In this Nukefact we address the questions under the subsection Reactor Operational Physics. This subsection is the eighth and final set of questions in the section on Reactor Theory. It is the most important subsection because the questions are, as the title indicates, operational type questions. These questions are most closely tied to what the student, when qualified as a reactor operator, carries into the control room. These questions provide the basis of fundamentals by which the operator makes decisions about how to respond to normal and abnormal events in the control room.
Of the one-hundred eighty-one (181) questions in the INPO Catalog pertaining to Reactor Operational Physics we find ninety-nine to be technically incorrect, that's 55% wrong.
The primary reasons for judging these questions to be technically incorrect can be summarized as follows:
a. source range count rate, reactor period and rod position*
b. source range count rate, reactor pressure, and steam demand
c. axial peaking factor, reactor period, and source range count rate
d. axial peaching factor, steam demand, and reactor pressure
Comment: The asterisk indicates the intended correct answer is Choice "a". This question, and Question #5 (not shown), are satisfactory. Question #1 is included here only for comparison with similar questions, Questions #2, #3, and #4, which are not considered acceptable. The correct choice, "a", emphasizes that it is crucial to reactor safety to monitor and control three important parameters directly related to the core nuclear status during reactor startup. The reason this action is necessary is because of the large positive reactivity change that is being introduced by withdrawal of control rods.
Question 2: Which of the following parameters should be closely monitored and controlled specifically during the approach to criticality?
a. steam demand
b. reactor period*
c. reactor pressure
d. axial peaking factor
Comment: The asterisk indicates the intended correct answer is choice "b". The question is technically incorrect because it can seriously mislead the student into believing that this is the only parameter that needs monitoring. For this question the choices provided must emphasize the importance of three parameters, as in Question #1. Delete the question.
Question 3: Which of the parameters should be closely monitored and controlled specifically during the approach to criticality?
a. reactor pressure
b. core exit thermocouple indications
c. source range count rate*
d. axial peaking factor
Comment: The asterisk indicates the intended correct answer is choice "c". The question is technically incorrect because it can seriously mislead the student into believing that this is the only parameter that needs monitoring. For this question the choices provided must emphasize the importance of three parameters, as in Question #1. Delete the question.
Question 4: Which of the following parameters should be closely monitored and controlled particularly during the approach to criticality?
a. turbine bypass value position
b. reactor pressure
c. turbine speed
d. rod position*
Comment: The asterisk indicates the intended correct answer is choice "d". The question is technically incorrect because it can seriously mislead the student into believing that this is the only parameter that needs monitoring. For this question the choices provided must emphasize the importance of three parameters, as in Question #1. Delete the question.
Question 6 : List four parameters or mechanisms that can affect reactivity during an approach to criticality and explain the effect of each.
Answer: Factors affecting reactivity are:
1. control rod position - Rod withdrawal reduces neutron absorption in poisons, inserting positive reactivity. Rods are used as the operators' reactivity control mechanism during startups.
2. reactor pressure - If the reactor is producing sufficient decay heat to generate steam bubbles, an increase in pressure will collapse the bubbles and insert positive reactivity by improving neutron moderation.
3. coolant temperature - Because of the negative temperature coefficient, an increase in coolant temperature decreases coolant density and inserts negative reactivity by reducing neutron moderation.
4. fuel temperature - Due to the Doppler effect, when the fuel temperature increases, negative reactivity is inserted because more neutrons are absorbed in the fuel (uranium-238) while slowing down.
5. xenon concentration - Xenon-135 is a fission product poison that increases shortly after shutdown, then decreases, inserting negative, then positive, reactivity.
Comment: The question is technically incorrect because each of the answers indicates that reactivity is inserted. Reactivity change is introduced in each case. In addition, during an approach to criticality fuel temperature change can only occur via a change in moderator temperature, which will be so small as to introduce a negligible reactivity change. Delete item #4 and restate the answer and as follows:
Answer: (revised) Factors affecting reactivity are:
1. control rod position - Rod withdrawal reduces neutron absorption in poisons, introducing a positive reactivity change. Rods are used as the operators' reactivity control mechanism during startups.
