Thermal Hydraulics

Question 1

Which one of the following is an example of significant radiative heat transfer?

Answer 1

Heat transfer from the fuel cladding to the reactor coolant through a stable vapor layer

Question 2

Refer to the drawing of a pool boiling curve (see figure below). In which region of the curve does the
most efficient form of heat transfer occur?

Answer 2

Region II

Question 3

Refer to the drawing of a pool boiling curve (see figure below).
Which region of the curve contains the operating point at which the hottest locations of a reactor
normally operate to transfer heat from the fuel cladding to the coolant at 100 percent power?

Answer 3

Region II

Question 4

Why does nucleate boiling improve heat transfer in a reactor core?

Answer 4

Heat is removed from the fuel rod as both sensible heat and latent heat of vaporization, and the
motion of the steam bubbles causes rapid mixing of the coolant

Question 5

Convection heat transfer improves when nucleate boiling begins on the surface of a fuel rod because:

Answer 5

the motion of the steam bubbles causes rapid mixing of the coolant.

Question 6

How does the convective heat transfer coefficient vary from the bottom to the top of a fuel assembly if
reactor coolant enters the fuel assembly as subcooled water and exits as superheated steam?

Answer 6

Increases, then decreases.

Question 7

Nucleate boiling affects heat transfer from a fuel rod primarily by…

Answer 7

improving the convective heat transfer from the fuel rod to the coolant.

Question 8

Subcooled water enters the bottom of an operating reactor core. As the water flows upward past the
fuel assemblies, steam bubbles form on the surface of a few fuel rods and are swept away.
If the coolant at the surface of the affected fuel rods had remained subcooled, average fuel temperature
in the affected fuel rods would have been __________ because single-phase convection is a
__________ efficient method of heat transfer than boiling.

Answer 8

higher; less

Question 9

Case 1: Subcooled reactor coolant enters the bottom of a fuel assembly in a reactor operating at
power. As the coolant flows upward through the fuel assembly, the water heats up and exits the fuel
assembly still subcooled.
Case 2: Same as above except that reactor pressure is decreased such that the coolant begins to boil
halfway up the fuel assembly, which results in a saturated steam-water mixture exiting the fuel
assembly.
Assume that departure from nucleate boiling is avoided in both cases and that power level does not
change. As compared to Case 1, the average fuel temperature for Case 2 will be __________ because
boiling is a __________ efficient method of heat transfer.

Answer 9

lower; more

Question 10

Subcooled reactor coolant enters the bottom of a fuel assembly and exits the top of the fuel assembly
as a saturated steam-water mixture. How does the convective heat transfer coefficient change as the
coolant travels upward through the fuel assembly?

Answer 10

Increases only

Question 11

Subcooled water enters a fuel assembly in a reactor operating at power. As the water flows upward
through the fuel assembly, the water begins to boil and exits the fuel assembly as a saturated
steam-water mixture.
If fuel assembly power is unchanged and system pressure is increased such that all of the water
remains subcooled, the average fuel temperature in the fuel assembly would be __________ because
boiling is a __________ efficient method of heat transfer

Answer 11

higher; more

Question 12

Initially, subcooled water is flowing into a fuel assembly with subcooled water exiting the fuel
assembly several degrees hotter than when it entered. No boiling is occurring in the fuel assembly.
Assume that fuel assembly thermal power and water flow rate remain the same.
System pressure is decreased, causing some of the water in contact with the fuel rods to boil during
transit through the fuel assembly, but the water exiting the fuel assembly remains subcooled.
Compared to the initial conditions, the average fuel temperature in the fuel assembly will be
__________; and the temperature of the water exiting the fuel assembly will be __________.

Answer 12

lower; higher

Question 13

Subcooled nucleate boiling is occurring along a heated surface. If the heat flux is increased slightly,
what will be the effect on the differential temperature (ΔT) between the heated surface and the fluid?
(Assume subcooled nucleate boiling is still occurring.)

