© Copyright 2014Operator Generic Fundamentals Operator Generic Fundamentals Thermodynamics –...

© Copyright 2014 Operator Generic Fundamentals

Operator Generic Fundamentals Thermodynamics – Thermal Hydraulics

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Thermal Hydraulics• Boiling heat transfer improves the heat transfer of fission heat to the

reactor coolant• In this module we will cover:

– Fuel channel flow characteristics– Fuel temperature profiles and core bypass flows– Natural circulation, what it is, how it works, and how to enhance

its effectiveness, operation during reactor accident conditions

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Figure: Modes of Heat Transfer

Introduction


Terminal Learning ObjectivesAt the completion of this training session, the trainee will demonstrate mastery of this topic by passing a written exam with a grade of 80 percent or higher on the following Terminal Learning Objectives (TLOs):

1. Explain the various types of boiling heat transfer.

2. Describe the basic reactor core thermal hydraulic properties.

3. Explain natural circulation and methods to enhance its effectiveness.

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Introduction


Boiling Heat Transfer

• Convection heat transfer describes the process of heat transfer due to fluid movement.

• Applicable for many fluid systems in a nuclear power plant.

• The transfer of heat from a solid to a fluid, or reverse, requires:

– Bulk motion of the fluid, and

– Diffusion and conduction of heat through fluid boundary layer in contact with the solid.

TLO 1

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TLO 1 – Explain the various types of boiling heat transfer.


Boiling Heat Transfer• In a PWR,

– Reactor Coolant System (RCS) at a pressure greater than saturation

– Convection heat transfer is primary means to remove heat from the nuclear fuel.

• Under certain conditions some form of boiling of the RCS at the fuel surface or within the coolant may occur.

TLO 1

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1. Describe the differences between boiling processes and other means of heat transfer.

2. Describe the process of nucleate boiling, bulk boiling, departure from nucleate boiling, and critical heat flux (CHF).

3. Describe the transition to partial film boiling.

4. Describe the transition to full film boiling.

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TLO 1

Enabling Learning Objectives for TLO 1



• Convection heat transfer is the primary means of heat transfer in liquids and gasses.

• In a PWR boiling of the liquid coolant on or near the heat transfer surfaces may take place.

• Boiling may improvement overall convection heat transfer rate, or…

– Detrimentally affect the heat transfer rate.

ELO 1.1

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ELO 1.1 – Describe the differences between boiling processes and other means of heat transfer.



• Convective heat transfer involves:

– Fluid motion and

– Diffusion and conduction of heat through the fluid boundary layer in contact with the solid.

• When boiling occurs at the boundary layer, a change of phase takes place.

• Steam bubbles occur at the boundary layer next to the heated surface,

– Immediately collapsing in the fluid, or

– Traveling further into the fluid main stream if saturation conditions exist.

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ELO 1.1



• Local boiling

– Steam bubbles form at the surface and

– Immediately collapse in the fluid

• Bulk boiling

– Boiling taking place when saturation conditions exist.

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ELO 1.1


Boiling Heat Transfer Regions

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ELO 1.1

Figure: Fluid Heat Transfer Regions



Knowledge Check

True or False?

Boiling on the heat transfer surface is never a good thing.

A. False

B. True

Correct Answer is A.

ELO 1.1

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Nucleate Boiling, DNB, and CHF

• Boiling heat transfer can be beneficial to improving convective heat transfer by

– Reducing fuel cladding temperature and

– Increasing safety margins

• BUT ... if boiling increases too much, its effects on reactor operation are very detrimental.

ELO 1.2

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ELO 1.2 – Describe nucleate boiling, bulk boiling, departure from

nucleate boiling, critical heat flux (CHF), and subcooling margin.


Nucleate Boiling

• Steam bubbles form at the fuel surface then break away into the main stream of RCS fluid.

• Surface heat transferred directly into the fluid stream by the steam bubbles collapsing in cooler fluid bulk temperatures.

• Rapid bubble collapse in RCS flow causes brisk mixing of coolant increasing convective heat transfer rate.

• Heat energy at the fuel surface is quickly and efficiently transferred to the RCS.

ELO 1.2

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Nucleate Boiling

• Heat is removed from the fuel rod as sensible heat and latent heat of vaporization– High temperature fuel cladding surface adds heat energy to the

RCS as a temperature change and

– Latent heat of vaporization transfers heat energy in the form of a phase change

ELO 1.2

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Nucleate Boiling

• Latent heat of vaporization is the formation of the steam bubbles, a phase change.

• More efficient than heat transfer by conduction and convection heat transfer alone.

• Nucleate boiling heat transfer is maximized with turbulent flow.

• Nucleate boiling is the region in which the hottest locations of the nuclear reactor operate.

ELO 1.2

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Subcooled Nucleate Boiling

• Nucleate boiling where the liquid bulk temperature is below saturation but temperature at the heat transfer surface is above saturation. – Steam bubbles forming at the heat transfer surface condense rapidly

in the cooler liquid; the net effect being no net generation of steam vapor.

• Steam bubbles have a tendency to form first at sites with surface imperfections such as scratches. – Known as nucleation sites they provide greater heat transfer than

smooth clean areas.

– Intensity of steam bubble formation, number and size, increases as the heat is increased.

ELO 1.2

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Subcooled Nucleate Boiling

• Other factors that affect steam bubble formation:– Saturation temperature of the liquid

– Latent heat of vaporization of the liquid

– Gases within the liquid

– Contact between bubbles and surface area

ELO 1.2

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Nucleate Boiling

ELO 1.2

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Convection Heat Transfer

ELO 1.2

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Figure: Convection Heat Transfer


Nucleate Boiling

ELO 1.2

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Figure: Nucleate Boiling Example


Bulk Boiling

• If system temperature increases or pressure decreases,

– Bulk fluid can reach saturation conditions.

• Steam bubbles entering the coolant channel do not collapse, but rather join together in forming larger steam bubbles.

• Provides adequate heat transfer provided steam bubbles do not interfere with keeping heat transfer surface continuously wetted with liquid.

• Bulk boiling is a type of nucleate boiling.

ELO 1.2

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ELO 1.2

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Departure from Nucleate Boiling

• With additional heat, steam bubbles begin to cover the entire heat transfer surface.

• At this transition point maximum heat flux occurs.

Figure: Critical Heat Flux / DNB

ELO 1.2

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Figure: Stages of Nucleate Boiling



ELO 1.2

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Critical Heat Flux (CHF)

• Heat flux associated with DNB

• Heat flux that causes DNB to occur for given pressure and temperature conditions.

• With increasing differential temperature heat flux reaches a turning point within the nucleate boiling region.

– A rapid increase in differential temperature between the heat transfer surface and the liquid

– Indicates the heat transfer surface loosing cooling, heating, and potentially causing damage, in this case the nuclear fuel.

