In this Nukefact we address the questions under the subsection Fission Product Poisons. Of the one hundred twenty-seven questions in the INPO Catalog pertaining to the Fission Product Poisons we find thirty-seven to be technically incorrect, that's 34% wrong folks. Nine of the erroneous questions deal with the definition of, the properties of, and the characteristics of fission product poisons. Twenty-six of the erroneous questions deal with Xenon transients but contain flawed terminology, as follows:
9 - general poor wording and sloppy terminology
8 - misuse of reactivity and reactivity change
2 - lack of definition of control rod "worth"
7 - failure to fully define initial and/or final conditions
This subsection is deficient in that there are no questions on the transient poison Samarium or on stable fission product buildup.
a. fissile materials
b. fission product poisons*
c. fissionable nuclides
d. burnable poisons
Comment: The asterisk indicates the intended correct answer is choice "b". The question is grammatically incorrect in using "or" where "are" is intended. The question is technically incorrect because it implies that some fission products are fissionable. There are no fission products that are fissionable by reactor neutrons. Restate the question and choices as follows:
Question 1: (revised) Fission fragments are daughters that have a substantial neutron absorption cross section are called
a. fissile materials
b. fission product poisons*
c. fissionable nuclides
d. burnable poisons
Question 2: A fission product poison is defined as a
a. neutron poison that is loaded into the core during fabrication to control power
b. fission product that absorbs a neutron and fissions
c. fission product that has a substantial neutron absorption cross section and does not fission*
d. fission product that emits a neutron sometime after the initial fission event
Comment: The asterisk indicates the intended correct answer is choice "c". The question is technically incorrect for the same reason given in question #1. Restate the question as follows:
Question 2: (revised) A fission product poison is defined as a
a. neutron poison that is loaded into the core during fabrication to control power
b. fission product that absorbs a neutron and fissions
c. fission product that has a substantial neutron absorption cross section*
d. fission product that emits a neutron sometime after the initial fission event
Question 3: Which of the following best defines the term "fission product poison?"
a. fission fragment or daughter that absorbs neutrons and does not fission*
b. fission fragment or daughter that absorbs neutrons and fissions
c. fission fragment or daughter that emits neutrons and does not fission
d. fission fragment or daughter that emits neutrons and fissions
Comment: The asterisk indicates the intended correct answer is choice "a". In addition to stating "best defines" where there is only one correct answer, the question is technically incorrect for the reason given in question #1. Restate the question as follows:
Question 3: (revised) Which of the following defines the term "fission product poison?"
a. fission fragment or daughter that absorbs neutrons*
b. fission fragment or daughter that absorbs neutrons and fissions
c. fission fragment or daughter that emits neutrons and does not fission
d. fission fragment or daughter that emits neutrons and fissions
Question 9: What are the substances in the correct order, from largest to smallest, of microscopic cross section (thermal neutrons) for capture?
a. U-235, H2O, Xe-135
b. U-235, Xe-135, H2O
c. Xe-135, U-235, H2O*
d. Xe-135, H2O, U-235
Comment: The asterisk indicates the intended correct answer is choice "c". The question is technically incorrect because H2O has no "microscopic" cross section. The wording is also atrocious. Restate the question as follows:
Question 9: (revised) Which sequence orders the nuclides from largest to smallest microscopic capture cross section for thermal neutrons?
a. U-235, U-238, Xe-135
b. U-235, Xe-135, U-238
c. Xe-135, U-235, U-238*
d. Xe-135, U-238, U-235
Question 16: Xenon 135 is considered a major fission product poison because it has a large
a. fission cross section
b. capture cross section*
c. elastic scatter cross section
d. inelastic scatter cross section
Comment: The asterisk indicates the intended correct answer is choice "b". The question is technically incorrect because it indicates that there is "one" reason, which is inconsistent with questions #10, #11, #12, and #17 which give two reasons, the second reason being large indirect-yield. Restate the question as follows:
Question 16: (revised) One reason that Xenon 135 is considered a major fission product poison is because it has a large
a. fission cross section
b. capture cross section*
c. elastic scatter cross section
d. inelastic scatter cross section
Question 17: State and explain two characteristics that make xenon 135 a significant fission product poison.
Answer: Xenon 135 is a significant fission product poison because of its large microscopic capture cross section and its large and variable abundance.
Because of its large capture cross section, an increase in xenon 135 concentration removes additional neutrons from the fission chain reaction, inserting negative reactivity, which the designer and operator must contend with.