Answer 13

Small increase in ΔT as vapor bubbles form and collapse.

Question 14

Which one of the following characteristics will enhance steam bubble formation in water adjacent to a
heated surface?

Answer 14

The presence of gases dissolved in the water.

Question 15

What type of boiling is described as follows?
The bulk temperature of the liquid is below saturation, but the temperature of the heat transfer surface
is above saturation. Vapor bubbles form at the heat transfer surface, but condense in the bulk liquid
so that no net generation of vapor is obtained.

Answer 15

Subcooled nucleate boiling

Question 16

Which one of the following is a characteristic of subcooled nucleate boiling but not saturated nucleate
boiling?

Answer 16

TBulk Coolant is less than TSat

Question 17

Which one of the following is a characteristic of saturated nucleate boiling but not subcooled nucleate
boiling?

Answer 17

TBulk Coolant equals TSat

Question 18

Which one of the following describes a reason for the increased heat transfer rate that occurs when
nucleate boiling begins on the surface of a fuel rod?

Answer 18

The motion of the steam bubbles causes rapid mixing of the coolant.

Question 19

Which one of the following modes of heat transfer is characterized by steam bubbles moving away
from a heated surface and collapsing in the bulk fluid?

Answer 19

Subcooled nucleate boiling

Question 20

Which one of the following characteristics will enhance steam bubble formation in the coolant
adjacent to a fuel rod?

Answer 20

Surface scratches or cavities in the fuel cladding.

Question 21

A nuclear power plant is currently shut down after several months of operation at 100 percent power.
The shutdown cooling system is in operation, maintaining an average reactor coolant temperature of
280°F. A pressure control malfunction causes reactor coolant pressure to slowly and continuously
decrease from 100 psia while reactor coolant temperature remains constant.
Which one of the following describes the location where nucleate boiling will first occur?

Answer 21

At a scratch on the surface of a fuel rod near the top of a fuel assembly.

Question 22

If departure from nucleate boiling occurs on the surface of a fuel rod, the surface temperature of the
fuel rod will…

Answer 22

increase rapidly.

Question 23

Which one of the following describes the heat transfer from a fuel rod experiencing departure from
nucleate boiling? (Note: ΔT refers to the difference between the fuel rod surface temperature and
the bulk coolant saturation temperature.)

Answer 23

Steam bubbles begin to blanket the fuel rod surface, causing a rapid increase in the ΔT for a given
heat flux

Question 24

Departure from nucleate boiling should not be allowed to occur in the core because…

Answer 24

as steam bubbles begin to blanket the fuel rod, its temperature rises sharply.

Question 25

Which one of the following is indicated by a rapid increase in the temperature difference between the fuel cladding and the bulk coolant?

Answer 25

Departure from nucleate boiling is occurring

Question 26

Which one of the following reactor coolant system parameters has the least effect on margin to departure from nucleate boiling?

Answer 26

Pressurizer level

Question 27

An adequate subcooling margin during a loss of coolant accident is the most direct indication that __________ is being maintained.

Answer 27

core cooling

Question 28

Which one of the following parameter changes will reduce the departure from nucleate boiling ratio?

Answer 28

Increasing reactor coolant temperature.

Question 29

Which one of the following will increase the departure from nucleate boiling ratio?

Answer 29

Increasing pressurizer pressure.

Question 30

A nuclear power plant is operating with the following initial conditions: • Reactor power is 45 percent in the middle of a fuel cycle. • Axial and radial power distributions are peaked in the center of the core. Assuming reactor power level does not change, which one of the following will increase the steady-state departure from nucleate boiling ratio?

Answer 30

Core xenon-135 builds up in proportion to the axial and radial power distribution with automatic rod control.

Question 31

A nuclear power plant is operating with the following initial steady-state conditions: • Reactor power is 45 percent in the middle of a fuel cycle. • Axial and radial power distributions are peaked in the center of the core. Which one of the following will decrease the steady-state departure from nucleate boiling ratio?

