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Flow Stagnation and Thermal Stratification in Natural Circulation Loops

Learn about the mechanisms that interrupt natural circulation flow in single-phase and two-phase loops, and the impact of stagnation on thermal stratification. Explore methods for calculating fluid mixing and plume behavior in the system.

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Flow Stagnation and Thermal Stratification in Natural Circulation Loops

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  1. Department of Nuclear Engineering & Radiation Health Physics Flow Stagnation and Thermal Stratification in Single and Two-Phase Natural Circulation Loops(Lecture T17) José N. Reyes, Jr. June 25 – June 29, 2007 International Centre for Theoretical Physics (ICTP) Trieste, Italy IAEA-ICTP Natural Circulation Training Course, Trieste, Italy, 25-29 June 2007

  2. Course Roadmap IAEA-ICTP Natural Circulation Training Course, Trieste, Italy, 25-29 June 2007

  3. Lecture Objectives • Describe the mechanisms by which natural circulation flow is interrupted in single-phase and two-phase loops • Identify the impact of loop stagnation on thermal stratification within the loop components. • Identify the methods that can be used to calculate fluid mixing and plume behavior in the system. IAEA-ICTP Natural Circulation Training Course, Trieste, Italy, 25-29 June 2007

  4. Outline • Introduction • Single-Phase Natural Circulation Stagnation Mechanisms • Loss of Heat Sink • Negatively Buoyant Regions • Two-Phase Natural Circulation Stagnation Mechanisms • Thermal Fluid Stratification and Plume Formation • Onset of Thermal Stratification • Axisymmetric Forced Plumes • Planar Plumes • Downcomer Plume Behaviour • Conclusions IAEA-ICTP Natural Circulation Training Course, Trieste, Italy, 25-29 June 2007

  5. Introduction • Under certain accident conditions in a PWR, natural circulation plays an important role in maintaining a thermally well-mixed system. • Pressurized Thermal Shock (PTS) • If cold borated water is injected into cold legs while cold leg flow rates are low, thermal stratification and plume formation in the reactor vessel downcomer can occur. • Should a pre-existing flaw in the vessel wall or welds exist at a location experiencing prolonged contact with a cold plume, while at high pressure, there is a potential for the flaw to grow into a “through-wall” crack. IAEA-ICTP Natural Circulation Training Course, Trieste, Italy, 25-29 June 2007

  6. Thermal Stratification in a PWR Cold Leg and Downcomer Plume Formation IAEA-ICTP Natural Circulation Training Course, Trieste, Italy, 25-29 June 2007

  7. Experimental Studies in APEX-CE • APEX-CE Integral System Test facility, at Oregon State University. • Model of a 2x4 loop Combustion Engineering PWR. • Reactor vessel with an electrically heated rod bundle • Pressurizer • 2 Inverted U-tube steam generators • 4 Cold legs and reactor coolant pumps • 2 Hot legs • Safety injection system • Length scale ratio 1:4 • Volume ratio of 1:274. • Decay powers ranging down from 6%. • Tests conducted after reactor scram with the reactor coolant pumps tripped in a natural circulation mode of operation. IAEA-ICTP Natural Circulation Training Course, Trieste, Italy, 25-29 June 2007

  8. Single-Phase Natural Circulation Stagnation Mechanisms Loss of Heat Sink Negatively Buoyant Loop Seal Fluid IAEA-ICTP Natural Circulation Training Course, Trieste, Italy, 25-29 June 2007

  9. Loss of Heat Sink(Steam Generator Reverse Heat Transfer) • Driving potential for natural circulation flow in a PWR: • Density and elevation differences between the thermal centers of the core (heat source) and the steam generators (heat sink). • If the the heat sink is lost, the driving potential is also lost. • Loss of heat sink can occur with a loss of main and auxiliary feedwater supplies followed by steam generator dryout, or • Main Steam Line Break (MSLB) in a single steam generator in a multi-loop plant. • Operators isolate the feedwater to the steam generators and close the main steam isolation valves. • “Broken” steam generator will continue to vent steam and depressurize. This results in a rapid cooling of the entire primary system fluid. • Primary loop fluid temperatures may drop below the secondary side temperatures of the isolated “unbroken” steam generator. • Loop flow stops on “unbroken” side of plant. IAEA-ICTP Natural Circulation Training Course, Trieste, Italy, 25-29 June 2007