Because of its abundance, its macroscopic capture cross section can be large. Furthermore, xenon concentration is power-dependent, so that every change in reactor power produces a transient in which xenon concentration, and therefore reactivity, is changed. (Reference 77, pages 4-11 and 4-24)
Comment: The answer is technically incorrect because it confuses cause (characteristic) and effect (abundance). The "large ... abundance" is a result of a Xenon 135 characteristic, not a characteristic in and of itself. The characteristic that creates a large abundance is the large indirect yield from fission. Furthermore, the Xenon does not "insert negative reactivity", it introduces a negative change in reactivity. Restate the question as follows:
Answer: (revised) Xenon 135 is a significant fission product poison because of its large microscopic capture cross section and its large indirect yield from fission.
Because of its large capture cross section, an increase in xenon 135 concentration removes additional neutrons from the fission chain reaction, inserting a negative change in reactivity, which the designer and operator must contend with.
Because of its high indirect yield, its macroscopic capture cross section can be large. Furthermore, xenon concentration is power-dependent, so that every change in reactor power produces a transient in which xenon concentration, and therefore reactivity, is changed.
Question 21: Following a reactor trip, xenon 135 concentration in the reactor will
a. initially decrease because xenon is produced directly from fission
b. initially increase due to the decay of iodine already in the core*
c. remain the same because the decay of iodine and xenon balance each other
d. decrease immediately, then slowly increase due to the differences in the half lives of iodine and xenon
Comment: The asterisk indicates the intended correct answer is choice "b". The question is technically incorrect because the initial conditions before scram are not defined and because choice b does not explain the Xenon increase. Restate the question as follows:
Question 21: (revised) Following a reactor trip from extended operation at 100% power, xenon 135 concentration in the reactor will
a. initially decrease because xenon is produced directly from fission
b. initially increase because xenon production by decay of iodine exceeds loss by xenon decay*
c. remain the same because the decay of iodine and xenon balance each other
d. decrease immediately, then slowly increase due to the differences in the half lives of iodine and xenon
Question 23: Which of the following pairs presents the two major methods for Xe-135 production in the core?
a. decay of fission products and activation of U-233
b. decay of Sm-149 and activation of oxygen
c. decay of iodine and fission*
d. decay of iodine and activation of oxygen
Comment: The asterisk indicates the intended correct answer is choice "c". The question is flawed because of a poor wording. Why "two major" ... are there other methods? Restate the question as follows:
Question 23: (revised) Which of the following represents the means for Xe-135 production in the core?
a. decay of fission products and activation of U-233
b. decay of Sm-149 and activation of oxygen
c. decay of iodine and direct fission yield*
d. decay of iodine and activation of oxygen
Question 25: The reactor has been shut down for two weeks following extended power operation. What control rod movement is required to maintain 10% stable power immediately after startup?
a. small amounts of rod insertion to compensate for LPRM chamber depletion
b. small amounts of rod withdrawal to compensate for samarium buildup
c. small amounts of rod insertion to compensate for installed poison 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 the use of "stable" power. The reactor period is stable for exponential power change. Restate the question as follows:
Question 25: (revised) The reactor has been shut down for two weeks following extended power operation. What control rod movement is required to maintain 10% power immediately after startup?
a. occasional intermittent rod insertion to compensate for LPRM chamber depletion
b. continuous rod withdrawal to compensate for samarium buildup
c. continuous rod rod insertion to compensate for installed poison burnout
d. occasional intermittent withdrawal to compensate for xenon buildup*
Question 29: A reactor has been operating at full power for several weeks. Xenon-135 is being produced as a fission product in approximately __________% of all fissions.
a. 0.3*
b. 3.0
c. 30
d. 100
Comment: The asterisk indicates the intended correct answer is choice "a". The question is flawed because it is ambiguous. Restate the question as follows:
Question 29: (revised) A reactor has been operating at full power for several weeks. Xenon-135 is being produced as a direct fission product in approximately __________% of all fissions.
a. 0.3*
b. 3.0
c. 30
d. 100
Question 33: Describe the xenon 135 removal mechanism in an operating reactor.
Answer: Xenon 135 is removed by two mechanisms in an operating reactor. With its half-life of 9.1 hours, xenon decays by beta- emission to cesium 135. In addition, because of its large microscopic capture cross-section, xenon 135 is "burned out" by the neutron flux in an operating reactor. Through neutron capture, xenon 135 is transformed to xenon 136, whose capture cross section is negligible. (Reference 77, page 4-12)
Comment: The question is flawed because it asks for the "removal mechanism" (singular) while the correct answer requires two mechanisms of removal. Restate the question as follows:
Question 33: (revised) Describe the xenon-135 removal mechanism(s) in an operating reactor.