2. reactor pressure - If the reactor is producing sufficient decay heat to generate steam bubbles, an increase in pressure will collapse the bubbles and introduce a positive reactivity change by improving neutron moderation.
3. coolant temperature - Because of the negative temperature coefficient, an increase in coolant temperature decreases coolant density and introduces a negative reactivity change by reducing neutron moderation.
4. xenon concentration - Xenon-135 is a fission product poison that increases shortly after shutdown, then decreases, introducing a negative reactivity change, then a positive reactivity change.
Question 8: For equal positive reactivity additions as a subcritical reactor approaches criticality, the neutron population
a. increase is greater when Keff is closer to 1.0*
b. increase is smaller when Keff is closer to 1.0
c. decreases linearly as Keff approaches 1.0
d. decreases exponentially as Keff approaches 1.0
Comment: The asterisk indicates the intended correct answer is choice "a". The question is technically incorrect because it specifies "equal positive reactivity additions", rather than equal positive reactivity changes. Reactivity is not being added. A change in reactivity, delta-rho, is being introduced. The question is not operational because there is no way for the operator to perform "equal" reactivity change. Commercial BWR's do not provide reactivity indication. This question, and others like it - #10, #11, #15, #17, #29, and #30 - can mislead the student as to what is possible during reactor startup. Delete the question.
Question 9: Control rods are withdrawn to a density of 95% with the reactor still subcritical. A control rod is notched out and the initial prompt jump is observed. Assuming the reactor stays subcritical the neutron population will
a. increase exponentially
b. stabilize at a new higher level*
c. decrease to the original value
d. stay constant
Comment: The asterisk indicates the intended correct answer is choice "b". The question is technically incorrect because of improper terminology. The power does not "stabilize", it reaches a higher equilibrium, or steady-state, level. Restate the question as follows:
Question 9: (revised) Control rods are withdrawn to a density of 95% with the reactor still subcritical. A control rod is notched out and the prompt jump is observed. Assuming the reactor stays subcritical, the neutron population will
a. increase exponentially
b. increase to a higher equilibrium level*
c. decrease to the original value
d. stay constant
Question 10: During a reactor startup, for equal reactivity changes as Keff approaches 1.0, steady-state neutron population will be reached
a. in a longer time*
b. in a shorter time
c. in the same amount of time
d. instantaneously
Comment: The asterisk indicates the intended correct answer is choice "a". The question is technically incorrect because there is no way for the operator to perform "equal" reactivity change. Power doubling produces the same effect and requires the same answer. This question, and others like it - #8, #11, #15, #17, #29 and #30 - can mislead the student as to what is possible during reactor startup. Restate the question as follows:
Question 10: (revised) During a power doubling reactor startup, as Keff approaches 1.0, steady-state neutron population will be reached
a. in a longer time*
b. in a shorter time
c. in the same amount of time
d. instantaneously
Question 11: A series of rod withdrawals is made during an approach to criticality. Assuming each withdrawal inserts the same amount of reactivity, describe the effect on reactor power (neutron count rate).
Answer: When rods are withdrawn, count rate increases sharply (the "prompt jump") as prompt neutrons respond to the insertion of positive reactivity, Count rate then gradually levels off a value determined by Keff and the source strength. Each subsequent withdrawal causes a larger increase in count rate and requires a longer time to reach the new equilibrium value.
Comment: The question is technically incorrect because it specifies the "insertion of the same amount of reactivity", rather than equal positive reactivity changes. The question is not operational because there is no way for the operator to perform "same amount" of reactivity change. This question, and others like it - #8, #10, #15, #17, #29, and #30 - can mislead the student as to what is possible during reactor startup. Delete the question.
