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Stratified Flow Phenomena, Graphite Oxidation, and Mitigation Strategies of Air Ingress Accident

Stratified Flow Phenomena, Graphite Oxidation, and Mitigation Strategies of Air Ingress Accident. INL / Chang Oh, U.S. PI. What happens following LOCA ? Depressurization Stratified Flow Diffusion Natural Convection T/H Safety Issues Core maximum temperature Potential core collapse

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Stratified Flow Phenomena, Graphite Oxidation, and Mitigation Strategies of Air Ingress Accident

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  1. Stratified Flow Phenomena, Graphite Oxidation, and Mitigation Strategies of Air Ingress Accident INL / Chang Oh, U.S. PI

  2. What happens following LOCA ? Depressurization Stratified Flow Diffusion Natural Convection T/H Safety Issues Core maximum temperature Potential core collapse Technical Requirements Accurate stratified flow modeling Accurate graphite oxidation and collapse modeling Accurate power distribution with neutronics model Background and Motivation

  3. To conduct experiments to supply information to model important phenomena in air-ingress accident, and code V&V. Effect of density-driven stratified flow on the air-ingress Oxidation and density variation of the graphite structures Internal pore area density of the graphite structures Effect of burn-off on the structural integrity of the graphite structures To develop a coupled neutronics and thermal-hydraulic capability in the GAMMA code Development of core neutronics model Coupling neutronic-thermal hydraulic tools Coupled core model V&V Objectives • To evaluate various methods for the mitigation • of air-ingress

  4. Project Organization Schematic diagram of all tasks involved Stratified Flow Study Task 1 (INL) Task 2 (INL) Validation Stratified Flow Experiment Stratified Flow Analysis (CFD & GAMMA) Models and Parameters Core Neutronic Model Advanced Graphite Oxidation Study Models and Parameters Task 7 (KAIST) Task 3 (INL) Coupling Neutronic Thermal Hydraulic Tool GAMMA Code Advanced Graphite Oxidation Study Models and Parameters Task 8 (KAIST) Task 5 (KAIST) Analysis Experiment of Burn-off In the Bottom Reflector Core Neutronic Model Task 9 (KAIST) Task 6 (KAIST) Task 4 (INL) Coupled Core Model V&V Structural Test of Burn- off Bottom Reflector Full Air-ingress Analysis Air-ingress Mitigation Study

  5. New Assumption on the Air-ingress Analysis • Stratified Flow in Air-ingress • Previous air-ingress analyses are all based on the assumption that the main air-ingress is dominated by molecular diffusion. • Previous analyses were performed using • 1-D and a vertical geometry • A new issue has been raised for the possibility of a convective flow driven by local density gradient. • After depressurization, there is large density differences between inside(Helium) and outside(Air) of vessel. • The density driven stratified flow can highly accelerate the whole air-ingress scenarios. Diffusion Assumption (40000 sec) Stratified Flow Assumption (60 sec)

  6. Density-Gradient Driven Stratified Flow (3) Density-driven Flow (1) Depressurization (2) Onset-of Flow Air ingress velocities by density driven flow • Volumetric flowrates of air and helium are the same. • At the interface, the shear stresses are the same between Air and Helium.

  7. Density Driven Stratified Flow Water and Salted Water Experiment Helium Air Salted Water Water

  8. New Scenario for Air Ingress Helium

  9. CFD Analysis on the Stratified Flow in VHTR 51,566 nodes Mesh (GAMBIT / FLUENT)

  10. Porous Media Parameters • Porous Media Approach • Core and Plenum were assumed to be porous media. • Porosity and Permeability should be determined. Porous Zones Additional Momentum Source Inertia resistance permeability

  11. Porous Media Parameters Porosity In the Core Core Hole Pattern (d = 1.58 cm, p = 3.27 cm) In the Lower Plenum Geometry of Lower Plenum (d = 0.212 m, p = 0.36 m)

  12. Porous Media Parameters • Flow Resistance Parameters in the Core • Empirically determined based on the friction correlations • Flow resistance in the radial was assumed to be infinitely large. In the Core Friction Factor in the Circular Channel

  13. Porous Media Parameters • Flow Resistance Parameters in the Lower Plenum • Flow resistance in the radial direction was calculated based on the friction data in the staggered array. • Flow resistance in the axial direction was calculated based on the friction data in the circular pipe flow. For axial direction In the Lower Plenum For radial direction Friction Factor in the Staggered Array

  14. Simulation of Stratified Flow Stratified Flow Simulation (by FLUENT 6.3) Initial Conditions Temperature Air-Mole Fraction Natural convection started about 160 sec after simulation.

