Operator Generic Fundamentals Thermodynamics – Thermal Hydraulics

Operator Generic Fundamentals Thermodynamics – Thermal Hydraulics

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. Figure: Modes of Heat Transfer Introduction

Boiling Heat Transfer TLO 1 – Explain the various types of boiling heat transfer. 1.1 Describe the differences between boiling processes and other means of heat transfer. 1.2 Describe the process of nucleate boiling, bulk boiling, departure from nucleate boiling, and critical heat flux (CHF), and subcooling margin. 1.3 Describe the transition to partial film boiling. 1.4 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. Most heat exchangers utilize convective heat transfer From outer laminar boundary layer to bulk coolant RCS flow in fuel channel might result in boiling on cladding surface Function of saturation temperature of RCS pressure Amount of heat added at that core location Related KA K1.01 Distinguish between boiling processes and other heat transfer mechanisms ELO 1.1

Convection Heat Transfer
Occurs when Tsurface < Tsat Consists of: Cool water heated Density decreases Fluid rises Replaced with denser cool water Convection heat transfer is Region 1 of the Pool Boiling Curve (shown in future slide). Figure: Convection Heat Transfer ELO 1.1

Boiling Heat Transfer Boiling Heat Transfer
Occurs when Tsurface > Tsat Steam bubbles occur at the boundary layer next to the heated surface Bubbles collapse in the subcooled fluid Bubbles might not collapse as Tbulk reaches Tsat Boiling results in: Higher convective (and overall) heat transfer coefficient Lower fuel temperature 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 Increasing safety margins BUT ... if boiling increases too much, its effects on reactor operation are very detrimental Fuel damage Release of fission products to RCS Pool Boiling Curve defines heat transfer regions 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 Images
Nucleate Boiling Examples NOTE: In the left picture it doesn’t appear as steam is being produced. This means the bubbles are mostly collapsing in the liquid. ELO 1.2

Nucleate Boiling Nucleate Boiling region consists of:
Subcooled nucleate boiling Tclad > Tsat > Tbulk Saturated nucleate boiling (also called bulk boiling) Tclad > Tsat < Tbulk Region II of Pool Boiling Curve Subcooled Nucleate Boiling is the formation and collapsing of steam bubbles Promotes mixing and motion of the coolant Greater energy transfer The second boiling heat transfer region, following natural convection region, is called nucleate boiling. The bubbles transfer the latent heat of vaporization to the bulk fluid. BUT, the majority of the heat transfer improvement is due to the increase in fluid flow at the boundary layer and its resultant improvement in the heat transfer coefficient. ELO 1.2

Boiling Heat Transfer Regions
Pool Boiling Curve Relationship of DT (Tclad- Tbulk) versus heat flux added Region I – Natural Convection Larger change in DT for flux added than Nucleate Boiling region Region II – Nucleate Boiling Region Subcooled Lower in fuel channel Saturated Near the tops of some hot fuel channels I II III IV The Natural Convection heat transfer process is not as efficient as subcooled nucleate boiling. Pay attention to the slope of the curve in this region versus the nucleate boiling region when the Pool Boiling Curve slide is shown. Figure: Fluid Heat Transfer Regions ELO 1.2

Subcooled Nucleate Boiling
Preferred method of nucleate boiling Steam bubbles form first at sites with surface imperfections/scratches Known as nucleation sites As heat flux is increased Intensity of steam bubble formation increases Number and size of bubbles increase Provided bulk coolant < saturation temperature Bubbles continue to collapse Be sure to note to the students this difference between surface imperfections in the cladding versus condenser tubes. Imperfections create areas that trap crud, rust or other debris that can/will reduce the rate of heat transfer. However, if these imperfections result in convective heat transfer becoming subcooled nucleate boiling, this is beneficial. There is a smaller change in Delta-T (from Tfuel to Tbulk) for a given amount of heat flux added during subcooled nucleate boiling. This will be shown in a future slide when the Pool Boiling curve is presented. A sample bank question will be shown in the review section to reinforce this concept. ELO 1.2

Subcooled Nucleate Boiling
Other factors that affect steam bubble formation: Temperature of the liquid Pressure of the liquid Flow rate of the liquid Rate of heat generation Gases within the liquid ELO 1.2

Saturated Nucleate Boiling
If system temperature increases or pressure decreases Bulk fluid can reach saturation conditions Steam bubbles do not collapse Bubbles coalesce to form “slugs” (form bigger bubbles) Sufficient heat transfer method, provided: Cladding surface is still wetted Not steam blanketed ELO 1.2

Figure: Fluid Heat Transfer Regions
Pool Boiling Curve I II III IV Saturated Nucleate boiling is the upper part of the nucleate boiling curve. Remind students that even though we might operate near the top of Region II in certain HOT channels, we are still within our limits (DNBR, discussed in upcoming slide). Point out the top of Region II is DNB – Departure from Nucleate Boiling (next slide). Figure: Fluid Heat Transfer Regions ELO 1.2

