Question 32: A reactor startup is in progress with Keff at 0.995 and stable source range indication. If Keff is increased to 0.997 by control rod withdrawal, reactor period will initially become __________ and then __________.
a. shorter; approach infinity*
b. shorter; continue to gradually shorten
c. longer; approach infinity
d. longer; continue to gradually lengthen
Comment: The asterisk indicates the intended correct answer is choice "a". The question is technically incorrect because improper terminology. "Stable source range indication" should be equilibrium subcritical multiplication. The units on reactor period should be indicated as seconds. Restate the question as follows:
Question 32: (revised) A reactor startup is in progress with Keff at 0.995 and equilibrium subcritical multiplication. If Keff is increased to 0.997 by control rod withdrawal, reactor period will initially become __________ and then __________.
a. shorter; approach infinite seconds*
b. shorter; continue to gradually shorten
c. longer; approach infinite seconds
d. longer; continue to gradually lengthen
Question 33: As criticallity is approached during a reactor startup, equal insertions of positive reactivity result in a _________ change in equilibrium count rate and a ___________ time to reach each new equilibrium.
a. greater, longer*
b. greater, shorter
c. smaller, longer
d. smaller, shorter
Comment: The asterisk indicates the intended correct answer is choice "a". The question is technically incorrect because it specifies "equal reactivity insertions" and is not operational because there is no way for the operator to perform "equal" reactivity change. This question, and others like it - #8, #10, #11, #15, #17, #18, #29, and #30 - can mislead the student as to what is possible during reactor startup. Delete the question.
Question 34: As the reactor approaches criticality, for a given reactivity addition, the time required to reach a constant neutron population will be (shorter/longer) and the time required for period to reach infinity will be (shorter/longer).
A. shorter, shorter
B. shorter, longer
C. longer, shorter
D. longer, longer*
Comment: The asterisk indicates the intended correct answer is choice "d". The question is technically incorrect because it specifies equal reactivity additions and is not operational because there is no way for the operator to perform "equal" reactivity change. Restate the question as follows:
Question 34: (revised) As the reactor approaches criticality using the power doubling procedure, the time required to reach a constant neutron population will be (shorter/longer) and the time required for the period to reach infinite seconds will be (shorter/longer).
A. shorter, shorter
B. shorter, longer
C. longer, shorter
D. longer, longer*
Question 35: As the reactor approaches criticality during a reactor startup it takes longer to reach an equilibrium neutron count rate after each control rod withdrawal due to the increased
a. fraction of fission neutrons leaking from the core
b. number of neutron generations required to reach a stable level*
c. length of time from neutron generation to absorption
d. fraction of delayed neutrons appearing as criticality is approached
Comment: The asterisk indicates the intended correct answer is choice "b". The question is technically incorrect for three reasons. First, the size of the reactivity changes introduced are not specified. Secondly, the answer is based on the lifecycle, or generation time, model to attempt to describe actual reactor behavior. In this model, the prompt and delayed neutrons are lumped into an average neutron with a so called generation time. This primitive lifecycle model is incapable of representing actual reactor behavior. Generation time does not appear in either the prompt jump or reactor period equation. Thirdly, choices "c" and "d" are also correct answers. Choice "d" is consistent with question #31. Restate the question as follows:
Question 35: (revised) As the reactor approaches criticality during a reactor startup that uses the power doubling procedure, it takes longer to reach an equilibrium neutron count rate after each control rod withdrawal due to the increased
a. fraction of fission neutrons leaking from the core
b. delayed neutron source strength*
c. prompt fission neutrons
d. effect of fast fissions on the multiplication process
Question 38: When the reactor is exactly critical, core reactivity is
a. greater than zero
b. equal to zero*
c. less than zero
d. undefined
Comment: The asterisk indicates the intended correct answer is choice "b". The question is technically incorrect because of improper terminology. No adjectives, such as "exactly", are required to define criticality. Criticality is a very specific and widely understood condition. Restate the question as follows:
Question 38: (revised) When a reactor is critical, core reactivity is
a. greater than zero
b. equal to zero*
c. less than zero
d. equal to unity
Question 39: When the reactor is exactly critical, reactivity is
a. greater than 1.0 delta-k/k
b. equal to 1.0 delta-k/k
c. less than 1.0 *
d. undefined
Comment: The asterisk indicates the intended correct answer is choice "c". The question is technically incorrect for the same reason given for Question #38. Also, no units are given for the so called correct choice. Restate the question as follows:
Question 39: (revised) When a reactor is critical, reactivity is
a. greater than 1.0 delta-k/k
b. equal to 1.0 delta-k/k
c. less than 1.0 delta-k/k*
d. undefined
Question 40: For a reactor that is exactly critical, and in which source neutrons are negligible, complete he following statement to best describe the change in neutron population in the core from one generation to the next generation. The neutron population __________ the neutron population in the previous generation.
