1. 1: Tokai Carbon Co. LTD., Japan 2: Oak Ridge National Laboratory, USA
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

2. Objective
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
Graphite materials in nuclear services undergo very significant modifications in various thermal and mechanical properties
These modifications are resulting from the irradiation-induced microstructural changes in graphite
c-axis
(swelling)
a-axis(shrink)
shrink in bulk
fatal cracks caused
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...
Relative Volume Change (%)
To characterize the microstructural changes of isotropic fine-grained graphite following the neutron irradiation to help develop graphite for improved radiation service life
Fast Neutron Fluence

3. Microstructural Analysis ~ Matrix
The microstructure of graphite
Constituents
Volume
Size
Distribution
Shape
Anisotropy
Degree of
Graphitization
Filler
2-MGEM -
TEM
RAMAN
Binder
Pore OM MP
Crack
SEM, TEM
Whole
XRD, ER
CTE
XRD
Filler cokes
Carbonized binder
Pores(Cracks)
Crack
Pore
Characteristic microstructures of graphite are determined by focusing on the attributes of these constituents.
2-MGEM : two-modulator generalized ellipsometry microscope
OM : Optical Microscope
MP : Mercury Porosimetry
ER : Electrical Resistivity

4. Microstructural Analysis ~ Schema to Goal
reported at INGSM 13
Microstructural analysis of pre-irradiated graphite
The microstructure of graphite
Changes during manufacturing process
Status of as-manufactured graphite
Filler cokes
Carbonized binder
Pores(Cracks)
Graphite Specification
Our goal
Crack
Pore
Identify each nuclear-grade graphite based on microstructural properties so that we can predict its behavior after irradiation
Raman microscopic identification
structural factor
shape factor
Correlate with the macroscopic properties
Microstructural analysis for post-irradiated graphite
this presentation
Changes during neutron irradiation

5. Microstructural Analysis ~ focal point
Place the focus on 2 constituents
Pores (distribution changes)
Considered to be the most important factor directly related to the dimensional changes on each graphite grade
(i.e. life time)
Cokes (orientation changes)
Found that even isotropic graphite show definite anisotropic dimensional changes
This may also be a great influence on the dimensional changes.
shown only in dimensional changes
more pronounced at higher fluence
Unirr.
Irrad.
CTE (RT-500C)
WG/AG = 1.04
DYM
WG/AG = 0.98
Irrad. Strain -
Significant
WG : with gravity, AG : across gravity

6. Material Information
Nuclear grade graphite G347A and G458A manufactured by Tokai Carbon are used for microstructural analysis
G347A and G458A are both fine-grained, high strength, isotropic graphite
G347A
G458A
100μm
Reference value
Grade
Bulk 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
This study
* RT~100℃
** RT~1000℃
* RT~100℃
** RT~1000℃

7. Pore distribution measurement
The changes of pore size and distribution is evaluated by optical image analysis
1.1mm
1.3mm
8 bit gray scale composite image is changed into a set of 256 colored histogram
Composite microstructural image(56 images)
Minimum evaluation area is 1.6μm2
Pixels
As existing pores do not reflect visible light, the dark region is recognized as pores
⇒ Pores are automatically identified and calculated
In this study pores less than 5μm2 were disregarded to remove any error by dot noises in the microscope
Gray Scale Level

8. Result : pore distribution
filled markers < 50μm2, open markers > 50μm2
G347A
Total pore number / (mm2) -1
Total pore number / (mm2) -1
Error bar = ±1σ
Error bar = ±1σ
Neutron Fluence (x1025 n/m2 [E>0.1MeV])
Neutron Fluence (x1025 n/m2 [E>0.1MeV])
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

9. Result : pore distribution
filled markers < 50μm2, open markers > 50μm2
Total pore area / μm2 (mm2) -1
Total pore area / μm2 (mm2) -1
Error bar = ±1σ
Error bar = ±1σ
Neutron Fluence (x1025 n/m2 [E>0.1MeV])
Neutron Fluence (x1025 n/m2 [E>0.1MeV])
Initially a rapid decrease of total pore area, but little change after that with increasing fluence
─ Change of the large pores (area > 50μm2) are the dominant trend
Large variation observed in pore area could be a result of a small measurement area

10. Result : pore distribution
Pre-irradiated data
overall mean
mean ( >50μm2)
mean ( <50μm2) ~
Aspect ratio / -
Mean Pore Area / μm2
Error bar = ±1σ ~
Error bar = ±1σ
Neutron Fluence (x1025 n/m2 [E>0.1MeV])
Neutron Fluence (x1025 n/m2 [E>0.1MeV])
Each graph showed cubic/quadratic -like changes as a function of neutron fluence respectively
─ 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

