Operator Generic Fundamentals Thermodynamics – Thermal Hydraulics

Operator Generic Fundamentals Thermodynamics – Thermal Hydraulics
Ensure students have calculators that are permitted for use on the Generic Fundamentals Examination. Operator Generic Fundamentals Thermodynamics – Thermal Hydraulics

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 Figure: Modes of Heat Transfer Introduction

Terminal Learning Objectives
At 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): Explain the various types of boiling heat transfer. Describe the basic reactor core thermal hydraulic properties. Explain natural circulation and methods to enhance its effectiveness. Introduction

Boiling Heat Transfer TLO 1 – Explain the various types of 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

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

Enabling Learning Objectives for TLO 1
Describe the differences between boiling processes and other means of heat transfer. Describe the process of nucleate boiling, bulk boiling, departure from nucleate boiling, and critical heat flux (CHF). Describe the transition to partial film boiling. Describe the transition to full film boiling. TLO 1

Boiling Heat Transfer ELO 1.1 – Describe the differences between boiling processes and other means of heat transfer. 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. Related KA K1.01 Distinguish between boiling processes and other heat transfer mechanisms ELO 1.1

Boiling 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. ELO 1.1

Boiling Heat Transfer Local boiling
Steam bubbles form at the surface and Immediately collapse in the fluid Bulk boiling Boiling taking place when saturation conditions exist. ELO 1.1

Boiling Heat Transfer Regions
This figure illustrates the relative heat flux (heat transfer rates) for the different heat transfer regions, including boiling. Natural convection Nucleate Boiling Partial Film Boiling Film boiling Figure: Fluid Heat Transfer Regions ELO 1.1

Boiling Heat Transfer Knowledge Check True or False?
Boiling on the heat transfer surface is never a good thing. A. False B. True Correct Answer is A. Correct Answer is A. Boiling heat transfer can enhance the heat transfer rates – sometimes it can reduce heat transfer rates. ELO 1.1

Nucleate Boiling, DNB, and CHF
ELO 1.2 – Describe nucleate boiling, bulk boiling, departure from nucleate boiling, critical heat flux (CHF), and subcooling margin. 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. Related KA’s K1.02 Describe means by which boiling affects convection heat transfer ; K1.03 Describe the processes of nucleate boiling, subcooled nucleate boiling, and bulk boiling ; K1.04 Describe DNB (departure from nucleate boiling) ; K 1.05 List the parameters that affect DNR and DNBR and describe their effect(s) ; K1.10 Define DNBR ; K1.06 Describe CHF (critical heat flux) ; K1.09 Describe burnout and burnout heat flux , K1.15 Define and describe subcooling margin (SCM) ELO 1.2

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. The first boiling heat transfer region, following natural convection, is called nucleate boiling. ELO 1.2

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 Latent heat of vaporization = change phase, liquid to steam ELO 1.2

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. Formation and collapsing of steam bubbles promotes mixing and motion of the coolant with greater energy transfer as small steam bubbles carry off the latent heat of vaporization. ELO 1.2

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

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

Nucleate Boiling Nucleate Boiling Examples ELO 1.2

Convection Heat Transfer
Figure: Convection Heat Transfer ELO 1.2

Nucleate Boiling Figure: Nucleate Boiling Example ELO 1.2

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

Bulk boiling is the upper part of the nucleate boiling curve. Figure: Fluid Heat Transfer Regions ELO 1.2

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 Critical Heat Flux / DNB Figure: Stages of Nucleate Boiling ELO 1.2

Transition from nucleate boiling to partial film boiling is the departure from nucleate boiling, commonly written as DNB. Figure: Fluid Heat Transfer Regions ELO 1.2

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

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

Figure: Transition / Partial Film Boiling
Transition / Partial Film Boiling - steam bubbles begin to cover the entire heat transfer surface. ELO 1.2

Departure from Nucleate Boiling Ratio
Describes the margin between actual and critical heat flux Referred to as DNBR. Mathematically: 𝐷𝑁𝐵𝑅= 𝐶𝑟𝑖𝑡𝑖𝑐𝑎𝑙 ℎ𝑒𝑎𝑡 𝑓𝑙𝑢𝑥 𝐴𝑐𝑡𝑢𝑎𝑙 ℎ𝑒𝑎𝑡 𝑓𝑙𝑢𝑥 𝑎𝑡 𝑎𝑛𝑦 𝑝𝑜𝑖𝑛𝑡 𝑎𝑙𝑜𝑛𝑔 𝑎 𝑓𝑢𝑒𝑙 𝑟𝑜𝑑 ELO 1.2