a. fluctuates above and below
b. is less than
c. is greater than
d. is equal to*
Comment: The asterisk indicates the intended correct answer is choice "d". In addition to poor terminology, "exactly critical" and "best describe", the question is technically incorrect because it uses the generation time model to explain actual reactor behavior. Restate the question as follows:
Question 40: (revised) For a reactor that is critical, and in which source neutrons are negligible, complete he following statement to describe the change in neutron population in the core from one prompt neutron lifetime to the next. The neutron population __________ the neutron population in the previous prompt neutron lifetime.
a. fluctuates above and below
b. is less than
c. is greater than
d. is equal to*
Question 42: With keff = 0.985, how much reactivity must be added to make the reactor critical?
a. 1.47% Delta-k/k
b. 1.50% Delta-k/k
c. 1.52% Delta-k/k*
d. 1.61% Delta-k/k
Comment: The asterisk indicates the intended correct answer is choice "c". The question is technically incorrect because of erroneous terminology, "reactivity is added" instead of "reactivity change is introduced". Also, the choices are indicated to represent reactivity instead of reactivity change. The final reactivity condition will be rhofinal = rhoinitial + delta-rho. Delta-rho, or reactivity change, is introduced to move from the inital reactivity condition to the final reactivity condition. The algebraic sign of the change should be provided. Delta-k/k is not delta-rho, it is rho. Restate the question as follows:
Question 42: (revised) With keff = 0.985, what reactivity change must be introduced to make the reactor critical?
a. +1.47% Delta-rho
b. +1.50% Delta-rho
c. +1.52% Delta-rho*
d. +1.61% Delta-rho
Question 43: With keff = 0.985, how much reactivity must be added to make the reactor critical?
a. 0.0148 Delta-k/k
b. 0.0150 Delta-k/k
c. 0.0152 Delta-k/k*
d. 0.0154 Delta-k/k
Comment: The asterisk indicates the intended correct answer is choice "c". The answer is technically incorrect for the same reason given for question #42. Restate the question as follows:
Question 43: (revised) With keff = 0.985, what reactivity change must be introduced to make the reactor critical?
a. +0.0148 Delta-rho
b. +0.0150 Delta-rho
c. +0.0152 Delta-rho*
d. +0.0154 Delta-rho
Question 44: A reactor startup is in progress at your plant. The reactor operator entered in his log that the initial source range count rate prior to control rod withdrawal was 10 cps on each source range instrument. Rods are pulled for criticality, pausing per the startup procedure to monitor the count rate. The reactor operator notices that counts have now stabilized at 20 cps. Complete the following statement that best describes the size of the reactivity addition. The reactivity addition was approximately
a. twice the total amount required to reach criticality
b. half the total amount required to reach criticality*
c. equal to the total amount required to reach critcality
d. a quarter of the total amount required to reach criticality
Comment: The asterisk indicates the intended correct answer is choice "b". The question is technically incorrect because of erroneous terminology, "reactivity addition" instead of "reactivity change is introduced". Also, the count rate does not stabilize. The use of "best describes" is inappropriate; there is only one correct answer. Restate the question as follows:
Question 44: (revised) A reactor startup is in progress at your plant. The reactor operator entered in his log that the initial source range count rate prior to control rod withdrawal was 10 cps on each source range instrument. Rods are pulled for criticality, pausing per the startup procedure to monitor the count rate. The reactor operator notices that counts have now reached equilibrium at 20 cps. Complete the following statement to describe the size of the reactivity change that was introduced. The reactivity change was approximately
a. twice the total change required to reach criticality
b. half the total change required to reach criticality*
c. equal to the total change required to reach critcality
d. a quarter of the total change required to reach criticality
Question 45: The reactor is critical and critical data has been recorded. Which of the following best describes reactor response to a subsequent withdrawal of control rods?