  10. SG#1 Temperature Hot Leg #1 Temperature Flows in Cold Legs #1 and #3 Resume Flows in Cold Legs #1 and #3 Stop APEX-CE MSLB Test (OSU-CE-0012)Comparison of Unaffected SG#1 and HL#1 Temperatures IAEA-ICTP Natural Circulation Training Course, Trieste, Italy, 25-29 June 2007

  11. Flows in Cold Legs #1 and #3 Resume Flows in Cold Legs #1 and #3 Stop APEX-CE MSLB Test (OSU-CE-0012)Cold Leg #1 and #3 Flow Rates IAEA-ICTP Natural Circulation Training Course, Trieste, Italy, 25-29 June 2007

  12. Negatively Buoyant Loop Seal Fluid • During Safety Injection, the cold borated water injected into cold legs spills into the loop seals. • This creates a cold liquid plug with a gravity head that resists loop flow-essentially adding an additional resistance term. • In multi-loop systems, flow is preferentially diverted to the adjacent cold leg through the SG lower channel head. • This can occur under single-phase or two-phase conditions. IAEA-ICTP Natural Circulation Training Course, Trieste, Italy, 25-29 June 2007

  13. Negatively Buoyant Loop Seal Fluid • OSU Flow Simulation of Loop Seal Cooling due to HPI Flow IAEA-ICTP Natural Circulation Training Course, Trieste, Italy, 25-29 June 2007

  14. Loop Seal Dowcomer on Vessel Side

  15. Asymmetric Loop Seal Cooling in Multi-Loop System (OSU-CE-008) • Loop Seal #4 Cools Early • Flow diverts to Cold Leg #2 IAEA-ICTP Natural Circulation Training Course, Trieste, Italy, 25-29 June 2007

  16. Asymmetric Loop Stagnationin Multi-Loop System (OSU-CE-008) IAEA-ICTP Natural Circulation Training Course, Trieste, Italy, 25-29 June 2007

  17. Two-Phase Natural CirculationStagnation Mechanisms SG Tube Voiding Negatively Buoyant Loop Seal Fluid IAEA-ICTP Natural Circulation Training Course, Trieste, Italy, 25-29 June 2007

  18. Steam Generator Tube Voiding • During a Small Break Loss of Coolant Accident (SBLOCA) in a PWR, steam generator tube draining will result in a gradual decrease in primary side natural circulation flow until it transitions to a boiling-condensing mode of operation. • As liquid mass is removed from the system, the loop void fraction increases. • Initial rise in loop flow rates due to increased density difference. • The loop flow reaches a maximum value when the two-phase buoyancy driving head is at its maximum. • Steam generator tubes begin to drain causing a decrease in flow rate because the distance between the core and steam generator thermal centers has decreased. • Longest tubes drain first. Loop flow ceases when shortest tubes begin to drain. IAEA-ICTP Natural Circulation Training Course, Trieste, Italy, 25-29 June 2007

  19. Cold Leg Flow Rates Versus Primary Side Inventory Stepped-Inventory Reduction Test (OSU-CE-0002) Increasing Void Fraction 1-Phase N/C IAEA-ICTP Natural Circulation Training Course, Trieste, Italy, 25-29 June 2007

  20. Asymmetric Steam Generator Tube Draining(Steam Generator #2 During SLOCA Test (OSU-CE-0008) Shortest Tubes Longest Tubes IAEA-ICTP Natural Circulation Training Course, Trieste, Italy, 25-29 June 2007

  21. Criteria for Onset of Cold LegThermal Stratification Modified Froude Number: 1. Theofanous, et al., (1984): 2. Reyes (2001): IAEA-ICTP Natural Circulation Training Course, Trieste, Italy, 25-29 June 2007

  22. Onset of Cold Leg Thermal Stratification Creare 1/5 Scale Data Ref 1 Ref 2 Well-Mixed Stratified IAEA-ICTP Natural Circulation Training Course, Trieste, Italy, 25-29 June 2007

