In this Nukefact we address the questions under the subsection Control Rods. Of the ninety-one questions in the INPO Catalog pertaining to control rods we find thirty-eight to be technically incorrect, that's 41% wrong folks.
And from the existing technical error it becomes evident that INPO hasn't a clue about the following:
1. the definition of differential rod worth
2. the definition of integral rod worth
3. that initial, and final, conditions in a question are important and must be defined for the student
The sloppiness in structuring the questions and in terminology is truly disturbing ... and appalling. Notably, after first presenting questions that incorrectly define both "differential" and "integral" rod worth, there are then a multitude of questions about the effect of changes in various core parameters on "control rod worth" ... which was never defined or explained as meaning differential, integral, or both.
a. low capture cross section for thermal neutrons
b. high capture cross section for thermal neutrons*
c. low fission cross section for fast neutrons
d. high fission cross section for fast neutrons
Comment: The asterisk indicates the intended correct answer is choice "b". The question is flawed because it asks for "properties" (plural) and provides choices that are only singular. Restate the question and choices as follows:
Question 10: (revised) What property of boron-10 makes it ideal for use in control rods?
a. low capture cross section for thermal neutrons
b. high capture cross section for thermal neutrons*
c. low fission cross section for fast neutrons
d. high fission cross section for fast neutrons
Question 17: The reactor is exactly critical below the point of adding heat. A control rod is withdrawn a short distance. Reactor power will
a. increase until the effect of rising temperature is seen*
b. increase during the rod motion, then return to its original value
c. increase to a new equilibrium value below the point of adding heat
d. decrease temporarily, then return to its original value
Comment: The asterisk indicates the intended correct answer is choice "a". The question is flawed because of the poor description provided in the correct choice "a". Critical is an "exact" condition; "exactly" critical adds nothing. Power does not increase "until" the effect of rising temperature is seen, whatever that means, it continues to rise for sometime thereafter. Restate the question as follows:
Question 17: (revised) The reactor is critical below the point of adding heat. A control rod is withdrawn a short distance. Reactor fission power will
a. increase on a stable rate until slowed, and turned, by the effect of rising temperature*
b. increase during the rod motion, then return to its original value
c. increase to a new equilibrium value below the point of adding heat
d. decrease temporarily, then return to its original value
Question 18: The reactor is exactly critical below the point of adding heat. A control rod is manually inserted for 5 seconds. Reactor power will
a. decrease to a shutdown power level low in the source (startup) range*
b. decrease temporarily, then return to the original value due to the resulting decrease in moderator temperature
c. decrease until inherent positive reactivity feedback causes the reactor to become critical at a lower neutron level
d. decrease temporarily, then return to the original value due to subcritical multiplication
Comment: The asterisk indicates the intended correct answer is choice "a". In addition to specifying "exactly" critical, the answer is technically incorrect because a 5 second insertion of a single rod will introduce only a small negative reactivity change. The reactor is still in close proximity to criticality. A low power level is not an indicator of whether the reactor is shut down. A "shutdown" power level does not result from a 5-second rod insertion. A reactor is not "shutdown" when a single rod is inserted for 5 seconds but rather when all control rods are "fully" inserted. Source multiplication following a 5-second rod insertion will be large. Depending on the non-fission source strength, power may settle to an equilibrium level either in the middle of the Source Range, in the upper Source Range, or in the lower Intermediate Range. Restate the question as follows:
Question 18: (revised) The reactor is critical below the point of adding heat. A control rod is manually inserted for 5 seconds. Reactor fission power will
a. decrease to an equilibrium subcritical multiplication power level in the source (startup) range*
b. decrease temporarily, then return to the original value due to the resulting decrease in moderator temperature
c. decrease until inherent positive reactivity feedback causes the reactor to become critical at a lower neutron level
d. decrease temporarily, then return to the original value due to subcritical multiplication