Answer 31

The operator decreases reactor coolant boron concentration by 5 ppm with no control rod motion.

Question 32

A nuclear power plant is operating with the following initial conditions: • Reactor power is 55 percent in the middle of a fuel cycle. • Axial and radial power distributions are peaked in the center of the core. Which one of the following will decrease the steady-state departure from nucleate boiling ratio?

Answer 32

Core xenon-135 depletes in proportion to the axial and radial power distribution with no control rod motion

Question 33

A nuclear power plant is operating with the following initial conditions: • Reactor power is 45 percent in the middle of a fuel cycle. • Axial and radial power distributions are peaked in the center of the core. Which one of the following will decrease the steady-state departure from nucleate boiling ratio?

Answer 33

A pressurizer malfunction decreases reactor coolant system pressure by 20 psig

Question 34

A reactor is shutdown with all control rods inserted. The reactor coolant system (RCS) is at normal operating temperature and pressure. Which one of the following will decrease the departure from nucleate boiling ratio for the reactor? (Assume the reactor remains shutdown.)

Answer 34

Reducing RCS flow rate by 3 percent.

Question 35

A nuclear power plant is operating with the following conditions: • Reactor power is 55 percent in the middle of a fuel cycle. • Axial and radial power distributions are peaked in the center of the core. Which one of the following will increase the steady-state departure from nucleate boiling ratio?

Answer 35

A reactor trip occurs and one control rod remains fully withdrawn from the core.

Question 36

A nuclear power plant is operating with the following initial conditions: • Reactor power is 45 percent in the middle of a fuel cycle. • Axial and radial power distributions are peaked in the center of the core. Which one of the following will increase the steady-state departure from nucleate boiling ratio?

Answer 36

A reactor trip occurs and one control rod remains fully withdrawn from the core.

Question 37

A reactor is shut down at normal operating temperature and pressure with all control rods inserted. Which one of the following will decrease the departure from nucleate boiling ratio for this reactor? (Assume the reactor remains shutdown.)

Answer 37

Decreasing reactor coolant pressure by 10 psig.

Question 38

Which one of the following parameter changes would move a reactor farther away from the critical heat flux?

Answer 38

Decrease reactor power.

Question 39

How does the critical heat flux vary from the bottom to the top of a typical fuel assembly during normal 100 percent power operation?

Answer 39

Decreases continuously.

Question 40

The heat flux that causes departure from nucleate boiling is the...

Answer 40

critical heat flux.

Question 41

The critical heat flux is the heat transfer rate per unit __________ of fuel rod that will initially cause __________.

Answer 41

area; departure from nucleate boiling

Question 42

How does critical heat flux (CHF) vary with core height during normal full power operation?

Answer 42

CHF decreases from the bottom to the top of the core.

Question 43

A reactor is operating at steady-state 75 percent power. Which one of the following parameter changes will cause the core to operate closer to the critical heat flux? (Assume reactor power does not change unless stated.)

Answer 43

Decrease reactor coolant flow rate by 5 percent.

Question 44

Which one of the following is most likely to result in fuel cladding damage?

Answer 44

Operating at a power level that exceeds the critical heat flux.

Question 45

A small increase in differential temperature at the fuel cladding-to-coolant interface causes increased steam blanketing and a reduction in heat flux. This describes which type of boiling?

Answer 45

Partial film boiling

Question 46

Refer to the drawing of a pool boiling curve (see figure below). Choose the region of the curve where transition boiling is the primary heat transfer process.

Answer 46

Region III

Question 47

Refer to the drawing of a pool boiling curve (see figure below). Which one of the points shown marks the onset of transition boiling?

Answer 47

B | Region 2

Question 48

Which one of the following describes the heat transfer conditions in a fuel assembly that is experiencing transition boiling?

Answer 48

Alternate wetting and drying of the fuel rod surface

Question 49

Which one of the following describes the conditions in a fuel assembly that is experiencing transition boiling?