Departure from Nucleate Boiling
Steam bubbles begin to cover the entire heat transfer surface Results in: Rapid drop in overall heat transfer coefficient Rapid increase in fuel pellet/cladding temperature Figure shows movement UP a flow channel by moving right. Region to the left (not shown) is the Natural Convection region where no boiling occurred (as Tcold enters flow channels). The two regions to the right of DNB are explained in upcoming slides. Figure: Stages of Boiling ELO 1.2

Critical Heat Flux (CHF)
CHF is: The heat flux where DNB occurs CHF is specific for a given core location and plant conditions For example: temperature, pressure, and flow CHF continually decreases as you move up the core (flow channel) Temperature increases as heat flux is added Margin to CHF/DNB normally protected by: Plant trip setpoints Operating Procedures – operators maintaining RCS pressure, RCS temperature and reactor power within prescribed limits for the existing flow conditions Tech Specs If necessary, while in Slide Show Mode, type 14 then press ENTER to return to the Pool Boiling Curve to show where CHF is on the curve. When finished, type 16 then press ENTER to return to this slide. If CHF reached: 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

Departure from Nucleate Boiling Ratio
Describes the margin between actual heat flux and critical heat flux Referred to as DNBR Mathematically: DNBR= Critical heat flux at any point along a fuel rod Actual heat flux at same point along the same fuel rod Maximum DNBR Bottom of core Maximum CHF and minimum actual heat flux (AHF) Minimum DNBR Some point slightly above core midplane Hotter water with sufficient actual heat flux FSAR Required DNBR Typically ranges between 1.3% and 1.6% FSAR – Final Safety Analysis Report (sometimes referred to as UFSAR – Updated…) More information on DNBR and it’s impact on core thermal limits will be covered in the next chapter. However, what this does show is that with DNBR limits of 1.3%, we must have trips that prevent us from getting to this value. That means we don’t normally operate at the top of Region II where the slope starts to drop off. ELO 1.2

Departure from Nucleate Boiling Ratio
Relationship of Critical Heat Flux and Actual Heat Flux can be shown graphically: As you move up the core: AHF increases, then decreases CHF decreases continually Maximum DNBR Bottom of core Minimum DNBR: Slightly above core midplane Top Core Location Midplane Peak AHF Text and arrows/lines animated in order to show relationships between AHF, CHF and DNBR As you move up the core the actual flux distribution in the core increases (peak just below core midplane) and then decreases (based on leakage) As you move up the core the critical heat flux continually decreases (water gets hotter and hotter, closer to saturation). Actual Heat Flux Critical Heat Flux Bottom Low High Flux (Actual and Critical) ELO 1.2

CHF/DNB Factors Critical Heat Flux decreases if: Flow Decreases
Reduction in RCS flowrate results in an increase in coolant temperature, reducing DNBR Flux Increases High local power densities produce higher heat flux, and higher coolant and cladding temperatures Temperature Increases Closer to saturation conditions, more bubble or slug flow, lower heat transfer, reducing DNBR Pressure Decreases Operating at lower pressures allows DNB to occur at lower temperatures Temperature increase and/or pressure decrease Shifts the boiling curve down and to the left ELO 1.2

Subcooling Margin (SCM)
Subcooling Margin = Tsat – Tact for the existing RCS pressure Subcooling Margin = Pact – Psat for the existing RCS temperature CHF increases with an increase in RCS subcooling If CHF increases and AHF is constant, DNBR increases Good indication of adequate core cooling during small break LOCA Since temperature is a function of power Maintain SCM by maintaining RCS pressure on program Subcooling Margin is also a great indication (post trip) of the type of accident that is occurring. Normal full power SCM might be around degrees. Normal uncomplicated trip SCM might be around 100 degrees. If the post trip SCM is > 100 degrees: steam line break. If the post SCM is < 100 degrees: SBLOCA or SGTR (depending on rad monitors). If the post trip SCM is essentially 0 degrees: LBLOCA ELO 1.2

Pool Boiling Curve Knowledge Check
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. Based on the slope of the curve, for a given amount of heat flux added, Region II results in the smallest Delta-T ELO 1.2

Knowledge Check Knowledge Check – NRC Bank
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. NRC Bank Question – P87 Analysis: A. As pressure decreases, the RCS moves closer to saturation conditions for the given temperature, thus closer to departure from nucleate boiling. Thus, the reactor is operating closer to critical heat flux. B. Reducing flow rate through the coolant channels causes the vapor bubbles formed a nucleation sites to be swept away at a reduced rate, thus steam bubbles can be produced at a lower rate before DNB occurs. Therefore, lowering flow rate through the core makes the reactor operate closer to critical heat flux. C. Reduced reactor power (heat flux) densities result in a lower temperature, thus further away to DNB. The further channel conditions are from DNB, margin to CHF increases (operating farther away from CHF). D. As temperature increases, the RCS moves closer to saturation conditions for the given pressure, thus closer to departure from nucleate boiling. The reactor is operating closer to critical heat flux. ELO 1.2

The departure from nucleate boiling (DNB) ratio is defined as the... actual heat flux divided by the critical heat flux. critical heat flux divided by the actual heat flux. actual core thermal power divided by the rated core thermal power. rated core thermal power divided by the actual core thermal power. Correct answer is B. Correct answer is B. NRC Question P89 Analysis: DNBR is defined as the critical heat flux divided by the actual heat flux at any point along a fuel rod. Therefore, the further away from critical heat flux, the higher the DNBR. ELO 1.2