a. Reactor power increases to or slightly above the point of adding heat and then levels off.*
b. Reactor power doesn't change, but reactor coolant temperature increases.
c. Reactor power increases until the operator drives rods back in to level off power.
d. Reactor power doesn't change, but fuel temperature increases.
Comment: The asterisk indicates the intended correct answer is choice "a". In addition to "best describes", the question is technically incorrect because the initial conditions are not specified, which determine what the response will be. The wording implies that the reactor power levels off above the POAH. This is incorrect. If there are ambient losses or slight steam flow, the reactor power will, eventually settle at a constant value that meets these demands. Since the coolant temperature will then settle to constant value, this reactor power level is just very slightly below the power required to generate sensible heating. It is very slightly below that required for the POAH. Choice "a" is not the correct answer and is inconsistent with question #82, #96, and #97. Restate the question as follows:
Question 45: (revised) The reactor is critical in the Source range and critical data has been recorded. Heat loss from the primary system is at 0.5% of rated power. Which of the following describes reactor response to a subsequent withdrawal of control rods?
a. Reactor power increases to slightly above the POAH, oscillates, and then settles at a power slightly less than the POAH.*
b. Reactor power doesn't change, but reactor coolant temperature increases until the operator makes a small rod insertion.
c. Reactor power increases until the operator drives rods back in to level off power.
d. Reactor power doesn't change, but fuel temperature increases and oscillates around the point of adding heat.
Question 46: During a reactor plant startup, with the reactor supercritical slightly below the point of adding heat, the reactor operator withdraws a control rod one additional notch. This rod motion should initially cause reactor power to
a. take a step increase*
b. slowly ramp upward
c. take a step decrease
d. ramp upward at a diminished rate
Comment: The asterisk indicates the intended correct answer is choice "a". The question is technically incorrect because of improper terminology. A notch control rod movement occurs over about a 2 second time interval, and can be considered to be a "step" change in reactivity. A step change in reactivity causes reactor power to undergo a "prompt jump", not a step change. Restate the question as follows:
Question 46: (revised) During a reactor plant startup, with the reactor supercritical slightly below the point of adding heat, the reactor operator withdraws a control rod one additional notch. This rod motion should initially cause reactor power to
a. undergo prompt jump*
b. slowly ramp upward
c. undergo prompt drop
d. ramp upward at a diminished rate
Question 47: During a reactor startup and after taking critical data, the operator withdraws a control rod one notch and stops. The rod withdrawal initially causes reactor power to take a
a. step increase*
b. ramp increase
c. step decrease
d. ramp decrease
Comment: The asterisk indicates the intended correct answer is choice "a". The question is technically incorrect for the same reason given for question #46. Restate the question as follows:
Question 47: (revised) During a reactor startup and after taking critical data, the operator withdraws a control rod one notch and stops. The rod withdrawal initially causes reactor power to take a
a. prompt jump*
b. ramp increase
c. prompt drop
d. ramp decrease
Question 48: Assume a reactor is critical at a power level below the point of adding heat. For a 0.01% delta-k/k positive reactivity addition, the reactor period will be
a. shorter at a higher reactor coolant temperature
b. longer at a higher reactor coolant temperature
c. shorter at EOL than at BOL*
d. longer at EOL than at BOL
Comment: The asterisk indicates the intended correct answer is choice "c". The question is technically incorrect because of improper terminology. The "positive reactivity change, delta-rho" is not a "delta-k/k positive reactivity addition". Restate the question as follows:.