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

Practice Question 1 Why does nucleate boiling improve heat transfer in a nuclear reactor core? The formation of steam bubbles at nucleation sites on the fuel clad allows greater heat transfer by conduction. The formation of steam bubbles at nucleation sites on the fuel clad promotes local radiative heat transfer and allows more heat transfer by convection. Heat removal from fuel rods as both sensible heat and latent heat of condensation with direct transferred to the coolant by radiative heat transfer. 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. Correct answer is 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. ELO 1.2

Practice Question 3 Many 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? Chemicals dissolved in the water The absence of ionizing radiation exposure to the water A highly polished heat transfer surface with minimal scratches or cavities The presence of gases dissolved in the water Correct answer is D. Correct answer is D. The presence of gases dissolved in the water 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? Decrease pressurizer pressure Decrease reactor coolant flow Decrease reactor power Increase reactor coolant temperature Correct answer is C. Correct answer is C. Note: There are additional knowledge questions for this objective at the end of the chapter, as hidden slides. ELO 1.2

Knowledge Check Knowledge Check – NRC Bank
The departure from nucleate boiling (DNB) ratio is the... actual heat flux divided by the critical heat flux at any point along a fuel rod. critical heat flux divided by the actual heat flux at any point along a fuel rod. core thermal power divided by the total reactor coolant mass flow rate. number of coolant channels that have reached DNB divided by the number of coolant channels that are subcooled. Correct answer is B. Correct answer is B. NRV Questions P89 ELO 1.2

Partial Film Boiling ELO 1.3 – Describe the transition to partial film boiling. The point where DNBR and CHF is reached transitions to the partial film boiling region. Related KA’s K1.07 Describe transition (partial film) boiling Figure: Fluid Heat Transfer Regions ELO 1.3

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

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). Figure: Partial Film Boiling ELO 1.3

Figure: Transition / Partial Film Boiling
ELO 1.3

Partial Film Boiling Knowledge Check
Which one of the following describes the conditions in a fuel coolant channel that is experiencing transition boiling? Complete steam blanketing of the fuel rod surface Subcooled nucleate boiling Saturated nucleate boiling Alternate wetting and drying of the fuel rod surface Correct answer is D. Correct answer is D. ELO 1.3

Film Boiling ELO 1.4 – Describe the transition to film boiling.
With heat transfer continuing to degrade, film boiling is the next heat transfer region. Related KA’s K1.08 Describe film boiling Figure: Fluid Heat Transfer Regions ELO 1.4

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

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

Film Boiling Knowledge Check – NRC Bank
Film boiling heat transfer is... the most efficient method of boiling heat transfer. heat transfer through a vapor blanket that covers the fuel cladding. heat transfer through an oxide film on the cladding. heat transfer being accomplished with no enthalpy change. Correct answer is B. Correct answer is B. NRV Question P88 ELO 1.4

TLO 1 Summary 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. Nucleate Boiling, DNB and CHF Nucleate boiling is the formation of small bubbles at a heat transfer surface. TLO 1

TLO 1 Summary Bubbles swept into coolant and collapse due to the coolant being a subcooled liquid. 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. 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

TLO 1 Summary 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

TLO 1 Summary – Review Questions
Identify the region of the curve where the most efficient form of heat transfer exists. Region IV Region III Region II Region I Correct answer is C. Correct answer is C. TLO 1 Explain the various types of boiling heat transfer. TLO 1

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? Region IV Region III Region II Region I Correct answer is C. Correct answer is C. TLO 1 Explain the various types of boiling heat transfer. TLO 1

Which one of the points shown represents the onset of transition boiling? A B C D Correct answer is B. Correct answer is B. TLO 1 Explain the various types of boiling heat transfer. TLO 1

Crossword Puzzle It’s crossword puzzle time!
Give students approximately 20 minutes to complete the puzzle. Review key with class after all students have completed. Summary

