Thermal Hydraulics

Question 1

Distinguish between boiling processes and other heat transfer mechanisms.

Answer 1

The characteristic that distinguishes the boiling
heat transfer process from other mechanisms is
the change of phase that occurs in the coolant.

Question 2

Describe nucleate boiling and film boiling.

Answer 2

Nucleate boiling is where bubbles begin to form at surface scratches
and irregularities on the bottom of the container.

Film boiling is where the differential temperature between the
surface and the bulk liquid is allowed to reach a high enough value
that the surface is covered with a steam blanket.

Question 3

Describe means by which boiling improves convection heat transfer.

Answer 3

Nucleate boiling bubbles disturb the stagnant water layer at the surface,
resulting in higher thermal conductivity of the water layer. Also, each bubble carries the energy of latent heat of vaporization absorbed during the phase change from water to steam. The enthalpy of the bubbles is much greater than the enthalpy
of the water heated by natural convection.

Question 4

Describe microconvection.

Answer 4

As nucleate boiling bubbles grow, their buoyancy makes them detach from the
surface and drift into the bulk liquid. This agitation or pumping action is known as
microconvection. It accounts for the majority of the energy transferred during boiling.

Question 5

Describe surface or cavity nucleation.

Answer 5

The heat input to the surface is increased until
the surface temperature is slightly above
the saturation temperature of the water.

Question 6

What factors affect bubble formation in a cavity?

Answer 6

Factors that affect bubble formation include the
saturation temperature of the fluid, cavity size,
latent heat of vaporization of the fluid,
noncondensable gas pressure in the nucleation
site, contact angle between the bubble and
surface, and the change in specific volume from
to vaporization.

Question 7

Draw a simple pool boiling curve.

Answer 7

Question 8

Describe natural convection heat transfer.

Answer 8

Natural convection is where the surface is heated and heat transfers to the water across the stagnant film because of the temperature difference
between the water and the surface. As the temperature of the water at the bottom of
the container increases, it becomes less dense and rises, being displaced by the cooler, denser water above.

Question 9

Relate nucleate boiling to the pool boiling curve.

Answer 9

The increased slope of the pool boiling curve in Region II indicates a greater heat transfer coefficient. This is the nucleate boiling region.

Question 10

Describe departure from nucleate boiling (DNB).

Answer 10

The point where the method of heat transfer changes. If the heat applied to the surface continues to increase, more bubbles form until the bubbles start to collect on areas of the surface. As the bubbles coalesce, a vapor film or steam blanket begins to cover the surface.

Question 11

Describe critical heat flux.

Answer 11

The point of maximum heat flux associated with DNB is called the
critical heat flux (CHF).

Question 12

Describe transition (partial film) boiling.

Answer 12

As temperature of the container is increased, heat
transfer is by conduction and radiation through
the vapor film. These methods of heat transfer
are inefficient in this temperature range, and lead
to a decrease in the rate of heat transfer. The
vapor film formed is highly unstable and may
move away from the bottom of the container,
allowing liquid to rewet the surface. The vapor
film may also spread until the entire surface is
covered with steam. This is called “transition”
or “partial film boiling” and is identified by the
instability of the heat transfer mechanism.

Question 13

Describe stable film boiling.

Answer 13

If the differential temperature between the
bottom of the container and the bulk liquid is
allowed to reach a high enough value, the bottom
of the container is covered with a steam blanket.
This is represented on the pool boiling curve as Region IV and is stable film boiling or dryout.

Question 14

Describe burnout and burnout heat flux.

Answer 14

The major form of heat transfer in dryout is
radiative heat transfer. If more heat is added in
this region, the bottom of the container will
probably fail. This is known as burnout. The
heat flux associated with the failure of the
container is known as the burnout heat flux.

Question 15

Describe the onset of transition boiling (OTB).

Answer 15

Also known as DNB.

Question 16

Describe rewetting temperature.

Answer 16

With decreasing heat flux, when the rewetting temperature is reached the steam blanket begins to collapse and a rapid transition from film boiling to nucleate boiling occurs.

Question 17

Describe subcooled nucleate boiling and bulk boiling.

Answer 17

Subcooled nucleate boiling in the fuel channel:

• BUBBLES FORM ON THE WALL, BUT COLLAPSE IN BULK COOLANT STREAM.
• NO NET VAPOR GENERATION.
• A VERY SMALL VOID FRACTION.
• OCCURS ABOVE FORCED CONVECTION REGION.

