Micro-mechanics on Nuclear Graphite

Dr. Dong Liu

EPSRC Postdoctoral Research Fellow

1851 Exhibition Brunel Research Fellow

Junior Research Fellow, Mansfield College

Department of Materials, University of Oxford, U.K.

Research Affiliate

Lawrence Berkeley National Laboratory, U.S.A.

Outline

Background

o The material

o Microstructure over multiple length-scales

o Irradiation damage in nuclear graphite

Micro-mechanical testing over multiple length-scales

o Ex situ and in situ nano-indentation

o In situ micro-cantilever testing

Key messages

Nuclear Graphiteo Graphite has been widely used as a moderator, reflector and fuel matrix in various

types of nuclear reactors, such as gas-cooled reactor (e.g. AGR, MAGNOX), Russian

RBMK reactors, high temperature gas cooled reactor (Dragon, Peach Bottom, AVR,

THTR-300, Fort St. Vain, HTTR, HTR-10 ) etc.

o Gilsocarbon graphite is used as moderators and structural components in operating

Advanced Gas-cooled Reactors (AGRs) in the UK; Life-limiting as it is not replaceable.

Nuclear Graphite

Filler particle

Micro-scale Mrozowski cracks

100 µm

500 µm 500 µm

Filler

Binder

Threshold image

• Macro-scale • Micro- and Nano- scale

Background: Microstructure

Room temperatureElevated temperature (1000 - 1200°C)

Raman spectroscopy

Crystal bonding

Neutron diffraction

Lattice strain

X-ray tomography

Micro-scale deformation

Macro-scale deformation

Background: Multiple length-scale

• Micro-crack closure from expansion in the c-direction and• Dimensional change from irradiation induced creep

[Equivalent DIDO Nickel Dose]

Marsden et al, International Materials Reviews, 2016

Fast Neutron Irradiation Effect on Graphite Properties: Dimensional change

Nightingale et al

• Dimensional changes are correlatedwith irradiation and temperature

• Expansion of c-direction as a functionof neutron flux at different temperature(1Mwd/At = thermal energy output forone tonne of nuclear fuel produced bya flux of 3.5x1020 n.m-2 in the reactor,this corresponds to about 3.1x1023

displacement/m2)

Fast Neutron Irradiation Effect on Graphite Properties: Dimensional change

Kelly & Rappeneau et al.

Fast Neutron Irradiation Effect on Graphite Properties: Thermal conductivity

• Comparison between theoretical andempirical values in the fractionalchange of the thermal resistance as afunction of neutron dose. K0 and K arethe thermal conductivity values beforeand after irradiation, respectively.

𝐾0

𝐾-1• Fractional change:

S. Ishiyama et al. Journal of Nuclear Materials 230 (1996) 1-7

Fast Neutron Irradiation Effect on Graphite Properties: modulus and strength

UKAEA data on near isotropic graphites irradiated in the Dounreay Fast Reactor, R. Price.

𝑆

𝑆0= (

𝐸

𝐸0)𝑘

Fast Neutron Irradiation Effect on Graphite Properties: modulus and strength

Micro-mechanical testing over multiple length-scales

200 µm

Setup 1: Nano-indentationNano Indenter G200Ex situ test

Setup 4: Micro-cantilever bendingTriangular section, 100-200 µm in lengthIn situ test

1 mm200 µm

Indenter

Graphite cantilevers

Setup 3:Micro-cantilever bendingRectangular section, 10-20 µm in length In situ test

Micro-cantilever

Loading probe

5 µm

10 µm

Indenter Graphite surface

Setup 2: Nano-indentationNano Indenter inside a SEMIn situ test

Liu et al. Journal of Nuclear Materials, 2017

Nano-indentation (Ex situ)

• Load control• Displacement changes dramatically • Large scatter in the modulus measurements (similar as in hardness)• Which of these data can we trust?