Question 12: While withdrawing control rods during a reactor startup, the count rate doubles. If the same amount of reactivity that caused the first doubling is added again, the count rate will __________ and the reactor will be _________.
a. double; subcritical
b. double; critical
c. more than double; supercritical
d. more than double; critical*
Comment: The asterisk indicates the intended correct answer is choice "d". The question is technically incorrect. "Reactivity is not added", rather positive reactivity change is introduced. In addition, the question is not operational. Restate the question as follows:
Question 12: (revised) While withdrawing control rods during a reactor startup, the count rate doubles. If a second positive reactivity change of the same magnitude as the first is introduced, the count rate will __________ and the reactor will be _________.
a. double; subcritical
b. double; critical
c. more than double; supercritical
d. more than double; critical*
Question 13: After a rod notch withdrawal during the approach to criticality, the neutron count rate will settle at a new, higher level and reactor period indication will (assume reactor remains subcritical)
a. not change
b. return to infinity*
c. remain at a constant positive value
d. remain at a constant negative value
Comment: The asterisk indicates the intended correct answer is choice "b". The question is technically incorrect because of improper wording. The units on reactor period are seconds. Restate choice "b" as follows:
Question 13: (revised) After a rod notch withdrawal during the approach to criticality, the neutron count rate will settle at a new, higher level and reactor period indication will (assume reactor remains subcritical)
a. not change
b. be at infinite seconds*
c. remain at a constant positive value
d. remain at a constant negative value
Question 14: Assume a reactor startup is in progress. Which of the following best describes control-rod withdrawal and startup instrument response as Keff approaches unity.
a. It requires a longer time for the neutron level to level out at the equilibrium level for each rod withdrawal.*
b. Reactor neutron level will continue to increase slightly even though the reactor period as returned to infinity.
c. Successively smaller changes in neutron level will result for identical changes in Keff.
d. For each control-rod withdrawal, the reactor will become supercritical momentarily, due to a prompt jump.
Comment: The asterisk indicates the intended correct answer is choice "a". In addition to stating "best describes" where there is only one correct answer, the question is flawed because of other poor wording ... in Choice "a", time for the neutron level to level out". "Has" is mispelled in Choice "b". The question is technically incorrect because the time required to reach equilibrium depends on the relative magnitude of reactivity change introduced by each withdrawal. Restate the question as follows:
Question 14: (revised) Assume a reactor startup is in progress, using power doubling. Which of the following describes startup instrument response as Keff approaches unity.
a. A longer time interval is required for the Source Range indicators to reach the new equilibrium level after each rod withdrawal.*
b. Reactor neutron level will continue to increase slightly even though the reactor period has returned to infinite seconds.
c. Successively smaller changes in neutron level will result for identical changes in Keff.
d. For each control rod withdrawal, the reactor will become supercritical momentarily, due to a prompt jump.
Question 15 : Assume your reactor is being taken critical by periodically withdrawing equal reactivity control-rod increments. Which of the following statements is correct as Keff approaches unity?
a. The neutron level change for successive rod increments 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.
Comment: The asterisk indicates the intended correct answer is choice "b". The question is technically incorrect because it specifies "equal reactivity increments" and is not operational because there is no way for the operator to perform "equal" reactivity change. This question, and others like it - #8, #10, #11, #17, #29 and #30 - can mislead the student as to what is possible during reactor startup. Restate the question as follows:
Question 15: (revised) A reactor is being taken critical by power doubling. Which of the following statements is correct as Keff approaches unity?
a. The neutron level change for successive rod increments 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.
Question 16 : Assume a reactor has a Keff greater than unity and is several decades below the point of adding heat. Which of the following describes the reactor response during this period time?
a. As power increases, the reactor period will increase due to moderator temperature increase.
b. Keff will return to unity shortly after control-rod withdrawal is terminated.
c. Below the heating range, a change in moderator temperature will have no effect on reactor period.
d. A constant positive period, with increasing neutron level, will occur only if the net reactivity is not changing.*
Comment: The asterisk indicates the intended correct answer is choice "d". The question is flawed because of poor wording. What does "during this period time" mean? In any case, use of "period of time" is inappropriate when discussing "reactor period." Restate the question as follows:
Question 16: (revised) Assume a reactor has a Keff greater than unity and is several decades below the point-of-adding-heat. Which of the following describes the reactor response prior to reaching the point-of-adding-heat.?
a. As power increases, the reactor period will increase due to moderator temperature increase.
b. Keff will return to unity shortly after control-rod withdrawal is terminated.
c. Below the heating range, a change in moderator temperature will have no effect on reactor period.
d. A constant positive period, with increasing neutron level, will occur if the reactivity remains constant.*
Question 17 : During a reactor startup, as Keff approaches unity, which one of the following statements is correct for equal positive reactivity additions.
a. The changes in neutron population are larger*.
b. As the neutron population increases, the number of neutrons lost per generation decreases.
c. The number of fast neutrons gained per generation increases more slowly.
d. A step increase in Keff increases the neutron population and therefore decreases the number of neutrons lost per generation.
