A reactor is critical below the point of adding heat. If control rods are manually inserted for 5 seconds, reactor power will decrease:
A. to a shutdown power level determined by subcritical multiplication.
B. temporarily, then return to the original value due to the resulting decrease in moderator temperature.
C. until inherent positive reactivity feedback causes the reactor to become critical at a lower neutron level.
D. temporarily, then return to the original value due to subcritical multiplication.
ANSWER: A.
Comment: The answer is technically incorrect because a 5 second insertion of a single rod will introduce only a small negative reactivity change. The reactor is still in close proximity to criticality. A low power level is not an indicator of whether the reactor is shut down. A "shutdown" power level does not result from a 5-second rod insertion. A reactor is not "shutdown" when a single rod is inserted for 5 seconds but rather when all control rods are "fully" inserted. Source multiplication following a 5-second rod insertion will be large. Depending on the non-fission source strength, power may settle to an equilibrium level either in the middle of the Source Range, in the upper Source Range, or in the lower Intermediate Range.
QID: B755 (P754) (TOPIC: 292005 KNOWLEDGE: K1.04 [3.5/3.5])
A reactor is exactly critical below the point of adding heat (POAH) during a normal reactor startup. If a control rod is manually withdrawn for 5 seconds, reactor power will:
A. increase to a stable critical power level below the POAH.
B. increase temporarily, then decrease and stabilize at the original value.
C. increase to a stable critical power level at the POAH.
D. increase temporarily, then decrease and stabilize below the original value.
ANSWER: C.
Comment: In addition to using "exactly" critical, the question is technically incorrect because of other erroneous terminology and because the indicated correct answer is not necessarily true. What is a "stable critical power level" ... a constant power level ? Stable is standard/common terminology for describing a constant off-critical reactor period, i.e. a condition of changing power with time. Now stable is used to define a constant power level, when "critical" is fully adequate. What occurs at the POAH will depend on the status of the system. If there is no load on the system (steam flow, or significant heat loss) heating will bring the reactor subcritical and it will remain subcritical. Power will decrease to an equilibrium level. If there is a heat load, heating will cause power to turn and undergo a damped oscillation as it levels off at a critical condition with power in balance with the load. The question, as stated, provides insufficient information to formulate an answer.
QID: B2254 (TOPIC: 292005 KNOWLEDGE: K1.04 [3.5/3.5])
A reactor is critical below the point of adding heat (POAH) during a reactor startup at the end of core life. Control rods are withdrawn for 20 seconds to establish a positive 30-second reactor period. Reactor power will increase:
A. continuously until control rods are reinserted.
B. and stabilize at a value slightly below the POAH.
C. temporarily, and then stabilize at the original value.
D. and stabilize at a value equal to or above the POAH.
ANSWER: D.
Comment: This question suffers some defects of B755. The necessary initiial conditions are not provided.
QID: B2554 (TOPIC: 292005 KNOWLEDGE: K1.04 [3.5/3.5])
A reactor is operating steady state at the point of adding heat (POAH) during a reactor startup near the beginning of core life. Reactor pressure is stable at 600 psig and main steam isolation valves are closed (no steam flow from reactor).
If a control rod is manually inserted for 5 seconds, and the reactor does not scram, when conditions stabilize, reactor power will be __________ and reactor vessel pressure will be __________.
A. at the POAH; 600 psig
B. at the POAH; less than 600 psig
C. less than the POAH; 600 psig
D. less than the POAH; less than 600 psig
ANSWER: B.
Comment: This question suffers some defects of B755. The necessary initiial conditions are not provided.
QID: B555 (TOPIC: 292005 KNOWLEDGE: K1.05 [2.5/2.6])
Rod density is a measure of the total number of control rod notches _______ the core divided by the total number of control rod notches _______ the core.
A. inserted into; available in
B. inserted into; withdrawn from
C. withdrawn from; available in
D. withdrawn from; inserted into
ANSWER: A.
Comment: The question is technically incorrect because control rod density is expressed as a percentage. The wording is poor. What are "available" notches, ones not inserted, one inserted, or total notches?
