Comments pertaining to questions 1048, 1648, 2148, 2248, and 2448 are slightly modified from the original posting on 3/15/01. Specifically, the indication that the four choices represent reactivity is correct. The five questions remain technically incorrect for the reasons given herein.
Which one of the following conditions describes a reactor that is exactly critical?
A. Keff = 1; delta-K/K = 0
B. Keff = 1; delta-K/K = 1
C. Keff = 0; delta-K/K = 0
D. Keff = 0; delta-K/K = 1
ANSWER: A.
Comment: The question is technically incorrect because it is not necessary to qualify the condition of criticality as "exact." Criticality is a unique condition, in and of itself.
QID: B1447 (P1346) (TOPIC: 292002 KNOWLEDGE: K1.08 [2.7/2.8])
The effective multiplication factor (Keff) can be determined by dividing the number of neutrons in the third generation by the number of neutrons in the ____________ generation.
A. first
B. second
C. third
D. fourth
ANSWER: B.
Comment: The question is technically incorrect because the "number of neutrons" in any generation is continually diminishing as the life cycle is traversed. In defining keff as a ratio of neutrons in successive generations, it is always necessary to specifically identify the point in the life cycle at which the ratio is taken.
QID: B1247 (N/A) (TOPIC: 292002 KNOWLEDGE: K1.09 [2.4/2.6])
Which one of the following combinations of core conditions at 35% power indicates the least amount of excess reactivity exists in the core?
| CONTROL ROD POSITION | REACTOR RECIRCULATION FLOW |
||
| A. | 50% inserted | 50% | |
| B. | 50% inserted | 25% | |
| C. | 25% inserted | 50% | |
| D. | 25% inserted | 25% |
ANSWER: C.
Comment: The question is technically incorrect because it uses meaningless terminology. On BWRs, control rod position is specified in terms of "control rod density", as used in question B1147.
QID: B1848 (P646) (TOPIC: 292002 KNOWLEDGE: K1.09 [2.4/2.6])
Which one of the following defines K-excess?
A. Keff - 1
B. Keff + 1
C. (Keff-1)/Keff
D. (1-Keff)/Keff
ANSWER: A.
Comment: This question is technically incorrect because (Keff - 1) does not define K-excess. (Keff - 1) defines delta-K, which can be used to determine the departure from criticality of any off-critical condition. K-excess is a hypothetical reactor condition with all control rods withdrawn. (Keff - 1) can be used to calculate the supercritical delta-K for this condition.
QID: B2048 (P1246) (TOPIC: 292002 KNOWLEDGE: K1.09 [2.4/2.6])
Which one of the following is a reason for installing excess reactivity (kexcess) in the core?
A. To compensate for burnout of Xe-135 and Sm-149 during power changes
B. To ensure the fuel temperature coefficient remains negative throughout core life
C. To compensate for the negative reactivity added by the power defect during a power increase
D. To compensate for the conversion of U-238 to Pu-239 over core life
ANSWER: C.
Comment: The question is technically incorrect because the power defect does not "add reactivity." Reactivity is a physical property of the core ... a measure of the nuclear state of one particular reactor condition, in terms of its deviation from criticality. An existing reactivity condition may be altered, or changed, but not by adding "reactivity." In addition, adding "negative" anything is usually considered to be removal or subtraction. As an analogy, an existing core temperature may be altered, or changed, but not by adding "temperature."
The difference between two nuclear states, as in this question, is:
|
50.1 |
and on rearranging terms:
 |
50.2 |
Notice that in moving to a new reactivity condition, rho2, reactivity change, delta-rho, is added to the initial reactivity, rho1. Or, conversely, notice that reactivity, rho, is definitely not added to the initial reactivity. Although use of terminology to indicate reactivity change is preferred, to say that an "increment" of reactivity is added is certainly better than saying reactivity is added.
QID: B648 (P1946) (TOPIC: 292002 KNOWLEDGE: K1.11 [3.2/3.3])
In a subcritical reactor, Keff was increased from 0.85 to 0.95 by rod withdrawal. Which one of the following is closest to the amount of reactivity that was added to the core?
A. 0.099 delta-K/K
B. 0.124 delta-K/K
C. 0.176 delta-K/K
D. 0.229 delta-K/K
ANSWER: B.
Comment: This question suffers the same defect as B2048. "Reactivity" is not added to the core; and there is no correct answer. The units shown for all choices (delta-k/k = reactivity) are incorrect because reactivity has no units. Reactivity is dimensionless. In this question, where numeric choices for reactivity change are given, they are indicated to represent reactivity. Mathematically, according to equation 50.2, this is wrong. Reactivity (rho) is never inserted into a reactor. A change in reactivity (delta-rho) is introduced.