ELO 1.2

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Critical Heat Flux (CHF)

• Limits boiling heat transfer use.

• Causes physical burnout of the heated surface materials due to:

– sudden inefficient heat transfer rate through a vapor film displacing the liquid adjacent to the heat transfer surface.

• When CHF occurs a large increase in the heat transfer surface

temperatures occurs.

ELO 1.2

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Partial Film Boiling

Figure: Transition / Partial Film Boiling

ELO 1.2

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Departure from Nucleate Boiling Ratio

• Describes the margin between actual and critical heat flux

– Referred to as DNBR.

• Mathematically:

ELO 1.2

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𝐷𝑁𝐵𝑅=𝐶𝑟𝑖𝑡𝑖𝑐𝑎𝑙h𝑒𝑎𝑡 𝑓𝑙𝑢𝑥

𝐴𝑐𝑡𝑢𝑎𝑙h𝑒𝑎𝑡 𝑓𝑙𝑢𝑥𝑎𝑡 𝑎𝑛𝑦𝑝𝑜𝑖𝑛𝑡 𝑎𝑙𝑜𝑛𝑔𝑎 𝑓𝑢𝑒𝑙𝑟𝑜𝑑


Subcooling Margin

• Equals the difference between actual RCS coolant temperature and coolant saturation temperature for the existing pressure.

– CHF increases with an increase in RCS subcooling or subcooling margin.

• Good indication of adequate core cooling (no boiling in core) during small loss-of-coolant accidents

– Maintaining a minimum subcooling margin is very important.

ELO 1.2

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Practice Question 1Why does nucleate boiling improve heat transfer in a nuclear reactor core?

A. The formation of steam bubbles at nucleation sites on the fuel clad allows greater heat transfer by conduction.

B. The formation of steam bubbles at nucleation sites on the fuel clad promotes local radiative heat transfer and allows more heat transfer by convection.

C. Heat removal from fuel rods as both sensible heat and latent heat of condensation with direct transferred to the coolant by radiative heat transfer.

D. Heat removal from the fuel rod as both sensible heat and latent heat of vaporization with the motion of the steam bubbles causing rapid mixing of the coolant.

Correct answer is D.

ELO 1.2

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Practice Question 3Many factors influence steam bubble formation as heat transfers to water adjacent to a heating surface. Which one of the following characteristics will enhance steam bubble formation?

A. Chemicals dissolved in the water

B. The absence of ionizing radiation exposure to the water

C. A highly polished heat transfer surface with minimal scratches or cavities

D. The presence of gases dissolved in the water


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ELO 1.2


Practice Question 7

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

A. Decrease pressurizer pressure

B. Decrease reactor coolant flow

C. Decrease reactor power

D. Increase reactor coolant temperature

Correct answer is C.

ELO 1.2

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Knowledge CheckKnowledge Check – NRC Bank

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

A. actual heat flux divided by the critical heat flux at any point along a fuel rod.

B. critical heat flux divided by the actual heat flux at any point along a fuel rod.

C. core thermal power divided by the total reactor coolant mass flow rate.

D. number of coolant channels that have reached DNB divided by the number of coolant channels that are subcooled.

Correct answer is B.ELO 1.2

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• The point where DNBR and CHF is reached transitions to the partial film boiling region.

ELO 1.3

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ELO 1.3 – Describe the transition to partial film boiling.




• If system pressure or flow decreases sufficiently, and/or heat transfer surface temperature increases:– Steam bubbles may start accumulating on the heat transfer

surface.

– As more bubbles are formed, they group together, covering small areas of the heat transfer surface with a film of steam.

• This is known as partial film boiling and will result in a series of wetting and drying out of the cladding surface.

ELO 1.3

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Partial Film Boiling • Steam has a lower convective heat transfer

coefficient than water– Steam film on the heat transfer surface acts to insulate the

surface reducing heat transfer capability.

– As the film area grows in size, the surface temperature increases dramatically, forcing the heat flux to decrease.

• This boiling heat transfer region is characterized by an– Increase in the heat transfer surface temperature (and ∆T) with,

– Decrease in heat flux (heat transfer rate).

ELO 1.3

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Figure: Partial Film Boiling



Figure: Transition / Partial Film Boiling

ELO 1.3

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Knowledge Check

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

A. Complete steam blanketing of the fuel rod surface

B. Subcooled nucleate boiling

C. Saturated nucleate boiling

D. Alternate wetting and drying of the fuel rod surface


ELO 1.3

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Film Boiling

• With heat transfer continuing to degrade, film boiling is the next heat transfer region.

ELO 1.4

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ELO 1.4 – Describe the transition to film boiling.



Film Boiling

• With heat flux decreasing and temperature of the heat transfer surface increasing from the reduced heat transfer, conditions could continue to degrade until a stable seam blanket covers the heat transfer surface.

• This insulating steam blanket prevents contact between the heat transfer surface and the flow channel liquid.

• The point of dryout occurs when the vapor blanket completely covers the heat transfer surface.

ELO 1.4

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Film Boiling • The point of dryout occurs when the vapor blanket completely covers

the heat transfer surface. – At this point further heat transfer is largely limited to ineffective

radiant heat transfer

– Heat transfer surface material will likely exceed its design limits, potentially undergoing failure and burnout.

• For a Nuclear Plant– This means clad damage/failure and release of fission product

gases into the reactor coolant system.

ELO 1.4

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Film Boiling

Knowledge Check – NRC Bank

Film boiling heat transfer is... A. the most efficient method of boiling heat transfer.

B. heat transfer through a vapor blanket that covers the fuel cladding.

C. heat transfer through an oxide film on the cladding.

D. heat transfer being accomplished with no enthalpy change.

Correct answer is B.

ELO 1.4

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1. Boiling Heat Transfer– Convective heat transfer is the primary means of heat transfer in

a PWR, and the heat transfer rate depends on the boiling condition at the heat transfer surface.

– This process involves fluid motion, and diffusion and conduction of heat through the fluid boundary layer in contact with the solid.

– A phase change takes place when boiling occurs at the boundary layer unlike pure convective liquid heat-transfer mechanisms. Steam bubbles occur at the boundary layer next to the heated surface, immediately collapsing in the liquid, or traveling further into the liquid main stream if saturation conditions exist.

2. Nucleate Boiling, DNB and CHF– Nucleate boiling is the formation of small bubbles at a heat

transfer surface.

TLO 1

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TLO 1 Summary


– Bubbles swept into coolant and collapse due to the coolant being a subcooled liquid.o Heat transfer is more efficient than for convection.

– Bulk boiling occurs when the bubbles do not collapse due to the coolant being at saturation conditions.

– Departure from nucleate boiling (DNB) occurs at the transition from nucleate to film boiling.

– Critical heat flux (CHF) is the heat flux that causes DNB to occur.

