Question 64 : Explain how, why, and over what time frame xenon 135 concentration changes following a scram from equilibrium conditions.
Answer: When the reactor scrams, production of xenon directly from fission and production of iodine effectively cease. However, xenon burnout also immediately ceases, and decay of the existing iodine causes xenon concentration to rise. After approximately eight to 10 hours, production of xenon from iodine decay is overcome by decay of xenon, and the xenon concentration begins to decrease. After approximately 80 hours, effectively all xenon will have decayed. (Reference 77, pages 4-21 and 4-22)
Comment: The question is technically incorrect because the initial conditions are not specified, which determine the answer to the question. Restate the question as follows:
Question 64: (revised) After 3 months of operation at 100% power, the reactor scrams. Explain how, why, and over what time frame xenon-135 concentration changes following this scram.
Question 71: A power reactor has been operating at full power for over 10 days when a scram occurs. Which of the following describes the behavior of xenon following the reactor scram?
a. A typical reactor cannot override peak xenon early in core life.
b. The core will have approximately 0% Delta-K/K from xenon approximately 41 hours after the scram.
c. The time for xenon to peak is approximately 17 to 21 hours after the scram.
d. The peak xenon reactivity worth is about -5% Delta-K/K in a typical reactor.*
Comment: The asterisk indicates the intended correct answer is choice "d". The question is technically incorrect because reactivity worth is represented as delta-k/k rather than delta-rho. Restate the question as follows:
Question 71: (revise) A power reactor has been operating at full power for over 10 days when a scram occurs. Which of the following describes the behavior of xenon following the reactor scram?
a. A typical reactor cannot override peak xenon early in core life.
b. The core will have approximately 0% Delta-K/K from xenon approximately 41 hours after the scram.
c. The time for xenon to peak is approximately 17 to 21 hours after the scram.
d. The negative reactivity change introduced by peak xenon is about -5% Delta-rho in a typical BWR reactor.*
Question 74: Compare control rod worths during a reactor startup from 100% peak xenon and a reactor startup from xenon-free conditions.
a. Center control rod worth will be higher during the peak xenon startup than during the xenon-free startup.
b. Peripheral control rod worth will be higher during the peak xenon startup than during the xenon-free startup.*
c. Both control rod worths will be the same regardless of core xenon conditions.
d. It is impossible to determine how xenon will affect the worth of center and peripheral control rods.
Comment: The asterisk indicates the intended correct answer is choice "b". The answer is technically incorrect because "control rod worth" is ambiguous terminology. Is the "worth" referred to the differential rod worth or the integral rod worth? From the information provided, it is not possible to restate the question.
Question 76: A reactor is scrammed from 100% power and equilibrium xenon concentration. Later, when xenon concentration has just peaked, the reactor is pulled critical. Which of the following statements is true?
a. To maintain criticality, the control rods will have to be rapidly inserted as xenon is burned out.
b. The reactivity added by the peaked xenon concentration results in more control rods having been withdrawn to achieve criticality.*
c. The control rod pattern to achieve criticality would be the same as that in a xenon-free reactor.
d. As criticality is achieved, xenon concentration will increase rapidly resulting in control rods having to be withdrawn to maintain criticality
Comment: The asterisk indicates the intended correct answer is choice "b". The question is technically incorrect because of poor terminology in the question and in choice c. Restate the question as follows:
Question 76: (revised) A reactor is scrammed from 100% power and equilibrium xenon concentration. Later, when xenon concentration has just peaked, a reactor startup attains criticality. Which of the following statements is true?
a. To maintain criticality, the control rods will have to be rapidly inserted as xenon is burned out.
b. The negative reactivity change introduced by the peak xenon concentration results in more control rods having been withdrawn to achieve criticality.*
c. The control rod pattern to achieve criticality would be the same as that in a xenon-free reactor.
