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Laser Flash Method

for Effective Thermal Diffusivity Measurement of Pebble Beds

CBBI-16

Portland, OR, USA

Sept. 9, 2011

Mu-Young Ahn1, Duck Young Ku1, Yi-Hyun Park1 , In-Keun Yu1, Seungyon Cho1

1 National Fusion Research Institute, Daejeon, Korea

Contents

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I

II

III

IV

Introduction

Laser Flash Method

Preliminary Experiments

& Construction of Laser Flash Apparatus

Summary & Future Works

KO HCSB TBM Design

The Helium Cooled Solid Breeder TBM consists of– Reduced Activation Ferritic/Martensitic steel as a structural material– High pressure helium as a coolant– Atmospheric pressure helium with 1% hydrogen as a purge gas– Three materials in breeding zone

• Li4SiO4 or Li2TiO3 pebble as a breeder• Be pebble as a multiplier• Graphite pebble as a reflector

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It is very important to know thermo-mechanical behavior of pebble bed for TBM design & qualification

– The breeding zone is packed in pebble form• The pebble size is around 1 mm (specific size and packing fraction for each material TBD)• Some properties of pebble bed is different from those of pebble itself (ex. effective heat

conductivity vs. heat conductivity)

– Pebble bed properties will change during the operation• Pebble bed is deformed by thermal stress• Local packing fraction is affected due to pebble moving

– Thermal & mechanical properties should be measured simultaneously• Thermal & mechanical properties are strongly coupled

Thermo-mechanics of Pebble Bed

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Tests for Pebble Bed & Objectives

Following tests for pebble bed are foreseen – Out-of-pile tests

• Thermal properties (Cp)• Thermo-mechanical properties (effective thermal conductivity, effective thermal expansion

rate, effective stress-strain correlation, thermal creep, pebble bed failure data, etc)• Wall interface data (heat conductance, friction coefficient, etc)

– In-pile tests

Construction of various facilities is planned and on-going– Thermo-mechanical data should be established for KO TBM and future breeding blankets– Specially the KO concept has unique feature of having graphite reflector

• No pebble bed data on graphite reflector pebble bed

Ojectives of the study– Preliminary results with laser flash method– Construction of laser flash apparatus with axial compression

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Contents

6

I

II

III

IV

Introduction

Laser Flash Method

Preliminary Experiments

& Construction of Laser Flash Apparatus

Summary & Future Works

Measurement Method?

Thermal conductivity vs. Effective thermal conductivity

From the NETZSCH viewgraph

Thermal Cond.Effective Thermal

Cond.Remark

Ceramic Breeder ~1 hot wire

Beryllium Multiplier 80~200 W/m/K 3~14 heat flow method / guarded hot plate

Graphite Reflector 100~150 W/m/K ? ?

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- Wide range of thermal properties

Laser Flash Method (1)

Laser flash method is a transient method widely used in industry– High accuracy and repeatibility with wide range of thermal diffusivity (conductivity) measurement– Absolute measurement technique (no calibration with other materials)– Small sample size and easy sample preparation– Short measurement time

– Heat capacity and density required for heat conductivity calculation– Sample thickness limitation mainly due to mathematical model and laser power

• Corrections required for radial and facial heat loss, and finite pulse effects (ex. Advanced Cape-Lehman approach)

• Sufficient laser power should be chosen

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Transient 1D Heat Equation

- initial condition (laser source)- boundary conditions (adiabatic)

From the NETZSCH viewgraph

Issues on Pebble Beds Measurement

Laser Flash Method?– Wall influence effect on sample thickness

• It is known that minimum thickness is required to behave as “pebble bed”• It is expected that the effective thermal property shows asymptotic behavior according to

increasing thickness• Is it possible to measure by LFM up to the minimum thickness?

• Preliminary test was performed to see feasibility regarding the thickness issues

– Thermal & mechanical properties are strongly coupled

• Compression load device & pebble holder were designed

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Contents

10

I

II

III

IV

Introduction

Laser Flash Method

Preliminary Experiments

& Construction of Laser Flash Apparatus

Summary & Future Works

Preliminary Experiments (1)

Preliminary experiments were performed to see feasibility regarding the thickness issue– The measurement was done at 25 oC in accordance with ASTM E-1461– 2mm graphite pebbles were packed in special crucibles covered with sapphire glass

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- Bad detector signals- No reproducible values

- Good detector signals- Reproducible values

Preliminary Experiments (2)

Preliminary experiments were performed to see feasibility regarding the thickness issue– Only ~3xD thickness shows the saturated value– Adopting 20 J/pulse laser makes pebble bed measurement over 10 mm possible

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- Bad detector signals- No reproducible values

- Good detector signals- Reproducible values

Construction of Laser Flash Apparatus (1)

The facility requirements for thermal properties– Effective thermal diffusivity (conductivity) is measured with axial compression load

• Graphite reflector pebble bed• Ceramic breeder pebble bed

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Parameter Value

Measurement  

   Materials Solid, Liquid, Powder, Pebble Bed (with pebble crucible)

   Temperature RT ~ 1500oC

Compression load Up to 10 MPa

Test section atmosphere Vacuum ~ 1.5 bar (filling or purging up to 200 ml/min)

Performance  

   Heat diffusivity range 0.01 ~ 1000 mm2/s

Heat conductivity range 0.1 ~ 2000 W/m/K

Accuracy ±5% or better for standard materials

   Repeatability ±3% or better for standard materials

Construction of Laser Flash Apparatus (2)

NETZSCH LFA 427 with special design – Compression load device– Sample holder for pebble bed

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Construction of Laser Flash Apparatus (3)

Compression load device – Pressure stamp exerts compression load on the load ring – Compression load is controlled pneumatically using MFC, EPC – Compression load is maintained at specified stress– Displacement is recorded by digital dial guage

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Construction of Laser Flash Apparatus (4)

Sample holder for pebble bed– Pebbles are confined by two sapphire discs and the container– Sapphire disc is coated by graphite for laser power absorption and emission– Maximum sample thickness is set to 10mm which is considered to be enough according to the

preliminary experiment results

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Construction of Laser Flash Apparatus (5)

The apparatus was constructed in 2010– Now commissioning is on-going

• Li2TiO3 measurement for comparison with previous studies• Background thermal expansion experiments

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Contents

18

I

II

III

IV

Introduction

Laser Flash Method

Preliminary Experiments

& Construction of Laser Flash Apparatus

Summary & Future Works

Summary & Future Works

Summary – Laser flash method was chosen as measurement method for effective thermal diffusivity

(conductivity)• Preliminary test was performed to investigate feasibility of laser flash method

– Laser flash apparatus was constructed and commissioning is on-going• Compression load device and sample holder were designed for pebble bed measurement• Li2TiO3 measurement for comparison purpose with previous studies and background

thermal expansion experiment are on-going

Future works– Validation of modified calculation logic to take thickness change into account is required– Validation of compression load(to reduce compression load variation) and displacement

measurement is necessary

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Thank you for your attention!

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