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

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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

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