A reactor is exactly 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 Control rods are withdrawn for 20 seconds to establish a 0.5 dpm startup rate. Reactor power will increase:
A. continuously until control rods are reinserted.
B. and stabilize at a value slightly below the POAH.
C. temporarily, then stabilize at the original value.
D. and stabilize at a value slightly above the POAH.
ANSWER: D.
Comment: In addition to using "exactly" critical, the question is technically incorrect because the indicated correct answer is not necessarily true. 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. Final power will then be at the POAH, not slightly above the POAH. The question, as stated, provides insufficient information to formulate an answer.
QID: P354 (B356) (TOPIC: 192005 KNOWLEDGE: K1.03 [3.5/3.6])
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 low in the source (startup) range.
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: P754 (B755) (TOPIC: 192005 KNOWLEDGE: K1.03 [3.5/3.6])
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 increase:
A. to a stable critical power level below the POAH.
B. temporarily, then decrease and stabilize at the original value.
C. to a stable critical power level at the POAH.
D. temporarily, then decrease and stabilize below the original value.
ANSWER: C.
Comment: The question suffers the same defect as P254. Initial conditions must be defined.
QID: P1754 (TOPIC: 192005 KNOWLEDGE: K1.03 [3.5/3.6])
A reactor is exactly critical at the point of adding heat (POAH) during a reactor startup at the end of core life. Then, control rods are manually withdrawn for 5 seconds. Assuming only ambient heat removal from the reactor coolant system (RCS), when conditions stabilize, reactor power will be __________ the POAH and RCS average temperature will be __________.
A. at; the same
B. at; higher
C. greater than; the same
D. greater than; higher
ANSWER: B.
Comment: The question uses unsatisfactory terminology. Critical is a very exact condition, by definition.
QID: P555 (B856) (TOPIC: 192005 KNOWLEDGE: K1.05 [2.8/3.1])
The total amount of reactivity added by a control rod position change from a reference height to any other rod height is called:
A. differential rod worth.
B. shutdown reactivity.
C. integral rod worth.
D. reference reactivity.
ANSWER: C.
Comment: The question is technically incorrect because it states that "reactivity is added", rather than a change in reactivity is introduced. For a detailed discussion of reactivity and reactivity change see QID:P1246 in section Reactor Theory - Neutron Life Cycle.
In addition, 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: P654 (TOPIC: 192005 KNOWLEDGE: K1.05 [2.8/3.1])
Integral control rod worth is the change in ____________ per ____________ change in rod position.
A. reactor power; total
B. reactivity; unit
C. reactor power; unit
D. reactivity; total
ANSWER: D.
Comment: This question suffers the same defect a P555. Integral rod worth is the reactivity change from fully inserted to fully withdrawn.
QID: P1354 (TOPIC: 192005 KNOWLEDGE: K1.05 [2.8/3.1])
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 point to another point.
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 P555. Reactivity is not inserted. In addition, integral rod worth is the reactivity change from fully inserted to fully withdrawn.
QID: P1471 (TOPIC: 192005 KNOWLEDGE: K1.05 [2.8/3.1])
Reactor power was ramped from 80% power to 100% power over 4 hours. The 80% conditions were as follows:
Reactor coolant system (RCS) boron concentration: 600 ppm
Control rod position: 110 inches
RCS average temperature: 575F
The 100% conditions are as follows:
RCS boron concentration: 580 ppm
Control rod position: 130 inches
RCS average temperature: 580F
Given the following reactivity coefficient/worth values, and neglecting fission product poison reactivity changes, what is the differential control rod worth?
Power coefficient: -0.03% delta-K/K/%
Moderator temperature coefficient: -0.02% delta-K/K/degree-F
Differential boron worth: -0.01% delta-K/K/ppm
A. -0.02% delta-K/K/inch
B. -0.025% delta-K/K/inch
C. -0.04% delta-K/K/inch
D. -0.05% delta-K/K/inch
ANSWER: A.
