Core Thermal Limits

Question 1

A nuclear power plant is operating at steady-state 80 percent power in the middle of a fuel cycle. All
control rods are fully withdrawn and in manual control. Core axial power distribution is peaked
below the core midplane.
Which one of the following will cause the maximum axial peaking (or hot channel) factor to initially
decrease?

Answer 1

Turbine load/reactor power is reduced by 10 percent.

Question 2

A reactor is operating at 80 percent power near the middle of a fuel cycle. All control rods are nearly
fully withdrawn and in manual control. Core axial power distribution is peaked below the core
midplane.
Which one of the following will increase the core maximum axial peaking (or hot channel) factor?
(Assume no operator action is taken unless stated, and that main turbine load and core xenon
distribution do not change unless stated.)

Answer 2

Reactor coolant system boron concentration is reduced by 15 ppm.

Question 3

A PWR core consists of 50,000 fuel rods; each fuel rod has an active length of 12 feet. The core is
producing 1,800 MW of thermal power. If the total heat flux hot channel factor (also called the total
core peaking factor) is 2.0, what is the maximum linear power density being produced in the core?

Answer 3

6.0 kW/ft

Question 4

A PWR core consists of 50,000 fuel rods; each fuel rod has an active length of 12 feet. The core is
producing 1,800 MW of thermal power. If the total heat flux hot channel factor (also called the total
core peaking factor) is 1.5, what is the maximum linear power density being produced in the core?

Answer 4

4.5 kW/ft

Question 5

A PWR core consists of 50,000 fuel rods; each fuel rod has an active length of 12 feet. The core is
producing 1,800 MW of thermal power. If the total heat flux hot channel factor (also called the total
core peaking factor) is 3.0, what is the maximum linear power density being produced in the core?

Answer 5

9.0 kW/ft

Question 6

A reactor is operating at 3,400 MW thermal power. The core linear power density limit is 12.2 kW/ft.
Given:
C The reactor core contains 198 fuel assemblies.
C Each fuel assembly contains 262 fuel rods, each with an active length of 12 feet.
C The highest total peaking factors measured in the core are as follows:
Location A: 2.5
Location B: 2.4
Location C: 2.3
Location D: 2.2
Which one of the following describes the operating conditions in the core relative to the linear power
density limit?

Answer 6

Locations A, B, and C have exceeded the linear power density limit while location D is operating
below the limit

Question 7

A reactor is operating at steady-state conditions in the power range with the following average
temperatures in a core plane:
Tcoolant = 550°F
Tfuel centerline = 1,680°F
Assume the fuel rod heat transfer coefficients and reactor coolant temperatures are equal throughout
the core plane. If the maximum total peaking factor in the core plane is 2.1, what is the maximum fuel
centerline temperature in the core plane?

Answer 7

2,923°F

Question 8

A reactor is operating at 3,300 MW thermal power. The core linear power density limit is 12.4 kW/ft.
Given:
C The reactor core contains 198 fuel assemblies.
C Each fuel assembly contains 262 fuel rods, each with an active length of 12 feet.
C The highest total peaking factors measured in the core are as follows:
Location A: 2.5
Location B: 2.4
Location C: 2.3
Location D: 2.2
Which one of the following describes the operating conditions in the core relative to the linear power
density limit?

Answer 8

Locations A and B have exceeded the linear power density limit while locations C and D are
operating below the limit.

Question 9

What is the basis for the limit on maximum linear power density (kW/ft)?

Answer 9

To provide assurance of fuel integrity.

Question 10

If a reactor is operated within the core thermal limits, then…

Answer 10

fuel cladding integrity is ensured.

Question 11

The 2,200°F maximum fuel cladding temperature limit is imposed because…

Answer 11

the rate of the zircaloy-steam reaction increases significantly at temperatures above 2,200°F.

Question 12

During normal operation, fuel cladding integrity is ensured by…

Answer 12

operating within core thermal limits.

Question 13

Maximum fuel cladding integrity is maintained by…

Answer 13

ensuring that actual heat flux is always less than critical heat flux

Question 14

Peaking (or hot channel) factors are used to establish a maximum reactor power level such that fuel
pellet temperature is limited to prevent __________ of the fuel pellets; and fuel cladding temperature
is limited to prevent __________ of the fuel cladding during most analyzed transients and abnormal
conditions.

Answer 14

melting; excessive oxidation

Question 15

Reactor thermal limits are established to…

Answer 15

ensure the integrity of the reactor fuel.

Question 16

Thermal limits are established to protect the reactor, and thereby protect the public during nuclear
power plant operations, which include…

Answer 16

normal, abnormal, and postulated accident operations only.

Question 17

Which one of the following describes the basis for the 2,200°F maximum fuel cladding temperature
limit?

Answer 17

The rate of the zircaloy-water reaction increases significantly at temperatures above 2,200°F.

Question 18

The initial stable parameters for a fuel rod segment are as follows:
Power density = 3 kW/ft
Tcoolant = 579°F
Tfuel centerline = 2,400°F
After a reactor power increase, the current stable parameters for the same fuel rod segment are as
follows:
Power density = 5 kW/ft
Tcoolant = 590°F
Tfuel centerline = ?
Assume the reactor coolant flow rate has not changed and the reactor coolant is not boiling. What is
the stable Tfuel centerline at the higher power level?

Answer 18

3,625°F

Question 19

Which one of the following describes the basis for the 2,200°F maximum fuel cladding temperature
limit?

Answer 19

The rate of the zircaloy-steam reaction increases significantly above 2,200°F.