QID: B1055 (TOPIC: 292005 KNOWLEDGE: K1.05 [2.5/2.6])
Rod density is a measure of the:
A. percentage of control rods inserted into the core.
B. percentage of control rods withdrawn from the core.
C. number of control rods fully inserted divided by the number of control rods fully withdrawn.
D. number of control rods fully withdrawn divided by the number of control rods fully inserted.
ANSWER: A.
Comment: The question is technically incorrect and contradicts B555. Choice A indicates that control rod density is based on "numbers" of control rods inserted ... and not on the number of notches inserted.
QID: B856 (P555) (TOPIC: 292005 KNOWLEDGE: K1.07 [2.4/2.6])
The total amount of reactivity added by a control rod position change from a reference point to any other rod height is called:
A. differential rod worth.
B. excess reactivity.
C. integral rod worth.
D. reference reactivity.
ANSWER: C.
Comment: The question is technically incorrect because the answer is wrong. As with temperature coefficients, differential rod worth is defined very specifically as the reactivity increase associated with a unit distance of rod withdrawal. Thus, differential rod worth is always carries a positive algebraic sign. Historically, integral rod worth was taken as the sum of the differential rod worth values from fully inserted to fully withdrawn. As the terminology implies, "integral" rod worth is the positive reactivity change that occurred if the single rod were withdrawn from fully inserted to fully withdrawn position. Integral rod worth is the total worth of a control rod. The algebraic sign of integral rod worth is also positive because the algebraic sign of each differential worth is positive.
Historically, both differential rod worth and integral rod worth were presented as curves. The differential rod worth curve was roughly bell-shaped, reflecting axial flux distribution and displaying delta-rho/unit position change on the ordinate and position on the abscissa, from fully inserted to fully withdrawn. The integral rod worth curve was S-shaped, displaying delta-rho on the ordinate and position on the abscissa, from fully inserted to fully withdrawn. In this question, an arbitrary rod position change creates nothing more than a reactivity change. Of course this reactivity change can be determined from an integral rod worth curve ... but ... this reactivity change is definitely NOT integral rod worth.
QID: B1555 (TOPIC: 292005 KNOWLEDGE: K1.07 [2.4/2.6])
As a control rod is withdrawn from notch position 00 (fully inserted) to notch position 48 (fully withdrawn) integral rod worth will:
A. decrease, then increase.
B. increase, then decrease.
C. decrease continuously.
D. increase continuously.
ANSWER: D.
Comment: This question suffers the same defect as B856. Integral rod worth is the reactivity difference between the fully inserted and fully withdrawn condition. There is no intermediate value.
QID: B1657 (P1555) (TOPIC: 292005 KNOWLEDGE: K1.07 [2.4/2.6])
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.
ANSWER: C.
Comment: This question suffers the same defect as B856. Integral rod worth is the reactivity difference between the fully inserted and fully withdrawn condition. There is no intermediate value.
QID: B1955 (TOPIC: 292005 KNOWLEDGE: K1.07 [2.4/2.6])
Which one of the following describes the change in magnitude (absolute value) of integral rod worth during the complete withdrawal of a fully-inserted control rod?
A. Increases, then decreases
B. Decreases, then increases
C. Increases continuously
D. Decreases continuously
ANSWER: C.
Comment: This question suffers the same defect as B856. Integral rod worth is the reactivity difference between the fully inserted and fully withdrawn condition. There is no intermediate value.
QID: B2755 (P1384) (TOPIC: 292005 KNOWLEDGE: K1.07 [2.4/2.6])
Integral rod worth is the:
A. change in reactivity per unit change in rod position.
B. rod worth associated with the most reactive control rod.
C. change in worth of a control rod per unit change in reactor power.
D. reactivity added by moving a control rod from a reference point to another point.
ANSWER: D.
Comment: This question suffers the same defect as B856. Integral rod worth is the reactivity difference between the fully inserted and fully withdrawn condition. There is no intermediate value. In addition, reactivity is not added.
QID: B2855 (P1354) (TOPIC: 292005 KNOWLEDGE: K1.07 [2.4/2.6])
Integral rod worth is the:
A. change in reactivity per unit change in rod position.
B. reactivity inserted by moving a control rod from a reference position to another position.
C. change in worth of a control rod per unit change in reactor power.
D. rod worth associated with the most reactive control rod.
ANSWER: B.