Answer 49

Alternate wetting and drying of the fuel rod surface.

Question 50

Refer to the drawing of a pool boiling curve (see figure below). Which one of the following describes the heat transfer conditions in a fuel assembly that is experiencing region III heat transfer?

Answer 50

Alternate wetting and drying of the fuel rod surface.

Question 51

Refer to the drawing of a pool-boiling curve (see figure below). With heat flux continuously increasing, the point at which the critical heat flux is reached (point B), marks the beginning of...

Answer 51

partial film boiling

Question 52

Refer to the drawing of a pool boiling curve (see figure below). Which one of the following regions represents the most unstable mode of heat transfer?

Answer 52

Region III

Question 53

Film boiling heat transfer is...

Answer 53

heat transfer through a vapor blanket that covers the fuel cladding

Question 54

Reactor power is increased sufficiently to cause steam blanketing of several fuel rods. This condition is being caused by...

Answer 54

departure from nucleate boiling.

Question 55

If the fission rate in a reactor core steadily increases, the mode of heat transfer that occurs immediately after the critical heat flux is reached is called...

Answer 55

transition boiling.

Question 56

Refer to the drawing of a pool boiling curve (see figure below). Which one of the points shown marks the smallest ΔT at which stable film boiling can exist?

Answer 56

Region 3

Question 57

Refer to the drawing of a pool boiling curve (see figure below). Which one of the following describes the conditions in a fuel assembly that is experiencing region IV heat transfer?

Answer 57

Complete steam blanketing of the fuel rod surface.

Question 58

During a loss of coolant accident, some fuel rods may experience stable film boiling. Which one of the following types of heat transfer from the fuel cladding will increase significantly when stable film boiling begins?

Answer 58

Radiation

Question 59

The departure from nucleate boiling (DNB) ratio is defined as the...

Answer 59

critical heat flux divided by the actual heat flux.

Question 60

In the definition of departure from nucleate boiling ratio, the term "actual heat flux" refers to the...

Answer 60

heat transfer rate per unit area at any point along the fuel rod.

Question 61

A reactor is operating at steady-state 100 percent power near the end of a fuel cycle with all control rods fully withdrawn. At what axial location in a typical fuel assembly will the maximum departure from nucleate boiling ratio occur?

Answer 61

At the bottom of the fuel assembly.

Question 62

If a reactor is operating with the departure from nucleate boiling ratio at its limit, which one of the following is indicated?

Answer 62

A small fraction of the fuel rods may be experiencing critical heat flux

Question 63

Core heat transfer rate is maximized by the presence of...

Answer 63

turbulent flow with nucleate boiling.

Question 64

The heat transfer coefficient for the core will be directly increased if: (Assume bulk coolant subcooling.)

Answer 64

nucleate boiling occurs in the coolant.

Question 65

Increasing the coolant flow rate through a reactor core affects the heat transfer rate from the fuel, because a higher coolant flow rate results in a __________ laminar film thickness and a __________ coolant temperature adjacent to the fuel.

Answer 65

smaller; lower

Question 66

Which one of the following will minimize core heat transfer?

Answer 66

Laminar flow with no nucleate boiling.

Question 67

A nuclear power plant is operating at 100 percent power. The reactor coolant subcooling margin will be directly reduced by…

Answer 67

increasing reactor coolant temperature.

Question 68

The difference between the actual temperature and the saturation temperature of a liquid is the...

Answer 68

subcooling margin.

Question 69

Which one of the following must be present to assure adequate core cooling following a small loss of coolant accident?

Answer 69

Subcooling margin greater than zero.

Question 70

Which one of the following will increase the reactor coolant system (RCS) subcooling margin with the reactor operating at full power?

Answer 70

Decreased RCS hot leg temperature.

Question 71

A 60°F/hour reactor coolant system (RCS) cooldown and depressurization with natural circulation is in progress. After one hour, the RCS subcooling margin will be minimum in the...