Question 48: (revised) Assume a reactor is critical at a power level below the point of adding heat. For a 0.01% delta-rho positive reactivity change, the reactor period will be
a. shorter at a higher reactor coolant temperature
b. longer at a higher reactor coolant temperature
c. shorter at EOL than at BOL*
d. longer at EOL than at BOL
Question 49: A __________________ with no further reactivity addition is an indication that a reactor has achieved criticality.
a. constant positive period*
b. slightly increasing period
c. constant negative period
d. positive, slightly decreasing period
Comment: The asterisk indicates the intended correct answer is choice "a". The question is technically incorrect because of improper terminology, "reactivity addition". The proper terminology to decribe a constant period is "stable period". This question can be misleading to the student in that the positive period is taken as an indicator of criticality (see Question #50). Restate the question as follows:.
Question 49: (revised) A __________________ with no further reactivity change is an indication that a reactor has achieved, and exceeded, criticality.
a. stable positive period*
b. slightly increasing period
c. stable negative period
d. positive, slightly decreasing period
Question 50: During a reactor startup, a positive 30-second reactor period is achieved with no further reactivity addition. The reactor is
a. exactly critical
b. supercritical*
c. subcritical
d. prompt critical
Comment: The asterisk indicates the intended correct answer is choice "b". The question is technically incorrect because of improper terminology, "reactivity addition". Restate the question as follows:.
Question 50: (revised) During a reactor startup, a stable +30-second reactor period is achieved with no further reactivity change. The reactor is
a. exactly critical
b. supercritical*
c. subcritical
d. prompt critical
Question 51: If a reactor is exactly critical and Keff is constant, and neglecting source neutrons, the fission rate
a. increases exponentially
b. increases linearly
c. is constant*
d. decreases linearly
Comment: The asterisk indicates the intended correct answer is choice "c". The question is technically incorrect because of improper terminology ... "exactly" critical. Restate the question as follows:
Question 51: (revised) If a reactor is critical, Keff is constant, and source neutrons are negligible, the fission rate
a. increases exponentially
b. increases linearly
c. is constant*
d. decreases linearly
Question 53: If a reactor is exactly critical, and source neutrons are negligible, the reactor period is
a. infinite*
b. -80 seconds
c. +80 seconds
d. zero
Comment: The asterisk indicates the intended correct answer is choice "a". The question is technically incorrect because of improper terminology, "exactly" critical. Also the units of reactor period, seconds, are not supplied for all choices. The accepted and standard unit for reactor period is "seconds". Restate the question as follows:
Question 53: (revised) If a reactor is critical, and source neutrons are negligible, the reactor period is
a. infinite seconds*
b. -80 seconds
c. +80 seconds
d. zero seconds
Question 54: If a reactor is exactly critical, and source neutrons are negligible, the neutron flux is
a. increasing
b. constant*
c. decreasing
d. fluctuating
Comment: The asterisk indicates the intended correct answer is choice "b". The question is technically incorrect because of improper terminology ... "exactly" critical. Restate the question as follows:
Question 54: (revised) If a reactor is critical, and source neutrons are negligible, the neutron flux is
a. increasing
b. constant*
c. decreasing
d. fluctuating
Question 55 Assume a reactor is critical at a power level below the point of adding heat. For an equal positive reactivity insertion, the reactor period would be:
a. shorter if the core were xenon-free
b. longer at EOL than at BOL
c. shorter at EOL than at BOL*
d. longer at higher moderator temperature
Comment: The asterisk indicates the intended correct answer is choice "c". The question is technically incorrect because of improper terminology. Introduction of a positive reactivity change is not a "positive reactivity insertion". Restate the question as follows:
Question 55: (revised) Assume a reactor is critical at a power level below the point of adding heat. For a given introduction of positive reactivity change, the reactor period will be:
a. shorter if the core were xenon-free
b. longer at EOL than at BOL
c. shorter at EOL than at BOL*
d. longer at higher moderator temperature
Question 56: Given a reactor that is critical below the point of adding heat, explain how the following parameter changes will affect reactor power.