Question 19: The reactor is exactly critical below the point-of-adding-heat. A control rod is inserted a short distance. Reactor power will
a. increase to a new higher level
b. increase temporarily then return to original value
c. decrease to a new lower value*
d. decrease temporarily then return to original value
Comment: The asterisk indicates the intended correct answer is choice "c". The question is flawed because of the use of "exactly" critical and the poor description of the final condition in choice "c". Restate the question as follows:
Question 19: (revised) The reactor is critical below the point-of-adding-heat. A control rod is inserted a short distance. Reactor fission power will
a. increase to a new higher level
b. increase temporarily then return to original value
c. decrease to equilibrium subcritical multiplication power level*
d. decrease temporarily then return to original value
Question 20: The reactor is exactly critical below the point of adding heat (POAH) during a normal reactor startup. If a control rod is manually withdrawn for 5 seconds, reactor power will
a. increase to a stable critical power level below the POAH
b. increase temporarily, then decrease and stabilize at the original value
c. increase to stable critical power level at the POAH*
d. increase temporarily, then decrease and stabilize below the original value
Comment: The asterisk indicates the intended correct answer is choice "c". In addition to using "exactly" critical, the question is technically incorrect because of other atrocious terminology and because the indicated correct answer is not necessarily true. What is a "stable critical power level" ... a constant power level ? Stable is standard/common terminology for describing a constant off-critical reactor period, i.e. a condition of changing power with time. Now we use stable to define a constant power level. Is there any end to such sloppy terminology or any question as to why confusion reigns? And, what is a "critical" power level suppose to explain? The reactor can be critical at an infinite number of power levels. What occurs at the POAH will depend on the status of the system. If there is no load on the system (steam flow, or significant heat loss) heating will bring the reactor subcritical and it will remain subcritical. Power will decrease to an equilibrium level. If there is a heat load, heating will cause power to turn and undergo a damped oscillation as it levels off at a critical condition with power in balance with the load. The question, as stated, provides insufficient information to formulate an answer. Restate the question as follows:
Question 20: (revised) The reactor is critical below the point of adding heat (POAH) during a normal reactor startup. Significant heat loss is occurring from the system. If a control rod is manually withdrawn for 5 seconds, reactor power will
a. increase on a positive rate, turn as heating occurs, and decrease to the Source Range as the reactor becomes subcritical
b. increase temporarily, then decrease and stabilize at the original value
c. increase on a positive rate, turn as heating occurs, and reestablish criticality at a power that balances heat loss*
d. increase temporarily, then decrease and stabilize below the original value
Question 23: The reactor is subcritical with all rods inserted. A center control rod is then fully withdrawn from the core. Neutron population will
a. increase to a new higher level*
b. increase temporarily then return to original value
c. decrease to a new lower value
d. decrease temporarily then return to original value
Comment: The asterisk indicates the intended correct answer is choice "a". The question is flawed because of a deficient description of the initial condition. Subcritical with all rods inserted does not define the rod configuration ... all rods are "fully" inserted. "New" is not a necessary descriptive term. Restate the question as follows:
Question 23: (revised) The reactor is shutdown with all rods fully inserted. A center control rod is then fully withdrawn from the core. Neutron population will
a. increase to a higher level because of greater equilibrium subcritical multiplication*
b. increase temporarily then return to original value
c. decrease to a lower equilibrium level
d. decrease temporarily then return to original value
Question 24: Rod density is defined as the
a. total inserted notches divided by the total available notches*
b. total withdrawn notches divided by the total available notches
c. amount of boron loading divided by the total possible boron loading in the control rod
d. amount of remaining active fuel loading divided by the total fuel loading in the fuel rod
Comment: The asterisk indicates the intended correct answer is choice "a". The question is technically incorrect because control rod density is expressed as a percentage. The wording is poor. What are "available" notches, ones not inserted or total notches? Restate the question as follows:
Question 24: (revised) Rod density expresses as a percentage the
a. inserted notches divided by the total core notches*
b. total withdrawn notches divided by the total available notches
c. amount of boron loading divided by the total possible boron loading in the control rod
d. amount of remaining active fuel loading divided by the total fuel loading in the fuel rod
Question 28: During a reactor shutdown, when control rods are being inserted, control rod density
a. increases until all the rods are inserted*
b. decreases until all the rods are inserted
c. increases only until 50% of the rods are inserted
d. decreases only until 50% of the rods are inserted
Comment: The asterisk indicates the intended correct answer is choice "a". The question is flawed because of a poor decription of the rod position. Restate the question as follows:
Question 28: (revised) During a reactor shutdown, when control rods are being inserted, control rod density
a. increases until all the rods are fully inserted*
b. decreases until all the rods are fully inserted
c. increases only until 50% of the rods are fully inserted
d. decreases only until 50% of the rods are fully inserted
Question 29: Rod density is a measure of the
a. percentage of control rods withdrawn from the core
b. percentage of control rods inserted into the core*
c. number of control rods withdrawn compared to the number of control rods fully inserted
d. control rod worth compared to the shutdown margin at 100 percent core flow
Comment: The asterisk indicates the intended correct answer is choice "b". The question is technically incorrect. Choice b indicates that control rod density is based on "numbers" of control rods inserted ... and not on the number of notches inserted. Restate the question as follows:
Question 29: (revised) Rod density defines the
a. percentage of total control rod notches withdrawn from the core
b. percentage of total core control rod notches inserted into the core*
c. number of control rods withdrawn compared to the number of control rods fully inserted
d. control rod worth compared to the shutdown margin at 100 percent core flow
Question 30: Rod density is a measure of the total number of control rod notches __________ the core compared to the total number of control rod notches __________ the core.
a. withdrawn from; available in
b. inserted into; available in*
c. withdrawn from; inserted into
d. inserted into; withdrawn from
Comment: The asterisk indicates the intended correct answer is choice "b". The question is technically incorrect for the same reason as question #24. Restate the question as follows:
Question 30: (revised) Rod density is a percentage measure of the number of control rod notches __________ the core out of the total control rod notches __________ .
a. withdrawn from; existing
b. inserted into; existing*
c. withdrawn from; inserted
d. inserted into; withdrawn
Question 34: From the list of choices below, select the choice that best completes the following definition of a reactor scram.
"A rapid, __________ insertion of __________ control rods in response to an abnormal condition"
a. full, specific
b. partial, specific
c. full, all*
d. partial, all
Comment: The asterisk indicates the intended correct answer is choice "c". The question is flawed because it asks for the best choice. There is only one correct answer. Restate the question as follows:
Question 34: (revised) From the list of choices below, select the choice that completes the following definition of a reactor scram.
"A rapid, __________ insertion of __________ control rods in response to an abnormal condition"
a. full, specific
b. partial, specific
c. full, all*
d. partial, all
Question 35: Differential control rod worth is defined as the change in ____________ per unit change in rod position.
a. reactivity*
b. reactor power
c. neutron flux
d. fast fissions
Comment: The asterisk indicates the intended correct answer is choice "a". The question is technically incorrect because, as for the moderator coefficient, differential rod worth is very specifically defined as being the change in reactivity associated with a unit of incremental rod "withdrawal". Restate the question as follows:
Question 35: (revised) Differential control rod worth is defined as the change in ____________ per unit withdrawal in rod position.
a. reactivity*
b. reactor power
c. neutron flux
d. fast fissions
Question 36: Differential control rod worth is the change in _________ per ________ change in rod position.
a. reactor power, total
b. reactivity, unit*
c. reactor power, unit
d. reactivity, total
Comment: The asterisk indicates the intended correct answer is choice "b". The answer is technically incorrect for the same reason given in question #35. Restate the question as follows:
Question 36: (revised) Differential control rod worth is the change in _________ per ________ withdrawal in rod position.
