NUKEFACTs #13 and #14 identified a few erroneous questions and/or answers as found in Training Center exam banks. In a letter to the NRC dated 1/8/97 we stated that 170 of the 650 questions (26%) in the INPO BWR Catalog were technically incorrect. Another 170 questions were flawed because of poor wording or terminology, making a total of over 50% of the catalog questions that were defective. In the next few issues of NUKEFACTS, we will specifically identify ALL of the defective questions. The numbering on each question is believed to be from an INPO BWR Catalog, vintage 1994/1995. If there is a later version of the INPO Catalog, you can rest assured that most of the erroneous questions are still present, not corrected but maybe renumbered. With INPO, improvement takes a considerable amount of time. As amazing as it may seem, many of the technically incorrect questions have existed in one form or another for 50 years.
Under the INPO Catalog section on Reactor Theory, there are seven subsections titled as follows:
Neutrons
Neutron Life Cycle
Reactor Kinetics and Neutron Sources
Reactivity Coefficients
Control Rods
Fisson Product Poisons
Reactor Operational Physics
In this Nukefact we address the questions under the subsection Neutrons. There are five questions out of a total of thirty-three in the Catalog that are identified as being technically incorrect, namely #6, #8, #12, #14, #16. Thus, in this subsection about 15% of the questions are technically incorrect. Another nine questions are identified as flawed. Why is this important? Because the Nuclear Training Centers teach to pass the License Exam. The tail is wagging the dog. The incorrect questions require that incorrect material be taught in the class room.
The ultimate loser is the student, and future Reactor Operator, who believes the material provided in the class room is legitimate. In NUKEFACT #13 we stated: "Reactor Operator training challenges many students because it confronts them with subjects outside their previous learning experience, e.g. the subject of Reactor Theory. Then, if the subject matter is of poor quality and laced with flawed concepts, several of which have been identified in NUKEFACTs, the instructor must employ smoke and mirrors in the ClassRoom, and the student struggles to gain even a muddled understanding of the subject. Finally, as the icing on the cake, a license exam must be passed that may contain erroneous test items." This constitutes a serious violation of reactor safety by the NRC.
Question 1: A neutron that is born within 10-14 seconds of a fission event is called a __________ neutron.
a. thermal
b. delayed
c. prompt*
d. fast
Comment: The asterisk indicates the intended correct answer is choice "c". The question is not technically incorrect but it is certainly flawed, and potentially confusing to the student. Neutrons may be classifed by more than one criterion, and the INPO Catalog uses four criteria, namely time of appearance after fission, energy at the time of release, average lifetime of the neutron, and the emitting entity or event. The neutron may be a prompt neutron, but it is also a high energy, fast neutron. There are two correct answers. Restate the question and choices as follows:
Question 1: (revised) A neutron that is born within 10-14 seconds of a fission event is called a __________ neutron.
a. thermal
b. delayed
c. prompt*
d. source
Question 3: __________ neutrons are born from first excited daughters generated from the fission process.
a. prompt
b. thermal
c. delayed*
d. fast
Comment: The asterisk indicates the intended correct answer is choice "c". The comment for question #1 applies. Restate the question as follows:
Question 3: (revised) __________ neutrons are born from first excited daughters generated from the fission process.
a. prompt
b. thermal
c. delayed*
d. slow
Question 4: 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 with an average kinetic energy of less than 1 MeV*
d. are responsible for the majority of U-235 fissions
Comment: The asterisk indicates the intended correct answer is choice "c". This question is included to illustrate an example of delayed neutrons being categorized according to energy, as per the comment for questions #1. Of course it also depends on what is meant by "responsible for the majority of fissions". Since the delayed neutrons act as source neutrons and initiate chain reactions, they are, in a very real sense, directly responsible for all fissions.