Partial Film Boiling ELO 1.3 – Describe the transition to partial film boiling. When DNB is reached core transitions to the partial film boiling region Also called Transition Boiling Alternate wetting and rewetting I II III IV Related KA’s K1.07 Describe transition (partial film) boiling Figure: 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 start to blanket cladding surface Not likely during normal plant operation Reactor Protection should trip plant before this occurs Could happen on LBLOCA Cladding uncovered on initial blowdown Surface rewetted when accumulators flood core Decay heat boils off water with slight uncovery possible Safety Injection system refills core Any rewetting of surface transitions core back to Region II Any loss of ECCS could transition to Stable Film Boiling (dryout) Partial Film Boiling can also happen on a Loss of Heat Sink event ELO 1.3

Partial Film Boiling Knowledge Check
Which one of the points shown represents the onset of transition boiling? A B C D Correct answer is B. Correct answer is B. NRC Bank Question – P1689 Analysis: NOTE: Point “B” is known as DNB, CHF, or OTB (Onset of Transition Boiling) At point B on the curve, as additional heat is added, bubbles spend more time on wall, results in the beginning a vapor blanket on the wall. This point between Region II and III is referred to the Departure from Nucleate Boiling (DNB). The heat flux at which DNB occurs is referred to as Critical Heat Flux (CHF), resulting in partial film boiling (also known as transition boiling). Note that Region III is very unstable; depending on fluid conditions, the wall can rewet or the vapor may expand to cover the entire heat transfer surface, resulting in movement into Region IV. 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. Alternate wetting and drying of the fuel rod surface. Steam bubbles form and collapse on the fuel rod surface. Steam bubbles form on the fuel rod surface and are swept away by subcooled bulk coolant. Correct answer is B. Correct answer is B. NRC Bank Question – P1987 Analysis: A. WRONG. This is Region IV, Stable Film Boiling B. CORRECT. Transition Boiling (also known as partial film boiling) occurs in Region III. Region III is very unstable; depending on fluid conditions, the wall can rewet moving back to Region II or the vapor may expand to cover the entire heat transfer surface, resulting in movement into Region IV. C. WRONG. This is not really an expected phenomenon. It might be indicative of going back and forth between Regions I and II. D. WRONG. This is indicative of Subcooled Nucleate Boiling, which occurs in Region II. ELO 1.3

Film Boiling ELO 1.4 – Describe the transition to film boiling.
If heat transfer continues to degrade, film boiling is the next heat transfer region Also called Stable Film Boiling I II III IV Related KA’s K1.08 Describe film boiling Figure: Fluid Heat Transfer Regions ELO 1.4

Film Boiling Bubbles produced so rapidly that they crowd the heated surface Cannot rewet the cladding surface Cladding surface quickly reaches “dryout” Results in cladding damage 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 an oxide film on the cladding. heat transfer being accomplished with no enthalpy change. heat transfer through a vapor blanket that covers the fuel cladding. Correct answer is D. Correct answer is D. NRC Question P88 Analysis: Region IV (Film Boiling) is characterized by a stable film boiling. A stable vapor film will cover the entire bottom of the pool/cladding. Note that this form of heat transfer is less effective than subcooled nucleate boiling. Radiative heat transfer will be prevalent due to the transfer of heat by electromagnetic radiation that arises due to the large temperature gradient. Though this is not an efficient form of heat transfer, the enthalpy of the bulk coolant will rise as heat is transferred. ELO 1.4

Reactor Core Thermal Hydraulic Properties
TLO 2 – Describe the basic reactor core thermal hydraulic properties. 2.1 Describe the heat transfer coefficient and effects from flowrate and phase change. 2.2 Explain fuel channel flow and heat transfer, including the following terms: Slug Flow Annular Flow Dryout Region Flow resistance 2.3 Draw a temperature profile from the centerline of a fuel pellet to the centerline of the flow channel. This section discusses RCS fuel channel flows through the core, core bypass flows, and temperature profiles of the fuel and fuel channels. TLO 2

Enabling Learning Objectives for TLO 2
2.4 Describe core bypass flow and purpose of adequate flow. 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 Flow rate in the channel TLO 2

Flowrate and Phase Change Effects on Heat Transfer
ELO 2.1 – Describe the convective heat transfer coefficient and effects from flowrate and phase change. Recall fuel heat transfer consists of: Conductive Heat Transfer From fuel centerline to pellet edge Across GAP Across cladding to outer laminar layer Convective Heat Transfer From outer laminar layer to bulk coolant Related KA K1.14 Describe effects of flowrate and phase change on the heat transfer coefficient ELO 2.1

