Micro-mechanics on Nuclear Graphite Dr. Dong Liu EPSRC Postdoctoral - - PowerPoint PPT Presentation

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

• The material
• Microstructure over multiple length-scales
• Irradiation damage in nuclear graphite

Micro-mechanical testing over multiple length-scales

• Ex situ and in situ nano-indentation
• In situ micro-cantilever testing

Key messages

Nuclear Graphite

• 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.

• 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 temperature Elevated 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 correlated with irradiation and temperature

• Expansion of c-direction as a function
• f neutron flux at different temperature

(1Mwd/At = thermal energy output for

• ne tonne of nuclear fuel produced by

a 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 and

empirical values in the fractional change of the thermal resistance as a function of neutron dose. K0 and K are the thermal conductivity values before and after irradiation, respectively.

𝐿0 𝐿 -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.

𝑇 𝑇0 = ( 𝐹 𝐹0 )𝑙

Fast Neutron Irradiation Effect on Graphite Properties: modulus and strength

Micro-mechanical testing over multiple length-scales

200 µm Setup 1: Nano-indentation Nano Indenter G200 Ex situ test Setup 4: Micro-cantilever bending Triangular section, 100-200 µm in length In situ test

1 mm

200 µm Indente r Graphite cantilevers Setup 3: Micro-cantilever bending Rectangular section, 10-20 µm in length In situ test Micro-cantilever Loading probe 5 µm 10 µm Indenter Graphite surface Setup 2: Nano-indentation Nano Indenter inside a SEM In 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 (mN)

0 2 4 6 8

Displacement (µm)

0 2 4 6 8 10 0 1 2 3

Displacement (µm)

0 2 4 6 8 10

Load (mN)

0 0.2 0.4 0.6 0.8 0 2 4 6 8 10

Load (mN) 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 3.5 10 20 30 40 50

E

E (GPa) Section size (m)

E = 29 4.5 GPa 1.75x1.75x11.5µm 1.25x1.25x8.80µm 2.0x2.0x17.0µm 3.25x3.25x21µm

1 2 3 4 50 100 150

E= 40.5 GPa

Cantilever 3 Linea fit

Load (N) Displacement (m) Slope=32.990.21

Load (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 bending Triangular section, 100-200 µm in length In situ test

1 mm

200 µm Indenter Graphite cantilevers Micro-cantilever bending Rectangular 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 µm Cantilever 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

20 40 60 0.0 0.5 1.0 1.5 2.0 2.5 3.0 20 40 60 0.0 0.5 1.0 1.5 2.0 2.5 3.0 20 40 60 0.0 0.5 1.0 1.5 2.0 2.5 3.0 20 40 60 0.0 0.5 1.0 1.5 2.0 2.5 3.0 20 40 60 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Load (mN) Displacement (m)

cycle 1

Load (mN) Displacement (m)

cycle 2

Load (mN) Displacement (m)

cycle 5

Load (mN) Displacement (m)

cycle 4

Load (mN) 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, 2017 Liu et al. Nature Communications, 2017

Indentation modulus

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

Weight loss Diameter (mm) Length (mm) Mass (g) Neutron dose DIDO 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 µm 40 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 matrix

Liu 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 testing
• In situ testing in a Dualbeam chamber
• Un-irradiated specimen as reference
• Micro-Raman analysis
• 60 nm penetration depth
• 1.5 µm laser spot
• 488 nm wavelength

Microstructural Characterisation

5 µm FEG-SEM image of a FIB cross-section

10 µm

Trench created by FIB in the middle

• f HOPG sample:
• Focused ion beam cross-sectioning
• 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

1 2 3 5 10 15 20 25 30 35

Load (N) Displacement (m)

Cantilever 1 Linear fit

Load-displacement curve

250 nm Fractured surface

• 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! 𝑇 𝑇0 = ( 𝐹 𝐹0 )𝑙

• R. Price

k = 0.5 to 1 For HOPG, k=1

UKAEA data on near isotropic graphites irradiated 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

200 400 600 1000 2000 3000

Counts Wavenumber (cm-1) Crossed polarisation Parallel polarisation

Unirradiated HOPG Raman spectrum G

1000 2000 3000

Intensity (a.u.) Wavenumber (cm

• 1)

760C_6.7dpa 633C_4.19dpa

Irradiated HOPG G D

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

0.2 0.4 0.6 0.8 1 1.2 1574 1576 1578 1580 1582 1584 ID/IG 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