Question 43 : As void fraction increases, control rod worth
a. increases
b. remains the same
c. decreases*
d. initially increases then decreases
Comment: The asterisk indicates the intended correct answer is choice "c". The question is technically incorrect for the reason given for question #41. It is not possible to restate the question.
Question 44: Control rod worth decreases with an increase in void fraction because there are __________ neutrons in the vicinity of the rod.
a. more fast
b. fewer fast
c. more thermal
d. fewer thermal*
Comment: The asterisk indicates the intended correct answer is choice "d". The question is technically incorrect for the same reason given for question #41. It is not possible to restate the question.
Question 45: As xenon concentration increases, control rod worth
a. increases
b. decreases*
c. stays the same
d. initially increases and then decreases
Comment: The asterisk indicates the intended correct answer is choice "b". The answer is technically incorrect for the same reason given for question #41. It is not possible to restate the question.
Question 46: Control rod worth decreases with a corresponding
a. increase in moderator temperature
b. decrease in voids
c. decrease in xenon concentration
d. increase in control rod density*
Comment: The asterisk indicates the intended correct answer is choice "d". The question is technically incorrect for the same reason given for question #41. It is not possible to restate the question.
Question 47: A correct statement regarding control rod worth during a reactor startup is that
a. central control rod worth will be higher during a peak xenon startup than during a xenon-free startup
b. peripheral control rod worth will be higher during a peak xenon startup than during a xenon-free startup*
c. peripheral control rod worth will be lower during a peak xenon startup than during a xenon-free startup
d. both control rod worths will be the same regardless of core xenon conditions
Comment: The asterisk indicates the intended correct answer is choice "b". The question is technically incorrect for the same reason given for question #41. It is not possible to restate the question.
Question 48: In regard to core parameters that affect control rod worth, which of the following statements is correct?
a. Control rod worth decreases when the temperature of the fuel decreases.
b. Control rod worth increases with an increase in voids.
c. Control rod worth increases with an increase in fast neutron flux.
d. Control rod worth decreases when approaching end of core life (EOL).*
Comment: The asterisk indicates the intended correct answer is choice "d". The question is technically incorrect for the same reason given for question #41. It is not possible to restate the question.
Question 49: Which of the following statements describes the relationship between control rod worth and moderator temperature change?
a. Control rod worth is not affected by moderator temperature change.
b. As moderator temperature increases, the neutrons travel farther during the slowing-down process; thus, control rod worth increases.*
c. As moderator temperature changes, control rod worth is inversely proportional, due to neutron velocity changes.
d. As moderator temperature increases, the fuel temperature increases, changing the Doppler; the control rod worth decreases.
Comment: The asterisk indicates the intended correct answer is choice "b". The question is technically incorrect for the same reason given for question #41. It is not possible to restate the question.
Question 50: The effect of a change in moderator temperature on control rod worth is to
a. increase control rod worth with a decrease in temperature
b. have little effect on control rod worth
c. increase control rod worth for an increase in temperature at beginning of life (BOL) but decrease control rod worth at end of life (EOL)
d. increase control rod worth with an increase in temperature*
Comment: The asterisk indicates the intended correct answer is choice "d". The question is technically incorrect for the same reason given for question #41. It is not possible to restate the question.
Question 51: Which one of the following statements explains changes in control rod worth as xenon concentration increases?
a. The xenon increase has no effect on control rod worth because worth is a function of position.
b. The xenon increase will decrease control rod worth because the xenon competes for thermal neutrons with the control rods.*
c. The xenon increase will increase the thermal utilization, and control rod worth increases.
d. The xenon increase does not affect control rod worth until the xenon peaks; after the peak, the control rod worth increases.
Comment: The asterisk indicates the intended correct answer is choice "b". The question is technically incorrect for the same reason as question #41. It is not possible to restate the question.
Question 52: An increase in which one of the following will increase control rod worth?
a. the percent voids
b. increasing fuel temperature
c. core age from 5,000 to 10,000 MWd/ton
d. moderator temperature*
Comment: The asterisk indicates the intended correct answer is choice "d". The question is technically incorrect for the same reason given for question #41. It is not possible to restate the question.
Question 54: Which one of the following will cause a decrease in the reactivity worth of a single control rod?
a. The xenon-135 concentration around the rod decreases.
b. The moderator temperature increases.
c. An adjacent control rod is withdrawn.
d. The void content around the rod increases.*
Comment: The asterisk indicates the intended correct answer is choice "d". The question is technically incorrect for the same reason as question #41. It is not possible to restate the question.
Question 60: If the void fraction surrounding centrally located fuel bundles increases, the worth of the associated control rod(s) will
a. decrease, because more neutrons are able to travel from one fuel bundle to the next without being absorbed by the control rod*
b. increase, because thermal neutrons will travel farther resulting in a larger fraction of thermal neutrons being absorbed by the control rod
c. decrease, because more neutrons are resonantly absorbed in the fuel as they are being thermalized resulting in fewer thermal neutrons to be absorbed by the control rod
d. increase, because control rods are epithermal neutron absorbers and neutrons remain at higher energies longer due to the longer slowing down length
Comment: The asterisk indicates the intended correct answer is choice "a". The question is technically incorrect for the same reason given in question #41. It is not possible to restate the question.