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

Flowrate and Phase Change Effects on Heat Transfer
Heat Transfer Rate (BTU/hr) Q fuel=UA∆T, or UA Tfuel−Tbulk coolant Area (A) is fixed – whether 12 ft rod or per foot Overall Heat Transfer Coefficient (U) can be improved by: Increasing flow Turbulent flow is better mixing of bulk coolant Better convective heat transfer coefficient (h) Breaks down laminar layer Smaller radius to conduct heat across Subcooled nucleate boiling occurring on cladding surface Recall from – Heat Transfer that the overall heat transfer coefficient (U) is made up of various parts depending on which heat transfer part is being discussed. NOTE – the biggest part of the overall heat transfer coefficient is the convective heat transfer coefficient (h), so even though this is discussing “U” since it is in the formula, the biggest part is the “h”. ELO 2.1

Reactor Core Thermal Hydraulic Properties
Knowledge Check – NRC Bank Core heat transfer rate is maximized 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 Analysis: Turbulent flow provides better mixing and enhances heat transfer. Nucleate boiling enhances heat transfer due to enhanced mixing and carrying Latent Heat of Vaporization into the bulk coolant. 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 always simple single-phase forced flow Post- accident flow is a combination of: Natural convection Multiple degrees of boiling flow 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 a hypothetical fuel pin, experiencing two phase flow. 1.9* 2.1* K1.17 Explain the necessity of determining 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 highly subcooled Cladding surface temperature < Tsat Heat transfer at the inlet takes place by convection no steam bubble formation ELO 2.2

Bubbly Flow As coolant continues flowing through the core
temperature increases reduces amount of subcooling closer to saturation bubbles start to form on fuel cladding imperfection sites bubbles swept away and collapse into the coolant flow Called Subcooled Nucleate Boiling ELO 2.2

Two-Phase Flow When Tbulk coolant reaches Tsat
bubbles no longer collapse Called Bulk Boiling or Saturated Nucleate Boiling This is the initiation of two-phase flow ELO 2.2

Slug Flow As coolant temperature continues to increase
Steam bubbles begin to coalesce into vapor slugs Void fraction (quality) increases Heat transfer continues at almost the same rate 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 This occurs higher in the flow channel Liquid still appears on cladding surface Vapor forms a continuous phase between fuel elements Lower velocity fluid flowing along the coolant channel walls Note that this doesn’t really occur in PWRs. Recall that there is only about 0.5% voiding in the core at 100% power. ELO 2.2

Mist Flow and Dryout Mist Flow
Majority of flow is vapor flow with a high quality Occurs at or beyond DNB Dryout The vapor core intensifies and more coolant flashes to steam Coolant acts as the fuel heat sink primary means of heat transfer shifts from convective heat transfer to radiative heat transfer if the heat removal does not occur fuel damage can and likely will result Figure on next slide shows the different regions of flow. 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 Head loss is the reduction in the total head of the fluid as it moves through a fluid system Friction between the fluid and the walls of the pipe Friction between adjacent fluid particles Turbulence caused by redirected flow Amount of head loss depends on Flow velocity, Pipe length, Pipe diameter, friction factor Recall Darcy’s Equation h f ≈ fL v 2 2D𝑔𝑐 This is a review of the concept of Headloss presented in – Fluid Statics and Dynamics. hf = Friction head loss f = Darcy resistance factor L = Length of the pipe D = Pipe diameter v = Mean velocity g = acceleration due to gravity 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. ELO 2.2

Two-Phase Fluid Flow Resistance
Two-phase flow is the simultaneous flow of both liquid 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 Note that there aren’t any bank questions requiring calculations relating to two-phase flow, just that two-phase flow usually results in more headloss. ELO 2.2

Single-phase coolant flow resistance in a reactor core is directly proportional to the square of coolant __________; and inversely proportional to __________. velocity; fuel assembly length temperature; fuel assembly length velocity; coolant channel cross-sectional area temperature; coolant channel cross-sectional area Correct answer is C. Correct answer is C. NRC Question – P1790 Analysis: WRONG. (1/2) Headloss is directly proportional to fuel assembly length. B. WRONG. (0/2) Headloss does not vary with the square of coolant temperature. Headloss is directly proportional to fuel assembly length. C. CORRECT. (2/2) Headloss is directly proportional to velocity squared. Coolant channel cross-sectional area is determined by piping diameter, thus headloss is inversely proportional to coolant channel cross-sectional area. D. WRONG. (1/2) Headloss does not vary with the square of coolant temperature. 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 section illustrates the radial temperature profile of the fuel to coolant Related KA’s K1.16 Draw the temperature profile from the centerline of a fuel pellet to the centerline of the flow channel Currently there aren’t any questions that require you to draw a temperature profile, but recognition of the heat transfer mediums and processes is important to understand. ELO 2.3

Uranium oxide pellet not very good conductive medium Large DT across pellet Gap pressurized with psia of helium Helps promote conductive heat transfer Protects pellet from pressure stress from 2250 psia RCS acting on cladding Cladding to outer laminar layer is also conductive heat transfer Results in large DT from pellet centerline to bulk coolant ≈ 2000oF average centerline temperature, ≈ 585oF Tave Pellet swells due to fission product production Clad creep due to RCS pressure Convective and radiative heat transfer across the gap is considered negligible with steady state operations. ELO 2.3