11. Result : pore distribution
Unirradiated data
overall mean
mean ( >50μm2)
mean ( <50μm2)
Ratio of Circularity / -
Fractal dimension / -
4πS/L2*
2Δ logL/ Δ logS* ~
Error bar = ±1σ
Error bar = ±1σ
Neutron Fluence (x1025 n/m2 [E>0.1MeV])
Neutron Fluence (x1025 n/m2 [E>0.1MeV])
These parameters are known to have the correlation with strength and fracture toughness
Each graph showed quadratic/cubic -like changes as a function of neutron fluence respectively
─ Curve shape depends on the mean value of large pores against overall mean
*L = pore perimeter, S = pore area

12. Conclusion : pore distribution
Characteristic changes shown during irradiation are mostly explained in the following model
extinction
equilibrium
generation
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?

13. Orientation measurement
Time-dependent intensity can be expressed as
Intensity(t) = Idc + IX0X0 + IY0Y0 + IX1X1 + IY1Y1 + IX0X1X0X1
+ IX0Y1X0Y1 + IY0X1Y0X1 + IY0Y1Y0Y1
Two of eight coefficients of the basis functions are used to investigate graphite orientation
i.e. IY0 = sin(2γ)N
IY1 = -con(2γ)N
tan(2γ) = - (IY0 / IY1)
N= I Y02 + I Y12
Sample
IY0, IY1 = coefficients of basis function,
γ = principal axis, N = diattenuation
2-MGEM
(2- Modulator Generalized Ellipsometry Microscope)
Preferential orientation of the principal axis in graphite is perpendicular to the c-axis (parallel to the graphite plane)
2-MGEM is configured as a reflection microscope at near-normal incidence and measures eight different parameters*
*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)

14. Orientation measurement
Principal axis angle collections are converted into histogram and normalized for calculation
histogram conversion γ
1-a
(amplitude)
Counts
Sample
Fast axis angle
2-MGEM
Each histogram was fitted with sign curve;
Count（nor.） = (1-a)sinn (2(x-b))+a
Measurement area : total 4mm2
Pixel size : 25 μm2
Sample Pixels : ~160000
Wavelength : 577nm
Graphite orientation was estimated by the value of amplitude for simple evaluation

15. Specimen surface Specimen surface is vital for optical measurement
normalization
G-band
All specimen used in this study encased in resin and polished.
D-band
*() standard deviation
G-FWHM
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
G-RS
Equipment : HR-800 Laser : Ar+ (514.5nm),
Measurement Grating : 1800gr/mm Range : 1100 – 1800cm-1
Point : 5 each sample Exposure : 5s x 3
Irradiation area : ☐10μm x 10
Polished surface is more appropriate (less damaged) than the other surface
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
- Exposed edges on surface are vital for 2-MGEM measurement

16. Result : Comparison with XRD method
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 z x y
Φ1mm x 8 mm
Counter
(fixed)
X-ray
Specimen(rotating)
θ002 z x φ
1.0
0.8
0.6
0.4
0.2
0.0 20 40 60 80
100
120
140
160
I (φ), norm.
gravity
φ (deg.)
Grade
Amplitude
XRD
2-MGEM*
G347A
0.18
0.33
Graphite A
0.24
0.54
2-MGEM : y-z plane was measured
2-MGEM method showed the same trend as XRD method indicating its availability for orientation evaluation

17. Result : orientation
Irradiated graphite (G347A, 750ºC) were evaluated by using 2-MGEM
Dimensional change / %
Amplitude / -
Measurement specimen
Neutron Fluence (x1025 n/m2 [E>0.1MeV])
Neutron Fluence (x1025 n/m2 [E>0.1MeV])
Increment of amplitude was observed with increasing neutron fluence
─ Internal orientation became more anisotropic after irradiation
─ This tendency well agree with the anisotropic dimensional changes

18. Conclusion : orientation
Specimen used in this study had sufficient and appropriate surface for optical analysis such as 2-MGEM
Preferential orientation z x y
with gravity
2-MGEM is of use in evaluating graphite orientation having the advantage in terms of sensitivity to preferential orientation compared with the XRD method
G347A showed more anisotropic properties after irradiation agreeing with anisotropic dimensional changes
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?
─ In our recent study with unirradiated graphite, BAFs obtained by XRD showed a
different trend than that of mechanical/thermal properties

19. Summary
Focused on the changes of pore distribution/ cokes orientation during irradiation
Pore changes observed as a function of neutron fluence showed similar trends as the dimensional changes
Future work for detailed analysis ;
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?
Future work for detailed analysis ;
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 .

20. Thank you for your attention