3. Partial Film Boiling– Liquid reaches partial film-boiling region when increased temperature

difference causes departure from nucleate boiling (DNB) and critical heat flux (CHF). This is detrimental to the heat transfer surfaces.

– In the partial film boiling region, steam bubbles grow and begin to combine and cover small areas of heat transfer surface with a film of steam.

TLO 1

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TLO 1 Summary


4. Film Boiling– Liquid could shift into the partial film boiling or transition region if

system pressure or flow decreases, or if temperature increases.

– When heat transfer conditions continue to degrade, a stable steam blanket covers the heat transfer surface with heat flux decreasing and temperature of the heat transfer surface increasing from the reduced heat transfer.

– This insulating steam blanket prevents contact between the heat transfer surface and the flow channel liquid.

– An increase in ∆T and a decrease in heat flux characterize this region. If conditions continue to degrade (ΔT continues to increase), eventually total film boiling will occur.

TLO 1

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TLO 1 Summary


Identify the region of the curve where the most efficient form of heat transfer exists.

A. Region IV

B. Region III

C. Region II

D. Region I


TLO 1

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TLO 1 Summary – Review Questions


Which region of the curve contains the operating point at which the hottest locations of the nuclear reactor operate to transfer heat from the cladding to the coolant at 100 percent power?

A. Region IV

B. Region III

C. Region II

D. Region I

Correct answer is C.TLO 1

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Which one of the points shown represents the onset of transition boiling?

A. A

B. B

C. C

D. D


TLO 1

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Crossword Puzzle

• It’s crossword puzzle time!

Summary

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• Fluid flow through the reactor core is important for producing power but also to keep the core cool.

• Fuel channel flow consists of single-phase and multiple forms of two-phase flow.

• Nucleate boiling is normal in the fuel channels & enhances heat transfer

• Departure from nucleate boiling (DNB) where CHF also occurs is detrimental to protecting the integrity of the fuel and especially the cladding.

TLO 2

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TLO 2 – Describe the basic reactor core thermal hydraulic properties.

Reactor Core Thermal Hydraulic Properties



1. Describe the heat transfer coefficient and effects from flowrate and phase change.

2. Explain fuel channel flow and heat transfer, including the following terms:

a. Slug Flow

b. Annular Flow

c. Dryout Region

d. Flow resistance

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

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TLO 2



1. Describe core bypass flow and purpose of adequate flow.

2. Draw the axial temperature and enthalpy profiles for a typical reactor coolant channel and describe how they are affected by the following:

a. Onset of nucleate boiling

b. Axial core flux

c. Inlet temperature

d. Heat generation rate

e. Flow rate in the channel

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TLO 2


• Reducing the thickness or effective thickness of the stagnant laminar flow layer at the heat transfer surface improves the convection heat transfer coefficient.

– Increased flow rate and two-phase flow enhances this.

ELO 2.1

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ELO 2.1 – Describe the heat transfer coefficient and effects from flowrate and phase change.

Flowrate and Phase Change Effects on Heat Transfer


Convective Heat Transfer Coefficient

• Defines heat transfer due to convection

• Represents the thermal resistance of a relatively stagnant layer of fluid between a heat transfer surface and the fluid medium.

• BTU/hr–ft2-°F.

• “Rate” of heat transfer per unit area per degree F.

ELO 2.1

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Flowrate and Phase Change Effects on Heat Transfer


Flowrate and Phase Change effects on Heat Transfer

Heat Transfer Rate• BTU/hr (

Heat Flux• Heat flux is the rate of heat transfer per unit area BTU/hr-ft2 (q).

• Relationship between Heat Transfer Coefficient and Heat Flux

ELO 2.1

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h= �̇�∆𝑇


Flowrate and Phase Change effects on Heat Transfer Laminar Flow

• Layers of water flow over one another at different speeds with virtually no mixing between layers.

• Fluid particles move in definite and observable paths or streamlines.

Turbulent Flow

• Irregular movement of particles of the fluid.

• No definite frequency as in wave motion.

• Particles travel in irregular paths with no observable pattern and no definite layers.

ELO 2.1

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Flowrate and Two-Phase Flow Effects on the Convection Heat Transfer Coefficient • Calculation of convection heat transfer effectiveness is difficult

– Analysis by observation and experimentation is used.

• Factors affecting convection heat transfer:

– Fluid velocity

– Fluid viscosity

– Heat flux

– Surface roughness

– Type of flow (single-phase/two-phase)

ELO 2.1

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Flowrate and Two-Phase Flow Effects on the Convection Heat Transfer Coefficient • Convective heat transfer coefficient for laminar flow is low compared

to turbulent flow.

– Due to turbulent flow thinning the stagnant fluid film layer on the heat transfer surface

• Reducing the effective thickness of the stagnant film layer increases the convection heat transfer coefficient.

ELO 2.1

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Flowrate and Two-Phase Flow Effects on the Convection Heat Transfer Coefficient Methods to Improve Convective Heat Transfer

• Higher fluid velocity - decrease laminar film thickness and lower temperature of the coolant adjacent to the fuel.

• Increased flow turbulence – thins out the stagnant laminar layer – for example, fuel assembly grid spacers increase turbulence.

• Increased fluid friction against the heat transfer surface to break up the laminar flow. Examples: roughness, surface imperfections, etc.

• Nucleate Boiling and two-phase flow

ELO 2.1

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Flowrate and Two-Phase Flow Effects on the Convection Heat Transfer Coefficient • Nucleate boiling improves heat transfer by removing heat from the

heat transfer surface (fuel clad) both as

– Sensible (no phase change) and

– Latent heat of vaporization

ELO 2.1

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Knowledge Check – NRC BankCore heat transfer rate maximizes by the presence of...

A. turbulent flow with no nucleate boiling.

B. laminar flow with nucleate boiling.

C. laminar flow with no nucleate boiling.

D. turbulent flow with nucleate boiling.


ELO 2.1

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Reactor Core Thermal Hydraulic Properties


Fuel Channel Flow

• RCS flow in the reactor is not simple single-phase forced flow.

– Undergoes nucleate boiling and

– Various two-phase flow types

– Normally enhances heat transfer from the fuel to the coolant.

ELO 2.2

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ELO 2.2 – Explain fuel channel flow and heat transfer, including the following terms: slug flow, annular flow, dryout region, and flow resistance.


Fuel Channel Flow

Convective Heat Transfer

• RCS coolant entering the fuel channel inlet is subcooled and pressurized

• Heat transfer at the inlet takes place by convection and flow is single-phase; no steam bubbles.

ELO 2.2

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Bubbly Flow

• Coolant flowing through the core increases in temperature, which reduces amount of subcooling.

– Closer to saturation.

• Small bubbles start to form on fuel cladding imperfection sites.

– Break away and collapse into the coolant flow.