Bulk boiling in the fuel channel:

• BUBBLES FORM ON THE WALL, BUT DO NOT COLLAPSE IN COOLANT STREAM.
• BULK COOLANT IS AT SATURATION TEMPERATURE.
• BULK BOILING OCCURRING, BUT BUBBLES ARE NOT YET COALESCING.
• QUALITY IS LOW.

Question 18

Classify the slug flow region along a hypothetical fuel channel experiencing two-phase flow.

Answer 18

As the water flows up the coolant channel, more heat is added and the bubbles in the coolant begin to coalesce into “slugs” of vapor. This condition is called “slug flow.” Since the boiling regime is still nucleate bulk boiling, slug flow is actually a
change in the flow condition from bubble flow. The heat transfer process is not significantly affected by this transition. In slug flow, the steam voids constitute a
significant fraction of the volumetric flow rate. As a result, the coolant velocity increases.

Question 19

Classify the annular flow region along a hypothetical fuel channel experiencing two-phase flow.

Answer 19

In annular flow, steam voids are so numerous that they combine and form a vapor
core in the coolant channel. The fuel is still covered with water, and nucleate bulk boiling takes place at the cladding surface.

Question 20

Classify the dryout or mist flow region along a hypothetical fuel channel experiencing two-phase flow

Answer 20

In the mist flow region, the fluid film flashes to steam. The coolant is a fog composed of vapor with small water droplets entrained in it. This is called dryout and is characterized by the breakdown of the heat transfer mechanism and a
temperature excursion.

Question 21

Classify the OTB point along a hypothetical

fuel channel experiencing two-phase flow

Answer 21

The point of DNB (departure from nucleate boiling) or OTB (onset of transition boiling) is the dividing point between annular flow and mist flow. This is when the fluid film inside the walls of the fuel channel ceases to exist. The surface of the fuel cladding is then cooled by the impingement of moisture droplets entrained in the vapor flow.

Note that the film heat transfer coefficient deteriorates at DNB. However, when the film layer ceases to exist, the velocity of the vapor and moisture droplets is very high. Therefore, the rate of heat transfer is still good.

Question 22

Describe effects along a fuel channel experiencing two-phase flow.

Answer 22

Subcooled boiling, bubble flow, slug flow, annualar flow and mist flow

Question 23

Describe the effects of flow rate and phase changed on the heat transfer coefficient.

Answer 23

Reactor coolant flowrate affects the heat transfer rate in the fuel channel. When heat is transferred from the fuel cladding to the coolant, it must pass through a laminar flow region before reaching the turbulent bulk flow region. The thickness of the laminar region is proportional to its resistance to heat flow. As flow rate increases, the outer portions of the laminar layer are disturbed, resulting in more turbulent flow, a thinner laminar layer and an increased heat transfer coefficient.

The heat transfer coefficient increases through the nucleate boiling stage, but decreases after DNB.

Question 24

Draw the radial temperature profile from the centerline of a fuel pellet to the centerline of the channel.

Answer 24

Question 25

Explain the reason for the shape of the radial temperature profile.

Answer 25

The energy produced in the fuel is transferred in
the form of heat by conduction and convection to
the reactor coolant. Essentially, conduction
takes place from the center of the fuel to the
outer surfaces of the clad. Convection takes
place to remove heat from the clad surface. The
heat production results in a temperature profile
for the fuel rod center to the reactor coolant

Question 26

Draw the axial temperature and enthalpy profile for a fuel rod and coolant channel.

Answer 26

Question 27

Describe how the axial temperature profile
is affected by the onset of nucleate boiling.

Answer 27

Heat transfer is improved by nucleate boiling, the bulk fluid temperature profile changes more rapidly with the onset of nucleate boiling.

Question 28

Describe how the axial temperature profile
is affected by axial core flux.

Answer 28

The coolant flowing next to a high power fuel
rod will experience a larger enthalpy rise than at
a lower power.

Question 29

Describe how the axial temperature profile
is affected by inlet temperature.

Answer 29

The coolant entering the core is usually assumed
to have the same enthalpy. As the coolant passes
along the fuel rods, it receives heat from the fuel
rods and the enthalpy increases.

Question 30

Describe how the axial temperature profile
is affected by heat generation rate.

Answer 30

heat generation rate is
proportional in the fuel rod.

Question 31

Describe how the axial temperature profile
is affected by flow rate in the channel.

Answer 31

It has been found through testing that the critical
heat flux required for DNB can be lowered by as
much as 40% when flow is oscillating. This
severely reduces the thermal limit and heat
generation rate along the length of the reactor
core.