Reactor Core Thermal Hydraulic Properties
TLO 2 – Describe the basic reactor core thermal hydraulic properties. 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. This section discusses RCS fuel channel flows through the core, core bypass flows, and temperature profiles of the fuel and fuel channels. TLO 2

Describe the heat transfer coefficient and effects from flowrate and phase change. Explain fuel channel flow and heat transfer, including the following terms: Slug Flow Annular Flow Dryout Region Flow resistance Draw a temperature profile from the centerline of a fuel pellet to the centerline of the flow channel. TLO 2

Describe core bypass flow and purpose of adequate flow. 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 Flow rate in the channel TLO 2

Flowrate and Phase Change Effects on Heat Transfer
ELO 2.1 – Describe the heat transfer coefficient and effects from flowrate and phase change. 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. Related KA K1.14 Describe effects of flowrate and phase change on the heat transfer coefficient ELO 2.1

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. Recall these terms used for convection heat transfer ELO 2.1

Flowrate and Phase Change effects on Heat Transfer
Heat Transfer Rate BTU/hr ( 𝑄) 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 Recall heat transfer rate and heat flux terms ℎ= 𝑄 ∆𝑇 ELO 2.1

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. Recall laminar and turbulent flow ELO 2.1

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) Convection heat transfer occurs from the motion and mixing of the molecules of a liquid; it involves the transfer of heat by the fluid flow along a solid heat transfer surface (fuel). Convection heat transfer effectiveness varies greatly by the fluid flow conditions, especially the laminar or stagnant layer directly in contact with the solid surface. ELO 2.1

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. Convective heat transfer coefficient for laminar flow is LOW compared to the coefficient for turbulent flow. ELO 2.1

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 Convective heat transfer coefficient helped by . . . ELO 2.1

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 Steam bubbles form at the heat transfer surface (phase change) and move into the fluid stream condensing (heat release) and causing increased mixing of the coolant. ELO 2.1

Reactor Core Thermal Hydraulic Properties
Knowledge Check – NRC Bank Core heat transfer rate maximizes by the presence of... turbulent flow with no nucleate boiling. laminar flow with nucleate boiling. laminar flow with no nucleate boiling. turbulent flow with nucleate boiling. Correct answer is D. Correct answer is D. NRC Question – P389 ELO 2.1

Fuel Channel Flow ELO 2.2 – Explain fuel channel flow and heat transfer, including the following terms: slug flow, annular flow, dryout region, and flow resistance. 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. Related KA’s K1.11 Classify slug flow region along a fuel pin, experiencing two phase flow. 1.9* 2.1*, K1.12 Describe annular flow region along a hypothetical fuel pin, experiencing two phase flow. 1.8* 1.9*, K1.13 Describe dryout region or mist flow region along ahypothetical fuel pin, experiencing two phase flow. 1.9* 2.1* K1.17 Explain the necessity of determing core coolant flow , K1.18 Describe the factors affecting single and two phase flow resistance ELO 2.2

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

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

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

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

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

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

Figure: Fuel Channel Flows
Point out: Forced Convection. Bubbly Flow, Slug Flow, Annular Flow, Dryout (transition and mist flow) Figure: Fuel Channel Flows ELO 2.2

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. Reynolds number = characterize different flow regimes within a similar fluids, such as laminar or turbulent. Very important guide and are widely used. Laminar flow occurs at low Reynolds numbers, characterized by smooth, constant fluid motion Turbulent flow occurs at high Reynolds numbers and is dominated by inertial forces, producing chaotic eddies, vortices and other flow instabilities. Matching the Reynolds numbers is not on its own sufficient to guarantee flow characteristics. Fluid flow is generally chaotic, and very small changes to shape and surface roughness can result in very different flows. ELO 2.2

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 = 𝑓𝐿 𝑣 2 2𝐷𝑔 Where: hf = Friction head loss f = Darcy resistance factor L = Length of the pipe D = Pipe diameter v = Mean velocity g = 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

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 Frictional loss is that part of the total head loss occurring as fluid flows through straight pipes; it is directly proportional to the length of pipe, the square of the fluid velocity, inversely proportional to the diameter of the pipe, and a term accounting for fluid friction called the friction factor. For all minor losses in turbulent flow, head loss varies as the square of the velocity. Most techniques for evaluating head loss due to friction use experimental evidence and the friction factor. ELO 2.2