Question 6: What type of a neutron has an average neutron generation time of 12.5 seconds?
a. prompt
b. delayed*
c. fast
d. thermal
Comment: The asterisk indicates the intended correct answer is choice "b". The question is technically incorrect because no neutrons have an average neutron generation time of 12.5 seconds. The mean life of the delayed neutrons is 12.5 seconds. This mean life consists primarily of the delay time before neutrom emission. A generation of neutrons includes all neutrons produced from fission, i.e. the prompt neutrons and the delayed neutrons. Restate the question as follows:
Question 6: (revised) What neutrons have a mean life of 12.5 seconds?
a. prompt
b. delayed*
c. fast
d. thermal
Question 7: Which of the following is the best definition of the term "prompt neutron"?
a. a high-energy neutron emitted from a neutron precursor, immediately after the fission process
b. a neutron with an energy level greater than 0.1 MeV, emitted in less than 10-4 seconds following a
. nuclear fission
c. a neutron emitted in less than 10-14 seconds following a nuclear fission*
d. a neutron emitted as a result of a gamma-n or alpha-n reaction
Comment: The asterisk indicates the intended correct answer is choice "c". Since the only correct answer is "c", it is unclear why the student is instructed to identify the best definition. Restate the question as follows:
Question 7: (revised)Which of the following defines a "prompt neutron"?
a. a high-energy neutron emitted from a neutron precursor, immediately after the fission process
b. a neutron with an energy level greater than 0.1 MeV, emitted in less than 10-4 seconds following a
. nuclear fission
c. a neutron emitted in less than 10-14 seconds following a nuclear fission*
d. a neutron emitted as a result of a gamma-n or alpha-n reaction
Question 8: The term "neutron generation time" is defined as the average time between
a. neutron absorption and 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
Comment: The asterisk indicates the intended correct answer is choice "c". 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. None of the choices is correct. Restate the question as follows:
Question 8: (revised) The term "neutron generation time" is defined as the average time between
a. neutron absorption and resulting fission
b. the production of a delayed neutron and subsequent neutron thermalization
c. fission events that initiate successive generations*
d. neutron thermalization and subsequent neutron absorption
Question 12: A __________ neutron has reached the same energy as its surrounding medium.
a. delayed
b. prompt
c. thermal*
d. fast
Comment: The asterisk indicates the intended correct answer is choice "c". The energy level of the medium depends on the reactor design. In a fast spectrum reactor, the equilibrium energy of the neutron is fast. In an intermediate spectrum reactor, the equilibrium energy of the neutron is intermediate. There are two correct answers to this question. Restate the question as follows:
Question 12: (revised) In a BWR, a __________ neutron has reached the same energy as its surrounding medium.
a. delayed
b. prompt
c. thermal*
d. fast
Question 14: A neutron that possesses the same kinetic energy as its surroundings is called a/an __________ neutron.
a. slow
b. intermediate
c. fast
d. thermal*
Comment: The asterisk indicates the intended correct answer is choice "d". As for the comment to question #12, there are at least three correct answers to this question. The structure of the question is also poor. The surroundings referred to probably have no kinetic energy, except of course 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. Restate the question as follows:
Question 14: (revised) In a BWR, a neutron that possesses kinetic energy in equilibrium with its surroundings is called a/an __________ neutron.
a. delayed
b. prompt
c. fast
d. thermal*
Question 15: In the definition of a thermal neutron, a relationship is established between the neutron and its environment. Which of the following statements best describes this relationship?
a. Thermal neutron possesses lower kinetic energy than its environment
b. Thermal neutron possesses same kinetic energy as its environment*
c. Thermal neutron possesses higher kinetic energy than its environment
d. Kinetic energy is not part of the relationship
Comment: The asterisk indicates the intended correct answer is choice "b". Here again, there is no best answer because there is only one correct answer. The comment of question #14 applies here to environment. In fact choice "d" could be true. Restate the question as follows:
Question 15: (revised) In the definition of a thermal neutron, a relationship is established between the neutron and its environment. Which of the following statements describes this relationship in a BWR?