Question 32

Explain the necessity of determining core coolant flow.

Answer 32

Reactor coolant flowrate also has a direct affect
on fuel temperature. The rate of heat transfer is
proportional to the reactor coolant flowrate and
the differential temperature between the fuel and
the coolant:

Question 33

Describe core bypass flow.

Answer 33

Approximately 3% to 4.5% of the flow bypasses the core:

a. Flows through the upper head for cooling purposes.
b. Flow enters the guide thimbles to cool the control rods.
c. Leakage flows from the vessel inlet nozzle directly to the vessel outlet nozzle, through the gap between the vessel and the barrel.
d. Flow is introduced between the baffle and the barrel to cool these components.
e. Flows in the gaps between the fuel assemblies on the core periphery and the
adjacent baffle wall.

Question 34

Explain the causes of natural circulation.

Answer 34

Heating and cooling of water changes the density of the coolant. As the
density decreases, a given volume of water has less mass. The heated water tends to rise, while the cooled water tends to fall.

The following conditions must exist for natural circulation:

• A density difference. In all practical systems, this density difference is produced by a temperature difference. The warmer fluid is less dense.
• A height difference. The cooler, denser fluid must be at a higher elevation than the warmer, less dense fluid.
• Fluids in physical contact with each other.

Question 35

Describe the means by which the operator
can determine if natural circulation flow exists.

Answer 35

The Reactor Coolant System differential temperature should be approximately
25% to 80% of full power as indicated by wide range resistance temperature detectors (RTDs).

The hot leg RTD should indicate either a steady or slowly decreasing value. This indicates that heat removal is in operation and the decay heat generated in the core is decreasing slowly, as it should.

Core exit thermocouples (CET or CETC) should also be monitored for a slowly decreasing value.

Steam generator pressure should follow reactor coolant temperatures. As average reactor coolant temperature decreases, steam pressure should also decrease.

Cold leg temperatures should indicate constant or slowly decreasing value.

Question 36

Describe the means by which the operator can enhance natural circulation.

Answer 36

Pressurizer level should be maintained at 50% or greater to ensure that no vapor products have formed in the loops.

The Reactor Coolant System should be maintained at least 15°F subcooled. 50°F
subcooling is desired, but at least 15°F subcooling is required.

The heat sink required is at least one steam generator. The Auxiliary Feedwater System should be used as necessary, to maintain narrow range level in one steam generator.

Question 37

Describe how gas binding affects natural circulation.

Answer 37

If excessive quantities of noncondensable gases, such as oxygen or hydrogen, exist in the reactor coolant, they will collect in the highest point in the RCS. The steam generator’s U-tubes are the highest point in the RCS, and an accumulation of
noncondensables in the U-tubes stops natural circulation flow.

Question 38

Describe the process of reflux boiling.

Answer 38

Refluxing is a heat and mass transfer process. Some of the steam produced by the core is condensed in the steam generator tubes and flows back down the hot legs to the core, thus transferring energy from the core to the steam generators. Refluxing continues until either the RCS temperature drops below the steam generator
temperature, resulting in a loss of condensation, or the core uncovers, resulting in a loss of steam production.

Question 39

Describe departure from nucleate boiling ratio (DNBR ).

Answer 39

DNBR is the ratio of critical heat flux to actual heat flux.

Question 40

Describe the parameters that affect DNB and DNBR and describe their effects.

Answer 40

If hot leg temperature were increased inadvertently, by boron dilution or rod withdrawal, saturated boiling could occur in the upper regions of the core, causing DNBR to decrease to a less conservative value.

If temperature is maintained and RCS pressure reduced, DNBR will decrease. A reduction in pressure shifts the boiling curve to the left. Thus, operating at lower pressures allows DNB to occur at lower temperatures.

At any power level, a reduction in RCS flowrate will result in an increase in coolant temperature, again reducing DNBR.

The fourth factor that reduces DNBR is high local power densities. High local power densities produce higher heat flux, and higher coolant and cladding temperatures. As a result, the heat transfer conditions more closely approach actual CHF conditions and the DNBR is reduced.

Question 41

Define and describe subcooling margin (SCM).

Answer 41

Subcooling margin is calculated from the difference between the saturation temperature of the coolant at the existing pressure and the actual coolant temperature.

Subcooling margin is the best indication that adequate core cooling is available during a small loss-of-coolant accident (LOCA). Subcooling in a PWR can be increased in two ways: either by decreasing reactor coolant temperature while
maintaining pressure constant, or by raising pressure while maintaining coolant temperature constant.