Pellet swells due to fission product production Larger radius of pellet, but smaller radius of gap Better heat transfer in gap with additional fission products Clad creep due to RCS pressure 2250 psia acting on cladding Net result over core life: Slightly lower overall radius for conductive heat transfer Slightly better heat transfer Slightly lower fuel temperatures ELO 2.3

Peak fuel temperatures Average approximately 2,000°F Peak as high as 4,400°F Melting at 5,200°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
Refer to the drawing of a fuel rod and adjacent coolant flow channel (see figure below). With a nuclear power plant operating at steady-state percent reactor power at the beginning of a fuel cycle, which one of the following has the greater temperature difference? Fuel pellet centerline to pellet surface Fuel pellet surface-to-cladding gap Zircaloy cladding Coolant laminar layer Correct answer is A. Correct answer is A. NRC Bank Question – P391 Analysis: The energy produced in the fuel is transferred in the form of heat by conduction (from the center of the fuel to the outer surfaces of the clad) and convection (from the clad surface to coolant) to the reactor coolant. Fuel rod centerline temperature is the highest; the minimum temperature is the bulk temperature of the reactor coolant. The largest temperature difference occurs across the fuel region. Temperature ranges between the fuel and moderator differ based on power level, but average between 500ºF and 3000ºF Delta-T. The uranium oxide pellet is not a very good conductive medium. However, it is better than the helium gap, but you must keep in mind it also has a much larger radius by which to conduct heat across. By virtue of that, the pellet has the largest temperature difference across it (also, the combined effects of pellet swell and clad shrink over fuel age squeeze down and then eliminate the gap). 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 enters the vessel through inlet nozzles located above the active fuel region, but, 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

Reactor Vessel and Internals Coolant Flow
Coolant exits through the outlet nozzles located on the same horizontal plane as the inlet nozzles Core flowpaths ≈ 94 percent of the total vessel flow The remaining 6 percent is bypass flow Driving force for bypass flow is pressure drop across core Point out to students number and paths given are for generic Westinghouse design, other vendors will differ. Also note that this is not “generic” testable information, it is just provided to give you a sense of where the 6% flows. The key thing tested by the NRC is the following: M-dot (vessel) = M-dot (core) + M-dot (bypass); therefore, Q-dot (vessel) = Q-dot (core) + Q-dot (bypass); with Q-dot (bypass) being negligible (0 BTU/hr) With this understanding, you can determine flow rates of any unknown. Figure: Reactor Vessel Internals (Westinghouse Design) ELO 2.4

Reactor Core Bypass Flow Paths
Nozzle bypass flow (1 percent) slight gap between the core barrel and outlet nozzles. 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 Figure: Reactor Vessel Internals (Westinghouse Design) ELO 2.4

Reactor Core Bypass Flow Paths
Baffle wall bypass flow (1/2 percent) Coolant flows between the inner core barrel wall and vertically mounted core baffle plates Provides for inner barrel wall and core baffle plates Head cooling bypass flow (1/2 percent) Coolant flows through flow holes in the core barrel support flange and the top support plate Prevents stagnation and cools the vessel head plenum area 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 ensure the plant operates within design boundaries to protect the public health and safety kW/ft limit (hot spot in core) Located at or slightly below core midplane due to axial/radial flux peaks Prevents fuel pellet melting DNBR limit Located above core midplane Prevents cladding oxidation 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

Onset of Nucleate Boiling
Nucleate Boiling increases heat transfer to the coolant Lower fuel temperatures Less ∆T between the cladding and the bulk coolant Lower thermal stresses Less likelihood of thermal design limits being exceeded ELO 2.5

Axial Core Flux 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 kW/ft also greatest in this area 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 Burnable and non-burnable poisons Axial power peaks normally occur around midplane DNBR concern if: Axial flux peaks near top of core Water closer to saturation Recall the graph of Actual Heat Flux and Critical Heat Flux. If the Actual Heat Flux starts peaking towards the top of the core, it gets MUCH closer to the Critical Heat Flux Draw this on the board if necessary. ELO 2.5

Inlet Temperature A constant power implies a constant core ∆T
An increase in inlet temperature means a higher outlet temperature and enthalpy Fuel and clad temperatures also increase accordingly Basically, maintain temperature/pressure on program in order to remain with the assumptions used in the plants accident analysis 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 Higher core exit temperatures and enthalpy Basically, do NOT exceed the thermal power rating of the core ELO 2.5

Flow Rate in the Channel
Reactor coolant flow in a PWR is relatively constant If mass flow rate decreases (for a constant Q-dot) ∆T must increase Normally, no change in RCS flow is allowed Immediate reactor trip on any change in flow However, some flow channels might have less flow than others These channels usually closest to design limits 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 Heat added in core is change in specific enthalpy of Tcold to Thot Enthalpy rise is dependent on core location Higher the ∆h, greater the kW added to RCS Hottest channel (total ∆h or kW) must be within thermal design limits Hottest channel might be different location than the “hot spot” 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

Coolant temperature constantly increases through the core Slope of Tcoolant greater near core midplane Tclad and Tfuel peaks slightly above core midplane Function of hotter water with higher flux Enthalpy profile similar to axial temperature profiles 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

