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 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 Graphite o 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

Background: Microstructure • • Micro- and Nano- scale Macro-scale Filler particle Threshold image Filler 100 µm Micro-scale Mrozowski cracks Binder 500 µm 500 µm

Background: Multiple length-scale Macro-scale deformation X-ray tomography Micro-scale deformation Neutron diffraction Lattice strain Raman spectroscopy Crystal bonding Elevated temperature (1000 - 1200 ° C) Room temperature

Fast Neutron Irradiation Effect on Graphite Properties: Dimensional change [Equivalent DIDO Nickel Dose] • Micro-crack closure from expansion in the c-direction and • Dimensional change from irradiation induced creep Marsden et al , International Materials Reviews, 2016

Fast Neutron Irradiation Effect on Graphite Properties: Dimensional change • Dimensional changes are correlated with irradiation and temperature • Expansion of c-direction as a function of neutron flux at different temperature (1Mwd/At = thermal energy output for one tonne of nuclear fuel produced by a flux of 3.5x10 20 n.m -2 in the reactor, this corresponds to about 3.1x10 23 displacement/m 2 ) Nightingale 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. 𝐿 0 𝐿 -1 • Fractional change: Kelly & Rappeneau et al.

Fast Neutron Irradiation Effect on Graphite Properties: modulus and strength S. Ishiyama et al . Journal of Nuclear Materials 230 (1996) 1-7

Fast Neutron Irradiation Effect on Graphite Properties: modulus and strength 𝑇 = ( 𝐹 UKAEA data on near isotropic graphites irradiated in the ) 𝑙 Dounreay Fast Reactor, R. Price. 𝑇 0 𝐹 0

Micro-mechanical testing over multiple length-scales

Setup 1: Setup 2: Nano-indentation Nano-indentation Nano Indenter G200 Nano Indenter inside a SEM Ex situ test In situ test Indenter Graphite surface 10 µm 200 µm Setup 4: Setup 3: Micro-cantilever bending Micro-cantilever bending Triangular section, 100-200 µm in length Rectangular section, 10-20 µm in length In situ test In situ test Loading probe Indente Graphite Micro-cantilever r cantilevers 5 µm 200 µm Liu et al . Journal of Nuclear Materials, 2017 1 mm

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) Indenter Indenter Indenter Sample surface Sample Sample 10 µm 10 µm surface surface 10 µm 0 2 4 6 8 10 0 2 4 6 8 10 0 2 4 6 8 10 Load (mN) Load (mN) Load (mN) 0 2 4 6 8 0 1 2 3 0 0.2 0.4 0.6 0.8 Displacement (µm) Displacement (µm) Displacement (µm) Liu et al . Journal of Nuclear Materials, 2017

In situ micro-cantilever bending • • • Step III Step I Step II • 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

Calibration: Glassy carbon • • • Specimens prior to failure Linear load-displacement relation Small variation in E with sample size 1.75x1.75x11.5µm 50 150 Cantilever 3 E = 29 4.5 GPa E Linea fit Fracture 40 1.25x1.25x8.80µm Slope=32.99  0.21 100 Load (µN) 30 E= 40.5 GPa E (GPa) Load (  N) 2.0x2.0x17.0µm 20 50 10 3.25x3.25x21µm 0 0 1.0 1.5 2.0 2.5 3.0 3.5 0 1 2 3 4 Displacement (µm) Displacement (  m) Section size (  m) • 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

In situ micro-mechanical testing: small cantilevers • 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 & large cantilevers Micro-cantilever bending Micro-cantilever bending Triangular section, 100-200 µm in length Rectangular section, 10-20 µm in length In situ test In situ test Loading probe Indenter Micro-cantilever Graphite cantilevers 5 µm 200 µm 1 mm ‘Small’ cantilever ‘Large’ cantilever Liu et al . Journal of Nuclear Materials, 2017

In situ micro-mechanical testing: large cantilevers 3.0 3.0 3.0 3.0 3.0 cycle 1 cycle 2 2.5 2.5 2.5 2.5 2.5 cycle 3 Loading probe cycle 4 2.0 2.0 2.0 2.0 2.0 cycle 5 Non-linear Load (mN) Load (mN) Load (mN) Load (mN) Load (mN) 1.5 1.5 1.5 1.5 1.5 Post-peak Loading arm length progressive 1.0 1.0 1.0 1.0 1.0 failure Cantilever specimen Linear 0.5 0.5 0.5 0.5 0.5 20 µm 0.0 0.0 0.0 0.0 0.0 0 0 0 0 0 20 20 20 20 20 40 40 40 40 40 60 60 60 60 60 Displacement (  m) Displacement (  m) Displacement (  m) Displacement (  m) Displacement (  m) Side surface of Cantilever root the triangle Triangular cross-section Fracture cantilever path b x c h yc 90° 20 µm 10 µm 30 µm Liu et al . Journal of Nuclear Materials, 2017

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

In situ micro-mechanical testing: irradiated PGA Filler particle • The irradiated PGA graphite (6018-12/3/3) Weight Diameter Length Mass Neutron dose Temp. loss (mm) (mm) (g) DIDO equiv. (n·cm -2 ) (K) 33.2 × 10 20 15% 12.1 7.1 1.01 560 Radiolytically-oxidised (CO 2 environment) PGA graphite samples from a Magnox reactor supplied by Magnox Ltd. Matrix 10 µm 40 µm Liu et al . Carbon, 2017

• Irradiated filler particle Irradiated filler particle • E = 40 to 86 GPa • σ f = 600 to 1300 MPa • Irradiated matrix • E ≤ 10 GPa • σ f ≤ 500 MPa • Unirradiated PGA graphite • E = 10 to 20 GPa • σ f = 200 to 500 MPa Irradiated matrix Liu et al . Carbon, 2017

Highly Oriented Pyrolytic Graphite HOPG An angular spread of the c-axes of the crystallites is of the order of 1 degree http://nanoprobes.aist-nt.com/apps/HOPGinfo.htm

Irradiated HOPG • Micro-mechanical testing o In situ testing in a Dualbeam chamber o Un-irradiated specimen as reference • Micro-Raman analysis Samples Temp (celcius) dpa o 60 nm penetration depth HOPG1 633 4.19 o 1.5 µm laser spot HOPG2 760 6.71 o 488 nm wavelength

Microstructural Characterisation o Focused ion beam cross-sectioning o The material is free of large pores FEG-SEM image of a FIB cross-section Trench created by FIB in the middle of HOPG sample: 10 µm 5 µm

Orientation of the basal plane to the loading direction SEM sample holder

Orientation of the basal plane to the loading direction

Load-displacement curve Fractured 35 surface Cantilever 1 30 Linear fit Plasticity? 25 Fractured Twinning Load (  N) at root 20 15 10 Elastic 5 250 nm 0 o Modulus and flexural 0 1 2 3 Displacement (  m) strength can be measured.

• Modulus/strength increased by a factor of about 2 after irradiation at 760°C for 6.71 dpa! 𝑇 = ( 𝐹 • R. Price ) 𝑙 𝑇 0 𝐹 0 k = 0.5 to 1 For HOPG, k=1 UKAEA data on near isotropic graphites irradiated in the Dounreay Fast Reactor

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

• Transformation from crystallite material to other Irradiated HOPG G 600 D G 633C_4.19dpa Intensity (a.u.) 400 Counts 200 0 0 1000 2000 3000 760C_6.7dpa Wavenumber (cm -1 ) Crossed polarisation Parallel polarisation 0 1000 2000 3000 -1 ) Unirradiated HOPG Wavenumber (cm Raman spectrum