Comment: This question suffers the same defect as B856. Integral rod worth is the reactivity difference between the fully inserted and fully withdrawn condition. There is no intermediate value. In addition, reactivity is not inserted.
QID: B2956 (TOPIC: 292005 KNOWLEDGE: K1.07 [2.4/2.6])
A reactor is operating at steady-state 50% power at the end of core life with all control systems in manual. The radial power distribution is symmetric and peaked in the center of the core, and the axial power distribution peak is slightly below the core midplane.
The tip of the most centrally-located control rod is currently located at the core midplane. The control rod is constructed of a homogeneous neutron absorber and the active neutron absorber length is exactly as long as the adjacent fuel assembly. The rod is manually inserted fully into the core, no other operator action is taken, and reactor power stabilizes at 42%. If, instead, the control rod had been withdrawn fully from its core midplane position, the reactor would have experienced:
A. a larger absolute change in integral control rod reactivity.
B. a smaller absolute change in integral control rod reactivity.
C. a larger absolute change in reactor shutdown margin.
D. a smaller absolute change in reactor shutdown margin.
ANSWER: A.
Comment: This question suffers the same defect as B856. Integral rod worth is the reactivity difference between the fully inserted and fully withdrawn condition. There is no intermediate value.
QID: B53 (TOPIC: 292005 KNOWLEDGE: K1.09 [2.5/2.6])
Which one of the following statements describes how changes in core parameters affect control rod worth (CRW)?
A. CRW decreases when the temperature of the fuel decreases.
B. CRW increases with an increase in voids.
C. CRW increases with an increase in fast neutron flux.
D. CRW decreases when approaching end of core life.
ANSWER: D.
Comment: The question is technically incorrect because "control rod worth" is an ambiguous term. With previous questions dealing with "differential rod worth" and "integral rod worth", the meaning of "control rod worth" in this question is an unknown. Are control rods being withdrawn during the cycle or are rod(s) at a fixed position?
QID: B1157 (TOPIC: 292005 KNOWLEDGE: K1.09 [2.5/2.6])
Which one of the following conditions will cause the associated individual control rod worth(s) to become more negative?
A. During a small power change, fuel temperature increases.
B. With the reactor shut down, reactor coolant temperature increases from 100 F to 200 F.
C. During a small power change, the percentage of voids increases.
D. During a control pattern adjust, the local thermal neutron flux surrounding a control rod decreases while the core average thermal neutron flux remains the same.
ANSWER: B.
Comment: This question suffers the same defects as B53. Control rod worth is not a defined term and there are no gradations of negativity. Negative is negative and positive is positive.
QID: B1456 (TOPIC: 292005 KNOWLEDGE: K1.09 [2.5/2.6])
Which one of the following events will cause control rod worth to become less negative?
A. Fuel temperature decreases as the fuel pellets come into contact with the fuel clad.
B. The moderator is heated from 170 F to 215 F 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.
ANSWER: C.
Comment: This question suffers the same defects as B1157. Control rod worth is not a defined term and there are no gradations of negativity. Negative is negative and positive is positive.
QID: B1556 (TOPIC: 292005 KNOWLEDGE: K1.09 [2.5/2.6])
If the void fraction surrounding centrally located fuel bundles decreases, the worth of the associated control rod(s) will:
A. increase, because the average neutron energy in the area of the affected control rod(s) increases.
B. increase, because less neutrons are resonantly absorbed in the fuel while they are being thermalized, resulting in more thermal neutrons available to be absorbed by the affected control rod(s).
C. decrease, because the diffusion length of the thermal neutrons decreases, resulting in fewer thermal neutrons reaching the affected control rod(s).
D. decrease, because neutrons will experience a shorter slowing down length, resulting in a larger fraction of thermal neutrons being absorbed by the fuel and less thermal neutrons available to be absorbed by the affected control rod(s).
ANSWER: B.
Comment: This question suffers the same defects as B53. Control rod worth is not a defined term.
