Microstructural Analysis of Nuclear-Grade Graphite Materials after Neutron Irradiation

1. 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 c-axis (swelling) 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 a-axis(shrink) fatal cracks caused shrink in bulk 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 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 • 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) Our goal Graphite Specification Identify each nuclear-grade graphite based on microstructural properties so that we can predict its behavior after irradiation Crack Pore Correlate with the macroscopic properties • structural factor • shape factor Raman microscopic identification this presentation Microstructural analysis for post-irradiated graphite • 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. • shownonlyindimensional changes • more pronounced at higher fluence 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 G458A G347A 100μm 100μm Reference value 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 1.3mm 8 bit gray scale composite image is changed into a set of 256 colored histogram Minimum evaluation area is 1.6μm2 Composite microstructural image(56 images) As existing pores do not reflect visible light, the dark region is recognized as pores ⇒ Pores are automatically identified and calculated Pixels 1.1mm 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 mean ( >50μm2) overall mean 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 mean ( >50μm2) overall mean mean ( <50μm2) Ratio of Circularity / - Fractal dimension / - 2ΔlogL/ Δ logS* 4πS/L2* 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 tan(2γ) = - (IY0 / IY1) i.e. IY0 = sin(2γ)N IY1 = -con(2γ)N Sample IY0,IY1 = coefficients of basis function, γ= principal axis, N = diattenuation 2-MGEM Preferential orientation of the principal axis in graphite is perpendicular to the c-axis (parallel to the graphite plane) (2- Modulator Generalized EllipsometryMicroscope) 2-MGEM is configured as a reflection microscope at near-normal incidenceand 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 G-RS Equipment : HR-800 Laser : Ar+ (514.5nm), 0.7mw@sample Measurement Grating : 1800gr/mm Range : 1100 – 1800cm-1 Irradiation area : ☐10μm x 10 Point : 5 each sample Exposure : 5s x 3 Polished surface ismore 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 Counter (fixed) 1.0 z 0.8 φ x θ002 I (φ), norm. gravity 0.6 X-ray θ002 0.4 θ002 0.2 Specimen(rotating) 0.0 φ(deg.) 20 40 60 80 100 120 140 160 y z 2-MGEM : y-z plane was measured x 2-MGEM method showed the same trend as XRD method indicating its availability for orientation evaluation Φ1mm x 8 mm

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 • 2-MGEM is of use in evaluating graphite orientation having the advantage in terms of sensitivity to preferential orientation compared with the XRD method with gravity z • G347A showed more anisotropic properties after irradiation agreeing with anisotropic dimensional changes y x • 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 withunirradiated 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 resolutionis 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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