ELO 2.2

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Two-Phase Flow

• If coolant temperature reaches saturation, the small bubbles no longer collapse, but remain in the coolant stream.

– Bulk boiling occurs but the bubbles do not combine

• This is the initiation of two-phase flow

ELO 2.2

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Slug Flow

• As heat transfer continues increasing coolant temperature,

– Coolant steam bubbles begin to coalesce into elongated vapor slugs

– Large void fractions occur as steam vapor occupies more volume.

– Heat transfer continues at almost the same rate and coolant velocity increases due to the large volume of slugs.

ELO 2.2

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Annular Flow

• Vapor slugs may combine within the coolant near the center of the coolant channel creating a vapor core in the coolant channel.

– This occurs higher in the flow channel.

• Some RCS coolant remains in contact with channel walls to remove heat.

• Vapor forms a continuous phase between fuel elements with lower velocity fluid flowing along the coolant channel walls.

ELO 2.2

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Dryout

• The vapor core intensifies and more coolant flashes to steam.

• A vapor cloud with small entrained water droplets forms.

• The ability to remove heat from the fuel greatly decreases.

• Low coolant flow or pressure enhances the possibility of dryout.

Coolant acts as the fuel heat sink; if the heat removal does not occur, fuel damage can and likely will result.

ELO 2.2

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Fuel Channel Flows

ELO 2.2

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Figure: Fuel Channel Flows


Single-Phase Fluid Flow Resistance

• Fluid friction (resistance or head loss) occurs with fluid flow.

• Depends on

– Flow velocity

– Pipe length and diameter

– Friction factor based on the roughness of the pipe and the Reynolds number

• Head loss measures the reduction in the total head of the fluid as it moves through a fluid system.

• Head loss is unavoidable in real fluids.

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ELO 2.2



Where:

hf = Friction head lossf = Darcy resistance factorL = Length of the pipeD = Pipe diameterv = Mean velocityg = acceleration due to gravity

The Darcy friction factor, f, is usually selected from a chart known as the Moody diagram. The Moody diagram is a family of curves that relate the friction factor, to Reynolds number, Re, and the relative roughness of a pipe, e/D.

ELO 2.2

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The Darcy Equation predicts the frictional energy loss in a pipe based on the velocity of the fluid and the resistance due to friction. It is used to calculate head loss due to friction in turbulent flow.

h 𝑓=𝑓𝐿𝑣2

2𝐷𝑔



• Head loss is present because of:

– Friction between the fluid and the walls of the pipe

– Friction between adjacent fluid particles as they move relative to one another

– Turbulence caused by redirected flow or by components such as piping entrances and exits, pumps, valves, flow reducers, and fittings

• Forced flow = when pumps do work on the fluid to compensate for head losses creating resistance to flow

• With no pumps = natural circulation flow

ELO 2.2

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• Two-phase flow is the simultaneous flow of liquid water and steam.

• Head loss is typically greater than single-phase for the same pipe dimensions and mass flow rates.

– Type of two-phase flow and velocity affect the friction losses.

• Two-phase flow losses determined experimentally by actual flow measurements.

ELO 2.2

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Two-Phase Fluid Flow Resistance


Where:

R = two-phase friction multiplier (no units)

Hf, two-phase = two-phase head loss due to friction (ft)

Hf, saturated liquid = single-phase head loss due to friction (ft)

ELO 2.2

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𝑅=𝐻 𝑓 , 𝑡𝑤𝑜− h𝑝 𝑎𝑠𝑒

𝐻 𝑓 ,𝑠𝑎𝑡𝑢𝑟𝑎𝑡𝑒𝑑 𝑙𝑖𝑞𝑢𝑖𝑑



• The friction multiplier (R) is much higher at lower pressures than at higher pressures

• Two-phase head loss can be many times greater than the single-phase head loss.

ELO 2.2

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𝑅=𝐻 𝑓 , 𝑡𝑤𝑜− h𝑝 𝑎𝑠𝑒

𝐻 𝑓 ,𝑠𝑎𝑡𝑢𝑟𝑎𝑡𝑒𝑑 𝑙𝑖𝑞𝑢𝑖𝑑


• R value analysis performed for multiple types of two-phase flows - most common include:

– Bubbly flow - dispersion of steam bubbles in a liquid – the onset of two-phase flow.

– Slug flow – steam bubbles grow, combine, and ultimately become of the same order of diameter as the tube; bullet-shaped bubbles characteristic of the slug-flow regime.

– Annular flow - the liquid is now distributed between a liquid film flowing up the channel wall and a vapor core within the coolant channel

ELO 2.2

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© Copyright 2014 Operator Generic FundamentalsELO 2.2

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Figure: Fuel Channel Heat Transfer Flows



Knowledge Check


Which one of the following will minimize core heat transfer?

A. Laminar flow with no nucleate boiling

B. Turbulent flow with no nucleate boiling

C. Laminar flow with nucleate boiling

D. Turbulent flow with nucleate boiling

Correct answer is A.

ELO 2.2

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• A large ∆T is required to transfer heat from

– The fuel pellet,

– Across the pellet to cladding gap,

– Thru the cladding gap,

– Across the cladding, then into the coolant.

• This session illustrates the radial temperature profile of the fuel and cladding.

ELO 2.3

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ELO 2.3 – Draw a temperature profile from the centerline of a fuel pellet to the centerline of the flow channel.

Radial Fuel Temperature Profile


• Reactor fuel fissions produce heat energy.

• Heat transfer from fuel to the RCS coolant via conduction and convection.

– Conduction occurs from the fuel center to the cladding outer surface.

• Helium gas pressurized gap between the fuel and the cladding increases in pressure over fuel element life from fission product gasses.

– These gases also transfer heat via conductivity.

ELO 2.3

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• As previously discussed:

– From the cladding to the coolant convection heat transfer occurs

– Heat transfer is enhanced with two-phase flow involving nucleate boiling and resulting turbulent flow.

ELO 2.3

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Figure: Radial Fuel Temperature Profile



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Peak fuel temperatures • Average approximately 2,000°F• Peak as high as 4,400° F• Melting at 4,800°F




Knowledge CheckAt 100 percent reactor power, the greatest temperature difference in a radial fuel temperature profile occur across the: (Assume the temperature profile begins at the fuel centerline.)

A. fuel pellet centerline to pellet surface.

B. fuel pellet surface-to-clad gap.

C. zircaloy cladding.

D. flow channel boundary (laminar) layer.


ELO 2.3

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Fuel Channel Flow


• Core bypass flow equalizes temperatures between the reactor vessel and the upper vessel head

• Used for cooling internal reactor vessel components

ELO 2.4

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ELO 2.4 – Describe core bypass flow and purpose of adequate flow.

