Question 51 Addressing each term in the six-factor formula, explain why a change in void fraction causes a change in core reactivity.
Answer: The effect of a increase in void fraction on the six-factor formula is: ... thermal utilization factor - Increases, because fewer neutrons are absorbed by the less-dense moderator. This is a significant effect, especially when boron is dissolved in the moderator.
Comment: The answer is technically incorrect in discussing the thermal utilization factor because an operating BWR does not have boron in the moderator.
Question 54: The void coefficient of reactivity (alpha-V) is defined as the
a. change in reactivity caused by a unit change in void quality
b. change in reactivity caused by a unit change in void fraction*
c. change in reactivity caused by a unit change in reactor vessel steam pressure
d. change in reactivity caused by a unit change in core flow
Comment: The asterisk indicates the intended correct answer is choice "b". The question is technically incorrect for the reason given in question #1. Restate the question as follows:
Question 54: (revised) The void coefficient of reactivity (alpha-V) is defined as the
a. change in reactivity caused by a unit change in void quality
b. change in reactivity caused by a 1% increase in void fraction*
c. change in reactivity caused by a 1 psig decrease in reactor vessel steam pressure
d. change in reactivity caused by a unit change in core flow
Question 56: Which of the following best describes the mechanism by which an increase in voids adds negative reactivity?
a. increases reactor pressure
b. diminishes fuel pellet temperature
c. increases neutron slowing down time*
d. alters the axial flux profile
Comment: The asterisk indicates the intended correct answer is choice "c". The question is technically incorrect for the reason given in question #6. The increase in voids causes a negative reactivity "change". Restate the question as follows:
Question 56: (revised) Which one of the following causes a negative reactivity change with increasing voids?
a. increasing reactor pressure
b. decreasing fuel pellet temperature
c. increasing neutron slowing down time*
d. change in the axial flux profile
Question 60: As the percent of the voids in the core increases, the void coefficient
a. becomes more negative*
b. remains unchanged beyond 15% void percent
c. remains the same
d. becomes less negative
Comment: The asterisk indicates the intended correct answer is choice "a". The question is technically incorrect for the reason given in question #9. Restate the question as follows:
Question 60: (revised) As the percent of the voids in the core increases, the negative void coefficient
a. becomes larger*
b. remains unchanged beyond 15% void percent
c. remains the same
d. becomes smaller
Question 62: The magnitude of the void coefficient will change with an increasing void fraction. Which statement most accurately describes how and why this change occurs?
a. more negative due to the voids building into areas of the core with higher neutron flux*
b. more negative due to larger change in moderator density at higher power levels
c. less negative due to greater absorption of neutrons in fuel since neutrons will spend a longer time in the thermal energy range
d. less negative due to the increased absorption of neutrons in U-238
Comment: The asterisk indicates the intended correct answer is choice "a". The question is technically incorrect for the reason given in question #9. The magnitude does not become more or less negative. Restate the question as follows:
Question 62: (revised) The magnitude of the negative void coefficient will change with an increasing void fraction. Which statement describes how and why this change occurs?
a. larger due to the voids building into areas of the core with higher neutron flux*
b. larger due to larger change in moderator density at higher power levels
c. smaller due to greater absorption of neutrons in fuel since neutrons will spend a longer time in the thermal energy range
d. smaller due to the increased absorption of neutrons in U-238
Question 70: Given a reactor at low power with the following coefficient magnitudes
Doppler = 1x10-5 delta-K/K/°F
Moderator temperature = 1x10-4 delta-K/K°F
Void = 1x10-3 delta-K/K/% void
If the fuel temperature increases 1000°F, the moderator temperature increases 400°F, and voids increase by 20 percent, what is the expected net change in reactivity?
a. -7 x 10-2 delta-K/K*
b. +4.2 x 10-2 delta-K/K
c. +7 x 10-2 delta-K/K
d. -2.8 x 10-1 delta-K/K
Comment: The asterisk indicates the intended correct answer is choice "a". The question is technically incorrect because the coefficients are indicated to represent rho/oF, instead of delta-rho/oF ... and because the choices of net change in reactivity are indicated to represent rho, instead of delta-rho. Restate the question as follows:
Question 70: (revised) Given a reactor at low power with the following coefficient magnitudes
Doppler = 1x10-5 delta-rho/°F
Moderator temperature = 1x10-4 delta-rho/°F
Void = 1x10-3 delta-rho/% void
Apply the appropriate algebraic sign to each coefficient. If the fuel temperature increases 1000°F, the moderator temperature increases 400°F, and voids increase to 20 percent of coolant volume, what is the expected net change in reactivity?
a. -7 x 10-2 delta-rho*
b. +4.2 x 10-2 delta-rho
c. +7 x 10-2 delta-rho
d. -2.8 x 10-1 delta-rho
Question 73: Which of the following operating conditions is associated with a less negative fuel temperature coefficient?
a. increase in moderator temperature
b. increase in fuel temperature*
c. increase in core age
d. increase in void concentration
Comment: The asterisk indicates the intended correct answer is choice "b". The question is technically incorrect for the reason given in question #9. Restate the question as follows:
Question 73: (revised) Which of the following operating conditions is associated with a smaller negative fuel temperature coefficient?
a. increase in moderator temperature
b. increase in fuel temperature*
c. increase in core age
d. increase in void concentration
Question 78: Rank the following reactivity coefficients in decreasing order of magnitude. State a typical value for each
moderator temperature
fuel temperature (Doppler)
void
Answer: Typical values, in decreasing order of magnitude, are as follows:
void coefficient: -1 x 10-3 delta-K/K/%
moderator temperature coefficient: -1 x 10-4 delta-K/K/°F
Doppler coefficient: -1 x 10-5 delta-K/K/°F
(Reference 27, volume II, chapter 4)
Comment: The question is technically incorrect for the reason given in question #3. Restate the answer as follows:
Answer: (revised) Typical values, in decreasing order of magnitude, are as follows:
void coefficient: -1 x 10-3 delta-rho/% voids
moderator temperature coefficient: -1 x 10-4 delta-rho/°Fmod
Doppler coefficient: -1 x 10-5 delta-rho/°Ffuel
Question 84: Which one of the following describes the net reactivity effect of a decrease in moderator temperature in an undermoderated reactor core?
a. Negative reactivity will be added because more thermal neutrons will be captured by the moderator.
b. Negative reactivity will be added because more neutron leakage will occur.
c. Positive reactivity will be added because less neutron leakage will occur.*
d. Positive reactivity will be added because less thermal neutrons will be captured by the moderator.
Comment: The asterisk indicates the intended correct answer is choice "c". The question is technically incorrect because "reactivity" will not be added. A reduction in moderator temperature will introduce a positive change in reactivity. Neutron leakage is not the only reactivity contributor in this case. In addition, what is a "net" reactivity change supposed to mean? Net compared to what? Restate the question as follows:
Question 84: (revised)Which one of the following describes the reactivity effect of a decrease in moderator temperature in an undermoderated reactor core?
a. A negative change in reactivity will occur because more thermal neutrons will be captured by the moderator.
b. A negative change in reactivity will occur because more neutron leakage will occur.
c. A positive change in reactivity will occur because less neutron leakage will occur.*
d. A positive change in reactivity will occur because fewer thermal neutrons will be captured by the moderator.
If you disagree with any of our comments on these questions, or would care to add further enlightenment, 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 Control Rods.