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Microstructural Analysis of Nuclear-Grade Graphite Materials after
Neutron Irradiation
K.Takizawa1, A.Kondo1, Y.Katoh2, G.E.Jellison2, A.A.Cambell2,1: Tokai Carbon Co. LTD., Japan
2: Oak Ridge National Laboratory, USA
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Objective
c-axis(swelling)
a-axis(shrink)
These modifications are resulting from the irradiation-induced microstructural changes in graphite
Fast Neutron Fluence
Rela
tive
Volu
me
Chan
ge (%
) It is essential to characterize the microstructures before and after irradiation and correlate the structural changes with the evolving macroscopic properties
Objective of this work is...0
shrink in bulk fatal cracks caused
To characterize the microstructural changes of isotropic fine-grained graphite following the neutron irradiation to help develop graphite for improved radiation service life
Graphite materials in nuclear services undergo very significant modifications in various thermal and mechanical properties
Graphite is used as the core structural material of a Very High Temperature gas-cooled Reactor which is one of the candidate next generation reactors
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Microstructural Analysis ~ Matrix
Characteristic microstructures of graphite are determined by focusing on the attributes of these constituents.
Constituents Volume SizeDistribution Shape Anisotropy Degree of
Graphitization
Filler 2-MGEM - - 2-MGEM TEMRAMAN
Binder 2-MGEM - - 2-MGEM TEMRAMAN
Pore OMMP
OMMP OM - -
Crack OMSEM, TEM
OMSEM, TEM
OMSEM, TEM
- -
Whole - - - XRD, ERCTE
XRDRAMAN
2-MGEM : two-modulator generalized ellipsometry microscopeOM : Optical MicroscopeMP : Mercury PorosimetryER : Electrical Resistivity
Filler cokes Carbonized binder Pores(Cracks)
The microstructure of graphite
CrackPore
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Microstructural Analysis ~ Schema to Goal
Filler cokes Carbonized binder Pores(Cracks)
The microstructure of graphite
CrackPore
— Changes during manufacturing process— Status of as-manufactured graphite
Graphite Specification
Identify each nuclear-grade graphite based on microstructural properties so that we can predict its behavior after irradiation
reported at INGSM 13
Raman microscopic identification structural factor shape factor
Correlate with the macroscopic properties
Microstructural analysis for post-irradiated graphite
this presentation
— Changes during neutron irradiation
Our goal
Microstructural analysis of pre-irradiated graphite
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Microstructural Analysis ~ focal point
Pores (distribution changes)
Cokes (orientation changes)
Considered to be the most important factor directly related to the dimensional changes on each graphite grade (i.e. life time)
Place the focus on 2 constituents
Found that even isotropic graphite show definite anisotropic dimensional changes
This may also be a great influence on the dimensional changes.
Unirr. Irrad.
CTE (RT-500C) WG/AG = 1.04 WG/AG = 1.04
DYM WG/AG = 0.98 WG/AG = 0.98
Irrad. Strain - Significant
WG : with gravity, AG : across gravity
• shown only in dimensional changes• more pronounced at higher fluence
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Material Information
* RT~100℃** RT~1000℃
Nuclear grade graphite G347A and G458A manufactured by Tokai Carbon are used for microstructural analysis
G347A G458A
100μm 100μm
GradeBulk
density(g/cm3)
Electrical resistivity(µΩ̜+•m)
Flexural strength(Mpa)
Coefficient of thermal expansion
(x10-6/℃)
Thermal conductivity(W/m ・ k)
Young’s modulus
(GPa)
G347A 1.85 11.0 49.0 4.2* (5.5**) 116 10.8
G458A 1.86 9.5 53.9 3.1* (4.4**) 139 11.3
* RT~100℃** RT~1000℃
Reference value
This study
G347A and G458A are both fine-grained, high strength, isotropic graphite
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Pore distribution measurement
Composite microstructural image(56 images)
1.1mm
1.3mm
Minimum evaluation area is 1.6μm2
As existing pores do not reflect visible light, the dark region is recognized as pores ⇒ Pores are automatically identified and calculated
8 bit gray scale composite image is changed into a set of 256 colored histogram
Gray Scale Level
Pixe
ls
The changes of pore size and distribution is evaluated by optical image analysis
In this study pores less than 5μm2 were disregarded to remove any error by dot noises in the microscope
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Result : pore distributionTo
tal p
ore
num
ber /
(mm
2 ) -1
G347A
Error bar = ±1σ
Tota
l por
e nu
mbe
r / (m
m2 )
-1
Error bar = ±1σ
filled markers < 50μm2, open markers > 50μm2
Change of the total number showed quadratic-like curve, similar to the dimensional changes
─ Change of the small pores (<50μm2) is the dominant behavior─ Extinction and generation of pores reaches equilibrium around 15x1025 n/m2
Neutron Fluence (x1025 n/m2 [E>0.1MeV]) Neutron Fluence (x1025 n/m2 [E>0.1MeV])