Reactor Core Bypass Flow


• RCS coolant enters the vessel through inlet nozzles located in a horizontal plane above the active fuel region and below the plane of the vessel flange.

• Coolant flows downward through the annular spaces between the vessel wall and the thermal shield and core barrel.

ELO 2.4

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Figure: Reactor Vessel Internals (Westinghouse Design)

Reactor Vessel and Internals Coolant Flow


• At the vessel bottom the coolant reverses direction to travel upward flowing through:

– Bottom support plate

– Intermediate diffuser plate

– Lower core plate

– Fuel assemblies

– Upper core plate and

– Into the core barrel outlet plenum.

ELO 2.4

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Figure: Lower Core Support Assembly



• Coolant exits through the vessel outlet nozzles located in the same horizontal plane as the inlet nozzles.

• Fuel heat transfer flowpaths constitute 94 percent of the total reactor coolant flow.

• The remaining 6 percent is bypass flow.

ELO 2.4

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• Nozzle bypass flow (1 percent)

– Short circuiting from the reactor inlet to the reactor outlet nozzle due to the slight gap between the core barrel and outlet nozzles.

• Vessel pressure drop is the driving force for this bypass flow.

ELO 2.4

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Reactor Core Bypass Flow Paths




• Control rod and instrument thimble bypass flows (4 percent)

– Flow enters the control rod guide thimbles at the dashpot section

– Flow enters instrument thimbles at the bottom of the fuel elements

– Flows upward and out of the core without removing any heat from fuel

• Driving force is pressure drop across the core.

ELO 2.4

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• Baffle wall bypass flow (1/2 percent)

– Coolant from the annular space between the core barrel and thermal shield bypasses through holes in the top of the core barrel

– Flows downward between the inner core barrel wall and vertically mounted core baffle plates

• Provides cooling/temp. equalizing for inner barrel wall and core baffle plates.

• Driving force is the pressure drop across the vessel.

ELO 2.4

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Figure: Top View of Core Barrel and Baffle Plates (Westinghouse Design)


• Head cooling bypass flow (1/2 percent)

– Reactor vessel inlet water passes through flow holes in the core barrel support flange and the top support plate.

– Prevents stagnation and cools the vessel head plenum area.

– After passing through the flow holes up into the vessel head plenum, returns to the outlet plenum via the upper internals (control rod guide tubes, support columns, etc.) exiting the vessel.

• Driving force is the pressure drop across the reactor.

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92

Figure: Top View of Core Barrel and Baffle Plates (Westinghouse Design)



Knowledge Check – NRC BankAdequate core bypass flow is needed to...

A. cool the excore nuclear instrument detectors.

B. provide reactor coolant pump minimum flow requirements.

C. prevent stratification of reactor coolant inside the reactor vessel lower head.

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


ELO 2.4

93

Reactor Core Bypass Flow


• Core thermal limits on core operating parameters ensure the plant operates within design boundaries to protect the public health and safety.

ELO 2.5

94

ELO 2.5 – Draw the axial temperature and enthalpy profiles for a typical reactor coolant channel and describe how they are affected by the following: onset of nucleate boiling, axial core flux, inlet temperature, heat generation rate, and flow rate in the channel.

Axial Temperature and Enthalpy Profiles


• Thermal limits ensure that:– There is “at least a 95-percent probability at a 95-percent

confidence level”

– Departure from nucleate boiling (DNB) does not occur on limiting (hottest) fuel rods during normal operation and operational transients, including any transient conditions arising from faults of moderate frequency.

• This is to ensure protection of the fuel cladding.

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95



• Thermal limits, calculated from – Plant safety analysis criteria set by the Nuclear Regulatory Commission

(NRC)

– Thermal power output

– RCS coolant flow rates

• Fuel pellet design and size affect power density and fuel temperature margins.

• Fuel cladding design, material and thickness affects the heat transfer rate from fuel pellet to coolant and capability to withstand internal pressure from fission product gases.

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96



Onset of Nucleate Boiling

• Nucleate Boiling has the effect of increasing heat transfer to the coolant. – Lower fuel temperatures,

– Less ∆T across the cladding,

– Lower thermal stresses.

• With improved heat transfer – Coolant temperatures and

– Enthalpy increase rates are greater – assuming constant power, coolant pressures and flows.

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Axial Core Flux

ELO 2.5

98

• Fission rate is directly proportional to the heat generation rate.

• In the regions of highest flux, the highest heat generation rate will occur.

• Coolant temperatures and enthalpy increases are greatest in these areas of highest flux.

Figure: Axial Flux Profile


Axial Core Flux

• Axial (and radial) flux distribution is affected by numerous items,– Number of control rods and their positions in the core

– Core geometry and size

– Fission product poisons, and

– Burnable and non-burnable poisons.

• Axial power peaks normally occur at midplane; however, can shift during power transients, fission product oscillations, or core aging.

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99


Inlet Temperature

• A constant power implies a constant core ∆T (flow also remains constant),

• An increase in inlet temperature means a higher outlet temperature and enthalpy. – Fuel and clad temperatures also increase accordingly.

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100


Heat Generation Rate

• Heat generation rate or reactor thermal power level is proportional to the fission rate. – Higher power levels mean higher fuel temperatures.

• In a PWR, RCS coolant flow remains constant; therefore higher power levels means– Higher core ∆Ts, with resulting

– Higher ∆Ts across the fuel, gap and cladding, and

– Higher core exit temperatures and enthalpy.

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Flow Rate in the Channel• Reactor coolant flow in a PWR is constant (some minor changes

from RCS temperature/ density changes); therefore, channel flow rates are also constant.

– However, if flow rate does change with a constant power level, core ∆T also changes.

• An important concept to remember is that core ∆T is proportional to core power since PWRs maintain a constant RCS flow rate.

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102


Nuclear Enthalpy Rise • As coolant passes along fuel rods, it receives heat from fuel rods,

increasing coolant temperature and enthalpy.

• Enthalpy rise is dependent on core location.

• Coolant flowing by higher power fuel rods will attain a higher enthalpy increase.

• It is important that the temporary localized enthalpy increase does not cause an increase to DNB.

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ELO 2.5

104

• The temperature of the RCS coolant increases throughout the length of the channel

• The rate of increase varies with the linear heat rate (power output per linear unit) of the channel

• Enthalpy rise of the RCS coolant (not shown on the figure) has basically the same shape and responses as temperature

• Refer to the definition of enthalpy, temperature is a measure of heat energy so these will be very similar

Figure: Axial Core Temperature Profiles



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105

Power density and linear heat rate will follow the neutron flux shape.

• Fuel cladding and the fuel temperatures are highest at the highest linear heat rates

• Somewhat higher in the upper axial region of the core due to higher coolant temperatures.

Figure: Axial Core Temperature Profiles



Knowledge Check

Fuel clad integrity is ensured by ________________ during normal operation.