Liu et al. Journal of Nuclear Materials, 2017

Nano-indentation (In situ)

10 µm 10 µm 10 µm

Indenter Sample surface

Sample surface

Sample surface

Indenter Indenter

Load

(m

N)

0 2 4 6 8Displacement (µm)

0

2

4

6

8

10

0 1 2 3Displacement (µm)

0

2

4

6

8

10

Load

(m

N)

0 0.2 0.4 0.6 0.8

0

2

4

6

8

1

0

Load

(m

N)

Displacement (µm)

Liu et al. Journal of Nuclear Materials, 2017

• Dualbeam workstation (FEI Helios NanoLab 600i Workstation)

• Force measurement system (FMS) (Kleindiek Nanotechnik)

• Workstation stage monitored and debris collected

• Calibrated against a spring standard and on glassy carbon

• Step I • Step II • Step III

In situ micro-cantilever bending

1.0 1.5 2.0 2.5 3.0 3.50

10

20

30

40

50 E

E (

GP

a)

Section size (m)

E = 29 4.5 GPa1.75x1.75x11.5µm

1.25x1.25x8.80µm

2.0x2.0x17.0µm

3.25x3.25x21µm

0 1 2 3 40

50

100

150

E= 40.5 GPa

Cantilever 3

Linea fit

Loa

d (

N)

Displacement (m)

Slope=32.990.21Lo

ad (

µN

)

Fracture

Displacement (µm)

Calibration: Glassy carbon

• Specimens prior to failure • Linear load-displacement relation • Small variation in E with sample size

• Repeatable modulus and flexural strength as measured at macro-scale

• Similar brittle fracture modes observed at micro-scale as in macro-size samples

Liu et al. Carbon, 2017

• Load-displacement curve for a cantilever with less surface defects showing the linear and non-linear stages prior to fracture;

• Cantilevers at this length-scale with varied surface defects that lead to scatter in the measured modulus and strength. Liu et al. Journal of Nuclear Materials, 2017

In situ micro-mechanical testing: small cantilevers

Micro-cantilever bendingTriangular section, 100-200 µm in lengthIn situ test

1 mm200 µm

IndenterGraphite

cantilevers

Micro-cantilever bendingRectangular section, 10-20 µm in length In situ test

Micro-cantilever

Loading probe

5 µm

Liu et al. Journal of Nuclear Materials, 2017

In situ micro-mechanical testing: small & large cantilevers

‘Small’ cantilever ‘Large’ cantilever

20 µmCantilever specimen

Loading probe

Loading arm length

20 µm

Fracture path

Cantilever root

10 µm 90°

Triangular cross-section

30 µm

Side surface of the triangle cantilever

xc

yc

b

h

0 20 40 600.0

0.5

1.0

1.5

2.0

2.5

3.0

0 20 40 600.0

0.5

1.0

1.5

2.0

2.5

3.0

0 20 40 600.0

0.5

1.0

1.5

2.0

2.5

3.0

0 20 40 600.0

0.5

1.0

1.5

2.0

2.5

3.0

0 20 40 600.0

0.5

1.0

1.5

2.0

2.5

3.0

Lo

ad

(m

N)

Displacement (m)

cycle 1

Lo

ad

(m

N)

Displacement (m)

cycle 2

Lo

ad

(m

N)

Displacement (m)

cycle 5

Lo

ad

(m

N)

Displacement (m)

cycle 4

Lo

ad

(m

N)

Displacement (m)

cycle 3

Linear

Non-linear

Post-peak progressive

failure

Liu et al. Journal of Nuclear Materials, 2017

In situ micro-mechanical testing: large cantilevers

Liu et al. Journal of Nuclear Materials, 2017Liu et al. Nature Communications, 2017

Indentation modulus

• The irradiated PGA graphite (6018-12/3/3)

Weightloss

Diameter(mm)

Length(mm)

Mass(g)

Neutron doseDIDO equiv. (n·cm-2)

Temp.(K)

15% 12.1 7.1 1.01 33.2 × 1020 560

Radiolytically-oxidised (CO2 environment) PGA graphite samples from a Magnox reactor supplied by Magnox Ltd.