d. As criticality is achieved, xenon concentration will increase rapidly resulting in control rods having to be withdrawn to maintain criticality
Question 78: A step change in reactor power from 100% power and equilibrium xenon concentration to 50% power is made using core flow. Control rod movement is then used to maintain reactor power at 50% as xenon concentration changes. Which of the following best describes control rod position five hours after the power change? Control rods will
a. have been withdrawn to overcome the xenon concentration increase*
b. have been inserted to overcome the xenon concentration decrease
c. not have been moved since xenon concentration has not changed
d. have been inserted then withdrawn to about the same position as xenon concentration varies
Comment: The asterisk indicates the intended correct answer is choice "a". The question is technically incorrect because of inaccurate terminology. Reactivity may be changed by a step function but reactor power responds by a prompt drop. Since this is not a scram, "rapid reduction" would by satisfactory for this question. As usual with "best", there is only one correct answer to this question. Restate the question as follows:
Question 78: (revised) A rapid reduction in reactor power from 100% power and equilibrium xenon concentration to 50% power is made using core flow. Control rod movement is then used to maintain reactor power at 50% as xenon concentration changes. Which of the following describes control rod position five hours after the power change? Control rods will
a. have been withdrawn sufficiently to overcome the xenon concentration increase*
b. have been inserted sufficiently to overcome the xenon concentration decrease
c. not have been moved since xenon concentration has not changed
d. have been inserted then withdrawn to about the same position as xenon concentration varies
Question 82: Following a reactor startup from a long-term shutdown, the plant is taken to 100% power. After reaching 100% power, what kind of reactivity, if any, will the operator need to add to compensate for xenon buildup (first 10 to 20 hours)?
a. positive*
b. negative
c. none, power is already at maximum
d. none, xenon is not flux-dependent
Comment: The asterisk indicates the intended correct answer is choice "a". The question is technically incorrect because of incorrect terminology. What kind of reactivity will the operator need to add? What "kinds" of reactivity are there? Besides, "add" and "xenon buildup" give the answer away. Restate the question as follows:
Question 82: (revised) Following a reactor startup from a long-term shutdown, the plant is taken to 100% power. After reaching 100% power, the operator must compensate for xenon change over the first 20 hours by
a. introducing positive reactivity change*
b. introducing negative reactivity change
c. introducing no reactivity change, power is already at maximum
d. introducing no reactivity change, xenon is not flux-dependent
Question 84: Which one of the following statements best describes xenon buildup following a xenon-free reactor startup?
a. Xenon concentration will remain constant until the thermal neutron flux reaches 1 x 1010 neutrons/cm2/sec.
b. Xenon concentration will immediately increase as the moderator temperature coefficient becomes positive.
c. Xenon concentration will remain constant until the iodine decay develops.
d. Xenon concentration will immediately increase due to fission yield production.*
Comment: The asterisk indicates the intended correct answer is choice "d". The question is flawed because of garbled wording in the correct choice d. By now you know where "best" belongs. Restate the question as follows:
Question 84: (revised) Which one of the following statements describes xenon buildup following a xenon-free reactor startup?
a. Xenon concentration will remain constant until the thermal neutron flux reaches 1 x 1010 neutrons/cm2/sec.
b. Xenon concentration will immediately increase as the moderator temperature coefficient becomes positive.
c. Xenon concentration will remain constant until the iodine decay develops.
d. Xenon concentration will increase immediately due to direct yield of Xenon-135 from fission.*
Question 85: Following a 7 day shutdown, a reactor startup is performed and the plant is taken to 100% power over a 16-hour period. After reaching 100% power, what type of reactivity will the operator need to add to compensate for xenon changes over the next 24 hours?
a. negative only
b. negative, then positive
c. positive, then negative
d. positive only*
Comment: The asterisk indicates the intended correct answer is choice "d". The question is technically incorrect for the same reason given for question #82, except now it's "types" of reactivity instead of "kinds" of reactivity. Restate the question as follows:
Question 85: (revised) Following a 7 day shutdown, a reactor startup is performed and the plant is taken to 100% power over a 16-hour period. After reaching 100% power, the operator needs compensate for xenon changes over the next 24 hours by
a. introducing negative reactivity change only
b. introducing negative reactivity change followed by introducing positive reactivity change
c. introducing positive reactivity change followed by introducing negative reactivity change
d. introducing positive reactivity change only*
Question 93:A reactor that has been operating at 100% power for about two weeks has power reduced to 50%. During the next four to six hours, what must the operator do to compensate for a change in xenon 135?
a. The operator does not have to be concerned with xenon, only fuel depletion.
b. The operator must add negative reactivity because xenon is rapidly decaying away.
c. The operator must add positive reactivity to compensate for xenon building in.*
d. The operator must add negative reactivity because xenon is rapidly building in.