Having said all this, we recommend attaching delta(delta-k/k) to the four choices listed, to indicate what the number represents. Delta-rho gives a correct description of what the value represents, delta-k/k does not ... it is incorrect. A question like this always offers a good opportunity to give plus and minus choices of the same magnitude to determine if the student understands how to evaluate reactivity change. The question, as stated, gives away the algebraic sign of the change. In any case, it is always good practice to include the algebraic sign of the reactivity change with the numeric value. The sloppy (and incorrect) terminology of this question raises a particularly concern, because understanding reactivity and reactivity change is essential to understanding reactor behavior.
QID: B748 (TOPIC: 292002 KNOWLEDGE: K1.12 [2.4/2.5])
With Keff equal to 0.983, how much reactivity must be added to make the reactor exactly critical? (Round answer to nearest 0.01% delta-K/K)
A. 1.70% delta-K/K
B. 1.73% delta-K/K
C. 3.40% delta-K/K
D. 3.43% delta-K/K
ANSWER: B.
Comment: As per question B247, it is not necessary to qualify the condition of criticality as "exact." In addition, this question suffers the same defect as B2048, "reactivity" is not added to the core. And per B648, there is no correct answer.
QID: B1548 (P446) (TOPIC: 292002 KNOWLEDGE: K1.12 [2.4/2.5])
With Keff = 0.987, how much reactivity must be added to make the reactor exactly critical? (Round answer to nearest 0.01% delta-K/K.)
A. 1.30% delta-K/K
B. 1.32% delta-K/K
C. 1.34% delta-K/K
D. 1.36% delta-K/K
ANSWER: B.
Comment: As per question B247, it is not necessary to qualify the condition of criticality as "exact." In addition, this question suffers the same defect as B2048, "reactivity" is not added to the core. And per B648, there is no correct answer.
QID: B1947 (P2447) (TOPIC: 292002 KNOWLEDGE: K1.12 [2.4/2.5])
With Keff = 0.985, how much positive reactivity is required to make the reactor exactly critical?
A. 1.487% delta-K/K
B. 1.500% delta-K/K
C. 1.523% delta-K/K
D. 1.545% delta-K/K
ANSWER: C.
Comment: As per question B247, it is not necessary to qualify the condition of criticality as "exact." In addition, this question suffers the same defect as B2048, "reactivity" is not added to the core, or required. And per B648, there is no correct answer.
QID: B2848 (P2146) (TOPIC: 292002 KNOWLEDGE: K1.12 [2.4/2.5])
With Keff = 0.982, how much positive reactivity is required to make the reactor critical?
A. 1.720% delta-K/K
B. 1.767% delta-K/K
C. 1.800% delta-K/K
D. 1.833% delta-K/K
ANSWER: D.
Comment: This question suffers the same defect as B2048. "Reactivity" is not added to the core, or required; and per B648, there is no correct answer.
QID: B1048 (TOPIC: 292002 KNOWLEDGE: K1.14 [2.6/2.9])
One hour ago, a reactor scrammed from 100% steady state power due to an instrument malfunction. All systems operated normally. Given the following absolute values of reactivities added since the scram, assign a (+) or (-) as appropriate and choose the current value of core reactivity.
Xenon = ( ) 1.0% delta-K/K
Fuel temperature = ( ) 2.0% delta-K/K
Control rods = ( ) 14.0% delta-K/K
Voids = ( ) 3.0% delta-K/K
A. -8.0% delta-K/K
B. -10.0% delta-K/K
C. -14.0% delta-K/K
D. -20.0% delta-K/K
ANSWER: B.
Comment: This question suffers the same defect as B2048. "Reactivity" is not added after the scram; in addition, per B648, the four contributors to reactivity change are improperly indicated as representing reactivity instead of delta-rho.
QID: B1248 (TOPIC: 292002 KNOWLEDGE: K1.14 [2.6/2.9])
Which one of the following will increase the reactivity margin to criticality in a subcritical reactor at 250 F?
A. Decay of Samarium-149
B. Increased core recirculation flow rate
C. Reactor coolant heatup
D. Control rod withdrawal
ANSWER: C.
Comment: The question is technically incorrect because "reactivity margin" is uncommon and undefined terminology. If it refers to "shutdown margin", it is inappropriate because the temperature is not 68F and the control rod positions are not specified. In any case, reactivity margin can be confused with shutdown margin. The question is about "subcritical reactivity."