Knowledge Check During normal operation, fuel cladding integrity is ensured by... 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. This bank question (P894) is actually tested in the next chapter’s material. This is presented to ensure comprehension of basic design limits as well as show the overlap of material. ELO 2.5

Natural Circulation TLO 3 – Explain natural circulation and methods to enhance its effectiveness. 3.1 Define natural circulation and thermal driving head. 3.2 Describe the indications of natural circulation flow. 3.3 Describe how natural circulation can be enhanced. 3.4 Describe the process of reflux boiling. 3.5 Explain the effect of gas binding on natural circulation. 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 event, the reactor coolant pumps will be unavailable for operation. In these cases, another method must be available - 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 Function of the: design height differences Tcold and Thot temperature differences Related KA K1.21 Explain the conditions which must exist to establish natural circulation * 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 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

On loss of forced circulation, reactor trips Fission heat essentially gone, but decay heat still exists The higher the decay heat, the more need for heat removal The higher the decay heat, the higher the thermal driving head The higher the thermal driving head, the greater the natural circ flow The greater the natural circ flow, the greater the heat removal As decay heat decreases, thermal driving head decreases Natural circ flow decreases ELO 3.1

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 No voiding in RCS hot leg or SG u-tubes Adequate Heat Sink (SG) Procedural requirements: SG level to cover tubes AFW flow to maintain that level ELO 3.1

Knowledge Check – NRC Bank Sustained natural circulation requires that the heat sink is __________ in elevation than the heat source and that there is a __________ difference between the heat sink and the heat source. lower; pressure lower; temperature higher; pressure higher; temperature Correct answer is D. Correct answer is D. NRC Question P1887 Analysis: The temperature difference between Tcold and Thot causes a density difference or “thermal driving head” that causes natural circulation (NC) to take place. The significant decay heat generated after a trip provides the heat source. The heat source must be at a lower elevation than the heat sink (by design). Conversely, the heat sink (S/G) must be located higher than the heat source (reactor) to allow flow of the less dense water heated in the reactor. Provided that a heat sink is available (adequate SG level), a temperature difference is required to maintain natural circulation flow. ELO 3.1

Indications of Natural Circulation Flow
ELO 3.2 – Describe the indications of natural circulation flow. Loss of forced flow results in a reactor trip Natural circulation (NC) flow cannot remove fission heat Plant procedures provide guidance for determining NC flow The following slides discuss the following indications of NC RCS temperature RCS ∆T SG pressure Subcooling margin Related KA K1.22 Describe means to determine if natural circulation flow exists. 4.2* 4.2* 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 ELO 3.2

RCS temperature, pressurizer level and pressure steady or decreasing If decay heat equals steaming rate – steady If decay heat less than steaming rate - decreasing RCS ∆T NC flow is considerably less than forced flow ∆T will initially increase following a reactor trip depending on the amount of decay heat NC ∆T may be almost as high as normal full power ∆T generates thermal driving head As decay heat decreases, ∆T will also decrease ELO 3.2

SG pressure During NC, SG’s act as the heat sink SG pressure should track cold leg saturation temperature If decay heat equals steaming rate – steady If decay heat less than steaming rate – decreasing On loss of NC flow SG pressure decreases Less heat transferred from hot side to cold side SG level increases Feeding > steaming SG indications on a loss of natural circ can be looked at from the viewpoint of a regular heat exchanger process. For example: If the mass flow rate of the hot fluid decreases, the cold outlet temperature decreases (less heat transfer from hot to cold). Since the SG system is a saturated system, this means a lower steam pressure. ELO 3.2

Subcooling Margin On loss of forced flow Core bypass flow is gone Upper head flow stagnates On rapid cooldown, upper head temps will lag RCS temps Voiding could occur in upper head, piping, or top of u-tubes Adequate Subcooling margin means NC is removing core heat Procedures provide NC cooldown limits to prevent this 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 nuclear power plant was operating at steady-state 100 percent power when a loss of offsite power occurred, resulting in a reactor trip and a loss of forced reactor coolant circulation. Thirty minutes later, reactor coolant system (RCS) hot leg temperature is greater than cold leg temperature and steam generator (SG) levels are stable. Which one of the following combinations of parameter trends, observed 30 minutes after the trip, indicates that natural circulation is occurring? (CET = core exit thermocouple) Correct answer is A. RCS Hot Leg Temperature RCS Cold Leg Temperature SG Pressures RCS CET Subcooling A. Decreasing Stable Increasing B. C. D. Correct answer is A. NRC Question P1492 Analysis: Let’s look at each column RCS Hot Leg Temperature: Right away you can throw out “B” and “D” because hot leg temperatures should be decreasing, not increasing. So now we are looking at Choices “A” or “C”. Cold Leg Temp/SG Pressure: Either “A” or “C” could be correct depending on decay heat input/steaming rates. Both choices are still looking good. RCS CET Subcooling: If NC exists, Core Exit Thermocouple temperatures should also be decreasing (similar to Hot Leg). “Temperature” decreasing, means “subcooling” INCREASING. This makes Choice “A” correct. 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 Decay heat from the core Adequate heat sink (SG) 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 NOTE: “Enhanced” also means “Restored”. On a loss of natural circulation flow, a variety of steps can be taken to “restore” this flow (depending on the reason it was lost). 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 Maintaining PZR level keeps hot legs full Review these conditions for a thermal driving head (natural circulation), keeping in mind that the elevation requirement is a design feature and cannot be “enhanced”. ELO 3.3