Question 20

Refer to the partial drawing of a fuel rod and coolant flow channel (see figure below).
Given the following initial core parameters:
Reactor power = 100 percent
Tcoolant = 500°F
Tfuel centerline = 3,000°F
What would the fuel centerline temperature be if the total fuel-to-coolant thermal conductivity
doubled? (Assume reactor power and Tcoolant are constant.)

Answer 20

1,750°F

Question 21

The pellet-to-cladding gap in fuel rod construction is designed to…

Answer 21

reduce internal cladding strain

Question 22

Refer to the partial drawing of a fuel rod and coolant flow channel (see figure below).
Given the following initial core parameters:
Reactor power = 100 percent
Tcoolant = 500°F
Tfuel centerline = 2,500°F
What would the fuel centerline temperature be if the total fuel-to-coolant thermal conductivity
doubled? (Assume reactor power and Tcoolant are constant.)

Answer 22

1,500°F

Question 23

A reactor is operating at steady-state 80 percent power with all control rods fully withdrawn and in
manual control. Compared to a 50 percent insertion of one control rod, a 50 percent insertion of a
group (or bank) of control rods will cause a __________ increase in the maximum axial peaking factor
and a __________ increase in the maximum radial peaking factor. (Assume reactor power remains
constant.)

Answer 23

larger; smaller

Question 24

Refer to the partial drawing of a fuel rod and coolant flow channel (see figure below).
Given the following initial core parameters:
Reactor power = 100 percent
Tcoolant = 500°F
Tfuel centerline = 2,700°F
What would the fuel centerline temperature be if the total fuel-to-coolant thermal conductivity
doubled? (Assume reactor power and Tcoolant are constant.)

Answer 24

1,600°F

Question 25

A reactor is operating at 80 percent power with all control rods fully withdrawn. Compared to a 50
percent insertion of a group (or bank) of control rods, a 50 percent insertion of a single control rod will
cause a __________ increase in the maximum axial peaking factor and a __________ increase in the
maximum radial peaking factor. (Assume reactor power remains constant.)

Answer 25

smaller; larger

Question 26

Which one of the following describes the fuel-to-coolant thermal conductivity for a fuel rod at the end
of a fuel cycle (EOC) when compared to the beginning of the same fuel cycle (BOC)?

Answer 26

Larger at EOC, due to reduction in gap between the fuel pellets and cladding.

Question 27

Refer to the partial drawing of a fuel rod and coolant flow channel (see figure below).
Given the following initial core parameters:
Reactor power = 80 percent
Tcoolant = 540°F
Tfuel centerline = 2,540°F
What would the fuel centerline temperature be if the total fuel-to-coolant thermal conductivity
doubled? (Assume reactor power and Tcoolant are constant.)

Answer 27

1,540°F

Question 28

Which one of the following describes the fuel-to-coolant thermal conductivity for a fuel rod at the
beginning of a fuel cycle (BOC) compared to the end of a fuel cycle (EOC)?

Answer 28

Smaller at BOC, due to a larger gap between the fuel pellets and cladding.

Question 29

Refer to the partial drawing of a fuel rod and coolant flow channel (see figure below).
Given the following initial core parameters:
Reactor power = 60 percent
Tcoolant = 560°F
Tfuel centerline = 2,500°F
What would the fuel centerline temperature be if the total fuel-to-coolant thermal conductivity
doubled? (Assume reactor power and Tcoolant are constant.)

Answer 29

1,530°F

Question 30

Refer to the partial drawing of a fuel rod and coolant flow channel (see figure below).
The reactor is shut down with the following parameter values:
Tcoolant = 320°F
Tfuel centerline = 780°F
What would the fuel centerline temperature be if the total fuel-to-coolant thermal conductivity
doubled? (Assume core decay heat level and Tcoolant are constant.)

Answer 30

550°F

Question 31

Refer to the partial drawing of a fuel rod and coolant flow channel (see figure below).
The reactor is shut down at the beginning of a fuel cycle with the following average parameter values:
Tcoolant = 440°F
Tfuel centerline = 780°F
What will the fuel centerline temperature be at the end of the fuel cycle with the same coolant
temperature and reactor decay heat conditions if the total fuel-to-coolant thermal conductivity
doubles?

Answer 31

610°F

Question 32

Refer to the partial drawing of a fuel rod and coolant flow channel (see figure below).
Given the following initial core parameters:
Reactor power = 50 percent
Tcoolant = 550°F
Tfuel centerline = 2,750°F
What will the fuel centerline temperature be if the total fuel-to-coolant thermal conductivity doubles?
(Assume reactor power and Tcoolant are constant.)

Answer 32

1,650°F

Question 33

Refer to the partial drawing of a fuel rod and coolant flow channel (see figure below).
Given the following initial stable core parameters:
Reactor power = 50 percent
Tcoolant = 550°F
Tfuel centerline = 2,250°F
Assume the total heat transfer coefficient and the reactor coolant temperature do not change. What
will the stable fuel centerline temperature be if reactor power is increased to 75 percent?

Answer 33

3,100°F

Question 34

Consider a new fuel rod operating at a constant power level for several weeks. During this period,
fuel pellet densification in the fuel rod causes the heat transfer rate from the fuel pellets to the cladding
to __________; this change causes the average fuel temperature in the fuel rod to __________.

Answer 34

decrease; increase

Question 35

If fuel pellet densification occurs in a fuel rod producing a constant power output, the average linear
power density in the fuel rod will __________ because pellet densification causes fuel pellets to
__________.

Answer 35

increase; shrink