  23. Fundamentals of Forced Plumes IAEA-ICTP Natural Circulation Training Course, Trieste, Italy, 25-29 June 2007

  24. Fundamentals of Forced Plumes(Entrainment Assumption) • G.I. Taylor’s Entrainment Assumption: Linear spread of the plume radius with axial position implies that the mean inflow velocity across the edge of the plume is proportional to the local mean downward velocity of the plume. IAEA-ICTP Natural Circulation Training Course, Trieste, Italy, 25-29 June 2007

  25. Fundamentals of Forced Plumes(Gaussian Velocity and Buoyancy Profiles) • Correlation of Velocity Distributions Measured in a Planar Jet (1934) IAEA-ICTP Natural Circulation Training Course, Trieste, Italy, 25-29 June 2007

  26. Fundamentals of Forced Plumes(Similarity of Velocity and Buoyancy Profiles) IAEA-ICTP Natural Circulation Training Course, Trieste, Italy, 25-29 June 2007

  27. Governing Equations Gaussian Plume Profile Equations Plume Mass: Velocity: Momentum: Buoyancy: Energy: Temperature: Governing Equations for AxisymmetricHPI Plumes IAEA-ICTP Natural Circulation Training Course, Trieste, Italy, 25-29 June 2007

  28. Dimensionless Balance Equations Dimensionless Groups Plume Mass: Momentum: Energy: Dimensionless Equations for Axisymmetric HPI Plumes IAEA-ICTP Natural Circulation Training Course, Trieste, Italy, 25-29 June 2007

  29. Decay Correlations for AxisymmetricHPI Plumes Entrainment Correlation (Theofanous, et al.): Temperature Decay Correlation (Theofanous, et al.): IAEA-ICTP Natural Circulation Training Course, Trieste, Italy, 25-29 June 2007

  30. Downcomer Planar Plumes

  31. Gaussian Plume Profile Equations Governing Equations Velocity: Plume Mass: Momentum: Buoyancy: Temperature: Energy: Governing Equations for Downcomer Planar Plumes IAEA-ICTP Natural Circulation Training Course, Trieste, Italy, 25-29 June 2007

  32. Dimensionless Balance Equations Dimensionless Groups Plume Mass: Momentum: Energy: Dimensionless Equations for Planar Downcomer Plumes IAEA-ICTP Natural Circulation Training Course, Trieste, Italy, 25-29 June 2007

  33. Plume Velocity Correlation (Kotsovinos): Dimensionless Plume Velocity Correlation: Convective Heat Transfer Correlation: Where: Correlations for Planar PlumeVelocity and Heat Transfer IAEA-ICTP Natural Circulation Training Course, Trieste, Italy, 25-29 June 2007

  34. Heat Transfer Correlation for Planar Plumes (Creare ½-Scale data) IAEA-ICTP Natural Circulation Training Course, Trieste, Italy, 25-29 June 2007

  35. Photographs of the IVO Transparent Test Loop.Cold Leg C flow rate = 66 gpm (4.2 liters/s)HPI flow in Cold Leg B = 6.6 gpm (0.42 liters/s) IAEA-ICTP Natural Circulation Training Course, Trieste, Italy, 25-29 June 2007

  36. Complexity of Downcomer Plume Behaviour • Downcomer Plumes are quite complex in Multi-Loop Systems. • Plume position is not steady • Plumes can merge • Does not lend itself to simple models • CFD Codes may be best method to predict downcomer plume behavior. IAEA-ICTP Natural Circulation Training Course, Trieste, Italy, 25-29 June 2007

  37. Complexity of Downcomer Plume Behaviour(STAR-CD Calculation)

  38. Conclusions • Natural Circulation Flow Interruption: • SG Reverse Heat Transfer • Loop Seal Cooling • SG Tube Voiding • Thermal Stratification can occur upon loss of N/C flow • Onset Criteria • Simple Models for Axisymmetric and Planar Plumes • CFD needed to predict complex multiple plume interactions. IAEA-ICTP Natural Circulation Training Course, Trieste, Italy, 25-29 June 2007

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