Answer 71

reactor vessel head

Question 72

A reactor coolant system (RCS) cooldown and depressurization is in progress on natural circulation following a loss of offsite power. The following conditions currently exist: RCS Tcold = 520°F, decreasing RCS Thot = 538°F, decreasing Pressurizer pressure = 2,000 psia, decreasing If the cooldown rate is being maintained at 50°F/hr, which one of the following locations is most likely to experience sustained steam voiding?

Answer 72

Reactor vessel head

Question 73

Which one of the following is most likely to result in steam bubble formation in the reactor vessel head while maintaining a 60°F subcooling margin in the hottest reactor coolant system (RCS) hot leg?

Answer 73

Performing a 50°F/hr RCS cooldown with natural circulation.

Question 74

Which one of the following is most likely to result in steam bubble formation in a reactor vessel head while maintaining a 40°F subcooling margin in the hottest RCS hot leg?

Answer 74

Performing a 50°F/hr RCS cooldown with natural circulation.

Question 75

A nuclear power plant maintains the reactor coolant system (RCS) cold leg temperature (Tcold) at 557°F from 0 percent to 100 percent power. At 100 percent power, the reactor differential temperature (Thot - Tcold) is 60°F. If this plant also maintains RCS pressure constant at 2,235 psig, which one of the following is the approximate RCS subcooling margin at 50 percent power?

Answer 75

66°F

Question 76

Assume that a 30°F subcooling margin is maintained in the reactor coolant system (RCS) hot legs during each of the following cooldown operations for a shutdown reactor. Which one of the following will maintain the greatest subcooling margin in the reactor vessel head?

Answer 76

Performing a 25°F/hr RCS cooldown with all reactor coolant pumps running

Question 77

Refer to the drawing of a fuel rod and adjacent coolant flow channel (see figure below). With a nuclear power plant operating at steady-state 100 percent reactor power at the beginning of a fuel cycle, which one of the following has the greater temperature difference?

Answer 77

Fuel pellet centerline-to-pellet surface

Question 78

During a plant cooldown and depressurization with forced circulation, reactor coolant system (RCS) loop flow indications and reactor coolant pump (RCP) motor current indications become erratic. These abnormal indications are most likely caused by...

Answer 78

RCP cavitation

Question 79

Single-phase coolant flow resistance in a reactor core is directly proportional to the square of coolant __________; and inversely proportional to __________

Answer 79

velocity; coolant channel cross-sectional area

Question 80

Refer to the drawing of a section of pipe that contains flowing subcooled water (see figure below). Given: • Pressure at P1 is 24 psig. • Pressure at P2 is 16 psig. • Pressure change due to change in velocity is 2 psig. • Pressure change due to change in elevation is 10 psig. The pressure decrease due to friction head loss between P1 and P2 is __________; and the direction of flow is from __________.

Answer 80

4 psig; right to left

Question 81

Refer to the drawing of a section of pipe that contains flowing subcooled water (see figure below). Given: • Pressure at P1 is 26 psig. • Pressure at P2 is 34 psig. • Pressure change due to change in velocity is 2 psig. • Pressure change due to change in elevation is 8 psig. The pressure decrease due to friction head loss between P1 and P2 is __________; and the direction of flow is from __________.

Answer 81

2 psig; left to right

Question 82

Refer to the drawing of a section of pipe that contains flowing subcooled water. (See figure below). Given: • Pressure at P1 is 30 psig. • Pressure at P2 is 32 psig. • Pressure change due to change in velocity is 2 psig. • Pressure change due to change in elevation is 2 psig. The pressure decrease due to friction head loss between P1 and P2 is __________; and the direction of flow is from __________.

Answer 82

2 psig; right to left

Question 83

Refer to the drawing of a section of pipe that contains flowing subcooled water (see figure below). Given: • Pressure at P1 is 34 psig. • Pressure at P2 is 20 psig. • Pressure change due to change in velocity is 2 psig. • Pressure change due to change in elevation is 8 psig. The pressure decrease due to friction head loss between P1 and P2 is __________; and the direction of flow is from __________.