Comment: The asterisk indicates the intended correct answer is choice "c". The question is technically incorrect because of erroneous terminology. The operator does not add "reactivity"; he creates a reactivity change by some action. Restate the question as follows:
Question 93: (revised) A reactor that has been operating at 100% power for about two weeks has power reduced to 50%. During the next four to six hours, what action must the operator take to compensate for change in xenon-135?
a. The operator does not have to be concerned with xenon, only fuel depletion.
b. The operator must add negative reactivity because xenon is rapidly decaying away.
c. The operator must introduce a positive reactivity change to compensate for xenon buildup.*
d. The operator must add negative reactivity because xenon is rapidly building in.
Question 95: A reactor has been operating at 50% power for about two weeks. Power is then ramped to 100% power in four hours. What must the operator do during the next two to three hours, after reaching 100% power, to compensate for a change in xenon 135?
a. Add negative reactivity because xenon is burning out.*
b. Add positive reactivity because xenon is building in.
c. Do nothing due to constant iodine concentration.
d. Add positive reactivity because xenon production exceeds xenon decay.
Comment: The asterisk indicates the intended correct answer is choice "a". The question is technically incorrect for the same reason given for question #93. Restate the question as follows.
Question 95: (revised) A reactor has been operating at 50% power for about two weeks. Power is then ramped to 100% power in four hours. What action must the operator take during the next two to three hours, after reaching 100% power, to compensate for a change in xenon-135?
a. Introduce negative reactivity change to counter increased xenon burn out.*
b. Introduce positive reactivity change to counter increased xenon build up.
c. Do nothing due to constant iodine concentration.
d. Add positive reactivity because xenon production exceeds xenon decay.
Question 99: Which of the following describes the behavior of xenon following a reactor scram?
a. The core will be virtually xenon-free three days following the scram.*
b. The peak xenon will be approximately 15 hours following the scram.
c. The gradual decrease in xenon concentration, after peak xenon, is due to xenon having a shorter half-life than iodine.
d. The xenon peak is caused by the elimination of xenon conversion to cesium.
Comment: The asterisk indicates the intended correct answer is choice "a". The question is technically incorrect because the initial conditions are not defined. If the reactor is xenon-free before the scram, then it is also xenon-free 3 seconds after scram. Restate the question as follows:
Question 99: (revised) Which of the following describes the behavior of xenon following a reactor scram after extended operation at 100% power?
a. The core will be virtually xenon-free three days following the scram.*
b. The peak xenon will be approximately 15 hours following the scram.
c. The gradual decrease in xenon concentration, after peak xenon, is due to xenon having a shorter half-life than iodine.
d. The xenon peak is caused by the elimination of xenon conversion to cesium.
Question 100: A reactor has been operating at full power for several days when a trip occurs. Xenon concentration is expected to ______ because ______.
a. decrease; it is flux-dependent
b. increase rapidly; the burnout term has decreased significantly*
c. not change; xenon is not flux-dependent
d. decrease slowly; xenon now is only removed by decay
Comment: The asterisk indicates the intended correct answer is choice "b". The question is technically incorrect because the correct answer, choice c, is not complete. Restate the question as follows:
Question 100: (revised) A reactor has been operating at full power for several days when a trip occurs. Xenon concentration is expected to ______ because ______.