Question 67: The reason for flux shaping is to
a. minimize the potential of moving high-worth control rods*
b. allow thermal leakage to control thermal power by keeping periphery control rods farther out of the core
c. equalize wear and required maintenance
d. reduce thermal leakage by keeping periphery control rods farther into the core
Comment: The asterisk indicates the intended correct answer is choice "a". The question is technically incorrect because "high worth" control rods is ambiguous and because, as indicated in question #69, there are three principal reasons for shaping the flux. Hence, choice "a" is not "The (only) reason". It is not possible to restate the question.
Question 68: Neutron flux shaping within a reactor core is designed to
a. minimize the effects of rod shadowing
b. ensure that more power is generated in the lower portion of the core
c. ensure that local core power limits are not exceeded*
d. minimize the effects of an ejected rod
Comment: The asterisk indicates the intended correct answer is choice "c". The question is flawed because, as indicated in question #69, there are three reasons for flux shaping. Restate the question as follows:
Question 68: (revised) One important reason for neutron flux shaping within a reactor core is to
a. minimize the effects of rod shadowing
b. ensure that more power is generated in the lower portion of the core
c. ensure that local core power limits are not exceeded*
d. minimize the effects of an ejected rod
Question 69: Describe the benefits that result from proper flux shaping.
Answer: In general, a properly shaped neutron flux profile is flat. This results in a relatively uniform distribution of power throughout the core, which has the dual benefits of equalizing fuel burnup and avoiding potentially limiting power peaks. Similarly, a uniform flux profile prevents significant peaks in control rod worth, facilitating operation and protection of the reactor.
Comment: The answer is technically incorrect because "control rod worth" is ambiguous. Restate the answer as follows:
Answer (revised): In general, a properly shaped neutron flux profile is flat. This results in a relatively uniform distribution of power throughout the core, which has the dual benefits of equalizing fuel burnup and avoiding potentially limiting power peaks. Similarly, a uniform flux profile prevents significant peaks in the differential control rod worth, facilitating operation and protection of the reactor.
Question 70: Which one of the following statements describes the purpose of neutron flux shaping?
a. to produce more power in the lower regions of the core so that excore instruments can monitor neutron levels
b. to increase the length of time required before refueling
c. to maintain a uniform power distribution and ensure power limits are not exceeded*
d. to minimize the effects of rod shadowing
Comment: The asterisk indicates the intended correct answer is choice "c". The question is technically incorrect because, as indicated in question #69, there are three reasons for flux shaping. Restate the question as follows:
Question 70: (revised) Which one of the following statements describes one important purpose of neutron flux shaping?
a. to produce more power in the lower regions of the core so that excore instruments can monitor neutron levels
b. to increase the length of time required before refueling
c. to maintain a uniform power distribution and ensure power limits are not exceeded*
d. to minimize the effects of rod shadowing
Question 75: Axial flux would be most affected by a one-notch position change in which of the following areas?
a. fully inserted to 1/4 withdrawn
b. 3/4 out to fully withdrawn*
c. 1/2 out to 3/4 withdrawn
d. 1/4 out to 1/2 withdrawn
Comment: The asterisk indicates the intended correct answer is choice "b". The question is flawed because of poor wording. The choices are not "areas", they are control rod position ranges. Restate the question as follows:
Question 75: (revised) Axial flux would be most affected by a one-notch position change in which of the following ranges?
a. fully inserted to 1/4 withdrawn
b. 3/4 out to fully withdrawn*
c. 1/2 out to 3/4 withdrawn
d. 1/4 out to 1/2 withdrawn
Question 81: Explain the difference between the positions of deep and shallow control rods.
Answer: A "deep" control rod is one that is inserted far into the reactor. Typically, a rod inserted two-thirds or further into the core is considered a deep rod.
Shallow rods are those inserted a relatively short distance. Typically, a rod inserted less than one-third into the core is considered a shallow rod.
Reference 25, page 23-3.
Comment: The question is technically incorrect because question #75, and others (not shown), uses one-forth core heights to identify deep and shallow rods while this question uses one-third core heights. The question cannot be restated.
Question 89: Which of the following events will cause control rod worth to decrease?
a. Fuel temperature decreases as the fuel pellets come into contact with the fuel clad.
b. The moderator is heated from 170F to 215F during a startup.
c. Reactor power is increased from 40% to 60% by withdrawing control rods.*
d. Early in core life, the concentration of burnable poison decreases.
Comment: The asterisk indicates the intended correct answer is choice "c". The question is technically incorrect for the reason given in question #41. It is not possible to restate the question.
Question 91: Which one of the following expresses the relationship between differential rod worth (DRW) and integral rod worth (IRW)?
a. IRW is the slope of the DRW curve.
b. IRW is the inverse of the DRW curve.
c. IRW is the sum of the DRWs between the initial and final control rod positions.*
d. IRW is the sum of the DRWs of all control rods at any specific control rod position.
Comment: The asterisk indicates the intended correct answer is choice "c". The question is technically incorrect for the same reason given in question #37 . Restate the question as follows:
Question 91: (revised) Which one of the following expresses the relationship between differential rod worth (DRW) and integral rod worth (IRW)?
a. IRW is the slope of the DRW curve.
b. IRW is the inverse of the DRW curve.
c. IRW is the sum of the DRWs between the fully inserted and fully withdrawn rod position.*
d. IRW is the sum of the DRWs of all control rods at any specific control rod position.