Temperature Difference An increase in the temperature difference (heat source to heat sink) increases the thermal driving force Commence a slow and deliberate cooldown Lowers SG pressure, Tcold temperature Increases temperature difference An understanding of the requirements/restrictions of natural circulation cooldown is very important. This will be discussed in greater detail regarding the BASES in your emergency response procedures. The following is presented as a big picture of these concerns: Loss of forced circulation usually means a loss of power (to RCPS) Loss of power usually means loss of condenser and loss of makeup capability Loss of condenser means steaming to atmosphere Loss of makeup capability means you might not be able to fill your makeup tank (CST, for example) This means you have limited time to steam your SG’s before running our of inventory CST supply provides “x” amount of time to maintain in Mode 3 (Hot Standby) with “x” amount of time to reach Mode 4 (get on shutdown cooling and off SGs) This will require certain a NC cooldown rate to achieve this Cooling down too fast might draw a bubble in the upper head because of your plant design This is why an understanding natural circulation is important! ELO 3.3

Fluids Must Be In Contact with Each Other Two-phase flow or vapor blockage can inhibit NC flow Gases can come out of solution and accumulate at top of u-tubes Maintaining subcooling margin/Pzr level can prevent these occurrences Keep in mind that normal natural circulation flow may consist of two-phase flow (to some extent). The water in the core must heat up to create the Delta-T required for natural circulation flow to start. This can lead to some two-phase flow with possible vapor blockage at the tops of the U-Tubes. All that might need to be done in this case is to increase the steaming rate to increase the Delta-T to sweep the tops of the tubes. If this problem was caused by voiding in the head or lowering PZR level, these issues must be restored before normal natural circulation flow returns. This condition (reflux boiling/condensation) is discussed in the next section. ELO 3.3

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 source and the heat sink. decreasing the temperature difference between the heat source and the heat sink. decreasing the elevation difference between the heat source and the heat sink. Correct answer is B. Correct answer is B. NRC Question P392 Analysis: A. WRONG. Increasing the elevation of the heat source (reactor) to equal that of the heat sink (S/G) will eliminate the thermal driving head which requires an elevation difference for flow to occur. B. CORRECT. The temperature difference between Tcold and Thot causes a density difference or “thermal driving head” that causes natural circulation (NC) to take place. Since the heat source strength is a function of decay heat, the only variable is the heat sink. By slowly increasing steaming rates from the intact steam generator(s), you will lower Tc, thus increasing the Delta-T between the heat source (Th) and the heat sink (Tc). C. WRONG. Decreasing the temperature difference between the heat source (reactor) and the heat sink (S/G) reduces the density difference, thus lowering the thermal driving head. D. WRONG. Maximizing the elevation difference between the heat source (reactor) and heat sink (S/G) will enhance NC flow; reducing this elevation difference lowers NC flow. ELO 3.3

Natural Circulation Two-Phase Flow and Reflux Boiling
ELO 3.4 – Describe the process of reflux boiling. 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 boiling boiler-condenser mode for once-through steam generators 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

Single-Phase Natural Circulation
Normal mode without inventory loss Flowpath is from hot legs through u-tubes to cold leg and back to core Hot leg to cold leg Dominant heat transfer mechanism Convective heat transfer 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 flow could be caused by: Inadequate heat sink Small inventory loss in RCS Initial flow path might still be hot legs to u-tubes to cold leg and back Recall that two-phase flow has more headloss Steam can also form at top of u-tubes Eventually flow through U-tubes might stop Flowpath then is (hot leg to hot leg) Out hot legs into hot leg side of SG condensing in SG falling back to core in hot leg 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

Reflux Boiling Natural Circulation
Reflux boiling starts when hot leg condensation is unable to pass completely through the steam generators to enter the cold legs and maintain flow. 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 Boiling during a Small Break LOCA (Voiding at top of SG Tubes) ELO 3.4

In PWRs with U-tube steam generators, reflux boiling (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 Boiling during a Small Break LOCA ELO 3.4

Removal of decay heat from the core during reflux boiling does not require Large mass flow rates Large primary to secondary temperature differences During reflux condensation loop mass flow rate is the secondary heat removal parameter vapor condensation as the primary method Even though core heat is still being removed Full natural circulation flow MUST be restored Recall possible inventory limitations Small mass flow rates and primary to secondary temperature differences are characteristic of the reflux condensation mode of natural circulation. ELO 3.4