Comment: The question is technically incorrect because the three reactivity coefficients and the four choices for differential rod worth are indicated to represent reactivity per unit parameter. In addition, as indicated in P555, differential rod worth is positive for control rod "withdrawal." None of the choices is correct.
QID: P2255 (TOPIC: 192005 KNOWLEDGE: K1.05 [2.8/3.1])
The reactor is operating at steady state 70% power with the following conditions:
RCS boron concentration: 600 ppm
Control rod position: 110 inches
RCS average temperature: 575F
Reactor power is increased to 100% over the next four hours. The 100% reactor power conditions are as follows:
RCS boron concentration: 590 ppm
Control rod position: 130 inches
RCS average temperature: 580F
Given the following reactivity coefficient/worth values, and neglecting fission product poison reactivity changes, what is the differential control rod worth?
Power coefficient: -0.3% delta-K/K/%
Moderator temperature coefficient: -0.2% delta-K/K/degree-F
Differential boron worth: -0.1% delta-K/K/ppm
A. 0.2% delta-K/K/inch
B. 0.25% delta-K/K/inch
C. 0.4% delta-K/K/inch
D. 0.5% delta-K/K/inch
ANSWER: C.
Comment: This question suffers the same defect as P1471. Coefficients and choices are incorrectly indicated to represent reactivity per unit and the algebraic sign for differential rod worth does not conform to convention.
QID: P1555 (B1657) (OPIC: 192005 KNOWLEDGE: K1.06 [2.6/2.9])
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 P555. Integral rod worth is the reactivity change from fully inserted to fully withdrawn.
QID: P2156 (TOPIC: 192005 KNOWLEDGE: K1.07 [2.5/2.8])
A reactor is operating at 80% power near the end of a fuel cycle with the controlling group of control rods inserted 5% into the core. Which one of the following will cause group differential rod worth to become less negative? (Consider only the direct effect of the indicated change.)
A. Burnable poison rods become increasingly depleted.
B. Core Xe-135 concentration decreases toward an equilibrium value.
C. Reactor coolant temperature is allowed to decrease from 575F to 570F.
D. Reactor power is decreased to 70% using control rods for control of RCS temperature.
ANSWER: C.
Comment: The question is technically incorrect because there are no gradations of negativity. Negative is negative and positive is positive. The question refers to the magnitude of the differential rod worth becoming "smaller." In addition, per comment in P555 differential rod worth is conventionally expressed as a positive value. It should be stipulated that the rod position does not change during the temperature reduction.
QID: P2254 (TOPIC: 192005 KNOWLEDGE: K1.07 [2.5/2.8])
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. RCS boron concentration increases by 5 ppm at 80% power with no rod motion.
C. Reactor power is decreased from 100% to 90% with no rod motion.
D. Early in core life, the concentration of burnable poison decreases.
ANSWER: B.
Comment: The question suffers the same defect as P2156. Negative is negative. In addition, the question is technically incorrect because "control rod worth" is undefined terminology.
QID: P2356 (B2356) (TOPIC: 192005 KNOWLEDGE: K1.07 [2.5/2.8])
A reactor startup is in progress from a cold shutdown condition. During the RCS heatup phase of the startup, control rod differential reactivity worth (delta-K/K per inch insertion) becomes _______ negative, and during the complete withdrawal of the initial bank of control rods, control rod differential reactivity worth becomes _______.
A. more; more negative and then less negative
B. more; less negative and then more negative
C. less; more negative during the entire withdrawal
D. less; less negative during the entire withdrawal
ANSWER: A.
Comment: The question suffers the same defect as P2156. Negative is negative. In addition, differential rod worth is incorrectly indicated to represent "reactivity" per inch and, per comment in P555, differential rod worth is conventionally expressed as a positive value. The question is confusing because during heatup the rods are apparently at a fixed position and during "complete withdrawal" the rods are moving.