Question 39: Which one of the following is the best explanation for the xenon-135 concentration change immediately following a power increase from equilibrium conditions?
a. Xenon concentration will initially decrease due to the Doppler coefficient increase, thus increasing the thermal utilization.
b. Xenon concentration will initially decrease due to the increased absorption of thermal neutrons by xenon-135.*
c. Xenon concentration will initially increase due to the large increase in production from fission.
d. Xenon concentration will initially increase due to the decrease in thermal utilization as core flow increases.
Comment: The asterisk indicates the intended correct answer is choice "b". The answer is technically incorrect because of the poor use of terminology. What is "a power increase from equilibrium conditions" supposed to mean ... equilibrium power ... equilibrium Xenon ... or equilibrium everything? Equilibrium can be applied to power as in subcritical multiplication and to Xenon as for steady-state Xenon. Whatever happened to "stable" power? In addition the initial conditions are inadequately defined. What "best" explanation"? ... there is only one correct choice. And, choice c is an incomplete explanation. Restate the question as follows:
Question 39: (revised) Which one of the following explains xenon-135 concentration change immediately following a power increase to 67% power after extended operation at 34% power.
a. Xenon concentration will initially decrease due to the Doppler coefficient increase, thus increasing the thermal utilization.
b. Xenon concentration will initially decrease due to the increased burnup loss while production increase from Iodine-135 decay occurs relatively slowly.*
c. Xenon concentration will initially increase due to the large increase in production from fission, while Xenon-135 losses remain relatively constant.
d. Xenon concentration will initially increase due to the decrease in thermal utilization as core flow increases.
Question 51: Which one of the following statements best explains why xenon peaks after a scram?
a. The existing iodine-135 decays to xenon-135, and the iodine at shutdown has a greater activity than xenon.*
b. The existing xenon-135 decays to iodine-135, and iodine at shutdown has a smaller activity than xenon.
c. Iodine-135 production increases after a scram, thereby increasing the concentration of xenon.
d. Xenon-135 increases due to an increase in the number of delayed neutrons after a scram.
Comment: The asterisk indicates the intended correct answer is choice "a". The question is technically incorrect because none of the choices explain why Xenon peaks. Choice a explains (rather poorly) why Xenon concentration increases after scram. However, the initial conditions before scram are not defined ... the core could be in a Xenon free condition. Restate the question as follows:
Question 51: (revised) If a reactor scram occurs after extended operation at 100% power, which one of the following explains why xenon concentration increases after a scram?
a. The existing iodine-135 continues to decay to xenon-135 with greater activity than the xenon decay and xenon-135 losses by burnup terminate.*
b. The existing xenon-135 decays to iodine-135, and iodine at shutdown has a smaller activity than xenon.
c. Iodine-135 production increases after a scram, thereby increasing the concentration of xenon.
d. Xenon-135 increases due to an increase in the number of delayed neutrons after a scram.
Question 53: Which one of the following statements best explains why xenon concentration will increase after a power decrease?
a. The migration length changes as moderator density increases.
b. Fewer thermal neutrons are available to burn out xenon.*
c. The amount of xenon from fission does not decrease until the reactor is on a positive period.
d. The amount of Xe-135 produced from Sm-149 decreases for six hours after power changes.
Comment: The asterisk indicates the intended correct answer is choice "b". The question is technically incorrect because the initial conditions are not defined and because choice b is only a partial explantation. Restate the question as follows:
Question 53: (revised) Which one of the following statements explains why xenon concentration increases after a power reduction from 80% to 35% power?
a. The migration length changes as moderator density increases.
b. Xenon loss is immediately reduced as fewer thermal neutrons are available to burn out xenon, while Xenon production from Iodine decay changes only very slowly.*
c. The amount of xenon from fission does not decrease until the reactor is on a positive period.
d. The amount of Xe-135 produced from Sm-149 decreases for six hours after power changes.
Question 54: Explain how, why, and over what time frames xenon 135 concentration changes when reactor power is increased. Assume equilibrium initial conditions.
Answer: When power is increased, an immediate increase in xenon burnout takes place, causing xenon concentration to decrease. The power increase also causes a rapid increase in the iodine production rate. After approximately four-to-six hours, the production rate of xenon from iodine decay, combined with production directly from fission, overcomes the increased burnout, causing xenon concentration to begin to increase. After 40 to 50 hours, xenon concentration will have reached a new equilibrium level above its initial level.
Comment: The question is technically incorrect because the initial and final conditions are not specified. What are the assumed "equilibrium" conditions? Restate the question as follows:
Question 54: (revised) After operation at 50% power for one week power is increased in 5 hours to 100% rated power. Explain how, why, and over what time intervals xenon 135 concentration changes occur after reactor power reaches 100%.