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Salt Cooled High Temperature Reactors and Porous Media Flow Modeling in RELAP5-3D

Salt Cooled High Temperature Reactors and Porous Media Flow Modeling in RELAP5-3D. Nicolas Zweibaum , Per F. Peterson Thermal Hydraulics Laboratory Department of Nuclear Engineering University of California, Berkeley 2011 RELAP 5 International Users Seminar Wednesday, July 27, 2011.

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Salt Cooled High Temperature Reactors and Porous Media Flow Modeling in RELAP5-3D

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  1. Salt Cooled High Temperature Reactors andPorous Media Flow Modeling in RELAP5-3D Nicolas Zweibaum, Per F. Peterson Thermal Hydraulics Laboratory Department of Nuclear Engineering University of California, Berkeley 2011 RELAP 5 International Users Seminar Wednesday, July 27, 2011

  2. Presentation Outline • Advanced High Temperature Reactor (AHTR) Technology Overview • Pebble Recirculation Experiment (PREX): Project Objective • PREX 3.1: Porous Media Flow for Buoyant Pebbles • Modeling of PREX 3.1 in RELAP5-3D: • Methodology • First Results (vs. Experimental Results) • Challenges Faced • Conclusions/Future Work

  3. Advanced High Temperature Reactor (AHTR) Technology Overview 410 MWe PB-AHTR 2008 UCBNE Senior Design Project

  4. Advanced High Temperature Reactors (AHTRs) combine two older technologies Coated particle fuel max. PB-AHTR temp 1600°C Liquid fluoride salt coolants Excellent heat transfer Transparent, clean fluoride salt Boiling point ~1400ºC Reacts very slowly in air No energy source to pressurize containment But high freezing temperature (459°C) And industrial safety required for Be AHTRs have uniquely large fuel thermal margin

  5. Liquid fluoride salts have fundamentally different properties than other reactor coolants • High volumetric heat capacity provides high thermal inertia • High power density, low pressure operation possible compared to helium cooled reactors • High efficiency, compact primary loop equipment compared to water cooled reactors • Transparent coolant, low thermal shock, low chemical reactivity, compact primary loop equipment compared to sodium cooled reactors • But high freezing temperature still requires safety systems to prevent and control slowly evolving overcooling transients

  6. The modular PB-AHTR is a compact pool-type reactor with passive decay heat removal

  7. Pebble Recirculation Experiment (PREX): Project Objective • Objective: • Develop analysis methods consistent with NRC licensing standards for the study of granular flow phenomena in pebble bed reactor cores. • Project Components: • Regulatory and Licensing Requirements • Simulation Methods and Predictive Capabilities • Experimental Results for New Core Designs

  8. Pebble Recirculation Experiment (PREX): Project Objective • Objective: • Develop analysis methods consistent with NRC licensing standards for the study of granular flow phenomena in pebble bed reactor cores. • Project Components: • Regulatory and Licensing Requirements • Simulation Methods and Predictive Capabilities • Experimental Results for New Core Designs Michael Laufer, Thermal Hydraulics Laboratory, UCB

  9. PREX 3:Pebble Recirculation in a Dry Test Section Objective: Small time step data collection from PREX 3 to develop local velocity field data.

  10. Pebble Recirculation Experiment (PREX): Project Objective • Objective: • Develop analysis methods consistent with NRC licensing standards for the study of granular flow phenomena in pebble bed reactor cores. • Project Components: • Regulatory and Licensing Requirements • Simulation Methods and Predictive Capabilities • Experimental Results for New Core Designs Michael Laufer, Thermal Hydraulics Laboratory, UCB

  11. PREX 3.1:Porous Media Flow for Buoyant Pebbles Objective: Evaluate applicability of porous media flow correlations for flow regimes in PB-ATHR.

  12. PREX 3.1 Closed Loop Flow Schematic Test Section modeled with RELAP5-3D

  13. PREX 3.1 Scaling Parameters

  14. Porous Media Flow Theory • Determination of Form Loss Coefficient for Flow through a Randomly Packed Bed of Spheres: Ergun Equation: for 1 < Re < 104 With:

  15. Porous Media Flow Theory • Determination of Form Loss Coefficient for Flow through a Randomly Packed Bed of Spheres: Ergun Equation: for 1 < Re < 104

  16. Porous Media Flow Theory • Determination of Form Loss Coefficient for Flow through a Randomly Packed Bed of Spheres: By integration over the length of the bed: Form Loss Coefficient Used in RELAP5-3D

  17. Modeling of PREX 3.1 in RELAP5-3D Methodology: Replicate the as-built geometry of the test section and the hydrodynamic parameters of the flow through the bed of pebbles.