a. Thermal neutron possesses kinetic energy below thermal equilibrium with the environment
b. Thermal neutron possesses kinetic energy in equilibrium with the environment*
c. Thermal neutron possesses kinetic energy in excess of thermal equilibrium with the environment
d. Kinetic energy is not part of the relationship
Question 16: 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
Comment: The asterisk indicates the intended correct answer is choice "b". The same comment applies as for question #12. Restate the question as follows:
Question 16: (revised) In a BWR, 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
Question 18: A neutron reaches thermal equilibrium with its surroundings through
a. collisions with atoms*
b. radioactive decay
c. compton scattering
d. radiolytic decomposition
Comment: The asterisk indicates the intended correct answer is choice "a". In the slowing down process, a fission neutron loses energy by collision with nuclei. The neutron does not interact with the entire atom. This is a good example of sloppy terminology that does nothing but confuse the student. Restate the question as follows:
Question 18: (revised) A fission neutron reaches thermal equilibrium with its surroundings through
a. collisions with nuclei*
b. radioactive decay
c. compton scattering
d. radiolytic decomposition
Question 20: Which or the following best describes the term "moderation"?
a. slowing down of neutrons*
b. absorption of neutrons
c. distribution of energy per unit time of the neutron flux
d. net vector distance between thermalization and absorption
Comment: The asterisk indicates the intended correct answer is choice "a". Here again, there is no best answer because there is only one correct answer. Restate the question as follows:
Question 20: (revised) Which or the following describes the term "moderation"?
a. slowing down of neutrons*
b. absorption of neutrons
c. distribution of energy per unit time of the neutron flux
d. net vector distance between thermalization and absorption
Question 21: The interaction in the reactor core that is most efficient in thermalizing neutrons for fission occurs with the
a. hydrogen atoms in the water molecules*
b. oxygen atoms in the water molecules
c. boron atoms in the control rods
d. zirconium atoms in the fuel claddint
Comment: The asterisk indicates the intended correct answer is choice "a". Here again, as in question #18, in the slowing down process, a fission neutron loses energy by collision with nuclei. The neutron does not interact with the entire atom. Restate the question as follows:
Question 21: (revised)The interaction in the reactor core that is most efficient in thermalizing neutrons for fission occurs with the
a. hydrogen nuclei in the water*
b. oxygen atoms in the water molecules
c. boron nuclei in the control rods
d. zirconium atoms in the fuel cladding
Question 25: A description of a good moderator is a moderator that is
a. dense and is composed of large atoms
b. not dense and is composed of large atoms
c. dense and is composed of small atoms*
d. not dense and is composed of small atoms
Comment: The asterisk indicates the intended correct answer is choice "c". Both the wording and terminology used in this question is poor and is 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 means "closely packed", it is easily confused with density, or mass per unit volume, which may completely alter the student response. In addition, the geometric size of the atom is not the property most directly related to moderating ability. The question should be restated as follows:
Question 25: (revised) A good moderator is one
a. whose atoms are closely packed and composed of heavy nuclei
b. whose atoms are loosely packed and composed of heavy nuclei
c. whose atoms are closely packed and composed of light nuclei*
d. whose atoms are loosely packed and composed of light nuclei
Question 28: The best neutron moderator is ____________ and is composed of ____________ atoms.
a. dense; large
b. not dense; large
c. dense; small*
d. not dense; small
Comment: The asterisk indicates the intended correct answer is choice "c". The same comment applies as given for question #25. The question should be restated as follows:
Question 28: (revised) The best neutron moderator has atoms that are ____________ packed and is composed of ____________ nuclei.
a. closely ; heavy
b. loosely; heavy
c. closely; light*
d. loosely; light
Question 29: The best neutron moderator is
a. dense and is composed of large atoms
b. not dense and is composed of large atoms
c. dense and is composed of small atoms*
d. not dense and is composed of small atoms
Comment: The asterisk indicates the intended correct answer is choice "c". This question is essentially a duplication of question #25, and the comment made pertaining to question #25 apply here. The question should be restated as follows:
Question 29: (revised) The best neutron moderator is one
a. whose atoms are closely packed and composed of heavy nuclei
b. whose atoms are loosely packed and composed of heavy nuclei
c. whose atoms are closely packed and composed of light nuclei*
d. whose atoms are loosely packed and composed of light nuclei
If you disagree with any of our comments on these questions, we would appreciate hearing from you. Our E-MAIL EXPRESS is just a click away. In the next issued of NUKEFACTS we will address questions dealing with The Neutron Life Cycle.