A. the primary system relief valves.

B. core bypass flow design.

C. Maximum core ∆T setpoint.

D. operation within core thermal limits.


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106


1. Flowrate and Phase Change effects on Heat Transfer

– Reducing effective thickness of stagnant laminar flow layer at heat transfer surface improves convection-heat transfer coefficient.

– Increased flow rate and two-phase flow reduce the stagnant laminar flow layer, and improves heat transfer.

– Convection heat transfer effectiveness varies greatly by fluid flow conditions, laminar or stagnant layer directly in contact with solid surface.

– Factors affecting convection heat transfer:o Fluid velocityo Fluid viscosityo Heat fluxo Surface roughnesso Type of flow (single-phase/two-phase)TLO 2

107

TLO 2 Summary


1. Flowrate and Phase Change effects on Heat Transfer (cont)The factors below contribute to reducing stagnant film layer thickness:

– Higher fluid velocity - this decreases laminar film thickness and lowers temperature of the coolant adjacent to the fuel (heat transfer surface).

– Increased flow turbulence - this thins out the stagnant laminar layer - for example, fuel assembly grid spacers increase flow turbulence.

– Increased surface roughness - this increases fluid friction against the heat transfer surface to break up the laminar flow. Examples include roughness, surface imperfections, etc.

– Boiling - this increases nucleate boiling and two-phase flow.

TLO 2

108

TLO 2 Summary


– Nucleate boiling improves heat transfer by removing heat from the heat transfer surface (fuel) both as sensible (no phase change) and latent heat of vaporization

2. Fuel Channel Flow

– Bubbly flow

– Two-Phase Flow

– Slug Flow

– Annular Flow

– Dryout

TLO 2

109

TLO 2 Summary

Figure: Fuel Channel Heat Transfer Flows


• Single-phase Fluid Flow Resistance– Friction between the fluid and the walls of the pipe

– Friction between adjacent fluid particles as they move relative to one another

– Turbulence caused by redirected flow or by components such as piping entrances and exits, pumps, valves, flow reducers, and fittings.

• Two-phase flow Resistance – Friction head loss is typically greater than single-phase for the

same conduit dimensions and mass flow rates.

– The type of two-phase flow and velocity factor affect the friction losses.

– Flow losses are experimentally determined by actual flow measurements. TLO 2

110

TLO 2 Summary


3. Radial Fuel Temperature Profile

– Heat is transferred from the fuel to the coolant by conduction and convection.

– Conduction occurs from the fuel through to the cladding.

– Convection occurs at the surface of the cladding.

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111


TLO 2 Summary


4. Core Bypass flow - fuel heat transfer flowpaths constitute 94 percent of the total reactor coolant flow. The remaining bypass flowpaths (6 percent of vessel flow) are:

– Nozzle bypass flow (1 percent - short circuiting from reactor inlet to outlet nozzle – equalize temperatures.

– Control rod and instrument thimble bypass flows (4 percent) – maintain cooling for control rod and instrument thimbles.

– Baffle wall bypass flow (1/2 percent) – cooling/temperature equalizing for inner core barrel wall and core baffle plates.

– Head cooling bypass flow (1/2 percent) - prevents stagnation and cools the vessel head plenum area.

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112

TLO 2 Summary


5. Axial Temperature and Enthalpy Profiles

– RCS coolant temperature increases throughout the entire length of the channel - rate of increase varies with the linear heat rate of the channel.

– Enthalpy rise of the RCS coolant has basically the same shape and responses as temperature.

TLO 2

113

– Power density and linear heat rate will follow the neutron flux shape.

– Fuel cladding and the fuel temperatures are highest at the highest linear heat rates - also higher in the upper axial region of the core due to higher coolant temperatures. Figure: Axial Core Temperature Profiles

TLO 2 Summary


– Thermal limits are established to ensure fuel cladding is not compromised during transients and DNB is not reached.

– Heat generation rate in a nuclear core is proportional to fission rate of the fuel and the thermal neutron flux.

TLO 2

114

TLO 2 Summary


Crossword Puzzle


Summary

115


Natural Circulation

• Reactor coolant flow is continuously required for cooling (after power operations).

• For PWRs, natural circulation provides passive heat removal capability, available regardless of power availability.

TLO 3

116

TLO 3 – Explain natural circulation and methods to enhance its effectiveness.



1. Define natural circulation and thermal driving head.

2. Describe the indications of natural circulation flow.

3. Describe how natural circulation can be enhanced.

4. Describe the process of reflux condensation.

5. Explain the effect of gas binding on natural circulation.

117

TLO 3


• Natural circulation is the circulation of fluid within piping systems or open pools due to the density changes caused by temperature differences.

• Natural circulation does not require any mechanical devices to maintain flow.

ELO 3.1

118

ELO 3.1 – Define natural circulation and thermal driving head.

Natural Circulation and Thermal Driving Head


• Fluid systems are possible to design such that pumps are not needed to provide circulation.

• PWR designs are primarily for forced circulation but have capability for natural circulation for decay heat removal.

• Natural circulation - motive force from density gradients and elevation changes.

– Heat sink in the loop located high

– Heat source at lower elevation

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119



• The heat source heats the fluid, causing its density to decrease.

• The heat sink cools the fluid, causing its density to increase.

• This establishes a fluid density difference in the loop, lower density in the lower heat source and higher density at the elevated heat sink.

• This density difference, gravity, and elevation differences, produces the driving force for fluid flow.

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120

Figure: Simplified Natural Circulation Loop



• Fluid density differences can be created by changes in temperature or by changes in phase (i.e. vapor/liquid), as is the case for two-phase fluid flows.

• The sum of the resistances in the components and interconnecting piping limits the flow rate.

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121



• The difference in density (temperature) and elevation between two fluid portions of a closed loop or pool is called the thermal driving head.

– Force that causes natural circulation to take place.

• Natural circulation can only happen if the conditions necessary to establish a thermal driving head exist.

• Once started, removal of any one of these necessary conditions will cause natural circulation to stop.

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Thermal Driving Head


• Conditions for natural circulation are:

– A temperature difference exists - to create the density difference.

– The heat source is at a lower elevation than the heat sink

– Fluids must be in contact with each other to create the flow path.

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123



• Consider a hot air balloon.

– Heating the air inside the balloon, causes it to expand, decreasing its density.

– Meanwhile the surrounding air, at a cooler temperature, has a higher density.

– Less dense air in the balloon, more dense outside the balloon.

– Since gravity relates to mass, it has less effect on the balloon air because of its lower density.

– Therefore the balloon weighs less than the surrounding air.

– Gravity “pulls” the heavier air down into the space occupied by the balloon, forcing the balloon to rise.

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124




Natural circulation flow can be enhanced by...

