tokai carbon co., ltd. global leader of carbon materials microstructural analysis of nuclear-grade...

TOKAI CARBON CO., LTD.Global Leader of Carbon Materials

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


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


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


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


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


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


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


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])


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


Result : pore distribution

Aspe

ct ra

tio /

-

Mea

n Po

re A

rea

/ μm

2


─ 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


~~

~~

overall meanmean ( >50μm2)

mean ( <50μm2)

Pre-irradiated data


Result : pore distributionRa

tio o

f Circ

ular

ity /

-

Frac

tal d

imen

sion

/ -

4πS/L2* 2Δ logL/ Δ logS*



*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


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


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)


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


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


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


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


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


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 .


Thank you for your attention

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