10 µm40 µm

Filler particle

Matrix

Liu et al. Carbon, 2017

In situ micro-mechanical testing: irradiated PGA

• Irradiated filler particle• E = 40 to 86 GPa• σf = 600 to 1300 MPa

• Irradiated matrix• E ≤ 10 GPa• σf ≤ 500 MPa

• Unirradiated PGA graphite

• E = 10 to 20 GPa• σf = 200 to 500 MPa

Irradiated filler particle

Irradiated matrixLiu et al. Carbon, 2017

HOPG

Highly Oriented Pyrolytic Graphite

http://nanoprobes.aist-nt.com/apps/HOPGinfo.htm

An angular spread of the c-axes of the crystallites is of the order of 1 degree

Samples Temp (celcius) dpa

HOPG1 633 4.19

HOPG2 760 6.71

Irradiated HOPG

• Micro-mechanical testingo In situ testing in a Dualbeam chambero Un-irradiated specimen as reference

• Micro-Raman analysis

o 60 nm penetration deptho 1.5 µm laser spoto 488 nm wavelength

Microstructural Characterisation

5 µm

FEG-SEM image of a FIB cross-section

10 µm

Trench created by FIB in the middle of HOPG sample:

o Focused ion beam cross-sectioning o The material is free of large pores

SEM sample holder

Orientation of the basal plane to the loading direction

Orientation of the basal plane to the loading direction

0 1 2 30

5

10

15

20

25

30

35

Load (

N)

Displacement (m)

Cantilever 1

Linear fit

565057005750580058505900595060006050610061506200625063006350640064506500

0.0

-0.1

-0.2

-0.3

-0.4

-0.5

-0.6

-0.7

-0.8

C

A

raw U/V

Load-displacement curve

250 nm

Fractured surface

o Modulus and flexural strength can be measured.

Fractured at root

Twinning

Elastic

Plasticity?

• Modulus/strength increased by a factor of about 2 after irradiation at 760°C for 6.71 dpa!

𝑆

𝑆0= (

𝐸

𝐸0)𝑘• R. Price

k = 0.5 to 1For HOPG, k=1

UKAEA data on near isotropic graphitesirradiated in the Dounreay Fast Reactor

J. Hinks et al IOP Journal of Physics Conference Series, 2012 Wen et al, Journal of Nuclear Materials, 2008

• Densification

• Transformation from crystallite material to other

0

200

400

600

0 1000 2000 3000

Co

un

ts

Wavenumber (cm-1)

Crossed polarisation Parallel polarisation

Unirradiated HOPGRaman spectrum

G

0 1000 2000 3000

Inte

nsity (

a.u

.)

Wavenumber (cm-1)

760C_6.7dpa

633C_4.19dpa

Irradiated HOPG

GD

Ferrari AC, Robertson J. Interpretation of Raman spectra of disordered and amorphous carbon. Phys Rev B, 2000

0

0.2

0.4

0.6

0.8

1

1.2

1574 1576 1578 1580 1582 1584

ID/I

G

G peak position

633C_4.19dpa

760_6.7dpa

unirradiated

• Transformation from crystallite material to other

HOPG2: 633°C for 4.19 dpa

Each image is about 60 µm wide

Key Messages

• Micro-scale testing can potentially describe the mechanical properties in graphite over

a rang of length-scales

• The filler particles and binder matrix react differently to neutron irradiation

• Elastic modulus and flexural strength in HOPG doubled at 760C 6.71 dpa

• These approaches could well be applied to target graphite materials

Acknowledgements

D.L. acknowledges the EPSRC fellowship grant: EP/N004493/1

D.L. acknowledges the Royal Commission for the Exhibition of 1851 Research Fellowship

Collaborators:

Idaho National Laboratory, USA

Dr. Joshua Kane, Dr. William Windes

Centre for Device Thermography and Reliability, Bristol, UK

Prof. Martin Kuball, Dr. James Pomeroy

National Physical Laboratory, UK

Dr. Ken Mingard, Dr. Mark Gee