a. decrease; xenon is flux-dependent
b. increase rapidly because Xenon loss from burnup decreases significantly, while production by Iodine decay decreases very slowly*
c. not change; xenon is not flux-dependent
d. decrease slowly because xenon now is only removed by decay while production from Iodine decay remains relatively constant
Question 105: One reason that the xenon concentration initially increases when a reactor is shut down is that xenon
a. is being produced from the decay of iodine 135*
b. is being produced from the spontaneous fission of uranium
c. decay has dropped off significantly with flux
d. is no longer being removed by ion exchange
Comment: The asterisk indicates the intended correct answer is choice "a". The question is technically incorrect for the same reason given for question #100 and because the initial conditions are not sufficiently defined. There isn't "one reason". Xenon is also being produced from Iodine before the shutdown. Restate the question as follows:
Question 105: (revised) The xenon concentration initially increases when a reactor is shut down from power operation because
a. xenon production from Iodine decay decreases slowly while xenon loss from burnup significantly reduced at shutdown*
b. xenon is being produced from the spontaneous fission of uranium
c. xenon decay has dropped off significantly with flux
d. xenon is no longer being removed by ion exchange
Question 109: Four hours after a reactor scram from full power, equilibrium xenon condition, the xenon concentration will be
a. lower than 100% equilibrium xenon, and will have added negative reactivity since the scram
b. higher than 100% equilibrium xenon, and will have added negative reactivity since the scram*
c. lower than 100% equilibrium xenon, and will have added positive reactivity since the scram
d. higher than 100% equilibrium xenon and will have added positive reactivity since the scram
Comment: The asterisk indicates the intended correct answer is choice "b". The question is technically incorrect because of erroneous terminology. Xenon will not have added negative reactivity, it will introduce a negative change in reactivity. Restate the question as follows:
Question 109: (revised) Four hours after a reactor scram from full power, equilibrium xenon condition, the xenon concentration will be
a. lower than 100% equilibrium xenon, and will have introduced negative reactivity change since the scram
b. higher than 100% equilibrium xenon, and will have introduced negative reactivity change since the scram*
c. lower than 100% equilibrium xenon, and will have introduced positive reactivity change since the scram
d. higher than 100% equilibrium xenon and will have introduced positive reactivity change since the scram
Question 111: The reactor is initially at 25% power with equilibrium xenon concentration. A step change of reactor power to 75% is made. Considering the effect on reactor power due to xenon concentration changes seven hours after the step change, and assuming no operator actions, which of the following statements is true? Reactor power will be
a. greater than 75% and decreasing slowly*
b. greater than 75% and increasing slowly
c. lower than 75% and decreasing slowly
d. lower than 75% and increasing slowly
Comment: This question is inappropriate because it is about power behavior and not fission product poisons. The question does not present a realistic operational situation and should be deleted.
Question 113: Initially the reactor is at 100% power with equilibrium xenon conditions when a reactor scram occurs. Four hours later, the reactor is brought critical. Which of the following statements is true concerning xenon impact on reactor control?
a. High xenon concentration at the periphery of the core will cause periphery rods to exhibit high-worth characteristics.
b. Peak thermal flux at the periphery of the core will cause periphery rods to exhibit high-worth characteristics.*
c. Peak thermal flux at the center of the core will cause center rods to exhibit high-worth characteristics.
d. Low xenon concentration at the center of the core will cause center control rods to exhibit high-worth characteristics.
Comment: The asterisk indicates the intended correct answer is choice "b". The question is technically incorrect because of ambiguous terminology. What does "exhibit high-worth characteristics" mean? Without additional information the question cannot be restated.
Question 114: A reactor has been operating at full power for 10 weeks when a scram occurs. Twenty-four hours later, the reactor is brought critical and power level is maintained on range 5 of the intermediate range monitors. To maintain a constant power level for the next several hours, control rods must be
a. inserted, because the critical reactor will cause a high rate of xenon burnout
b. maintained at the present height as xenon establishes its equilibrium value for this power level
c. inserted, because xenon will approximately follow its normal decay curve*
d. withdrawn, because xenon concentration is increasing toward equilibrium
Comment: The asterisk indicates the intended correct answer is choice "c". The question is technically incorrect because of poor wording. In the correct choice c, the rods are "inserted" to where? ... and what does "approximately follow its normal decay curve" mean? Restate the question as follows:
Question 114: (revised)
A reactor has been operating at full power for 10 weeks when a scram occurs. Twenty-four hours later, the reactor is brought critical and power level is maintained on range 5 of the intermediate range monitors. To maintain a constant power level for the next several hours, control rods must be
a. inserted a few notches periodically, because the critical reactor will cause a high rate of xenon burnout
b. maintained at the present height as xenon establishes its equilibrium value for this power level
c. inserted a few notches periodically, because xenon will be decaying from a peak condition*
d. withdrawn a few notches periodically, because xenon concentration is increasing toward equilibrium
Question 115: A reactor has been shut down for two weeks after six months of full power operation. A reactor startup is performed and reactor power is stabilized at 10%. What control rod movement is required to maintain 10% stable power over the next two hours?
a. small amounts of rod insertion to compensate for samarium depletion
b. small amounts of rod withdrawal to compensate for samarium buildup
c. small amounts of rod insertion to compensate for xenon burnout
d. small amounts of rod withdrawal to compensate for xenon buildup*
Comment: The asterisk indicates the intended correct answer is choice "d". The question is technically incorrect because of poor terminology and wording. Again we have "stable" power. And what are "small amounts" of rod withdrawal? Restate the question as follows:
Question 115: (revised) A reactor has been shut down for two weeks after six months of full power operation. A reactor startup is performed and reactor power is raised to 10%. What control rod movement is required to maintain 10% power over the next two hours?