A. increasing the elevation of the heat source to equal that of the heat sink.

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

C. decreasing the temperature difference between the heat sink and the heat source.

D. decreasing the elevation difference between the heat source and the heat sink.


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125



Indications of Natural Circulation Flow

• Forced flow in a PWR maintains reactor coolant circulation unless

– A loss of plant power occurs causes inoperability of the reactor coolant pumps

– An accident response requires stopping the reactor coolant pumps

• Natural circulation flow is not adequate for power operation

– Very limited compared to forced flow

• This lesson explains how natural circulation can be identified.

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126

ELO 3.2 – Describe the indications of natural circulation flow.



• If force flow is not available operators are required to verify natural circulation flow.

• Plant procedures provide specific guidance and indication for determining natural circulation flow or core cooling by other methods.

• The following slides illustrate indications available for natural circulation verification.

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127


Indications of Natural Circulation Flow • RCS temperature, pressurizer level and pressure steady or

decreasing

– If core cooling is occurring or maintained constant this indicates that natural circulation could be functioning to remove core decay heat.

– Temperature, pressure and level increasing would be indication of a loss of natural circulation flow.

• Reactor coolant system ∆T

– Natural circulation flow is considerably less than forced flow.

– ∆T will initially increase following a reactor trip; depending on the amount of decay heat

– ∆T may be almost as high as normal full power.

– As decay heat decreases, ∆T will also decrease.

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128



• Steam generator pressure

– If natural circulation exists, the steam generators will act as a heat sink for the reactor.

– SG pressure should approximately track cold leg saturation temperature.

– If natural circulation flow does not exist, the steam generators are no longer acting as a heat sink or a link to the reactor; SG pressure decreases at an atypical higher rate.

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129



• Subcooling margin

– Adequate subcooling margin is another indication of adequate core cooling.

– In addition to the other indications, subcooling is indication of natural circulation functioning to remove core heat

• RCS temperature not increasing

– A loss of subcooling margin could indicate steam voiding in the upper portions of the reactor or accumulation of steam in portions of the RCS piping preventing (blocking) natural circulation flow.

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130




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 __________.

A. decrease; increaseB. decrease; remain constantC. increase; increaseD. increase; remain constant


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131


Enhancing Natural Circulation Flow

• Natural circulation only occurs if a thermal driving head exists.

• Once started, removal of any one of the necessary conditions for a thermal driving head will cause natural circulation to stop.

• Enhancing these conditions can improve the stability and effectiveness of natural circulation flow.

ELO 3.3

132

ELO 3.3 – Describe how natural circulation can be enhanced.


Enhancing Natural Circulation Flow • Recall that the following conditions are necessary for natural

circulation:

– A temperature difference exists; heat source and heat sink to create the density difference.

– The heat source is at a lower elevation than the heat sink; lower density to rise, higher density to sink.

– Fluids must be in contact with each other; creates the flow path.

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133


Enhancing Natural Circulation Flow Temperature Difference

• An increase in the temperature difference (heat source to heat sink) increases the thermal driving force – more stable flow.

• Steam flow cooling the steam generators and the hot reactor heat source produces the necessary thermal driving head.

– Continuous removal of heat by the steam generators (steam flow) is necessary to maintain natural circulation flow.

Reducing steam generator cooling by reducing or stopping steam flow will eventually equalize the ∆T resulting in natural circulation flow stopping.

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134



The Heat Source at Lower Elevation

• The reactor plant design has the steam generators (heat sink) physically higher than the reactor (heat source).

– This design accommodates natural circulation flow.

• If the elevation difference could be increased this would enhance natural circulation flow.

ELO 3.3

135

Reactor plant design is fixed


Enhancing Natural Circulation Flow Fluids Must Be In Contact with Each Other• If flow path obstructions or blockage exists, then natural circulation

cannot occur.

• Conditions such as two-phase flow or vapor blockage can occur to inhibit natural circulation flow.

• Avoiding gas intrusion that could inhibit flow and maintaining the fluid subcooled to reduce chances of steam voiding are important.

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136




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

A. minimize the RCS volume.

B. maximize the RCS flow rate during forced circulation.

C. ensure a maximum RCS loop transit time.

D. ensure RCS natural circulation flow can be established.


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137


Natural Circulation Two-Phase Flow and Reflux Condensation

• Natural circulation is likely to be essential to core decay heat removal for certain types accidents or transients in a PWR

– e.g., small break LOCAs or operational transients involving loss of reactor coolant pumps

• Understanding its response to abnormal reactor plant conditions is important.

ELO 3.4

138

ELO 3.4 – Describe the process of reflux condensation.


Natural Circulation Two-Phase Flow and Reflux Condensation • Natural circulation cooling is the primary means of removing reactor

decay heat (shutdown reactor) in PWRs following the loss of reactor coolant pumps (forced circulation) during operational transients or following certain accidents.

• The loss of reactor coolant pumps may result from

– Loss of offsite power

– Pump failure, or

– Operator action based on abnormal or emergency procedures.

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139


Natural Circulation Two-Phase Flow and Reflux Condensation • Depending on the primary (RCS) loop fluid inventory, three distinct

modes of natural circulation cooling are possible:

– Single-phase (liquid only)

– Two-phase (liquid/steam vapor continuous)

– Reflux condensation (or boiler-condenser mode for once-through steam generators)

ELO 3.4

140


• Single-phase natural circulation is the mode without inventory loss; ex. pump power failure with no loss of coolant.

• Two-phase natural circulation can occur with RCS inventory levels lower than normal single-phase flow, but not a major loss of inventory.

• With a significant reduction in RCS inventory, loop circulation breaks down and reflux condensation occurs.

– Cooling occurs primarily by steam condensation in the steam generator and subcooled liquid returned to the reactor

– Mass flowrate heat removal is limited

– This mode remains capable of removing decay heat, but w/o loop circulation.

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141

Single-Phase Natural Circulation


142

Figure: Natural Circulation Loop in PWR

Single-Phase Natural Circulation


• Two-phase natural circulation is the continuous flow of fluid and steam vapor.

• Vapor generated in the core enters the hot leg and flows along with the saturated liquid to the steam generator

– A portion of the steam vapor is condensed.

• Density gradients in the two-phase mode occur from liquid temperature difference and steam voids in the primary loops.

• Mass flow rate is the primary heat removal parameter in two-phase natural circulation.

ELO 3.4

143

Two-Phase Natural Circulation


Reflux Condensation Natural Circulation

• Reflux condensation starts when hot leg condensation is unable to

pass completely through the steam generators to enter the cold legs

and maintain flow.ELO 3.4

144

Figure: Start of the shift from Two-Phase NC Flow to Reflux Condensation during a Small Break LOCA (Voiding at top of SG Tubes)



ELO 3.4

145

Figure: Reactor Vessel Level Loss with Reflux Condensation during a Small Break LOCA



• Removal of decay heat from the core during reflux condensation does not require

– Large mass flow rates, or

– Large primary to secondary temperature differences.