QID: B1648 (TOPIC: 292002 KNOWLEDGE: K1.14 [2.6/2.9])
A reactor scrammed from 100% steady state power due to an instrument malfunction 16 hours ago. All systems operated normally. Given the following absolute values of reactivities added since the scram, assign a (+) or (-) as appropriate and choose the current value of core reactivity.
Xenon = ( ) 1.5% delta-K/K
Fuel temperature = ( ) 2.5% delta-K/K
Control rods = ( ) 14.0% delta-K/K
Voids = ( ) 3.5% delta-K/K
A. -6.5% delta-K/K
B. -9.5% delta-K/K
C. -11.5% delta-K/K
D. -13.5% delta-K/K
ANSWER: B.
Comment: This question suffers the same defect as B2048. "Reactivity" is not added after the scram; in addition, per B648, the four contributors to reactivity change are improperly indicated as representing reactivity instead of delta-rho.
QID: B1748 (TOPIC: 292002 KNOWLEDGE: K1.14 [2.6/2.9])
Twelve (12) hours ago, a reactor scrammed from 100% steady state power due to an instrument malfunction. All systems operated normally. Given the following absolute values of reactivities added since the scram, assign a (+) or (-) as appropriate and choose the current value of core reactivity.
Xenon = ( ) 2.0% delta-K/K
Fuel temperature = ( ) 2.5% delta-K/K
Control rods = ( ) 14.0% delta-K/K
Voids = ( ) 4.5% delta-K/K
A. -5.0% delta-K/K
B. -9.0% delta-K/K
C. -14.0% delta-K/K
D. -23.0% delta-K/K
ANSWER: B.
Comment: This question suffers the same defect as B2048. "Reactivity" is not added after the scram; in addition, per B648, the four contributors to reactivity change are improperly indicated as representing reactivity instead of delta-rho.
QID: B2148 (TOPIC: 292002 KNOWLEDGE: K1.14 [2.6/2.9])
A reactor scrammed from 100% steady-state power 36 hours ago due to an instrument malfunction 16 hours ago. All systems operated normally. Given the following absolute values of reactivities added since the scram, assign a (+) or (-) as appropriate and choose the current value of core reactivity.
Xenon = ( ) 1.0% delta-K/K
Fuel temperature = ( ) 2.0% delta-K/K
Control rods = ( ) 14.0% delta-K/K
Voids = ( ) 3.0% delta-K/K
A. -8.0% delta-K/K
B. -10.0% delta-K/K
C. -14.0% delta-K/K
D. -20.0% delta-K/K
ANSWER: A.
Comment: This question suffers the same defect as B2048. "Reactivity" is not added after the scram; in addition, per B648, the four contributors to reactivity change are improperly indicated as representing reactivity instead of delta-rho.
QID: B2248 (TOPIC: 292002 KNOWLEDGE: K1.14 [2.6/2.9])
Sixteen hours ago, a reactor scrammed from 100% steady state power due to an instrument malfunction. All systems operated normally. Given the following absolute values of reactivities added since the scram, assign a (+) or (-) as appropriate and choose the current value of core reactivity.
Xenon = ( ) 2.0% delta-K/K
Fuel temperature = ( ) 3.0% delta-K/K
Control rods = ( ) 12.0% delta-K/K
Voids = ( ) 4.0% delta-K/K
A. -5.0% delta-K/K
B. -7.0% delta-K/K
C. -9.0% delta-K/K
D. -11.0% delta-K/K
ANSWER: B.
Comment: This question suffers the same defect as B2048. "Reactivity" is not added after the scram; in addition, per B648, the four contributors to reactivity change are improperly indicated as representing reactivity instead of delta-rho.
QID: B2448 TOPIC: 292002 KNOWLEDGE: K1.14 [2.6/2.9])
A reactor scrammed from 100% steady state power due to an instrument malfunction 30 hours ago. All systems operated normally. Given the following absolute values of reactivities added since the scram, assign a (+) or (-) as appropriate and choose the current value of core reactivity.
Xenon = ( ) 1.5% delta-K/K
Fuel temperature = ( ) 2.5% delta-K/K
Control rods = ( ) 14.0% delta-K/K
Voids = ( ) 3.5% delta-K/K
A. -6.5% delta-K/K
B. -9.5% delta-K/K
C. -11.5% delta-K/K
D. -13.5% delta-K/K
ANSWER: A.
Comment: This question suffers the same defect as B2048. "Reactivity" is not added after the scram; in addition, per B648, the four contributors to reactivity change are improperly indicated as representing reactivity instead of delta-rho.