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Result : pore distributionTo
tal p
ore
area
/ μm
2 (m
m2 )
-1
Error bar = ±1σ Error bar = ±1σ
filled markers < 50μm2, open markers > 50μm2
Large variation observed in pore area could be a result of a small measurement area─ Change of the large pores (area > 50μm2) are the dominant trend
Initially a rapid decrease of total pore area, but little change after that with increasing fluence
Neutron Fluence (x1025 n/m2 [E>0.1MeV]) Neutron Fluence (x1025 n/m2 [E>0.1MeV])To
tal p
ore
area
/ μm
2 (m
m2 )
-1
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Result : pore distribution
Aspe
ct ra
tio /
-
Mea
n Po
re A
rea
/ μm
2
Error bar = ±1σ Error bar = ±1σ
─ The influence of large pores became bigger with extinction of small pores
Each graph showed cubic/quadratic -like changes as a function of neutron fluence respectively
─ Curve shape depends on the mean value of large pores against overall mean
Neutron Fluence (x1025 n/m2 [E>0.1MeV]) Neutron Fluence (x1025 n/m2 [E>0.1MeV])
~~
~~
overall meanmean ( >50μm2)
mean ( <50μm2)
Pre-irradiated data
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Result : pore distributionRa
tio o
f Circ
ular
ity /
-
Frac
tal d
imen
sion
/ -
4πS/L2* 2Δ logL/ Δ logS*
Neutron Fluence (x1025 n/m2 [E>0.1MeV]) Neutron Fluence (x1025 n/m2 [E>0.1MeV])
Error bar = ±1σ Error bar = ±1σ
*L = pore perimeter, S = pore area
Each graph showed quadratic/cubic -like changes as a function of neutron fluence respectively
~~
overall meanmean ( >50μm2)
mean ( <50μm2)
These parameters are known to have the correlation with strength and fracture toughness
─ Curve shape depends on the mean value of large pores against overall mean
Unirradiated data
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Conclusion : pore distribution Characteristic changes shown during irradiation are mostly explained in the following model
• Distributional changes during irradiation were closely related to the change of small pores
─ Since the influence of large pores became bigger with extinction of small pores, curve shape depends on the mean value of large pores against overall mean
─ Both the vertexes of quadratic-like curve and the inflection points of cubic-like curve appeared around 15x1025 n/m2 which was close to the mean turn around of G347A (300~900ºC)
• Each result DID NOT show the evident temperature dependence
─ Undetectable submicron sized pores, such as Mrozowski cracks, have a temperature dependence and a great influence on dimensional changes?
extinction generationequilibrium
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Orientation measurement
2-MGEM
2-MGEM is configured as a reflection microscope at near-normal incidence and measures eight different parameters*
Two of eight coefficients of the basis functions are used to investigate graphite orientation
Sample
i.e. IY0 = sin(2γ)N
IY1 = -con(2γ)N
tan(2γ) = - (IY0 / IY1)
IY0, IY1 = coefficients of basis function,γ = principal axis, N = diattenuation
Time-dependent intensity can be expressed as
Intensity(t) = Idc + IX0X0 + IY0Y0 + IX1X1 + IY1Y1 + IX0X1X0X1
+ IX0Y1X0Y1 + IY0X1Y0X1 + IY0Y1Y0Y1
Preferential orientation of the principal axis in graphite is perpendicular to the c-axis (parallel to the graphite plane)
*G. E. Jellison, Jr. and J. D. Hunn, “Optical anisotropy measurements of TRISO nuclear fuel particle cross-sections: The method” J. Nuclear Mat. 372, 36-44, (2008)
(2- Modulator Generalized Ellipsometry Microscope)
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Orientation measurement
2-MGEM
Sample
γhistogram conversion
Graphite orientation was estimated by the value of amplitude for simple evaluation
Fast axis angle
Coun
ts
Count ( nor. ) = (1-a)sinn (2(x-b))+a
Principal axis angle collections are converted into histogram and normalized for calculation
1-a(amplitude)
Each histogram was fitted with sign curve;
Measurement area : total 4mm2
Pixel size : 25 μm2
Sample Pixels : ~160000Wavelength : 577nm
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Specimen surface
ID/IG G-FWHM / cm-1 G-RS / cm-1
Block 0.320 22.92 (0.98)* 1580.6
Powder 0.217 21.67 (0.48)* 1581.3
Polished 0.609 21.17 (0.22)* 1582.9
*() standard deviation
The lowest value of G-FWHM attributed to the highest graphitized structure- Polishing effect was even lower compared with binder effect on powder
The highest value of G-RS indicates a possibility of having the highest graphitization (T.B.D)- Need to use Ne-bright line for calibration to obtain an accurate value of Raman Shift
The highest value of Raman R comes from not structural defect but edge exposed on the surface
Equipment : HR-800 Laser : Ar+ (514.5nm), 0.7mw@sampleMeasurement Grating : 1800gr/mm Range : 1100 – 1800cm-1
Point : 5 each sample Exposure : 5s x 3 Irradiation area : ☐10μm x 10
normalization
All specimen used in this study encased in resin and polished.