QID: B2356 (P2356) (TOPIC: 292005 KNOWLEDGE: K1.09 [2.5/2.6])
A reactor startup is in progress from a cold shutdown condition. During the heatup phase of the startup, control rod differential reactivity worth (delta-K/K per inch insertion) becomes _______ negative, and during the power increase from 20% to full power, control rod differential reactivity worth becomes _______ negative.
A. more; less
B. more; more
C. less; less
D. less; more
ANSWER: A.
Comment: This question is technically incorrect because differential rod worth is indicated to represent "reactivity" per inch "insertion", because negative has no gradations, and because no indication is given as to whether the control rod is moving during the heatup and power increase.
QID: B2656 (P1556) (TOPIC: 292005 KNOWLEDGE: K1.09 [2.5/2.6])
As moderator temperature increases, the magnitude of differential rod worth will:
A. increase due to longer neutron migration length.
B. decrease due to reduced moderation of neutrons.
C. increase due to decreased resonance absorption of neutrons.
D. decrease due to decreased moderator absorption of neutrons.
ANSWER: A.
Comment: The question is technically incorrect because it does not indicate whether the control rod is at a fixed position or is moving during the heatup.
QID: B2857 (TOPIC: 292005 KNOWLEDGE: K1.09 [2.5/2.6])
The reactor is operating at 85% power with control rod X-Y inserted 20%. Which one of the following will cause the differential control rod worth of control rod X-Y to become more negative? (Assume that control rod X-Y remains 20% inserted for each case.)
A. Fuel temperature increases as fission product gases accumulate in a fuel rod.
B. Reactor vessel pressure drifts from 900 psig to 880 psig.
C. Core Xe-135 builds up in the lower half of the core.
D. An adjacent control rod is fully withdrawn from the core.
ANSWER: D.
Comment:The question is technically incorrect because "negative" has no gradations and because the rod position is improperly defined as "percent inserted." (Note the stipulation that the control rod remains in a fixed position, an important qualifier that is lacking in many other questions of this type.
QID: B2957 (N/A) (TOPIC: 292005 KNOWLEDGE: K1.09 [2.5/2.6])
If the void fraction surrounding several centrally located fuel bundles increases, the worth of the associated control rod(s) will:
A. decrease, because the average neutron energy in the fuel bundles decreases, resulting in fewer neutrons traveling from within the fuel bundles to the affected control rod(s).
B. decrease, because more neutrons are resonantly absorbed in the fuel while they are being thermalized, resulting in fewer thermal neutrons available to be absorbed by the affected control rod(s).
C. increase, because the diffusion length of the thermal neutrons increases, resulting in more thermal neutrons traveling from within the fuel bundles to the affected control rod(s).
D. increase, because neutrons will experience a longer slowing down length, resulting in a smaller fraction of thermal neutrons being absorbed by the fuel and more thermal neutrons available to be absorbed by the affected control rod(s).
ANSWER: B.
Comment: This question suffers the same defects as B53. Control rod worth is not a defined term.
QID: B179 (TOPIC: 292005 KNOWLEDGE: K1.10)
Which one of the following is a reason for flux shaping?
A. To minimize the worth of individual control rods by evenly distributing the flux radially
B. To reduce the reverse power effect during rod withdrawal by peaking the flux at the top of the core
C. To equalize control rod drive mechanism wear and control rod blade neutron burnout
D. To increase the effectiveness of power control rods by peaking the flux at the bottom of the core
ANSWER: A.
Comment: This question suffers the same defects as B53. Control rod worth is not a defined term.
QID: B2054 (P2056) (TOPIC: 292005 KNOWLEDGE: K1.10 [2.8/3.3])
Neutron flux shaping in a reactor core reduces radial power peaking:
A. in the center of the core caused by a high number density of fuel assemblies.
B. at the periphery of the core caused by moderator reflection of thermal leakage neutrons.
C. throughout the core caused by uneven burnout of control rod poison material.
D. throughout the core caused by loading fuel assemblies of various fuel enrichments.
ANSWER: A.
Comment: This question is technically incorrect because of creative terminology. What is a "high number density of fuel assemblies?"