a. intermittent insertion of a few notches to compensate for samarium depletion
b. intermittent withdrawal of a few notches to compensate for samarium buildup
c. intermittent insertion of a few notches to compensate for xenon burnout
d. intermittent withdrawal of a few notches to compensate for xenon buildup*
Question 116: A reactor that has been operating at 100% power for about two weeks has power reduced to 50% in one hour. To compensate for the change in xenon-135 during the next four hours, the operator must add
a. negative reactivity to compensate for xenon building in
b. negative reactivity because xenon is rapidly decaying away
c. positive reactivity to compensate for xenon building in*
d. positive reactivity because xenon is rapidly decaying away
Comment: The asterisk indicates the intended correct answer is choice "c". The question is technically incorrect because of poor terminology. The operator does not "add positive reactivity", rather the operator introduces a positive reactivity change. There is double and unnecessary use of "compensate" in the question and in the choices. Restate the question as follows:
Question 116: (revised) A reactor that has been operating at 100% power for about two weeks has power reduced to 50% in one hour. To compensate for the change in xenon-135 during the next four hours, the operator must introduce
a. a negative reactivity change because of xenon build up
b. a negative reactivity change because xenon is rapidly decaying
c. a positive reactivity change because of xenon build up*
d. a positive reactivity change because xenon is rapidly decaying
Question 117: A correct statement regarding control-rod worth during a reactor startup under xenon-free conditions is that
a. central control-rod worth will be higher during the peak xenon startup than during the xenon-free startup
b. peripheral control-rod worth will be higher during the peak xenon startup than during the xenon-free startup*
c. peripheral control-rod worth will be lower during peak xenon startup than during the 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 because of poor terminology. Choice c is unnecessarily convoluted ... making it easy to misread. What does "Control-rod worth" mean? With the information provided, the question cannot be restated.
Question 118: Four hours after a reactor trip from a long-term, steady-state, 100% power run, the reactor has been taken critical and is to be maintained at 1% to 2% power. Which one of the following operator actions is required?
a. Add positive reactivity because xenon is building in.*
b. Add negative reactivity because xenon is building in.
c. Add negative reactivity because xenon is decaying away.
d. Add positive reactivity because xenon is decaying away.
Comment: The asterisk indicates the intended correct answer is choice a. The question is technically incorrect because of erroneous terminology. The operator does not "add positive reactivity", rather the operator introduces positive change in reactivity. Restate the question as follows:
Question 118: (revised) Four hours after a reactor trip from a long-term, steady-state, 100% power run, the reactor has been taken critical and is to be maintained at 1% to 2% power. Which one of the following operator actions is required?
a. intermittently introduce positive reactivity change to compensate for xenon build up.*
b. intermittently introduce negative reactivity change to compensate for xenon build up.
c. intermittently introduce negative reactivity change to compensate for xenon decay.
d. intermittently introduce positive reactivity change to compensate for xenon decaying.
Question 123: A reactor has been operating at 100% power for two months when a reactor scram occurs. Four hours later, the reactor is critical and stable at 10% power.
Which one of the following operator actions is required to maintain reactor power at 10% over the next 24 hours?
a. Add positive reactivity during the entire period.
b. Add negative reactivity during the entire period.
c. Add positive reactivity, then negative reactivity.*
d. Add negative reactivity, then positive reactivity.
Comment: The asterisk indicates the intended correct answer is choice c. The question is technically incorrect because of erroneous terminology. The operator does not "add positive reactivity", rather the operator introduces positive change in reactivity. "Stable" power is incorrect. Restate the question as follows:
Question 123: (revised) A reactor has been operating at 100% power for two months when a reactor scram occurs. Four hours later, the reactor is critical at 10% power.
Which one of the following operator actions is required to maintain reactor power at 10% over the next 24 hours?
a. intermittently introduce positive reactivity change during the entire period.
b. intermittently introduce negative reactivity change during the entire period.
c. intermittently introduce positive reactivity change as xenon builds to a peak, and then introduce negative reactivity change as xenon decays from the peak.*
d. intermittently introduce negative reactivity change as xenon builds to a peak, and then introduce positive reactivity change as xenon decay from the peak.