Department of Nuclear Engineering & Radiation Health Physics IAEA-ICTP Natural Circulation Training Course, Trieste, Italy, June 2007Flow Stagnation & Thermal Stratification in NC Loops (T17) - Reyes 1 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 Department of Nuclear Engineering & Radiation Health Physics

IAEA-ICTP Natural Circulation Training Course, Trieste, Italy, June 2007Flow Stagnation & Thermal Stratification in NC Loops (T17) - Reyes 2 Course Roadmap

Department of Nuclear Engineering & Radiation Health Physics IAEA-ICTP Natural Circulation Training Course, Trieste, Italy, June 2007Flow Stagnation & Thermal Stratification in NC Loops (T17) - Reyes 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.

Department of Nuclear Engineering & Radiation Health Physics IAEA-ICTP Natural Circulation Training Course, Trieste, Italy, June 2007Flow Stagnation & Thermal Stratification in NC Loops (T17) - Reyes 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

Department of Nuclear Engineering & Radiation Health Physics IAEA-ICTP Natural Circulation Training Course, Trieste, Italy, June 2007Flow Stagnation & Thermal Stratification in NC Loops (T17) - Reyes 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.

Department of Nuclear Engineering & Radiation Health Physics IAEA-ICTP Natural Circulation Training Course, Trieste, Italy, June 2007Flow Stagnation & Thermal Stratification in NC Loops (T17) - Reyes 6 Thermal Stratification in a PWR Cold Leg and Downcomer Plume Formation

Department of Nuclear Engineering & Radiation Health Physics IAEA-ICTP Natural Circulation Training Course, Trieste, Italy, June 2007Flow Stagnation & Thermal Stratification in NC Loops (T17) - Reyes 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.

Department of Nuclear Engineering & Radiation Health Physics IAEA-ICTP Natural Circulation Training Course, Trieste, Italy, June 2007Flow Stagnation & Thermal Stratification in NC Loops (T17) - Reyes 8 Single-Phase Natural Circulation Stagnation Mechanisms Loss of Heat Sink Negatively Buoyant Loop Seal Fluid

Department of Nuclear Engineering & Radiation Health Physics IAEA-ICTP Natural Circulation Training Course, Trieste, Italy, June 2007Flow Stagnation & Thermal Stratification in NC Loops (T17) - Reyes 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.

Department of Nuclear Engineering & Radiation Health Physics IAEA-ICTP Natural Circulation Training Course, Trieste, Italy, June 2007Flow Stagnation & Thermal Stratification in NC Loops (T17) - Reyes 10 APEX-CE MSLB Test (OSU-CE-0012) Comparison of Unaffected SG#1 and HL#1 Temperatures Flows in Cold Legs #1 and #3 Stop Flows in Cold Legs #1 and #3 Resume SG#1 Temperature Hot Leg #1 Temperature

Department of Nuclear Engineering & Radiation Health Physics IAEA-ICTP Natural Circulation Training Course, Trieste, Italy, June 2007Flow Stagnation & Thermal Stratification in NC Loops (T17) - Reyes 11 APEX-CE MSLB Test (OSU-CE-0012) Cold Leg #1 and #3 Flow Rates Flows in Cold Legs #1 and #3 Stop Flows in Cold Legs #1 and #3 Resume

Department of Nuclear Engineering & Radiation Health Physics IAEA-ICTP Natural Circulation Training Course, Trieste, Italy, June 2007Flow Stagnation & Thermal Stratification in NC Loops (T17) - Reyes 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.

Department of Nuclear Engineering & Radiation Health Physics IAEA-ICTP Natural Circulation Training Course, Trieste, Italy, June 2007Flow Stagnation & Thermal Stratification in NC Loops (T17) - Reyes 13 Negatively Buoyant Loop Seal Fluid OSU Flow Simulation of Loop Seal Cooling due to HPI Flow

Loop Seal Dowcomer on Vessel Side

Department of Nuclear Engineering & Radiation Health Physics IAEA-ICTP Natural Circulation Training Course, Trieste, Italy, June 2007Flow Stagnation & Thermal Stratification in NC Loops (T17) - Reyes 15 Asymmetric Loop Seal Cooling in Multi-Loop System (OSU-CE-008) Loop Seal #4 Cools Early Flow diverts to Cold Leg #2

Department of Nuclear Engineering & Radiation Health Physics IAEA-ICTP Natural Circulation Training Course, Trieste, Italy, June 2007Flow Stagnation & Thermal Stratification in NC Loops (T17) - Reyes 16 Asymmetric Loop Stagnation in Multi-Loop System (OSU-CE-008)

Department of Nuclear Engineering & Radiation Health Physics IAEA-ICTP Natural Circulation Training Course, Trieste, Italy, June 2007Flow Stagnation & Thermal Stratification in NC Loops (T17) - Reyes 17 Two-Phase Natural Circulation Stagnation Mechanisms Negatively Buoyant Loop Seal Fluid SG Tube Voiding

Department of Nuclear Engineering & Radiation Health Physics IAEA-ICTP Natural Circulation Training Course, Trieste, Italy, June 2007Flow Stagnation & Thermal Stratification in NC Loops (T17) - Reyes 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.

