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TOKAI CARBON CO., LTD. Global Leader of Carbon Materials Microstructural Analysis of Nuclear-Grade Graphite Materials after Neutron Irradiation K.Takizawa.

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Presentation on theme: "TOKAI CARBON CO., LTD. Global Leader of Carbon Materials Microstructural Analysis of Nuclear-Grade Graphite Materials after Neutron Irradiation K.Takizawa."— Presentation transcript:

1 TOKAI CARBON CO., LTD. Global Leader of Carbon Materials Microstructural Analysis of Nuclear-Grade Graphite Materials after Neutron Irradiation K.Takizawa 1, A.Kondo 1, Y.Katoh 2, G.E.Jellison 2, A.A.Cambell 2, 1: Tokai Carbon Co. LTD., Japan 2: Oak Ridge National Laboratory, USA

2 TOKAI CARBON CO., LTD. Global Leader of Carbon Materials Objective c-axis (swelling) a-axis(shrink) These modifications are resulting from the irradiation-induced microstructural changes in graphite Fast Neutron Fluence Relative Volume Change (%) 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

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

4 TOKAI CARBON CO., LTD. Global Leader of Carbon Materials Microstructural Analysis ~ Schema to Goal Filler cokes Carbonized binder Pores(Cracks) The microstructure of graphite Crack Pore —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

5 TOKAI CARBON CO., LTD. Global Leader of Carbon Materials 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 DYMWG/AG = 0.98 Irrad. Strain-Significant WG : with gravity, AG : across gravity shown only in dimensional changes more pronounced at higher fluence

6 TOKAI CARBON CO., LTD. Global Leader of Carbon Materials Material Information * RT~100 ℃ ** RT~1000 ℃ Nuclear grade graphite G347A and G458A manufactured by Tokai Carbon are used for microstructural analysis G347A G458A 100μm Grade Bulk density (g/cm 3 ) Electrical resistivity (µΩ ̜ m) Flexural strength (Mpa) Coefficient of thermal expansion (x10 -6 / ℃ ) Thermal conductivity (W/m ・ k) Young’s modulus (GPa) G347A * (5.5**) G458A * (4.4**) * RT~100 ℃ ** RT~1000 ℃ Reference value This study G347A and G458A are both fine-grained, high strength, isotropic graphite

7 TOKAI CARBON CO., LTD. Global Leader of Carbon Materials Pore distribution measurement Composite microstructural image(56 images) 1.1mm 1.3mm Minimum evaluation area is 1.6μm 2 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 Pixels The changes of pore size and distribution is evaluated by optical image analysis In this study pores less than 5μm 2 were disregarded to remove any error by dot noises in the microscope

8 TOKAI CARBON CO., LTD. Global Leader of Carbon Materials Result : pore distribution Total pore number / (mm 2 ) -1 G347A Error bar = ±1σ Total pore number / (mm 2 ) -1 Error bar = ±1σ filled markers 50μm 2 Change of the total number showed quadratic-like curve, similar to the dimensional changes ─ Change of the small pores (<50μm 2 ) is the dominant behavior ─ Extinction and generation of pores reaches equilibrium around 15x10 25 n/m 2 Neutron Fluence (x10 25 n/m 2 [E>0.1MeV])

9 TOKAI CARBON CO., LTD. Global Leader of Carbon Materials Result : pore distribution Total pore area / μm 2 (mm 2 ) -1 Error bar = ±1σ filled markers 50μm 2 Large variation observed in pore area could be a result of a small measurement area ─ Change of the large pores (area > 50μm 2 ) are the dominant trend Initially a rapid decrease of total pore area, but little change after that with increasing fluence Neutron Fluence (x10 25 n/m 2 [E>0.1MeV]) Total pore area / μm 2 (mm 2 ) -1

10 TOKAI CARBON CO., LTD. Global Leader of Carbon Materials Result : pore distribution Aspect ratio / - Mean Pore Area / μm 2 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 (x10 25 n/m 2 [E>0.1MeV]) ~ ~ ~ ~ overall mean mean ( >50μm 2 ) mean ( <50μm 2 ) Pre-irradiated data

11 TOKAI CARBON CO., LTD. Global Leader of Carbon Materials Result : pore distribution Ratio of Circularity / - Fractal dimension / - 4πS/L 2 * 2Δ logL/ Δ logS* Neutron Fluence (x10 25 n/m 2 [E>0.1MeV]) Error bar = ±1σ *L = pore perimeter, S = pore area Each graph showed quadratic/cubic -like changes as a function of neutron fluence respectively ~ ~ overall mean mean ( >50μm 2 ) mean ( <50μm 2 ) 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

12 TOKAI CARBON CO., LTD. Global Leader of Carbon Materials 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 15x10 25 n/m 2 which was close to the mean turn around of G347A (300~900 º C) 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? extinctiongenerationequilibrium

13 TOKAI CARBON CO., LTD. Global Leader of Carbon Materials 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. I Y0 = sin(2 γ ) N I Y1 = -con(2 γ ) N I Y0, I Y1 = coefficients of basis function, γ = principal axis, N = diattenuation Time-dependent intensity can be expressed as Intensity(t) = I dc + I X0 X0 + I Y0 Y0 + I X1 X1 + I Y1 Y1 + I X0X1 X0X1 + I X0Y1 X0Y1 + I Y0X1 Y0X1 + I Y0Y1 Y0Y1 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)

14 TOKAI CARBON CO., LTD. Global Leader of Carbon Materials Orientation measurement 2-MGEM Sample γ histogram conversion Graphite orientation was estimated by the value of amplitude for simple evaluation Fast axis angle Counts Count ( nor. ) = (1-a)sin n (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 4mm 2 Pixel size : 25 μm 2 Sample Pixels : ~ Wavelength : 577nm

15 TOKAI CARBON CO., LTD. Global Leader of Carbon Materials Specimen surface I D /I G G-FWHM / cm- 1 G-RS / cm -1 Block (0.98)* Powder (0.48)* Polished (0.22)* *() 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), Measurement Grating : 1800gr/mm Range : 1100 – 1800cm -1 Point : 5 each sample Exposure : 5s x 3 Irradiation area : ☐ 10 μm 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

16 TOKAI CARBON CO., LTD. Global Leader of Carbon Materials Result : Comparison with XRD method Grade Amplitude XRD2-MGEM* G347A Graphite A 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 2-MGEM method showed the same trend as XRD method indicating its availability for orientation evaluation I (φ), norm. φ (deg.) Counter (fixed) X-ray Specimen(rotating) θ 002 z x φ MGEM : y-z plane was measured gravity

17 TOKAI CARBON CO., LTD. Global Leader of Carbon Materials Result : orientation Neutron Fluence (x10 25 n/m 2 [E>0.1MeV]) Dimensional change / % Measurement specimen Amplitude / - Irradiated graphite (G347A, 750 º C) 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

18 TOKAI CARBON CO., LTD. Global Leader of Carbon Materials 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 gravity Specimen used in this study had sufficient and appropriate surface for optical analysis such as 2-MGEM

19 TOKAI CARBON CO., LTD. Global Leader of Carbon Materials Summary Focused on the changes of pore distribution/ cokes orientation during irradiation  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.

20 TOKAI CARBON CO., LTD. Global Leader of Carbon Materials Thank you for your attention


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