• Small mass flow rates and primary to secondary temperature differences are characteristic of the reflux condensation mode of natural circulation.

• During reflux condensation, loop mass flow rate is the secondary heat removal parameter with vapor condensation as the primary method.

ELO 3.4

146



ELO 3.4

147

Figure: PWR Liquid Distribution during Reflux Condensation


Natural Circulation Two-phase Flow and Reflux CondensationKnowledge Check – NRC BankA 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.)

A. Reactor core steam production results in two-phase coolant entering the hot leg and being delivered to the steam generators.

B. Hot leg steam quality is so high that the steam generators cannot fully condense it and two-phase coolant is returned to the reactor vessel via the cold leg.

C. Hot leg condensation is unable to pass completely through the steam generators to enter the cold legs.

D. The steam generators are no longer able to condense any of the steam contained in the hot leg.

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148



• Non-condensable gas in the primary loop may impede or even stagnate the natural circulation flow.

– Significantly reducing or terminating the heat removal capability of the steam generators for single-phase and two-phase modes of natural circulation cooling.

• Top of U-tubes is a good place for gas to collect.

– Affecting two-phase and reflux condensation by reductions in the effectiveness of reflux condensation

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149

ELO 3.5 – Explain the effect of gas binding on natural circulation.

Natural Circulation Gas Binding


• Gas in the steam generator tubes may:

– Cause a redistribution of condensation locations, and

– Influence the amount of liquid returning to the loops (and reactor) via the down-side of the U-tubes.

• Potential for non-condensable gas having a considerable influence on the effectiveness of the natural circulation heat removal process.

ELO 3.5

150



• Non-condensable gases may enter the primary system through:

– Safety injection operation

– Fuel degradation (Helium and fission gases)

– Hydrogen from the pressurizer vapor space

– Dissolved air in the refueling water (source of safety injection)

– Nitrogen from accumulators (following discharge)

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• For non-condensable gases to affect natural circulation, it is necessary that they travel and collect in the upper elevations of the primary loops.

• Two-phase flow is more tolerant of non-condensable gases than single-phase natural circulation.

– But, two-phase natural cooling may still be negatively affected.

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Knowledge CheckCan the accumulation of non-condensable gases in the SG U‑tubes prevent Natural Circulation from being re-established once the tubes have been refilled by injection flow?

Correct answer is: Yes, because safety injection will not necessarily purge the RCS of non-condensable gases.

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153


1. Natural Circulation and Thermal Driving Head– Natural circulation flow is circulation of a fluid without the use of

mechanical devices.

– Forced circulation flow is circulation of a fluid through a system by pumps.

– Thermal driving head is the driving force for natural circulation caused by the difference in density and elevation of two fluids.

o There must be a heat sink and a heat source.

o The heat source must be located below the heat sink.

o Flowpaths must exist between the warm fluid and the cold fluid.

– The greater the temperature difference, the higher the natural circulation flow rate.

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TLO 3 Summary


2. Indications of Natural Circulation Flow:

– RCS temperature, pressurizer level, and pressure: If core cooling is occurring or maintained constant with no forced flow (RCPs), these indicators would hold steady or decrease

– Reactor coolant system ∆T will initially increase following a reactor trip; depending on the amount of decay heat ∆T may be almost as high as normal full power. As decay heat decreases, decreasing ∆T indicates natural circulation flow is cooling the RCS.

– Steam generator pressure should approximately track cold leg saturation temperature.

– Adequate subcooling margin is another indication of adequate core cooling

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TLO 3 Summary


3. Enhancing Natural Circulation Flow:

– An increase in the temperature difference (heat source to heat sink) increases the thermal driving force for natural circulation flow

– Continuous removal of heat by a heat sink must exist at the low temperature area.

– The greater the elevation difference, the greater propensity for natural circulation flow.

– Obstruction or blockages to the natural circulation flowpath must be avoided.

– Fluid should remain subcooled to prevent steam voiding.

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TLO 3 Summary


4. Natural Circulation Two-phase Flow and Reflux Condensation

– Natural circulation consists of three distinct modes of cooling:o Single-phase (liquid only)o Two-phase (liquid/steam vapor continuous) o Reflux condensation (or boiler-condenser mode for once-

through steam generators)

– Single-phase natural circulation mode without inventory loss - driven by density gradients and elevation differences - heat transfer mechanism is convection.

– Two-phase natural circulation, continuous fluid and steam vapor flow, decreasing RCS inventory levels.

o Density gradients from both the temperature difference and the steam voids - mass flow rate is the primary heat removal parameter.

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TLO 3 Summary


• Reflux condensation - significant reduction in RCS inventory, loop circulation breaks down.

– Cooling is provided primarily by steam condensation in the steam generator and subcooled liquid returned to the reactor.

– In PWRs with U-tube SGs occurs when single-phase vapor generated in the core flows through the hot leg piping to the steam generators, and condensed in both the up-flow and down-flow sides of the SG U-tubes.

– Condensate in the up-flow tubes drains back via the hot leg and eventually back to the vessel along the bottom of the hot leg piping; this is a countercurrent flow of liquid and vapor on the hot leg side.

– On the down-flow side liquid and any uncondensed steam flows into the cold leg pump suction piping.

– Very effective due to the high latent heat associated with condensation.

– Does not require large mass flow rates or large primary to secondary temperature differences.

– Small mass flow rates and primary to secondary temperature differences are characteristic of the reflux condensation mode of natural circulation.

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TLO 3 Summary


5. Natural Circulation Gas Binding– Non-condensable gas in the primary loop may impede or even

stagnate the natural circulation flow. (single- or two-phase)

o Non-condensable gas in the steam generator tubes during reflux condensation mode may cause a redistribution of the condensation locations and influence the amount of liquid returning to the loops via the down-side of the U-tubes

– Non-condensable gases can be introduced into the primary system through:

o Safety injection operation

o Fuel degradation (Helium and fission gases)

o Hydrogen from the pressurizer vapor space

o Dissolved air in the refueling water (source of safety injection)

o Nitrogen from accumulators (following discharge)TLO 3

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TLO 3 Summary


– For non-condensable gases to affect natural circulation it is necessary that they travel and collect in the upper elevations of the primary loops.

– Two-phase flow is more tolerant of non-condensable gases than single-phase natural circulation.

TLO 3

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TLO 3 Summary


Crossword Puzzle


Summary

161


Now that you have completed this module, you should be able to demonstrate mastery of this topic by passing a written exam with a grade of 80 percent or higher on the following TLOs:

1. Explain the various types of boiling heat transfer.

2. Describe the basic reactor core thermal hydraulic properties.

3. Explain natural circulation and methods to enhance its effectiveness.

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Thermal Hydraulics Summary

Summary

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