a. coolant temperatures decreases 3°F
b. xenon concentration decreases
c. a single control rod moves in one notch
Answer: With the reactor critical below the point of adding heat (POAH), any positive reactivity insertion will result in a power increase to the POAH. Any negative reactivity insertion will reduce power to a subcritical equilibrium level.
a. A decrease in coolant temperature inserts positive reactivity due to the negative temperature coefficient. Power will increase.
b. A decrease in xenon concentration removes poison from the core, inserting positive reactivity. Power will increase.
c. A rod insertion inserts negative reactivity. Power will decrease. (Reference 73, chapter 6 and 7)
Comment: The answer is technically incorrect because of improper terminology. Reactivity is not inserted. A reactivity change in introduced. Restate the question as follows:
Answer: (revised) With the reactor critical below the point of adding heat (POAH), any introduction of positive reactivity change will result in a power increase to the POAH. Any introduction of negative reactivity change will reduce power to a subcritical equilibrium level.
a. A decrease in coolant temperature introduces a positive reactivity change due to the negative temperature coefficient. Power will increase.
b. A decrease in xenon concentration removes poison from the core, introducing a positive reactivity change. Power will increase.
c. A rod insertion introduces negative reactivity change. Power will decrease.
Question 59: A reactor that has been shut down for one week is being started up. Control rods have been withdrawn to make Keff equal to exactly one. If no further reactivity insertions are made over the next several minutes, describe how reactor power will change.
Answer: During a reactor startup, criticality will typically be attained with power in the source range, i.e., at a level where source neutron effects are observable.
The fission process in a critical reactor is exactly self-sustaining, but the continual addition of source neutrons will cause neutron population, and therefore power, to increase linearly.
Note that this linear increase will appear as a "tapering-off" on the logarithmic source range instrument. (Reference 73, page 8-54)
Comment: The answer is technically incorrect because whether or not source neutrons are significant depends on the reactivity rate as criticality is approached. The faster the rate, the more significant the non-fission source neutrons because delayed neutrons lag further behind the total neutron population. In a BWR the reactivity rate is so low that the non-fission source neutrons are almost always insignificant on attaining criticality. A linear increase in power will rarely, if ever, be seen or recognized. In addition, the question, as posed, is not operational or consistent with several other questions about identification of criticality. Delete the question. The linear increase in power is not important.
Question 63: A reactor is being started up with a stable 100-second period and power entering the intermediate range. Assuming no operator action, which of the following is true?
a. Reactor period will remain constant through all ranges of intermediate range indication.
b. As heat production in the reactor exceeds ambient losses, the temperature of the moderator increases, adding negative reactivity, and reactor period goes to infinity.*
c. As heat production in the reactor exceeds ambient losses, the resulting fuel temperature increase adds positive reactivity to counteract the negative reactivity added by increased moderator temperature.
d. Prior to reaching the point of adding heat, fuel temperature increases, adding positive reactivity, which causes period to become shorter and shorter until a scram occurs on short period.
Comment: The asterisk indicates the intended correct answer is choice "b". The question is technically incorrect because of incorrect terminology ... "reactivity is added" instead of "introduces a positive or negative reactivity change". However, note the use of "stable" 100-second period. This is correct. And the reason that the expression "stable" power should be avoided. Units of seconds should be included with all reactor period values. Restate the question as follows:
Question 63: (revised) A reactor is being started up with a stable 100-second period and power entering the intermediate range. Assuming no operator action, which of the following is true?
a. Reactor period will remain constant through all ranges of the Intermediate range indication.
b. As heat production in the reactor exceeds ambient losses, the resulting moderator temperature increase introduces a negative reactivity change, which causes the reactor period to decelerate to infinite seconds.*
c. As heat production in the reactor exceeds ambient losses, the resulting fuel temperature increase introduces a positive reactivity change, which counteracts the negative reactivity change introduced by increased moderator temperature.
d. Prior to reaching the point of adding heat, the resulting fuel temperature increase introduces a positive reactivity change, which causes period to become shorter and shorter until a scram occurs on short period.