a. reactor power, total
b. reactivity, unit*
c. reactor power, unit
d. reactivity, total
Question 37: Integral rod worth is the
a. change in reactivity per unit change in rod position
b. reactivity inserted by moving a control rod from a reference point to another point*
c. change in worth of a rod per unit change in reactor power
d. reactivity inserted by a rod on a power change
Comment: The asterisk indicates the intended correct answer is choice "b". The question is technically incorrect because the answer is wrong. Historically, integral rod worth was taken as the sum of the differential rod worth values from fully inserted to fully withdrawn. As the terminology implies, integral rod worth was the positive reactivity change that occurred if the single rod were withdrawn from fully inserted to fully withdrawn position. The algebraic sign of integral rod worth is positive because the algebraic sign of each differential worth is positive, by definition.
Historically, both differential rod worth and integral rod worth were presented as curves. The differential rod worth curve was roughly bell-shaped, reflecting axial flux distribution and displaying delta-rho/unit position change on the ordinate and position on the abscissa, from fully inserted to fully withdrawn. The integral rod worth curve was S-shaped, displaying delta-rho on the ordinate and position on the abscissa, from fully inserted to fully withdrawn. Choice "b" is nothing more than the reactivity change associated with an arbitrary rod position change. Of course this reactivity change can be determined from an integral rod worth curve ... but ... this reactivity change is definitely NOT integral rod worth. Restate the question as follows:
Question 37: (revised) Integral rod worth is the
a. change in reactivity per unit change in rod position
b. reactivity change caused by withdrawal of a single control rod from its fully inserted to fully withdrawn position*
c. change in worth of a rod per unit change in reactor power
d. reactivity inserted by moving a control rod from a reference point to another point
Question 38: The change in reactivity per unit change in rod position is called
a. total rod worth
b. integral rod worth
c. differential rod worth*
d. partial rod worth
Comment: The asterisk indicates the intended correct answer is choice "c". The question is technically incorrect for the same reason given in question # 35. Restate the question as follows:
Question 38: (revised) The change in reactivity per unit withdrawal in rod position is called
a. total rod worth
b. integral rod worth
c. differential rod worth*
d. partial rod worth
Question 39: The total amount of reactivity added by a control rod position change from a reference point to any other rod height is called
a. differential rod worth
b. excess reactivity
c. integral rod worth*
d. reference reactivity
Comment: The asterisk indicates the intended correct answer is choice "c". The question is technically incorrect for the same reason as given in question #37. Restate the question as follows:
Question 39: (revised) The reactivity change caused by withdrawing a single control rod from fully inserted to fully withdrawn is called
a. differential rod worth
b. excess reactivity
c. integral rod worth*
d. reference reactivity
Question 41: During a reactor startup and heatup, as the moderator temperature increases from 175°F to 545°F, control rod worth will
a. increase until moderator temperature reaches 545°F*
b. decrease until moderator temperature reaches 545°F
c. increase until moderator temperature reaches 300°F
d. decrease until moderator temperature reaches 300°F
Comment: The asterisk indicates the intended correct answer is choice "a". The question is technically incorrect because "control rod worth" is an ambiguous term and because the conditions of the heatup are not sufficiently defined. With previous questions dealing with "differential rod worth" and "integral rod worth", the meaning of "control rod worth" in this question is a mystery. Does the question refer to a single rod are to all rods? Are control rods being withdrawn during the heatup or are rod(s) at a fixed position? If fixed, how is the heatup accomplished? Because of the total dearth of information provided in this question, and others like it that follow, it is not possible to restate the question.
Question 42: Control rod worth increases as moderator temperature increases due to a
a. shorter slowing down length and thermal diffusion length
b. longer slowing down length and thermal diffusion length*
c. decrease in thermal neutron population
d. shorter thermal migration length
Comment: The asterisk indicates the intended correct answer is choice "b". The question is technically incorrect for the reason given in question #41. It is not possible to restate the question.