Answer 83

4 psig; right to left

Question 84

Refer to the drawing of a section of pipe that contains flowing subcooled water (see figure below). Given: • The pressure at P1 is 20 psig. • The pressure at P2 is 20 psig. • The pressure change caused by the change in velocity is 2 psig. • The pressure change caused by the change in elevation is 8 psig. The pressure decrease due to friction head loss between P1 and P2 is __________; and the direction of flow is from __________.

Answer 84

6 psig; right to left

Question 85

A reactor is producing 3,400 MW of thermal output with a reactor vessel differential temperature (ΔT) of 60°F and a reactor vessel mass flow rate of 1.4 x 108 lbm/hr. If core ΔT is 63.6°F, what is core bypass mass flow rate? (Assume bypass flow ΔT equals 0°F.)

Answer 85

7.92 x 106 lbm/hr

Question 86

A reactor is producing 3,400 MW of thermal output with a reactor vessel differential temperature (ΔT) of 60°F and a reactor vessel mass flow rate of 1.0 x 108 lbm/hr. If core ΔT is 63.6°F, what is core bypass mass flow rate? (Assume bypass flow ΔT equals 0°F.)

Answer 86

5.66 x 106 lbm/hr

Question 87

A reactor is producing 3,400 MW of thermal output with a reactor vessel differential temperature (ΔT) of 60°F and a reactor vessel mass flow rate of 1.1 x 108 lbm/hr. If core ΔT is 63.6°F, what is core bypass mass flow rate? (Assume bypass flow ΔT equals 0°F.)

Answer 87

6.23 x 106 lbm/hr

Question 88

Adequate core bypass flow is needed to...

Answer 88

equalize the temperatures between the reactor vessel and the reactor vessel upper head

Question 89

Which one of the following describes a function of core bypass flow?

Answer 89

Provides cooling to various reactor vessel internal components.

Question 90

Which one of the following is a function of core bypass flow?

Answer 90

Provides mixing of coolant in the reactor vessel head.

Question 91

Maximizing the elevation difference between the core thermal center and the steam generator thermal center and minimizing flow restrictions in the reactor coolant system (RCS) piping are features of nuclear power plant designs that...

Answer 91

ensure RCS natural circulation flow can be established.

Question 92

Which one of the following must exist for natural circulation flow to occur?

Answer 92

The heat sink must be located higher than the heat source.

Question 93

The driving head for natural circulation flow through the core is developed by differences in __________ between the hot leg and the cold leg

Answer 93

water density

Question 94

If the steam generator thermal centers were at the same elevation as the reactor core thermal center, natural circulation flow in the reactor coolant system would...

Answer 94

not occur.

Question 95

A reactor is shut down with natural circulation core cooling. Decay heat generation is equivalent to 1.0 percent of rated thermal power. Stable natural circulation mass flow rate is 1,000 gpm. When decay heat generation decreases to 0.5 percent of rated thermal power, stable natural circulation flow rate will be approximately...

Answer 95

794 gpm

Question 96

A reactor is shut down with natural circulation core cooling. Decay heat generation is equivalent to 1.0 percent of rated thermal power. Core differential temperature (ΔT) has stabilized at 16°F. When decay heat generation decreases to 0.5 percent of rated thermal power, core ΔT will be approximately...

Answer 96

10°F

Question 97

Sustained natural circulation requires that the heat sink is __________ in elevation than the heat source and that there is a __________ difference between the heat sink and the heat source.

Answer 97

higher; temperature

Question 98

Which one of the following conditions must occur to sustain natural convection in a fluid system?

Answer 98

A density change in the fluid.

