UC Berkeley

Nicolas Zweibaum, Per F. PetersonThermal Hydraulics Laboratory

Department of Nuclear EngineeringUniversity of California, Berkeley

2011 RELAP 5 International Users Seminar

Wednesday, July 27, 2011

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

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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

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Advanced High Temperature Reactor (AHTR) Technology Overview

410 MWe PB-AHTR2008 UCBNE Senior Design Project

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Advanced High Temperature Reactors (AHTRs) combine two older technologies

Liquid fluoride salt coolantsExcellent heat transferTransparent, clean fluoride saltBoiling point ~1400ºCReacts very slowly in airNo energy source to pressurize containmentBut high freezing temperature (459°C)And industrial safety required for Be

1600°C

AHTRs have uniquely large fuel thermal margin

max.PB-AHTR

temp

Coated particle fuel

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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

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

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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

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• 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

Pebble Recirculation Experiment (PREX): Project Objective

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PREX 3:Pebble Recirculation in a Dry Test Section

Objective:Small time step data collection from PREX 3 to develop local velocity field data.

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• 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

Pebble Recirculation Experiment (PREX): Project Objective

UC Berkeley

PREX 3.1:Porous Media Flow for Buoyant Pebbles

Objective:Evaluate applicability of porous media flow correlations for flow regimes in PB-ATHR.

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PREX 3.1 Closed Loop Flow Schematic

Test Section modeled with RELAP5-3D

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PREX 3.1 Scaling Parameters

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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:

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Porous Media Flow Theory

• Determination of Form Loss Coefficient for Flow through a Randomly Packed Bed of Spheres:

Ergun Equation: for 1 < Re < 104

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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 CoefficientUsed in RELAP5-3D

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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.

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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):

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Modeling the As-Built Geometry of PREX 3.1 in RELAP5-3D

Mesh interval: x = 3.70 cm

y1 = 5.06 cm

y2 = 3.34 cm

y3 = 4.93 cm

Bed Depth: 14.32 cm

Meshing in RELAP5-3D Model

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Replicate the Hydrodynamic Parameters of the Experiment Run Label 110228-R08

Pebble Injection Lines Flow (gpm)In_P_1 3 In_P_2 3 In_P_3 3

Estimated Bypass Fraction 0,247

Note: Bypass fraction is estimated for the flow in the Pebble Injection Lines only, not the total inflow to the test section.

Inside Reflector - Injection SurfaceIn_R_1 0 In_R_2 0 In_R_3 7/8In_R_4 1 7/8In_R_5 0 In_R_6 0

Outside Reflector - Injection SurfaceIn_L_1 1 7/8In_L_2 3 3/8In_L_3 5 In_L_4 3 3/8

Outside Reflector - Suction SurfaceOut_L_1 5 7/8Out_L_2 5 7/8Out_L_3 5 7/8Out_L_4 6 3/8In_P_1,2,3

In_L_1

In_L_2

In_L_3

In_L_4

Out_L_1

Out_L_2

Out_L_3Out_L_4

In_R_1

In_R_2In_R_3

In_R_4

In_R_5

In_R_6

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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

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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 fromComsol Multiphysics

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Experimental Conditions of Run 4 (Axial Flow)

In_P_1,2,3

In_L_3

In_L_4

Out_L_1

Out_L_2

Out_L_3Out_L_4

In_R_3

In_R_4

Run Label 110228-R04

Pebble Injection LinesIn_P_1 3 1/2In_P_2 3 1/2In_P_3 3 1/2Estimated Bypass Fraction 0,335

Inside Reflector - Injection SurfaceIn_R_1 0 In_R_2 0 In_R_3 1 In_R_4 1 In_R_5 0 In_R_6 0

Outside Reflector - Injection SurfaceIn_L_1 0 In_L_2 0 In_L_3 3 In_L_4 2

Outside Reflector - Suction SurfaceOut_L_1 3 1/4Out_L_2 3 1/8Out_L_3 3 1/4Out_L_4 3 3/8

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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%).

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Parametric Study: Influence of Meshing Fineness

Results have converged using 27 axial multi-dimensional volumes.

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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).

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Verification of the distortion with a simple model

• COMSOL Multiphysics solves the Ergun equation exactly.• RELAP5-3D overpredicts the pressure drop across the bed (+30%).

Upward flow of water (0.4 kg/s) through a randomly packed bed of spheres (0.15*0.15*1.0m rectangular column)

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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?

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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

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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)

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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)

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Questions/Suggestions?