In this Nukefact we address the questions under the subsection Fuel Depletion and Burnable Poisons. Of the seventeen questions in the INPO Catalog pertaining to Fuel Depletion and Burnable Poisons we find nine to be technically incorrect, that's 53% wrong folks. Of the nine incorrect questions, five are about burnable poisons, three are about kexcess behavior over the life cycle, and one concerns the effect of k-excess on control rod position. There are no questions in this section that address other important aspects of Fuel Depletion and Burnable Poisons ... including such topics as Megawatt-Days/Metric-Ton, typical end-of-cycle percent fuel depletion, plutonium production during the cycle, tons of fuel loaded, and fuel enrichment values.
a. accommodate control rod depletion that occurs over core life
b. increase the amount of fuel that can be loaded into the core*
c. balance the production of xenon 135 that occurs in the core
d. ensure that the reactor will always operate in an undermoderated condition
Comment: The asterisk indicates the intended correct answer is choice "b". The question is technically incorrect because it implies there is only one reason that burnable poisons are used. Questions #2 and #6 give three reasons that burnable poisons used in the BWR core ... as follows, provide more uniform power density, allow higher fuel enrichment in initial core load, provide neutron flux shaping. Restate the question as follows:
Question 3: (revised) One reason that burnable poisons are placed in a reactor core to
a. accommodate control rod depletion that occurs over core life
b. increase the amount of fuel that can be loaded into the core*
c. balance the production of xenon 135 that occurs in the core
d. ensure that the reactor will always operate in an undermoderated condition
Question 4 : Burnable poisons are loaded into the core to
a. allow the initial core to have excess reactivity to extend core life*
b. allow shallow rods to compensate for core burnout
c. reduce the amount of time required for xenon to peak following a scram
d. provide for flux shaping in areas of deep rods
Comment: The asterisk indicates the intended correct answer is choice "a". The question is technically incorrect because it implies there is only one reason for using burnable poison, because of improper terminology, and because the indicated correct choice is improperly explained. Fuel cells are "loaded" into the reactor vessel. Excess reactivity is not "loaded" into the core. Burnable poison is incorporated into the fuel pellets during manufacture. The burnable poison does not "allow" the initial core to have excess reactivity to extend core life. The burnable poison actually reduces the amount of k-excess present in an intial core or a reload core. And, standard terminology for the hypothetical condition of all rods full out is k-excess, not excess reactivity. Restate the question and choices as follows:
Question 4: (revised) One reason that burnable poison is used in a BWR core is to
a. compensate for increased fuel loading which extends core life*
b. allow shallow rods to compensate for core burnout
c. reduce the amount of time required for xenon to peak following a scram
d. provide for flux shaping in areas of deep rods
Question 5: Burnable poisons are loaded into the core to
a. increase the excess reactivity that can be loaded into the core during each refueling*
b. reduce the rod shadowing effect between shallow rods early in core life
c. ensure the moderator coefficient of reactivity remains negative throughout core life
d. provide for flux shaping in areas of deep rods during high power operation
Comment: The asterisk indicates the intended correct answer is choice "a". The question is technically incorrect for the same reasons given in questions #4. Restate the question and choices as follows:
Question 5: (revised) One reason that burnable poison is used in the BWR core is to
a. compensate for the increased k-excess of higher fuel loadings that extend cycle life*
b. reduce the rod shadowing effect between shallow rods early in core life
c. ensure the moderator coefficient of reactivity remains negative throughout core life
d. provide for flux shaping in areas of deep rods during high power operation
Question 6: Which of the following lists the reasons for using burnable poisons in an operating reactor?
1. Provide more uniform power density
2. Counteract the effects of control rod burnout
3. Allow higher fuel enrichment of initial core load.
4. Provide neutron flux shaping
a. 1, 2, 3
b. 1, 2, 4
c. 1, 3, 4*
d. 2, 3, 4
Comment: The asterisk indicates the intended correct answer is choice "c". The question is technically incorrect because it is poorly worded. The use of "initial core load" in choice "3" is misleading because it can be interpreted as being applicable to only the very first core loading. The presence of burnable poison in each subsequent core reload serves the same purpose. Restate choice "3" as follows:
Question 6: (revised) Which three of the following are reasons for using burnable poisons in a BWR core?
