Micro-mechanics on Nuclear Graphite

Transcription

1 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.

2 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 o Ex situ and in situ nano-indentation In situ micro-cantilever testing Key messages

3 o o 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.

4 Nuclear Graphite

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

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

7 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

8 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

9 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. Fractional change: K 0 K -1 Kelly & Rappeneau et al.

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

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

12 Micro-mechanical testing over multiple length-scales

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

14 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

15 Load (mn) Load (mn) Load (mn) Nano-indentation (In situ) Indenter Sample surface Indenter Indenter Sample Sample 10 µm surface 10 µm surface 10 µm Displacement (µm) Displacement (µm) Displacement (µm) Liu et al. Journal of Nuclear Materials, 2017

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

17 1.75x1.75x11.5µm 1.25x1.25x8.80µm 2.0x2.0x17.0µm 3.25x3.25x21µm Load ( N) Load (µn) Calibration: Glassy carbon Specimens prior to failure Linear load-displacement relation Small variation in E with sample size Cantilever 3 Linea fit Slope= E= 40.5 GPa Fracture E (GPa) Displacement ( m) (µm) Repeatable modulus and flexural strength as measured at macro-scale Similar brittle fracture modes observed at micro-scale as in macro-size samples E E = GPa Section size ( m) Liu et al. Carbon, 2017

18 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

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

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

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

22 In situ micro-mechanical testing: irradiated PGA Filler particle The irradiated PGA graphite ( /3/3) Weight loss Diameter (mm) Length (mm) Mass (g) Neutron dose DIDO equiv. (n cm-2) Temp. (K) 15% Radiolytically-oxidised (CO2 environment) PGA graphite samples from a Magnox reactor supplied by Magnox Ltd. Matrix 40 µm 10 µm Liu et al. Carbon, 2017

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

24 Highly Oriented Pyrolytic Graphite HOPG An angular spread of the c-axes of the crystallites is of the order of 1 degree

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

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

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

28 Orientation of the basal plane to the loading direction

29 Load-displacement curve Load ( N) C Twinning Elastic Cantilever 1 Linear fit Plasticity? Displacement ( m) Fractured at root raw U/V Fractured surface 250 nm o Modulus and flexural strength can be measured.

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

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

32 Counts Transformation from crystallite material to other Irradiated HOPG G Intensity (a.u.) 633C_4.19dpa D G Wavenumber (cm -1 ) 760C_6.7dpa Crossed polarisation Parallel polarisation Unirradiated HOPG Raman spectrum Wavenumber (cm -1 )

33 ID/IG Transformation from crystallite material to other C_4.19dpa 760_6.7dpa unirradiated G peak position Ferrari AC, Robertson J. Interpretation of Raman spectra of disordered and amorphous carbon. Phys Rev B, 2000

34 HOPG2: 633 C for 4.19 dpa Each image is about 60 µm wide

35 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

36 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