Two-Phase Fluid Flow Resistance
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

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) 𝑅= 𝐻 𝑓, 𝑡𝑤𝑜−𝑝ℎ𝑎𝑠𝑒 𝐻 𝑓, 𝑠𝑎𝑡𝑢𝑟𝑎𝑡𝑒𝑑 𝑙𝑖𝑞𝑢𝑖𝑑 One method for determining the two-phase friction loss uses a two-phase friction multiplier (R) with an equivalent single-phase flow. R is the ratio of the two-phase head loss and evaluated head loss using saturated liquid (single-phase) properties. ELO 2.2

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. 𝑅= 𝐻 𝑓, 𝑡𝑤𝑜−𝑝ℎ𝑎𝑠𝑒 𝐻 𝑓, 𝑠𝑎𝑡𝑢𝑟𝑎𝑡𝑒𝑑 𝑙𝑖𝑞𝑢𝑖𝑑 ELO 2.2

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

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 Figure: Fuel Channel Heat Transfer Flows ELO 2.2

Knowledge Check Knowledge Check – NRC Bank
Which one of the following will minimize core heat transfer? Laminar flow with no nucleate boiling Turbulent flow with no nucleate boiling Laminar flow with nucleate boiling Turbulent flow with nucleate boiling Correct answer is A. Correct answer is A. NRC Question – P1691 ELO 2.2

Radial Fuel Temperature Profile
ELO 2.3 – Draw a temperature profile from the centerline of a fuel pellet to the centerline of the flow channel. 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. Related KA’s K1.16 Draw the temperature profile from the centerline of a fuel pellet to the centerline of the flow channel ELO 2.3

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. Convective and radiative heat transfer across the gap is considered negligible with steady state operations. ELO 2.3

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

This figure illustrates the temperature profile from the center of the fuel pellet to the RCS coolant bulk temperature Inform students they may have to draw this. Figure: Radial Fuel Temperature Profile ELO 2.3

Peak fuel temperatures Average approximately 2,000°F Peak as high as 4,400° F Melting at 4,800°F The highest temperature is at the centerline of the fuel (Tcl); this is on the order of an average temperature of 2000°F, with peak temperatures as high as 4400°F. Lowest temperature is reactor coolant; around 540°F to 580°F depending on the PWR type and power level. Temperature drop Ts is from the lower velocity laminar fluid flow along the channel walls as compared to the more turbulent two-phase flow occurring in the main stream of the coolant. Figure: Radial Fuel Temperature Profile ELO 2.3

Fuel Channel Flow Knowledge Check
At 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.) fuel pellet centerline to pellet surface. fuel pellet surface-to-clad gap. zircaloy cladding. flow channel boundary (laminar) layer. Correct answer is A. Correct answer is A. ELO 2.3

Reactor Core Bypass Flow
ELO 2.4 – Describe core bypass flow and purpose of adequate flow. Core bypass flow equalizes temperatures between the reactor vessel and the upper vessel head Used for cooling internal reactor vessel components Related KA’s K1.19 Describe core bypass flow , K1.20 Explain the need for adequate core bypass flow ELO 2.4

Reactor Vessel and Internals Coolant 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. The reactor vessel acts to guide the reactor coolant (RCS) into a physical flow path with the fuel rods for core heat removal. Other flowpaths exist for reactor vessel component cooling and as "leakage" past metal to metal contact points between the vessel and internals components. All RCS coolant bypassing fuel rods identified as "bypass" flow. Figure: Reactor Vessel Internals (Westinghouse Design) ELO 2.4

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. Figure: Lower Core Support Assembly ELO 2.4

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. Point out to students number and paths given are for generic Westinghouse design, other vendors will differ. Figure: Reactor Vessel Internals (Westinghouse Design) ELO 2.4

Reactor Core Bypass Flow Paths
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. Figure: Reactor Vessel Internals (Westinghouse Design) ELO 2.4

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. Figure: Reactor Vessel Internals (Westinghouse Design) ELO 2.4

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. Figure: Top View of Core Barrel and Baffle Plates (Westinghouse Design) ELO 2.4

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. Figure: Top View of Core Barrel and Baffle Plates (Westinghouse Design) ELO 2.4