1. Provide more uniform power density
2. Counteract the effects of control rod burnout
3. Allow higher fuel enrichment to increase the core fuel load and endurance.
4. Provide neutron flux shaping
a. 1, 2, 3
b. 1, 2, 4
c. 1, 3, 4*
d. 2, 3, 4
Question 9: What is the definition of the term burnable poison?
a. isotopes manufactured into the fuel with large-scatter macroscopic cross sections
b. thermal neutron-absorbing material added to the fuel, during the manufacturing process*
c. neutron-absorber materials produced in the fuel by fast neutron absorption
d. fast neutron-absorbing material loaded into the upper third of the core to aid in slowing down neutrons
Comment: The asterisk indicates the intended correct answer is choice "b". The question is technically incorrect because the indicated correct choice is an incomplete definition of a burnable poison. Restate the choice "b" as follows:
Question 9: (revised) What is the definition of the term burnable poison?
a. isotopes manufactured into the fuel with large-scatter macroscopic cross sections
b. thermal neutron-absorbing material added to the fuel to increase fuel loading and endurance, to flatten core power distribution, and to shape flux*
c. neutron-absorber materials produced in the fuel by fast neutron absorption
d. fast neutron-absorbing material loaded into the upper third of the core to aid in slowing down neutrons
Question 10: Refer to Figure 2.7-1: The decrease in Keff from point 1 to point 2 is caused by
a. buildup of fission product poisons*
b. burnout of burnable poisons
c. initial heat-up of the reactor
d. burnout of fuel during startup physics testing
Question 10: (revised) Refer to Figure 2.7-1: The decrease in Keff from point 1 to point 2 is caused by
a. buildup of Samarium-149*
b. burnout of burnable poisons
c. initial heat-up of the reactor
d. burnout of fuel during startup physics testing
Question 11: Refer to Figure 2.7-1: The change in Keff from point 2 to point 3 is caused by
a. burnout of fission product poisons
b. burnout of burnable poisons*
c. depletion of fuel
d. depletion of control rods
Comment: The asterisk indicates the intended correct answer is choice "b". The question is technically incorrect because the indicated correct choice is an incomplete explanation. Burnout of burnable poison is occurring throughout most of the life cycle. Restate the choices as follows:
Question 11: (revised) Refer to Figure 2.7-1: The change in Keff from point 2 to point 3 is caused by
a. negative delta-k of fission product poisons exceeding the positive delta-k of fuel depletion
b. positive delta-k from burnable poison depletion exceeding the negative delta-k of fuel depletion*
c. positive delta-k of fuel depletion exceeding the negative delta-k of burnable poison depletion
d. positive delta-k from control rod depletion exceeding the negative delta-k of fuel depletion
Question 12: Refer to Figure 2.7-1: The change in Keff from point 3 to point 4 is caused by
a. burnout of fission product poisons
b. burnout of burnable poisons
c. depletion of fuel*
d. depletion of control rods
Comment: The asterisk indicates the intended correct answer is choice "c". The question is technically incorrect for the same reason given in question #11. Depletion of fuel is occurring throughout the life cycle. Restate the choices as follows:
Question 12: (revised) Refer to Figure 2.7-1: The change in Keff from point 3 to point 4 is caused by
a. negative delta-k of fission product poisons exceeding the positive delta-k of fuel depletion
b. positive delta-k from burnable poison depletion exceeding the negative delta-k of fuel depletion
c. positive delta-k of fuel depletion exceeding the negative delta-k of burnable poison depletion*
d. positive delta-k from control rod depletion exceeding the negative delta-k of fuel depletion
Question 16: Just prior to refueling, control rods are nearly fully withdrawn at 100 percent power. After refueling, the control rods are inserted much farther into the core at 100 percent power.
Which one of the following is the reason for the change in full power control rod position: (BOL = beginning of core life. EOL = end of core life.)
a. Reactivity from power defect at BOL is much greater than at EOL.
b. Reactivity from void coefficient at EOL is much greater than at BOL.
c. The excess reactivity in the core at BOL is much greater than at EOL.*
d. The integral control rod worth at EOL is much greater than at BOL.
Comment: The asterisk indicates the intended correct answer is choice "c". The question is technically incorrect because of flawed terminology. The standard, and accepted, definition of the hypothetical condition of rods fully withdrawn is k-excess. Restate choice "c" as follows:
Question 16: (revised) Just prior to refueling, control rods are nearly fully withdrawn at 100 percent power. After refueling, the control rods are inserted much farther into the core at 100 percent power.
Which one of the following is the reason for the change in full power control rod position: (BOL = beginning of core life. EOL = end of core life.)
a. Reactivity from power defect at BOL is much greater than at EOL.
b. Reactivity from void coefficient at EOL is much greater than at BOL.
c. The k-excess of the core at BOL is much greater than at EOL.*
d. The integral control rod worth at EOL is much greater than at BOL.