Department of Nuclear Engineering & Radiation Health Physics IAEA-ICTP Natural Circulation Training Course, Trieste, Italy, June 2007Flow Stagnation & Thermal Stratification in NC Loops (T17) - Reyes 19 Cold Leg Flow Rates Versus Primary Side Inventory Stepped-Inventory Reduction Test (OSU-CE-0002) Increasing Void Fraction 1-Phase N/C

Department of Nuclear Engineering & Radiation Health Physics IAEA-ICTP Natural Circulation Training Course, Trieste, Italy, June 2007Flow Stagnation & Thermal Stratification in NC Loops (T17) - Reyes 20 Asymmetric Steam Generator Tube Draining (Steam Generator #2 During SLOCA Test (OSU-CE-0008) Shortest Tubes Longest Tubes

Department of Nuclear Engineering & Radiation Health Physics IAEA-ICTP Natural Circulation Training Course, Trieste, Italy, June 2007Flow Stagnation & Thermal Stratification in NC Loops (T17) - Reyes 21 Criteria for Onset of Cold Leg Thermal Stratification 1. Theofanous, et al., (1984): Modified Froude Number: 2. Reyes (2001):

Department of Nuclear Engineering & Radiation Health Physics IAEA-ICTP Natural Circulation Training Course, Trieste, Italy, June 2007Flow Stagnation & Thermal Stratification in NC Loops (T17) - Reyes 22 Onset of Cold Leg Thermal Stratification Creare 1/5 Scale Data Well-Mixed Stratified Ref 1 Ref 2

Department of Nuclear Engineering & Radiation Health Physics IAEA-ICTP Natural Circulation Training Course, Trieste, Italy, June 2007Flow Stagnation & Thermal Stratification in NC Loops (T17) - Reyes 23 Fundamentals of Forced Plumes

Department of Nuclear Engineering & Radiation Health Physics IAEA-ICTP Natural Circulation Training Course, Trieste, Italy, June 2007Flow Stagnation & Thermal Stratification in NC Loops (T17) - Reyes 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.

Department of Nuclear Engineering & Radiation Health Physics IAEA-ICTP Natural Circulation Training Course, Trieste, Italy, June 2007Flow Stagnation & Thermal Stratification in NC Loops (T17) - Reyes 25 Correlation of Velocity Distributions Measured in a Planar Jet (1934) Fundamentals of Forced Plumes (Gaussian Velocity and Buoyancy Profiles)

Department of Nuclear Engineering & Radiation Health Physics IAEA-ICTP Natural Circulation Training Course, Trieste, Italy, June 2007Flow Stagnation & Thermal Stratification in NC Loops (T17) - Reyes 26 Fundamentals of Forced Plumes (Similarity of Velocity and Buoyancy Profiles)

Department of Nuclear Engineering & Radiation Health Physics IAEA-ICTP Natural Circulation Training Course, Trieste, Italy, June 2007Flow Stagnation & Thermal Stratification in NC Loops (T17) - Reyes 27 Governing Equations for Axisymmetric HPI Plumes Gaussian Plume Profile Equations Velocity: Buoyancy: Temperature: Governing Equations Plume Mass: Momentum: Energy:

Department of Nuclear Engineering & Radiation Health Physics IAEA-ICTP Natural Circulation Training Course, Trieste, Italy, June 2007Flow Stagnation & Thermal Stratification in NC Loops (T17) - Reyes 28 Dimensionless Equations for Axisymmetric HPI Plumes Dimensionless Balance Equations Plume Mass: Momentum: Energy: Dimensionless Groups

Department of Nuclear Engineering & Radiation Health Physics IAEA-ICTP Natural Circulation Training Course, Trieste, Italy, June 2007Flow Stagnation & Thermal Stratification in NC Loops (T17) - Reyes 29 Decay Correlations for Axisymmetric HPI Plumes Entrainment Correlation (Theofanous, et al.): Temperature Decay Correlation (Theofanous, et al.):

Downcomer Planar Plumes

Department of Nuclear Engineering & Radiation Health Physics IAEA-ICTP Natural Circulation Training Course, Trieste, Italy, June 2007Flow Stagnation & Thermal Stratification in NC Loops (T17) - Reyes 31 Governing Equations for Downcomer Planar Plumes Gaussian Plume Profile Equations Velocity: Buoyancy: Temperature: Governing Equations Plume Mass: Momentum: Energy:

Department of Nuclear Engineering & Radiation Health Physics IAEA-ICTP Natural Circulation Training Course, Trieste, Italy, June 2007Flow Stagnation & Thermal Stratification in NC Loops (T17) - Reyes 32 Dimensionless Equations for Planar Downcomer Plumes Dimensionless Balance Equations Plume Mass: Momentum: Energy: Dimensionless Groups

Department of Nuclear Engineering & Radiation Health Physics IAEA-ICTP Natural Circulation Training Course, Trieste, Italy, June 2007Flow Stagnation & Thermal Stratification in NC Loops (T17) - Reyes 33 Correlations for Planar Plume Velocity and Heat Transfer Plume Velocity Correlation (Kotsovinos): Dimensionless Plume Velocity Correlation: Convective Heat Transfer Correlation: Where:

Department of Nuclear Engineering & Radiation Health Physics IAEA-ICTP Natural Circulation Training Course, Trieste, Italy, June 2007Flow Stagnation & Thermal Stratification in NC Loops (T17) - Reyes 34 Heat Transfer Correlation for Planar Plumes (Creare ½-Scale data)

Department of Nuclear Engineering & Radiation Health Physics IAEA-ICTP Natural Circulation Training Course, Trieste, Italy, June 2007Flow Stagnation & Thermal Stratification in NC Loops (T17) - Reyes 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)

Department of Nuclear Engineering & Radiation Health Physics IAEA-ICTP Natural Circulation Training Course, Trieste, Italy, June 2007Flow Stagnation & Thermal Stratification in NC Loops (T17) - Reyes 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.

Complexity of Downcomer Plume Behaviour (STAR-CD Calculation)

Department of Nuclear Engineering & Radiation Health Physics IAEA-ICTP Natural Circulation Training Course, Trieste, Italy, June 2007Flow Stagnation & Thermal Stratification in NC Loops (T17) - Reyes 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.