  18. Modeling the As-Built Geometry of PREX 3.1 in RELAP5-3D • 3 Zones: Diverging, Constant Cross-Section, Converging • 9*9 meshing of each zone • Only keep cells filled at more than 50% (from previous modeling of PB-AHTR Annular Core):

  19. Modeling the As-Built Geometry of PREX 3.1 in RELAP5-3D Meshing in RELAP5-3D Model Dy3 = 4.93 cm Dy2 = 3.34 cm Dy1 = 5.06 cm Bed Depth: 14.32 cm Mesh interval: Dx = 3.70 cm

  20. Replicate the Hydrodynamic Parameters of the Experiment Out_L_4 Out_L_3 Out_L_2 Out_L_1 In_R_6 In_R_5 In_L_4 In_R_4 In_L_3 In_R_3 In_R_2 In_L_2 In_R_1 In_L_1 In_P_1,2,3

  21. Replicate the Hydrodynamic Parameters of the Experiment • Boundary Conditions: • Uniform velocity at each inlet/outlet (except defueling chute): • Divide velocity by the corresponding number of meshes in RELAP5-3D • Subtract by-pass flow around the core from the flow entering the pebble injection line (high uncertainty, but experiment being rebuilt) • Constant pressure (atmospheric) at defueling chute • Porous Media Flow: • Use calculated form loss coefficients for x- and z-direction flows: • Only Reynolds-independent term between meshes in one volume • Re-dependent and Re-independent terms at junctions between volumes

  22. Modeling of PREX 3.1 in RELAP5-3D First Results (vs. Experimental Results): Run the simulation for axial flow and compare to experimental results from Run 4 (February 28, 2011) Velocity Field from Comsol Multiphysics

  23. Experimental Conditions of Run 4 (Axial Flow) Out_L_4 Out_L_3 Out_L_2 Out_L_1 In_L_4 In_R_4 In_L_3 In_R_3 In_P_1,2,3

  24. Simulation Results from Run 4 RELAP5-3D Model • Ergun correlation (implemented in COMSOL Multiphysics) overpredicts the pressure drop across the bed (+21%), due to the wall effect in the experiment (flow resistance is much lower near the wall due to the ordered packing of the spheres). • The RELAP5-3D model greatly overpredicts the pressure drop across the bed (+133%).

  25. Parametric Study: Influence of Meshing Fineness Results have converged using 27 axial multi-dimensional volumes.

  26. Parametric Study: Influence of Various Parameters • Junction area factor has a great influence on the results. • Porosity (volume factor) and wall friction have a 2nd order influence on the results. • Wall roughness and turbulent friction factor have no influence on the results (laminar flow).

  27. Verification of the distortion with a simple model Upward flow of water (0.4 kg/s) through a randomly packed bed of spheres (0.15*0.15*1.0m rectangular column) • COMSOL Multiphysics solves the Ergun equation exactly. • RELAP5-3D overpredicts the pressure drop across the bed (+30%).

  28. Modeling of PREX 3.1 in RELAP5-3D Challenges Faced: How to improve the model in order to fit experimental results in simple, precisely determined conditions?

  29. Problems Faced while Modeling Run 4 • What correlations should be used for porous medium flow? • Is Ergun the best equation for this model? • Re < 1,400: it should be • Are we using the appropriate form of the form loss coefficient for RELAP? • Multi-dimensional equation to take cross-flow into account • Is the meshing fine enough to accurately model the test section? • Results have converged when refining the meshing • How should the by-pass flow around the core be treated? • First approximation: subtract estimated value from inlet flow • Experiment is being rebuilt to eliminate by-pass flow around the core

  30. Problems that May Appear when Modeling other Runs • Cross-flow issues (how to deal with multi-dimensional pressure drop), more important for non-axial flow • Instrumentation inaccuracy (Versa mount flowmeters measure volumetric flow rates for all of the injection lines to an accuracy of 5%) (second order issue)

  31. Future Work • Refine/tune the model in its current state (properly account for form loss, porosity distribution next to the walls, etc.) • Compare to experimental results for more runs when the data is available (including cross-flow) • Anticipate modeling of annular core • Implement visualization tools for pressure and flow distribution (will help to spot errors)

  32. Questions/Suggestions?

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