Reactor Core Bypass Flow
Knowledge Check – NRC Bank Adequate core bypass flow is needed to... cool the excore nuclear instrument detectors. provide reactor coolant pump minimum flow requirements. prevent stratification of reactor coolant inside the reactor vessel lower head. equalize the temperatures between the reactor vessel and the reactor vessel upper head. Correct answer is D. Correct answer is D. NRC Question P590 ELO 2.4

Axial Temperature and Enthalpy Profiles
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. Core thermal limits on core operating parameters ensure the plant operates within design boundaries to protect the public health and safety. Related KAs Sketch the axial temperature and enthalpy profiles for a typical reactor coolant channel and describe how they are affected by the following: K1.26 Onset of nucleate boiling; K1.27 Axial core flux, K1.28 Inlet temperature; K1.29 Heat generation rate, K1.30 Flow rate in the channel ELO 2.5

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. ELO 2.5

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. ELO 2.5

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. ELO 2.5

Axial Core Flux 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. Axial flux distribution depicts neutron flux (fission rate) from the bottom to the top of the core. Core power production (per unit) is highest at the midplane of the core; implies fuel and cladding temperatures are also highest at this point. Figure: Axial Flux Profile ELO 2.5

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. ELO 2.5

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. ELO 2.5

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. ELO 2.5

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. An example, if RCS coolant flow decreases with constant power, the BTUs transferred must remain constant. but with the mass flow rate decreased, core exit temperatures will increase to transfer the same number of BTUs of heat energy. This implies an increase in core ∆T, higher fuel temperatures, increased fuel and clad ∆Ts. ELO 2.5

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. Definition of Enthalpy - thermodynamic quantity equivalent to the total heat content of a system. It is equal to the internal energy of the system plus the product of pressure and volume. ELO 2.5

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 The temperature of the RCS coolant increases throughout the entire length of the channel. The rate of increase varies with the linear heat rate (power output per linear unit such as a foot) 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 ELO 2.5

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. 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 ELO 2.5

Knowledge Check Fuel clad integrity is ensured by ________________ during normal operation. the primary system relief valves. core bypass flow design. Maximum core ∆T setpoint. operation within core thermal limits. Correct answer is D. Correct answer is D. ELO 2.5

TLO 2 Summary 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: Fluid velocity Fluid viscosity Heat flux Surface roughness Type of flow (single-phase/two-phase) TLO 2 Describe the basic reactor core thermal hydraulic properties. TLO 2

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

Figure: Fuel Channel Heat Transfer Flows
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 Fuel Channel Flow Bubbly flow Two-Phase Flow Slug Flow Annular Flow Dryout TLO 2 Describe the basic reactor core thermal hydraulic properties. Figure: Fuel Channel Heat Transfer Flows TLO 2

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

TLO 2 Summary 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. TLO 2 Describe the basic reactor core thermal hydraulic properties. Figure: Radial Fuel Temperature Profile TLO 2

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

TLO 2 Summary 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. 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. TLO 2 Describe the basic reactor core thermal hydraulic properties. Figure: Axial Core Temperature Profiles TLO 2

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 Describe the basic reactor core thermal hydraulic properties. TLO 2

Natural Circulation TLO 3 – Explain natural circulation and methods to enhance its effectiveness. 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. Normally, the reactor coolant pumps pump water through the reactor core at a very high mass flow rate. Sometimes such as a loss of power or other emergency situations where the reactor coolant pumps are not available for operation. In these cases, another method must be available - natural circulation. TLO 3

Define natural circulation and thermal driving head. Describe the indications of natural circulation flow. Describe how natural circulation can be enhanced. Describe the process of reflux condensation. Explain the effect of gas binding on natural circulation. TLO 3

Natural Circulation and Thermal Driving Head
ELO 3.1 – Define natural circulation and thermal driving head. 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. Related KA K1.21 Explain the conditions which must exist to establish natural circulation * ELO 3.1

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 ELO 3.1

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. Heat sink is located higher than heat source Figure: Simplified Natural Circulation Loop ELO 3.1

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. ELO 3.1

Thermal Driving Head 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. ELO 3.1

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. ELO 3.1

Thermal Driving Head 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. ELO 3.1

Knowledge Check – NRC Bank Natural circulation flow can be enhanced by... increasing the elevation of the heat source to equal that of the heat sink. increasing the temperature difference between the heat sink and the heat source. decreasing the temperature difference between the heat sink and the heat source. decreasing the elevation difference between the heat source and the heat sink. Correct answer is B. Correct answer is B. NRC Question P392 ELO 3.1