Question 99

A reactor is shut down with natural circulation core cooling. Decay heat generation is equivalent to 1.0 percent of rated thermal power. Core differential temperature (ΔT) has stabilized at 16°F. When decay heat generation decreases to 0.333 percent of rated thermal power, core ΔT will be approximately...

Answer 99

8°F.

Question 100

A reactor is shut down with natural circulation core cooling. Decay heat generation is equivalent to 1.0 percent of rated thermal power. Core differential temperature (ΔT) has stabilized at 13°F. When decay heat generation decreases to 0.5 percent of rated thermal power, core ΔT will be approximately...

Answer 100

8°F.

Question 101

A reactor is shut down with natural circulation core cooling. Decay heat generation is equivalent to 1.0 percent of rated thermal power. Stable natural circulation flow rate is 800 gpm. When decay heat generation decreases to 0.5 percent of rated thermal power, stable natural circulation flow rate will be approximately...

Answer 101

635 gpm

Question 102

Sustained natural circulation requires that the heat source is __________ in elevation than the heat sink; and that there is a __________ difference between the heat source and the heat sink.

Answer 102

lower; temperature

Question 103

A nuclear power plant was operating at steady-state 100 percent power when a loss of offsite power occurred, resulting in a reactor trip and a loss of forced reactor coolant circulation. Thirty minutes later, reactor coolant system (RCS) hot leg temperature is greater than cold leg temperature and steam generator (SG) levels are stable. Which one of the following combinations of parameter trends, observed 30 minutes after the trip, indicates that natural circulation is occurring? (CET = core exit thermocouple)

Answer 103

RCS Hot Leg RCS Cold Leg SG RCSCET Temperature Temperature Pressures Subcooling Decreasing Stable Stable Increasing

Question 104

A nuclear power plant was operating at steady-state 100 percent power when a loss of offsite power occurred, resulting in a reactor trip and a loss of forced reactor coolant circulation. Two hours later, reactor coolant system (RCS) hot leg temperature is greater than cold leg temperature and steam generator (SG) levels are stable. Which one of the following combinations of parameter trends, observed two hours after the trip, indicates that natural circulation is not occurring? (CET = core exit thermocouples)

Answer 104

RCS Hot Leg RCS Cold Leg SG RCSCET Temperature Temperature Pressures Subcooling Stable Stable Decreasing Decreasing

Question 105

A reactor had been operating at 100 percent power for 3 months when a loss of offsite power occurred, causing a reactor trip and a loss of forced reactor coolant flow. If forced reactor coolant flow is not restored, which one of the following describes the relationship between reactor coolant hot leg and cold leg temperatures one hour after the reactor trip?

Answer 105

Hot leg temperature will be greater than cold leg temperature because natural circulation cooling flow occurs in the same direction as forced reactor coolant flow.

Question 106

A reactor is shut down at normal operating temperature and pressure with all reactor coolant pumps stopped. Stable natural circulation cooling is in progress with a minimum of 50°F subcooling. Which one of the following, if increased, will not affect natural circulation flow rate?

Answer 106

Reactor coolant pressure

Question 107

Fully-developed natural circulation flow rate will be greatest when...

Answer 107

all reactor coolant pumps stop at the same time as the reactor trip.

Question 108

Natural circulation flow can be enhanced by...

Answer 108

increasing the temperature difference between the heat source and the heat sink.

Question 109

Which one of the following will enhance natural circulation flow in the reactor coolant system?

Answer 109

Steam generator level is increased.

Question 110

A nuclear power plant was operating at a constant power level for the last two weeks when a loss of offsite power occurred, which caused a reactor trip and a loss of forced reactor coolant flow. Natural circulation reactor coolant flow developed and stabilized 30 minutes after the trip. Which one of the following combinations of initial reactor power and post-trip steam generator pressure will result in the greatest stable natural circulation flow rate 30 minutes after the trip?

