assume reactivity additions are equal, and the reactor remains subcritical.)
A. The prompt jump in count rate and the increase in count rate will be the same.
B. The prompt jump in count rate will be greater with Keff at 0.99, but the increase in count rate will be the same.
C. The prompt jump in count rate will be the same, but the increase in count rate will be greater with Keff at 0.99.
D. The prompt jump in count rate will be greater, and the increase in count rate will be greater with Keff at 0.99.
ANSWER: D.
Comment: This question suffers the same defect as P65. Reactivity is not added and the question does not pose an "operational" situation. In addition, a "prompt jump in count rate" will not occur unless the reactivity change is a step change.
QID: P1766 (TOPIC: 192008 KNOWLEDGE: K1.03 [3.9/4.0])
A reactor startup is in progress with the reactor currently subcritical. Which one of the following describes the change in count rate resulting from a short control rod withdrawal with Keff at 0.99 as compared to an identical control rod withdrawal with Keff at 0.95? (Assume reactivity additions are equal, and the reactor remains subcritical.)
A. Both the prompt jump in count rate and the increase in stable count rate will be the same.
B. Both the prompt jump in count rate and the increase in stable count rate will be smaller with Keff at 0.95.
C. The prompt jump in count rate will be smaller with Keff at 0.95, but the increase in stable count rate will be the same.
D. The prompt jump in count rate will be the same, but the increase in stable count rate will be smaller with Keff at 0.95.
ANSWER: B.
Comment: This question suffers the same defect as P565. Reactivity is not added. In addition, per P1166, a prompt jump requires a step change in reactivity.
QID: P2466 (B2465) (TOPIC: 192008 KNOWLEDGE: K1.03 [3.9/4.0])
A reactor startup is being performed by adding equal amounts of positive reactivity and waiting for neutron population to stabilize. As the reactor approaches criticality, the numerical change in stable neutron population after each reactivity addition __________, and the time required for the neutron population to stabilize after each reactivity addition ___________.
A. increases; remains the same
B. increases; increases
C. remains the same; remains the same
D. remains the same; increases
ANSWER: B.
Comment: This question suffers the same defect as P65. Reactivity is not added and the question does not pose an "operational" situation.
QID: P2467 (TOPIC: 192008 KNOWLEDGE: K1.03 [3.9/4.0])
A reactor startup is in progress. The reactor is slightly subcritical with a constant startup rate of 0.0 decades per minute (dpm). A short control rod insertion will cause the reactor startup rate indication to rapidly decrease (become negative), and then:
A. gradually become more negative until neutron population reaches equilibrium, then stabilize.
B. gradually become less negative and return to 0.0 dpm.
C. stabilize until neutron population reaches the prestartup equilibrium level, then return to 0.0 dpm.
D. stabilize at -1/3 dpm until delayed neutrons are no longer a significant contributor to the neutron population, and then return to 0.0 dpm.
ANSWER: B.
Comment: The question is unsatisfactory because of poor wording. How does the startup rate "decrease" if it is already 0.0 dpm? (The startup rate indication will move down scale to a negative rate that continues to accelerate during the rod insertion. On termination of the rod insertion, the negative startup rate will decelerate as it returns to 0.0 dpm.) In addition, the question is technically incorrect because there are no gradations of negativity. Negative is negative and positive is positive. Choice B refers to the magnitude of the startup rate becoming "smaller."
QID: P266 (B1566) (TOPIC: 192008 KNOWLEDGE: K1.04 [3.8/3.8])
During a reactor startup, the operator adds 1.0% delta-K/K of positive reactivity by withdrawing control rods, thereby increasing equilibrium source range neutron level from 220 cps to 440 cps. To raise equilibrium source range neutron level to 880 cps, an additional ______________ of positive reactivity must be added.
A. 4.0% delta-K/K
B. 2.0% delta-K/K
C. 1.0% delta-K/K
D. 0.5% delta-K/K
ANSWER: D.
Comment: This question suffers the same defect as P565. Reactivity is not added. In addition, the four choices are incorrectly indicated to represent reactivity rather than delta-rho.
QID: P566 (TOPIC: 192008 KNOWLEDGE: K1.04 [3.8/3.8])
During a reactor startup, control rods are withdrawn such that 1.05% delta-K/K of reactivity is added. Before the withdrawal Keff was 0.97 and count rate was 500 cps. Which one of the following will be the approximate final steady-state count rate following the rod withdrawal?
A. 750 cps
B. 1000 cps
C. 2000 cps
D. 2250 cps
ANSWER: A.
Comment: This question suffers the same defect as P565. Reactivity is not added.
QID: P1770 (B1665) (TOPIC: 192008 KNOWLEDGE: K1.04 [3.8/3.8])
Refer to the drawing of a 1/M plot (see figure below). The least conservative approach to criticality is represented by curve _____ and could possibly be the result of recording count rates at ________ time intervals after incremental fuel loading steps than for the situations represented by the other curves.
A. A; shorter
B. A; longer
C. C; shorter
D. C; longer
Comment: The question is technically incorrect because when far subcritical the time required to reach equilibrium is extremely short. The primary reasons that curve "C" is non-linear are that 1/M is plotted against "fuel loaded", rather than against keff and that the shape is effected by source-detector geometry distortions.
QID: P1866 (B2266) (TOPIC: 192008 KNOWLEDGE: K1.04 [3.8/3.8])
As a reactor approaches criticality during a reactor startup it takes longer to reach an equilibrium neutron count rate after each control rod withdrawal due to the increased:
A. length of time required to complete a neutron generation.
B. number of neutron generations required to reach a stable neutron level.
C. length of time from neutron birth to absorption.
D. fraction of delayed neutrons being produced as criticality is approached.
ANSWER: B.
Comment: The question is technically incorrect because "neutron generations" are a fictitious entity associated with the primitive life cycle model, which cannot accurately describe actual reactor behavior. The explanation should be based on the one-delay group model.
QID: P1867 (TOPIC: 192008 KNOWLEDGE: K1.04 [3.8/3.8])
During a reactor startup, the first reactivity addition caused the count rate to increase from 20 to 40 cps. The second reactivity addition caused the count rate to increase from 40 to 80 cps. Assume keff was 0.92 prior to the first reactivity addition. Which one of the following statements describes the magnitude of the reactivity additions?
A. The first reactivity addition was approximately twice as large as the second.
B. The second reactivity addition was approximately twice as large as the first.
C. The first and second reactivity additions were approximately the same.
D. There is not enough data given to determine the relationship between reactivity values.
ANSWER: A.
Comment: This question suffers the same defect as P565. Reactivity is not added.
QID: P2367 (B2366) (TOPIC: 192008 KNOWLEDGE: K1.04 [3.8/3.8])
Refer to the drawing of three 1/M plots (see figure below). A core refueling is in progress with an installed neutron source. During the early stages of the refueling, reactor criticality would be predicted earliest by curve ______ and could possibly be the result of using nuclear instrumentation that is located too ______ the neutron source.
A. A; far from
B. A; close to
C. C; far from
D. C; close to
Comment: This question is technically incorrect because the shape of Curve A, which is representative of PWR loading curves, is due to the fact that the initial fuel assemblies loaded cause a greater change in keff than do late assemblies. The 1/M curve is linear only when plotted against keff.