Indications of Natural Circulation Flow
ELO 3.2 – Describe the 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. Related KA K1.22 Describe means to determine if natural circulation flow exists. 4.2* 4.2* ELO 3.2

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. ELO 3.2

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. ELO 3.2

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. ELO 3.2

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. ELO 3.2

Knowledge Check – NRC Bank 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 __________. decrease; increase decrease; remain constant increase; increase increase; remain constant Correct answer is A. Correct answer is A. NRC Question P2493 ELO 3.2

Enhancing Natural Circulation Flow
ELO 3.3 – Describe how natural circulation can be enhanced. 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. Related KA K1.23 Describe means by which natural circulation can be enhanced ELO 3.3

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. Review these conditions for a thermal driving head (natural circulation) ELO 3.3

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. An increase in the temperature difference (heat source to heat sink) increases the thermal driving force for natural circulation flow, resulting in increased and more stable flow. The reactor heat source may be heating initially up from a lack of cooling; however, without increasing the steam flow from the steam generators to produce a cooler heat sink, an adequate thermal driving head does not exist. ELO 3.3

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. Reactor plant design is fixed ELO 3.3

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. To take advantage of the natural movement of warm and cool fluids there must exist a flow path. The next sections discuss gas binding and two-phase and reflux condensation modes of natural circulation cooling. ELO 3.3

Knowledge Check – NRC Bank 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... minimize the RCS volume. maximize the RCS flow rate during forced circulation. ensure a maximum RCS loop transit time. ensure RCS natural circulation flow can be established. Correct answer is D. Correct answer is D. NRC Question P91 ELO 3.3

Natural Circulation Two-Phase Flow and Reflux Condensation
ELO 3.4 – Describe the process of 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. Related KA K1.24 Describe the process of reflux boiling (boiler condenser process) Investigation and studies of the March 1979 accident at TMI-2 increased understanding of the functionality of natural circulation cooling to remove core decay heat, especially during accident situations. ELO 3.4

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. ELO 3.4

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) Remember: Natural circulation occurs in a PWR whenever circulation forces caused by differences in loop fluid densities and elevations are sufficient to overcome the flow resistance of loop components (steam generators, reactor coolant pumps, etc.). Accident conditions may result in loss of RCS fluid inventory ELO 3.4

Single-Phase Natural Circulation
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. ELO 3.4

Single-Phase Natural Circulation
Single-phase natural circulation is the flow of essentially subcooled primary liquid driven by liquid density differences within the primary loop. The dominant heat transfer mechanism is convection, making loop flow rate the most important parameter governing heat removal. When the cooled primary fluid flows back to the reactor vessel, the cooling cycle is completed. Figure: Natural Circulation Loop in PWR ELO 3.4

Two-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

Reflux Condensation Natural Circulation
This figure illustrates the two-phase natural circulation flow degrading with the top of the SG U-tube voided with steam, eventually blocking of flow. Figure: Start of the shift from Two-Phase NC Flow to Reflux Condensation during a Small Break LOCA (Voiding at top of SG Tubes) 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

In PWRs with U-tube steam generators, reflux condensation occurs when vapor generated in the core flows through the hot leg piping to the steam generators and condenses in both the up-flow and down-flow sides of the steam generator 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. This heat transfer method is effective due to the high latent heat associated with steam condensation. Figure: Reactor Vessel Level Loss with Reflux Condensation during a Small Break LOCA ELO 3.4

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

Figure: PWR Liquid Distribution during Reflux Condensation ELO 3.4

Natural Circulation Two-phase Flow and Reflux Condensation
Knowledge Check – NRC Bank 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.) Reactor core steam production results in two-phase coolant entering the hot leg and being delivered to the steam generators. 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. Hot leg condensation is unable to pass completely through the steam generators to enter the cold legs. The steam generators are no longer able to condense any of the steam contained in the hot leg. Correct answer is C. NRC Question P2692 Correct answer is C. ELO 3.4

Natural Circulation Gas Binding
ELO 3.5 – Explain the effect of gas binding on natural circulation. 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 Related KA K1.25 Describe how gas binding affects natural circulation ELO 3.5

