If you are an RO or SRO license applicant who will be taking, or has recently taken, the GFE, use the following listing of defective questions to verify grading. If a question is defective in any way, it is not valid examination question and cannot be included in the scoring. You have solid grounds for appeal and for corrective action. The NRC will not voluntarily correct these defective questions. Persistent challenging of the questions containing technical error will benefit future applicants.
The NRC Question Bank uses the same categories as the INPO Catalog, namely:
Neutrons
Neutron Life Cycle
Reactor Kinetics and Neutron Sources
Reactivity Coefficients
Control Rods
Fisson Product Poisons
Fuel Depletion and Burnable Poisons
Reactor Operational Physics
In this Nukefact the erroneous questions in the NRC BWR exam bank section on Neutrons are identified.
The term neutron generation time is defined as the average time between:
A. neutron absorption and the resulting fission.
B. the production of a delayed neutron and subsequent neutron thermalization.
C. neutron absorption producing a fission and absorption of resultant neutrons.
D. neutron thermalization and subsequent neutron absorption.
ANSWER: C.
Comment: This question appears in the INPO catalog and is technically incorrect. The neutron generation time is the time interval required by a generation of fictious neutrons to traverse one life cycle, which consists of six distinct interactions that occur in sequence over time. The neutron generation time starts at the instant of fission, when the generation of neutrons is born, and ends at the instant these neutrons create fissions to initiate the next generation of neutrons. Absorption, as referred to in Choice C, can occur in the epithermal resonances and in thermal non-fission interactions. Both of these interactions occur prior to completion of the life cycle, short of a full neutron generation time. None of the choices is correct.
QID: B245 (P545) (TOPIC: 292001 KNOWLEDGE: K1.02 [3.0/3.1])
Delayed neutrons are neutrons that:
A. have reached thermal equilibrium with the surrounding medium.
B. are born within 10 -14 seconds of the fission event.
C. are born at the lowest average kinetic energy of all fission neutrons.
D. are responsible for the majority of U-235 fissions.
ANSWER: C.
Comment: A similar question is included in the INPO catalog and is unsatisfactory because there are two correct answers. Since neutrons may be classifed by more than one criterion, including time of appearance after fission, energy at the time of release, average lifetime of the neutron, and the emitting entity or event, special care must taken to ensure that only one correct answer is provided. For example, a newly emitted neutron may be a delayed neutron, but it is also a fast neutron. In this question delayed neutrons are being categorized according to energy. However, since the delayed neutrons act as source neutrons and initiate chain reactions, in a very real sense they are directly responsible for all fissions, per Choice D.
QID: B1146 (P1945) (TOPIC: 292001 KNOWLEDGE: K1.02 [3.0/3.1])
Which one of the following types of neutrons has an average neutron generation lifetime of 12.5 seconds?
A. Prompt
B. Delayed
C. Fast
D. Thermal
ANSWER: B.
Comment: This question appears in the INPO catalog and is technically incorrect because of improper terminology. There is no such thing as an "average neutron generation lifetime." The fictitious average neutrons in the primitive life cycle model exist for a "generation time." A generation of average neutrons includes all neutrons produced from fission, i.e. the prompt neutrons and the delayed neutrons. Generation time does not exhibit a constant value of 12.5 seconds. Generation time is a variable, dependent on the mix of precursor atoms in the precursor inventory.
Prompt neutrons exist on average for a unique lifetime, which is very short and relatively constant. Delayed neutrons have a lifetime that is primarily a function of the delay time before neutron emission by the precursor atoms. The mean life of the delayed neutrons is 12.5 seconds at steady state, either equilibrium subcritical multiplication or criticality. For transient power conditions, the mean life is a variable, dependent on the mix of precursor atoms in the precursor inventory. "Generation time" is not a property that applies to either prompt or delayed neutrons.
QID: B1545 (P1145) (TOPIC: 292001 KNOWLEDGE: K1.02 [3.0/3.1])
Which one of the following is a characteristic of a prompt neutron?
A. Born with an average kinetic energy of 0.1 MeV
B. Emitted by the excited nucleus of a fission product daughter
C. Accounts for more than 99% of fission neutrons
D. Released an average of 13 seconds after the fission event
ANSWER: C.
Comment: This question is unsatisfactory because it is ambiguous and because Choice C is a characteristic of the fission process, not of the prompt neutron. If Choice C refers to the neutrons born from fission, then since (1 - beta) = .9935, Choice C is correct. If Choice C refers to the fraction of prompt neutrons in the population, then Choice C is incorrect because the fraction of delayed neutrons in the population is always (beta - rho). For transient power decay with large negative reactivity, the fraction of delayed neutrons in the population can easily exceed one percent, meaning that the fraction of prompt neutrons in the population is less than 99%.