QID: P2655 (TOPIC: 192005 KNOWLEDGE: K1.07 [2.5/2.8])
Which one of the following will cause group differential control rod worth to become less negative? (Assume the affected group of control rods remains 10% inserted for each case.)
A. During long-term full power operation, fuel temperature decreases as the fuel pellets come into contact with the fuel clad.
B. The reactor coolant system is cooled from 170 F to 120 F in preparation for a core refueling.
C. Core Xe-135 builds up in the lower half of the core.
D. Early in core life, the concentration of burnable poison decreases.
ANSWER: B.
Comment: The question suffers the same defect as P2156. Negative is negative. In addition, per comment in P555, differential rod worth is conventionally expressed as a positive value.
QID: P2854 (TOPIC: 192005 KNOWLEDGE: K1.07 [2.5/2.8])
The reactor is operating at 85% power with the controlling group of control rods inserted 10%. Which one of the following will cause group differential control rod worth to become more negative? (Assume reactor power and control rod position remain constant for each case.)
A. Fuel temperature increases as fission product gasses accumulate in a fuel rod.
B. RCS average temperature drifts from 580 F to 575 F.
C. Core Xe-135 builds up in the lower half of the core.
D. RCS boron concentration is increased by 5 ppm .
ANSWER: C.
Comment: The question suffers the same defect as P2156. Negative is negative. In addition, per comment in P555, differential rod worth is conventionally expressed as a positive value.
QID: P556 (TOPIC: 192005 KNOWLEDGE: K1.08 [2.7/2.9])
A plant is operating at 80% power with manual rod control. It has been determined that power distribution is excessive in the lower half of the core. Which one of the following will shift power distribution toward the upper half of the core? (Assume no additional operator actions.)
A. Reducing power to 40%
B. Withdrawing control rods
C. Borating the reactor coolant system
D. Diluting the reactor coolant system
ANSWER: B.
Comment: The question is technically incorrect because it does not indicate what is to compensate the reactivity change caused by rod withdrawal, or what core parameter is allowed to change. In addition, "withdrawing control rods" is too vague. How far are rods moved out? The initial and final position of the control rods should be specified.
QID: P2056 (B2054) (TOPIC: 192005 KNOWLEDGE: K1.08 [2.7/2.9])
Neutron flux shaping in a reactor core reduces radial power peaking:
A. in the center of the core caused by the 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 uneven burnout of fuel assemblies.
ANSWER: A.
Comment: This question is technically incorrect because of creative terminology. What is a "high number density of fuel assemblies" supposed to mean?
QID: P455 (TOPIC: 192005 KNOWLEDGE: K1.10 [3.0/3.3])
Which one of the following describes why most of the power is produced in the lower half of a reactor core that has been operating at 100% power for several weeks with all control rods withdrawn at the beginning of core life?
A. Xenon concentration is lower in the lower half of the core.
B. The moderator to fuel ratio is lower in the lower half of the core.
C. The fuel loading in the lower half of the core contains a higher U-235 enrichment.
D. The moderator temperature coefficient of reactivity is adding less negative reactivity in the lower half of the core.
ANSWER: D.
Comment: This question suffers the same defect as P555. Reactivity is not added. In addition, rods should be specified as "fully" withdrawn.
QID: P2656 (TOPIC: 192005 KNOWLEDGE: K1.10 [3.0/3.3])
A reactor has been operating at 100% power for 3 weeks, with all control rods fully withdrawn, shortly after a refueling outage. Which one of the following describes why most of the power is being produced in the lower half of the core?
A. The fuel loading in the lower half of the core contains a higher U-235 enrichment.
B. Reactor coolant boron is adding more negative reactivity in the upper half of the core.
C. There is a greater concentration of Xe-135 in the upper half of the core.
D. The moderator temperature coefficient of reactivity is adding more negative reactivity in the upper half of the core.
ANSWER: D.
Comment: This question suffers the same defect as P555. Reactivity is not added.