Comment: The asterisk indicates the intended correct answer is choice "a". The question is technically incorrect because it specifies "equal reactivity additions" and is not operational because there is no way for the operator to perform "equal" reactivity change. This question, and others like it - #8, #10, #11, #15, #18, #29, and #30 - can mislead the student as to what is possible during reactor startup. Delete the question.
Question 18: A reactor startup is being performed by adding equal amounts of positive reactivity and waiting for the neutron level to stabilize. Successive stable levels ________ critical will ______________________.
a. near; nearly be the same
b. far from; be large
c. near; be twice the previous level
d. far from; be small*
Comment: The asterisk indicates the intended correct answer is choice "d". The question is technically incorrect because "reactivity is added", rather than reactivity change being introduced, and because of erroneous terminology. The adjective "stable" has special meaning when associated with reactor rate. It (stable) should be reserved for that purpose to avoid confusion. Reactor power can be characterized as constant, at steady state, or as being at equilibrium subcritical multiplication level. And for the correct choice "d", what does "successive stable levels far from critical will be small" mean? This question, and others like it - #8, #10, #11, #15, #17, #29, and #30 - can mislead the student as to what is possible during reactor startup. Delete the question.
Question 20: Select the equation used to determine the subcritical multiplication factor of a reactor.
a. M = (Keff - 1)/Keff
b. M = 1/ (1 - Keff)*
c. M = (initial counts)/(final counts)
d. M = (final counts - initial counts)/(final counts)
Comment: The asterisk indicates the intended correct answer is choice "b". The question is technically incorrect because of ambiguous terminology. The question does not identify which parameter, M or Keff, is being referred to as the "subcritical multiplication factor". However, based on the fact that M appears on the left-hand-side of each equation choice, and as verified by the wording of Question #22, M is being referred to as the "subcritical multiplication factor". Operationally, the subcritical multiplication factor, M, cannot be determined from Choice "c" because the value of Keff is not known for the reactor. In fact, once the subcritical multiplication factor, M, is established from count rate measurements, a value for Keff can then be calculated form Choice"b", but may not be reliable. Rather than calculate the value of Keff, the subcritical multiplication factor is used to generate a 1/M loading curve or a 1/M startup curve. Restate the question as follows:
Question 20: (revised) If the subcritical multiplication factor, M, has been determined from count rate measurements, select the equation that can be used to determine the effective multiplication factor of a reactor.
a. M = (Keff - 1)/Keff
b. M = 1/ (1 - Keff)*
c. M = (Keff*initial counts)/(final counts)
d. M = (final counts - initial counts)/(Keff)
Question 22: Prior to withdrawing a control rod, the source range count rate is 220 cps. If the reactivity addition produces a subcritical multiplication factor of 2.0, what is the final steady-state count rate following this evolution?
a. 110 cps
b. 320 cps
c. 440 cps*
d. 520 cps
Comment: The asterisk indicates the intended correct answer is choice "c". The question is technically incorrect because the "subcritical multiplication factor" at any point during rod withdrawal is based on a reference count rate with all rods fully inserted. The question does not indicate this to be the case and is therefore potentially misleading in that it implies that the subcritical multiplication factor is the ratio of any two count rates during the withdrawal sequence. Restate the question as follows:
Question 22: (revised) With all rod fully inserted, the source range count rate is 220 cps. If after the first rod withdrawal increment a subcritical multiplication factor of 2.0 is calculated, what was the final steady-state count rate following this first rod withdrawal?
a. 110 cps
b. 320 cps
c. 440 cps*
d. 520 cps
Question 23: Prior to withdrawing a control rod, the source range count rate is 210 cps. Following the withdrawal, the final steady-state count rate is 420 cps. What is the subcritical multiplication factor?
