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 PWR exam bank section on Neutrons are identified.
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 classified 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: P845 (B1945) (TOPIC: 192001 KNOWLEDGE: K1.02 [2.4/2.5])
Delayed neutrons are the 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 certain fission fragments.
D. are responsible for the majority of U-235 fissions.
ANSWER: C.
Comment: This question suffers the same defect as P545. There are two correct answers.
QID: P1145 (B1545) (TOPIC: 192001 KNOWLEDGE: K1.02 [2.4/2.5])
Which one of the following is a characteristic of a prompt neutron?
A. Born with an average kinetic energy below 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: P1945 (B1146) (TOPIC: 192001 KNOWLEDGE: K1.02 [2.4/2.5])
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 neutrons alone, or to delayed neutrons alone.
QID: P2645 (B2645) (TOPIC: 192001 KNOWLEDGE: K1.02 [2.4/2.5])
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 P545. There are two correct answers.
QID: P2845 (TOPIC: 192001 KNOWLEDGE: K1.02 [2.4/2.5])
During a time interval in a typical commercial nuclear reactor operating at the beginning of core, 103 delayed neutrons were emitted. Approximately how many prompt neutrons were emitted during this same time interval?
A. 1.5 x 105
B. 6.5 x 106
C. 1.5 x 107
D. 6.5 x 108
ANSWER: A.
Comment: This question suffers the same defect as P1145. Insufficient information is provided. There is no correct answer.