QID: B1845 TOPIC: 292001 KNOWLEDGE: K1.02 [3.0/3.1]
Delayed neutrons are neutrons that:
A. have reached thermal equilibrium with the surrounding medium.
B. are born as thermal neutrons.
C. are born at a lower average kinetic energy than most other fission neutrons.
D. are responsible for the majority of U-235 fissions.
ANSWER: C.
Comment: This question suffers the same defect as B245, namely there are two correct answers, Choice C and Choice D.
QID: B1945 (P845) TOPIC: 292001 KNOWLEDGE: K1.02 [3.0/3.1]
Delayed neutrons are neutrons that:
A. have reached thermal equilibrium with the surrounding medium.
B. are born within 10 -14 seconds of the fission event.
C. are produced from the radioactive decay of specific fission fragments.
D. are responsible for the majority of U-235 fissions.
ANSWER: C.
Comment: This question suffers the same defect as B245, namely there are two correct answers, Choice C and Choice D.
QID: B2645 (P2645) TOPIC: 292001 KNOWLEDGE: K1.02 [3.0/3.1]
As compared to prompt neutrons, delayed neutrons:
A. are more likely to leak out of the core.
B. are more likely to cause fission of U-238.
C. are more likely to become thermal neutrons.
D. are responsible for the majority of U-235 fissions.
ANSWER: C.
Comment: This question suffers the same defect as B245, namely there are two correct answers, Choice C and Choice D.
QID: B345 (N/A) (TOPIC: 292001 KNOWLEDGE: K1.03 [2.7/2.7])
A neutron that possesses the same kinetic energy as its surroundings is called a/an ____________________ neutron.
A. slow
B. intermediate
C. resonant
D. thermal
ANSWER: D.
Comment: This question, which appears in the INPO catalog, is technically incorrect because the structure of the question is poor. Except for primary coolant flow, the surroundings referred to probably have no kinetic energy, other than our speed of 1100 mph resulting from the earth's rotation + 67,000 mph as earth orbits the sun + 486,000 mph as our solar system circles the galaxy + a few mph for expansion of the universe. Both the neutron and its surroundings always possess this kinetic energy.
Since many of the questions in this exam bank are generic, it would be helpful to the student if such questions pertaining specifically to a BWR are so qualified. The neutron energy level depends on the reactor design. In a fast reactor, the equilibrium energy of the neutron is fast. In an intermediate reactor, the equilibrium energy of the neutron is intermediate. In a thermal reactor, the equilibrium energy of the neutron is thermal. There are two correct answers to this question, Choice B and Choice D. The reactor type must be specified.
QID: B545 (N/A) (TOPIC: 292001 KNOWLEDGE: K1.03 [2.7/2.7])
A neutron is "thermal" when:
A. its kinetic energy is in the 1 eV to 1,000 eV energy range.
B. it is in energy equilibrium with the moderating medium.
C. it is released from the fission of a U-235 atom.
D. its cross-section for absorption in the fuel undergoes a sudden decrease.
ANSWER: B.
Comment: This question suffers the same defect as B345; equilibrium energy depends on the reactor design.
QID: B246 (TOPIC: 292001 KNOWLEDGE: K1.04 [3.2/3.2])
The interaction in the reactor core that is most efficient in thermalizing neutrons occurs with the:
A. hydrogen atoms in the water molecules.
B. oxygen atoms in the water molecules.
C. helium atoms in the fuel pins.
D. zirconium atoms in the fuel cladding.
ANSWER: A.
Comment: A similar question appeared in the INPO catalog and is unsatisfactory because the scattering interaction occurs with the hydrogen nucleus, not the hydrogen atoms in the water molecules.
QID: B346 (TOPIC: 292001 KNOWLEDGE: K1.05 [2.4/2.6])
The best neutron moderator is __________ and is composed of __________ atoms.
A. dense; large B. not dense; large C. dense; small D. not dense; small
ANSWER: C.
Comment: The question, which also appears in the INPO catalog, is unsatisfactory. Both the wording and terminology are poor and likely to confuse the student. The question, as stated, refers to both the spatial arrangement of the atoms and to a physical property of the atom itself, in a manner that lacks clarity. "Dense" is not technical terminology found in most nuclear engineering texts. Although it can mean "closely packed", it is easily confused with a more common definition of density, as mass per unit volume, which may completely alter the student response. For example, lead, Pb, is dense but is a poor moderator. In addition, the geometric size of the atom is never a property related to moderating ability. A good moderator is any one of a few light elements, namely those with nuclei containing but few nucleons.