a. 0.5
b. 1.0
c. 1.5
d. 2.0*
Comment: The asterisk indicates the intended correct answer is choice d. The question is technically incorrect for the same reason as given for question #22. Restate the question as follows:
Question 23: (revised) With all control rods fully inserted, the source range count rate is 210 cps. Following the first rod withdrawal increment, the final steady state count rate is 420 cps. What is the subcritical multiplication factor?
a. 0.5
b. 1.0
c. 1.5
d. 2.0*
Question 28: Which one of the following statements describes count rate characteristics after a 5 second control rod withdrawal with the reactor very close to criticality? Assume the reactor remains subcritical.
a. The count rate will rapidly increase (prompt jump) then gradually increase to a stable value.*
b. The count rate will rapidly increase (prompt jump) then gradually decrease to the previous value.
c. The count rate will rapidly increase (prompt jump) to a stable value.
d. There will be no change in count rate until criticality is achieved.
Comment: The asterisk indicates the intended correct answer is choice "a". The question is technically incorrect because a five second control rod withdrawal does not produce a "prompt jump". Prompt jump is usually associated with rapid rod movements of one or, at most, two seconds duration. A power increase over a time interval of 5 seconds is better characterized as a small, rapid increase. Here, the terminology is particularly poor. The "stable value" at the end of choice "a" can easily be interpreted by the student as "stable rate", which would render the choice incorrect. Restate the question as follows:
Question 28: (revised) Which one of the following statements describes count rate characteristics after a 5 second control rod withdrawal with the reactor very close to criticality? Assume the reactor remains subcritical.
a. The count rate will rapidly increase during rod motion and then gradually increase to a higher level of equilibrium subcritical multiplication.*
b. The count rate will rapidly increase during rod motion and then gradually decrease to the previous value.
c. The count rate will rapidly increase to a new higher level of equilibrium subcritical multiplication.
d. There will be no change in count rate until criticality is achieved.
Question 29: During a reactor startup, equal increments of reactivity are added and the count rate is allowed to reach equilibrium each time. Choose the statement that best describes what is observed.
a. The time required to reach equilibrium is longer each time.*
b. The time required to reach equilibrium is shorter each time.
c. The change in equilibrium count rate is the same each time.
d. The change in equilibrium count rate is smaller each time.
Comment: The asterisk indicates the intended correct answer is choice "a". The question is technically incorrect because it specifies "equal reactivity increments" and is not operational because there is no way for the operator to perform "equal" reactivity change. This question, and others like it - #8, #10, #11, #15, #17 and #30 - can mislead the student as to what is possible during reactor startup. Restate the question as follows:
Question 29: (revised) During a power doubling reactor startup, the count rate is allowed to reach equilibrium each time. Choose the statement that describes what is observed.
a. The time required to reach equilibrium is longer each time.*
b. The time required to reach equilibrium is shorter each time.
c. The change in equilibrium count rate is the same each time.
d. The change in equilibrium count rate is smaller each time.
Question 30: During a reactor startup, equal increments of reactivity are added and the count rate is allowed to reach equilibrium each time. Choose the statement that best describes what is observed.
a. The time required to reach equilibrium is the same each time.
b. The time required to reach equilibrium is shorter each time.
c. The change in equilibrium count rate is greater each time.*
d. The change in equilibrium count rate is the same each time.
Comment: The asterisk indicates the intended correct answer is choice "c". The question is technically incorrect because it specifies "equal reactivity increments" and is not operational because there is no way for the operator to perform "equal" reactivity change. This question, and others like it - #8, #10, #11, #15, #17, #18, and #29 - can mislead the student as to what is possible during reactor startup. Delete the question.
Question 31: During a reactor startup, as Keff approaches 1.0, it takes longer to reach an equilibrium neutron count rate due to the increased effect of
a. prompt neutrons
b. delayed neutrons*
c. fast neutrons
d. slow neutrons
Comment: The asterisk indicates the intended correct answer is choice "b". The question is technically incorrect because the reactivity change increments are not specified. Restate the question as follows:
Question 31: (revised) During a reactor startup using the power doubling procedure, as keff approaches 1.0, it takes longer to reach an equilibrium neutron count rate due to the increased effect of
a. prompt neutrons
b. delayed neutrons*
c. fast neutrons
d. slow neutrons