Answer 110

Initial Post-trip Steam Reactor Power Generator Pressure 100 percent 1,000 psia

Question 111

A nuclear power plant was operating at a constant power level for the last two weeks when a loss of offsite power occurred, which caused a reactor trip and a loss of forced reactor coolant flow. Natural circulation reactor coolant flow developed and stabilized 30 minutes after the trip. Which one of the following combinations of initial reactor power and post-trip steam generator pressure will result in the smallest stable natural circulation flow rate 30 minutes after the trip?

Answer 111

Initial Post-trip Steam Reactor Power Generator Pressure 25 percent 1,100 psia

Question 112

A nuclear power plant was operating at steady-state 100 percent power when a loss of offsite power occurred, which caused a reactor trip and a complete loss of forced reactor coolant flow. Natural circulation reactor coolant flow developed and stabilized approximately 30 minutes after the trip. Which one of the following combinations of reactor power history and post-trip steam generator pressure will result in the greatest stable natural circulation flow rate?

Answer 112

Days At Post-trip Steam Full Power Generator Pressure 100 percent 1,000 psia

Question 113

A few minutes ago, a nuclear power plant experienced a loss of offsite power that caused a reactor trip and a loss of all reactor coolant pumps. Natural circulation flow is currently developing in the reactor coolant system (RCS). Which one of the following operator actions will promote the development of natural circulation in the RCS?

Answer 113

Establish and maintain steam generator water level high in the normal operating range.

Question 114

A nuclear power plant was operating at steady-state 100 percent power when a sustained loss of offsite power occurred, which caused a reactor trip and a complete loss of forced reactor coolant flow. Which one of the following combinations of reactor power history and post-trip steam generator pressure will result in the smallest stable natural circulation flow rate?

Answer 114

Days At Post-trip Steam 100 Percent Power Generator Pressure 10 1, 100 psia

Question 115

During the reflux boiling method of core cooling, steam from the reactor core is condensed in the __________ side of a steam generator and flows back into the core via the __________. (Assume the steam generators contain U-tubes.)

Answer 115

hot leg; hot leg

Question 116

Which one of the following describes the method of core heat removal during reflux core cooling following a loss of coolant accident?

Answer 116

Convection with natural circulation coolant flow.

Question 117

A nuclear power plant is experiencing natural circulation core cooling following a loss of coolant accident. Which one of the following, when it first occurs, marks the beginning of reflux core cooling? (Assume the steam generators contain U-tubes.)

Answer 117

Steam condensation in the hot legs is unable to pass completely through the steam generators to enter the cold legs.

Question 118

A reactor coolant system natural circulation cooldown is in progress with steam release from the steam generator (SG) atmospheric steam relief valves (operated in manual control). If high point voiding interrupts natural circulation, which one of the following will occur? (Assume feedwater flow rate, SG relief valve position, and core decay heat level are constant.)

Answer 118

SG level will increase and SG pressure will decrease.

Question 119

A reactor coolant system natural circulation cooldown is in progress with steam release from the steam generator (SG) atmospheric steam relief valves (operated in manual control). Assume feedwater flow rate, SG relief valve position, and core decay heat level are constant. If high point voiding interrupts natural circulation, SG levels will gradually __________; and core exit thermocouple indications will gradually __________

Answer 119

increase; increase

Question 120

A reactor coolant system (RCS) cooldown on natural circulation is in progress. The cooldown rate is being controlled by releasing steam from the steam generator (SG) atmospheric relief valves in Manual control. If voids interrupt the RCS natural circulation flow, which one of the following will occur? (Assume feedwater flow rate, SG relief valve positions, and decay heat level are constant.)

Answer 120

SG pressure will decrease and core exit thermocouple (CET) temperatures will increase.

Question 121

A reactor coolant system natural circulation cooldown is in progress with steam release from the steam generator (SG) atmospheric steam relief valves (operated in manual control). Assume feedwater flow rate, SG relief valve position, and core decay heat level remain constant. If high point voiding interrupts natural circulation, SG steam flow rate will __________ and core exit thermocouple temperatures will __________.

Answer 121

decrease; increase