  15. Turbulence Models

  16. Air-ingress Analysis • Multi-step Approach for Air-ingress Analysis • Stratified flow phase was solved by CFD code (FLUENT). • Depressurization and Diffusion/natural convection phase were solved by GAMMA code. CFD Code System Code 1. Depressurization Analysis Data Transfer 3. Natural Convection Analysis 2. Stratified Flow Analysis Data Transfer FLUENT Simulation GAMMA Simulation

  17. Air Ingress Analysis - Results Temperature (Bottom Reflector) Temperature (Core) Corrosion (Lower Plenum) Stratified Flow Assumption Diffusion Assumption

  18. Experimental Plan - 1 • Isothermal Experiment in the Horizontal Circular Pipe (TEST-1) • Focused on the separate effect of stratified flow phenomena • A simple scaling method used for pipe sizing and test conditions. 1~1.5 m 1m 0.5~1.0 m Tube diameter = 20 cm

  19. Summary of Scaling Results (ratio = scaled down/full-scale) Experimental Plan - 1 • Scaling Analysis of Stratified Flow in a Simple Channel Countercurrent stratified flow behavior in the VHTR hot duct (Turner(1973)) By Reynolds number similitude By gas law

  20. Experimental Plan - 2 • Non-isothermal Test (TEST-2) • Focused on the coupling effect of stratified flow and natural convection. • On-set of natural convection is the main measuring parameter. 0.8~1.0 m Pipe diameter 5 cm

  21. Experimental Plan - 2 Fluent Simulation for Density Driven Air-ingress Experiment 10 sec 20 sec 40 sec 80 sec 300 sec 400 sec 410 sec 420 sec Onset of natural convection occurred at around 400 sec.

  22. Experimental Plan - 2 FLUENT Results for Density Driven Air-ingress Experiment (at Valve-3) Time vs. Temperature Time vs. Flow-rate onset of natural convection onset of natural convection Flow rate and Temperature can be used as signals for onset-natural-circulation.

  23. Experimental Plan – 3 and 4 • Non-isothermal Test (TEST-3, TEST-4) • Focused on the coupling effects • Stratified Flow + Natural Convection + Porous Media + Chemical Reaction • Basic Experimental Procedures are the same as TEST 2 Metal (TEST-3) or Graphite (TEST-4) Non-isothermal test with Structures Non-isothermal test with Pebbles

  24. Experiment on the Oxidized Graphite Fracture • Experimental Set-up • The experiment was performed at 650 oC for uniform oxidation. • The test procedure and set-up is based on ASTM standard test method. • IG-110 and H451 graphite was used for testing. Sample load and holder

  25. Experiment on the Oxidized Graphite Fracture Normal Compressive Stress vs. Burn-off old data old data H-451 IG-110 New data New data

  26. Graphite Surface Area Density • Graphite Surface Area Density (Unoxidized Initial Value) • The graphite surface area density was calculated from the BET surface area measured by previous investigations.

  27. Effect of Graphite Burn-off on the Oxidation Rate • Graphite Oxidation Rate and Burn-off • The reaction rate increases with the increasing burn-off in the beginning because of the increase of pore size and porosity open. • The reaction rate decreases at high burn-off because the pores join together, decreasing the reaction surface area.

  28. Modeling of Graphite Oxidation and Fracture in Air-ingress Reference Reactors (GT-MHR 600 MWt) Core Graphite Structure

  29. Estimation of Corrosion Depth by GAMMA code • Burn-offrefers to the oxidation of the graphite’s internal body, causing reduction of density, leading to reduction of stiffness and mechanical strength. • Corrosion refers to oxidation taking place on the outer surface exposed to airflow. The corrosion decreases the cross-sectional area available to support the weight leading to stress concentration. Most Seriously Damaged!

  30. Detailed Geometry of the Supporting Block Coolant channels from core Lower reflector blocks

  31. Oxidized State Results Compressive stress distribution on plenum head, 6.5 days after ONC Corrosion Progression 1/6 cyclic symmetry unit of the modified plenum head for each day * The specified time is the elapsed time after natural convection.

  32. Oxidized State Results Failure occurs 5.5to 6 days after the start of natural convection

  33. Task 5 - Experimental facility Task Progress Kinetics Completed activity Planned activity • Activation energy • Order of reaction Graphite selection Experimental facility Bottom reflector Burn-off model • IG-110 • IG-430 • NBG-18 • NBG-25 Mass transfer • Design • Installation • Heat/mass analogy Other effects • Geometry • Burn-off • Moisture Schematics of Experimental Facility

  34. Task 5 - Test Condition Picture of the Test Section Graphite

  35. Task 5 – Kinetics (I) Effect of Temperature on Oxidation Rate Effect of Oxygen Concentration on Oxidation Rate

  36. Task 5 – Kinetics (II)

  37. Task 6 - Task Progress Task Progress Completed activity Planned activity Fresh graphite Failure test Oxidized graphite • Mechanical test • Structural analysis • Uniform oxidation • Non-uniform oxidation Structure • Support column • Support block • Other components GAMMA Failure Model Development Data collection • Estimation of burn-off

  38. Task 6 - Bottom structures Bottom reflector components and condition Schematics of oxidation trend in graphite support columns Graphite support columns, GT-MHR 600MWth

  39. Task 6 - Test facility Hardened steel plate Machine cross head Air outlet Vent Thermocouple Clear Plastic safety shield Test specimen Graphite specimen Insulating material Compression block Spherical block Support plate Air inlet Distributor Picture of the electric furnace Picture of the failure test facility

  40. Task 6 – Compressive and buckling strength of fresh IG-110 column Compressive strength of IG-110 Buckling point: Buckling strength

  41. Task 6 - Compressive and buckling strength of oxidized IG-110 column Normalized compressive strength of oxidized graphite Normalized compressive and buckling strength of oxidized graphite columns. The strength of oxidized graphite column under axial load:

  42. Task 6 - Fracture load changes of oxidized complicated-shape samples Table of complicated-shape sample test Fracture load changes of oxidized complicated-shape samples

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