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. Non-condensable gas in the steam generator primary side (RCS) during two-phase or reflux condensation modes of natural circulation has the potential effect of the gas affecting the condensation process in the steam generator. ELO 3.5

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) ELO 3.5

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. ELO 3.5

Knowledge Check Can 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. Correct answer is Yes, because safety injection will not necessarily purge the RCS of non-condensable gases. ELO 3.5

TLO 3 Summary 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. There must be a heat sink and a heat source. The heat source must be located below the heat sink. Flowpaths must exist between the warm fluid and the cold fluid. The greater the temperature difference, the higher the natural circulation flow rate. TLO 3 Explain natural circulation and methods to enhance its effectiveness. TLO 3

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

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

TLO 3 Summary Natural Circulation Two-phase Flow and Reflux Condensation Natural circulation consists of three distinct modes of cooling: Single-phase (liquid only) Two-phase (liquid/steam vapor continuous) 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. Density gradients from both the temperature difference and the steam voids - mass flow rate is the primary heat removal parameter. TLO 3 Explain natural circulation and methods to enhance its effectiveness. TLO 3

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

TLO 3 Summary Natural Circulation Gas Binding
Non-condensable gas in the primary loop may impede or even stagnate the natural circulation flow. (single- or two-phase) 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: 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) TLO 3 Explain natural circulation and methods to enhance its effectiveness. TLO 3

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

Thermal Hydraulics Summary
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: Explain the various types of boiling heat transfer. Describe the basic reactor core thermal hydraulic properties. Explain natural circulation and methods to enhance its effectiveness. Review TLOs with class to ensure comprehension of this topic. Summary

Review Questions Nucleate boiling affects heat transfer from a fuel rod primarily by ... increasing the conductive heat transfer from the fuel rod to the coolant. increasing the convective heat transfer from the fuel rod to the coolant. decreasing the conductive heat transfer from the fuel rod to the coolant. decreasing the convective heat transfer from the fuel rod to the coolant. Correct answer is B. ELO 1.2

Review Questions The bulk temperature of the liquid is below saturation, but the temperature of the heat transfer surface is above saturation. Steam bubbles form at the heat transfer surface, but condense in the cold liquid so that no net generation of steam occurs. This is Bulk boiling Subcooled nucleate boiling Total film boiling Partial film boiling Correct answer is B. ELO 1.2

Review Questions If departure from nucleate boiling occurs in the core, the surface temperature of the fuel clad will... increase rapidly. decrease rapidly. increase gradually. decrease gradually. Correct answer is A. ELO 1.2

Review Questions If ∆T is the temperature difference between the fuel rod clad surface and the bulk coolant, which one of the following describes the heat transfer from a fuel rod experiencing departure from nucleate boiling? Steam bubbles begin to blanket the fuel rod clad, causing a rapid increase in the ∆T for a given heat flux. Steam bubbles completely blanket the fuel rod clad, causing a rapid decrease in the ∆T for a given heat flux. Steam bubbles begin to form on the fuel rod clad, causing a rapid decrease in the heat flux from the fuel rod for a given ∆T. Steam bubbles completely blanket the fuel rod clad, causing a rapid increase in the heat flux from the fuel rod for a given ∆T. Correct answer is A. ELO 1.2

Review Questions An adequate subcooling margin during a loss of coolant accident is the most direct indication that _______ is being maintained. steam generator water level pressure level core cooling subcriticality Correct answer is C. ELO 1.2

Review Questions A nuclear power plant is operating at 100 percent power. The reactor coolant subcooling margin is directly reduced by: increasing reactor coolant temperature. increasing pressurizer pressure. increasing reactor coolant flow. increasing pressurizer level. Correct answer is A. ELO 1.2

Operator Generic Fundamentals Thermodynamics – Thermal Hydraulics

Similar presentations

Presentation on theme: "Operator Generic Fundamentals Thermodynamics – Thermal Hydraulics"— Presentation transcript:

Similar presentations

About project

Feedback

Log in

Auth with social network:

Operator Generic Fundamentals Thermodynamics – Thermal Hydraulics

Similar presentations

Presentation on theme: "Operator Generic Fundamentals Thermodynamics – Thermal Hydraulics"— Presentation transcript:

Similar presentations

About project

Feedback