What is the definition of a "burnable poison?"
A. Isotopes manufactured into the fuel with large-scatter macroscopic cross sections to improve neutron thermalization.
B. Thermal neutron absorbing material added to the fuel during manufacturing to increase initial core fuel load.
C. Thermal neutron absorbing material produced in the non-fissionable fuel isotopes by fast neutron absorption.
D. Fast neutron absorbing material loaded into the upper one-third of the core to aid in flattening the thermal neutron flux.
ANSWER: B.
Comment: The question is technically incorrect because it implies there is only one reason that burnable poisons are used. There are at least three reasons that burnable poisons are used in the BWR core, namely to provide more uniform power density, to allow higher fuel enrichment in initial core load, to provide neutron flux shaping. The wording of Choice B is poor.
QID: B136 (TOPIC: 292007 KNOWLEDGE: K1.01 [2.9/3.1])
Burnable poisons are placed in a reactor core to:
A. increase the amount of fuel that can be loaded into the core.
B. accommodate control rod depletion that occurs over core life.
C. compensate for the buildup of xenon-135 that occurs over core life.
D. ensure that the reactor will always operate in an undermoderated condition.
ANSWER: A.
Comment: This question suffers the same defect as B64. There are at least three reasons for using burnable poisons.
QID: B264 (TOPIC: 292007 KNOWLEDGE: K1.01 [2.9/3.1])
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.
ANSWER: A.
Comment: 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.
QID: B564 (P264) (TOPIC: 292007 KNOWLEDGE: K1.03 [2.4/2.7])
Just prior to refueling, control rods are nearly fully withdrawn at 100% power. After refueling, the control rods are inserted much farther into the core at 100% power. Which one of the following is the reason for the change in full power control rod position?
A. Reactivity from power defect at beginning of core life (BOL) is much greater than at end of core life (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.
ANSWER: C.
Comment: The question is technically incorrect because of improper terminology. Standard terminology for the hypothetical condition of all rods full out is k-excess, not excess reactivity.
QID: B1163 (P1264) (TOPIC: 292007 KNOWLEDGE: K1.03 [2.4/2.7])
Refer to the drawing of Keff versus core age (see figure below). The decrease in Keff from point 1 to point 2 is primarily caused by:
A. depletion of fuel.
B. burnout of burnable poisons.
C. initial heat-up of the reactor.
D. buildup of fission product poisons.
Comment: The question is technically incorrect because the indicated correct choice is too general. Fission product poisons build up throughout the entire life cycle. The primary reason for the decrease in Keff from point 1 to point 2 is the build up of steady state Samarium-149.
QID: B1364 (P1864) (TOPIC: 292007 KNOWLEDGE: K1.03 [2.4/2.7])
Refer to the drawing of Keff versus core age (see figure below). The change in Keff from point 2 to point 3 is caused by:
A. depletion of fuel.
B. depletion of control rods.
C. burnout of burnable poisons.
D. burnout of fission product poisons.
Comment: The question is technically incorrect because the indicated correct choice is too general. Burnout of burnable poison is occurring throughout most of the fuel cycle. The positive reactivity change from poison depletion exceeds the negative reactivity change from fuel depletion.
QID: B1563 (P1563) (TOPIC: 292007 KNOWLEDGE: K1.03 [2.4/2.7])
Refer to the drawing of Keff versus core age (see figure below). The major cause for the change in Keff from point 3 to point 4 is:
A. depletion of U-235.
B. depletion of U-238.
C. burnout of burnable poisons.
D. buildup of fission product poisons.
Comment: The question is technically incorrect because the indicated correct choice is too general. Depletion of fuel is occurring throughout the life cycle. The negative reactivity change from fuel depletion exceeds the positive reactivity change from burnable poison depletion.