Specimen surface is vital for optical measurement
Polished surface is more appropriate (less damaged) than the other surface
- Exposed edges on surface are vital for 2-MGEM measurement
G-band
G-FWHM
D-band
G-RS
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Result : Comparison with XRD method
GradeAmplitudeXRD 2-MGEM*
G347A 0.18 0.33Graphite A 0.24 0.54
Orientation function I (φ) based on the diffraction curves of (002) were calculated by using X-ray diffractometer to compare with the results obtained by 2-MGEM measurement
The diffraction pattern of (002) for two characteristic graphite (G347A, Graphite A) were obtained as a function of rotating angle of these specimen
zx
y
Φ1mm x 8 mm
2-MGEM method showed the same trend as XRD method indicating its availability for orientation evaluation
I (φ
), no
rm.
φ (deg.)
Counter(fixed)
X-ray
Specimen(rotating)
θ002
θ002
zx
θ002
φ
1.0
0.8
0.6
0.4
0.2
0.020 40 60 80 100 120 140 160
2-MGEM : y-z plane was measured
gravity
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Result : orientation
Neutron Fluence (x1025 n/m2 [E>0.1MeV])Neutron Fluence (x1025 n/m2 [E>0.1MeV])
Dim
ensi
onal
cha
nge
/ %
Measurement specimen Ampl
itude
/ -
Irradiated graphite (G347A, 750ºC) were evaluated by using 2-MGEM
Increment of amplitude was observed with increasing neutron fluence
─ This tendency well agree with the anisotropic dimensional changes─ Internal orientation became more anisotropic after irradiation
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Conclusion : orientation
─ In our recent study with unirradiated graphite, BAFs obtained by XRD showed a different trend than that of mechanical/thermal properties
• G347A showed more anisotropic properties after irradiation agreeing with anisotropic dimensional changes
• 2-MGEM is of use in evaluating graphite orientation having the advantage in terms of sensitivity to preferential orientation compared with the XRD method
• 2-MGEM measurement indicated the possibility that rearrangement of cokes particles occurred by shrinkage
It is still unclear…
• Mechanical/thermal measurements showed isotropic properties even after irradiation
─ Crystallographic orientation is not vital for mechanical/thermal anisotropic properties?
Preferential orientation
z
x
y
with
gra
vity
• Specimen used in this study had sufficient and appropriate surface for optical analysis such as 2-MGEM
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Summary
Focused on the changes of pore distribution/ cokes orientation during irradiation
Future work for detailed analysis ;
Future work for detailed analysis ;
Pore changes observed as a function of neutron fluence showed similar trends as the dimensional changes
MicroTomography (XCT) measurements are underway that show similar trends as the optical image analysis. To investigate the thermal sensitive element with statistical volume, however, we have to come up with another methodology because XCT does not have a sufficiently high resolution to capture the submicron sized features.
Shrinkage during irradiation lead to rearrangement of internal crystals strengthening the originally existing preferential orientation?
2-MGEM work with higher resolution is now preparing to evaluate more in detail. Further supplemental investigations are also being done to confirm this result and method in detail. XRD measurement to obtain pole figures is currently underway .
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