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
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Contents
I
Introduction
II
Laser Flash Method
III
IV
Preliminary Experiments
& Construction of Laser Flash Apparatus
Summary & Future Works
1
KO HCSB TBM Design
 The Helium Cooled Solid Breeder TBM consists of
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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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Thermo-mechanics of Pebble Bed
 It is very important to know thermo-mechanical behavior of pebble bed for TBM design &
qualification
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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)
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Pebble bed properties will change during the operation
• Pebble bed is deformed by thermal stress
• Local packing fraction is affected due to pebble moving
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Thermal & mechanical properties should be measured simultaneously
• Thermal & mechanical properties are strongly coupled
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Tests for Pebble Bed & Objectives
 Following tests for pebble bed are foreseen
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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
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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
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Preliminary results with laser flash method
Construction of laser flash apparatus with axial compression
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Contents
I
Introduction
II
Laser Flash Method
III
IV
Preliminary Experiments
& Construction of Laser Flash Apparatus
Summary & Future Works
5
Measurement Method?
 Thermal conductivity vs. Effective thermal conductivity
Thermal Cond.
Effective Thermal Cond.
Remark
~1
hot wire
Ceramic Breeder
Beryllium Multiplier
80~200 W/m/K
3~14
heat flow method /
guarded hot plate
Graphite Reflector
100~150 W/m/K
?
?
- Wide range of thermal properties
From the NETZSCH viewgraph
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Laser Flash Method (1)
 Laser flash method is a transient method widely used in industry
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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
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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
Transient 1D Heat Equation
- initial condition (laser source)
- boundary conditions (adiabatic)
From the NETZSCH viewgraph
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Issues on Pebble Beds Measurement
 Laser Flash Method?
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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
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Thermal & mechanical properties are strongly coupled
• Compression load device & pebble holder were designed
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Contents
I
Introduction
II
Laser Flash Method
III
IV
Preliminary Experiments
& Construction of Laser Flash Apparatus
Summary & Future Works
9
Preliminary Experiments (1)
 Preliminary experiments were performed to see feasibility regarding the thickness issue
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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
- Bad detector signals
- No reproducible values
- Good detector signals
- Reproducible values
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Preliminary Experiments (2)
 Preliminary experiments were performed to see feasibility regarding the thickness issue
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Only ~3xD thickness shows the saturated value
Adopting 20 J/pulse laser makes pebble bed measurement over 10 mm possible
- Bad detector signals
- No reproducible values
- Good detector signals
- Reproducible values
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Construction of Laser Flash Apparatus (1)
 The facility requirements for thermal properties
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Effective thermal diffusivity (conductivity) is measured with axial compression load
• Graphite reflector pebble bed
• Ceramic breeder pebble bed
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
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Construction of Laser Flash Apparatus (2)
 NETZSCH LFA 427 with special design
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Compression load device
Sample holder for pebble bed
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Construction of Laser Flash Apparatus (3)
 Compression load device
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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
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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
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Now commissioning is on-going
• Li2TiO3 measurement for comparison with previous studies
• Background thermal expansion experiments
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Contents
I
Introduction
II
Laser Flash Method
III
IV
Preliminary Experiments
& Construction of Laser Flash Apparatus
Summary & Future Works
17
Summary & Future Works
 Summary
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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
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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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