Figure: PWR Liquid Distribution during Reflux Boiling ELO 3.4

Natural Circulation Two-phase Flow and Reflux Boiling
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 Analysis: A. WRONG. Reflux boiling is marked by steam voids in the hot leg being unable to make the u-tube bend. Initially when saturation conditions are reached in the hot leg, any vapor formation would be initially condensed in the steam generator u-tubes. These are not the first indications of reflux boiling. B. WRONG. Reflux boiling occurs when natural circulation flow is insufficient for the reactor coolant to make the bend in the steam generator u-tubes. This distractor discusses some of the hot leg water being condensed and returned to the reactor via the cold leg. Reflux boiling occurs when some boiling occurs in the hot leg, is cooled in the steam generator u-tube (but unable to make the bend), and is returned to the reactor. C. CORRECT. Reflux boiling is caused by the increased delta-T of the coolant lowering the density of Thot, causing some flow to enter the tubes of the SG. Flow is insufficient to make the U-Tube bend so it falls back into the hot leg and into the core (providing some cooling). D. WRONG. Reflux boiling occurs due to natural circulation flow not being large enough to make the bend in the steam generator u-tube. It does NOT occur due to loss of steam generator heat sink, as this distractor discusses. If a loss of steam generator heat sink occurred, reflux boiling could not occur because no cooling in the steam generator would occur. 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 Collect at top of the u-tubes Called “high point voiding” 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) Related KA K1.25 Describe how gas binding affects natural circulation ELO 3.5

High Point Voiding results in: Potential loss of natural circulation flow Reflux boiling/condensation now the heat removal process Loss of reduction is natural circulation flow results in: SG pressure decreasing SG level increasing Core exit thermocouple (CET) temperatures increasing ELO 3.5

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 are constant. If high point voiding interrupts natural circulation, SG levels will gradually __________; and core exit thermocouple indications will gradually __________. decrease; increase decrease; decrease increase; increase increase; decrease Correct answer is C. Correct answer is C. NRC Question P793 Analysis: High point voiding results in significantly reduced flow in the RCS due to the steam bubbles in the hot leg reducing natural circulation flow. Lower natural circulation flow rate produces less heat transfer from the reactor coolant system to the secondary. Reduced primary-to-secondary heat transfer results in less boil-off of the saturated mixture inside the steam generator. With less boil-off combined with constant feed flow and atmospheric steam dump valve position, steam generator water level will rise. Also, less heat transfer into the secondary results in steam generator pressure (and temperature, due to saturated conditions) lowering. As less heat is transferred from the primary to secondary, with a given decay heat level, core exit thermocouple indication will begin to rise because the heat is being produced at the same rate, but being transferred to the secondary at a lesser rate. ELO 3.5

NRC KA to ELO Tie KA # KA Statement RO SRO ELO K1.01 Distinguish between boiling processes and other heat transfer mechanisms. 2.8 3.0 1.1 K1.02 Describe means by which boiling affects convection heat transfer. 1.2 K1.03 Describe the processes of nucleate boiling, subcooled nucleate boiling, and bulk boiling. 3.1 K1.04 Describe DNB (departure from nucleate boiling). 3.3 K1.05 List the parameters that affect DNB and DNBR and describe their effect(s). 3.4 3.6 K1.06 Describe CHF (critical heat flux). 2.9 K1.07 Describe transition (partial film) boiling. 2.6 1.3 K1.08 Describe film boiling. 1.4 K1.09 Describe burnout and burnout heat flux. 2.3 2.4 K1.10 Define DNBR. K1.11 Classify slug flow region along a fuel pin, experiencing two phase flow. 1.9 2.1 2.2 K1.12 Describe annular flow region along a hypothetical fuel pin, experiencing two phase flow. 1.8 K1.13 Describe dryout region or mist flow region along a hypothetical fuel pin, experiencing two phase flow. K1.14 Describe effects of flowrate and phase change on the heat transfer coefficient. 2.7 K1.15 Define and describe subcooling margin (SCM). 3.8 K1.16 Draw the temperature profile from the centerline of a fuel pellet to the centerline of the flow channel. K1.17 Explain the necessity of determining core coolant flow. 3.2 K1.18 Describe the factors affecting single- and two-phase flow resistance. 2.5 K1.19 Describe core bypass flow. K1.20 Explain the need for adequate core bypass flow. K1.21 Explain the conditions which must exist to establish natural circulation. 3.9 4.2 K1.22 Describe means to determine if natural circulation flow exists. K1.23 Describe means by which natural circulation can be enhanced. 4.1 K1.24 Describe the process of reflux boiling (boiler condenser process). K1.25 Describe how gas binding affects natural circulation. 3.5 K1.26 Sketch 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 K1.27 Sketch the axial temperature and enthalpy profiles for a typical-reactor coolant channel and describe how they are affected by the following: Axial core flux K1.28 Sketch the axial temperature and enthalpy profiles for a typical-reactor coolant channel and describe how they are affected by the following: Inlet temperature K1.29 Sketch the axial temperature and enthalpy profiles for a typical-reactor coolant channel and describe how they are affected by the following: Heat generation rate K1.30 Sketch the axial temperature and enthalpy profiles for a typical-reactor coolant channel and describe how they are affected by the following: Flow rate in the channel Some of the NRC Bank questions related to K1.18 (Pipe flow diagrams) are really more related to Total System Pressure concepts covered in – Fluid Statics and Dynamics.

Operator Generic Fundamentals Thermodynamics – Thermal Hydraulics

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Operator Generic Fundamentals Thermodynamics – Thermal Hydraulics

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