1. Mechanism of Proton Irradiation-Induced Creep of Ultra-Fine Grain Graphite ZXF-5Q A.A. Campbell & G.S. Was INGSM-15 September 18, 2014 Research Supported by: US DOE under NERI Contract # FC07-06ID14732 INL under Contract # DE-AC07-05ID14517

2. Outline Objective Experimental Methodology Results Discussion Conclusions 2

3. Objective Determine the mechanism of proton irradiation- induced creep for an ultra-fine grain graphite This work is published in: A.A. Campbell & G.S. Was, “Proton Irradiation- Induced Creep of Ultra-Fine Grain Graphite”, Carbon, 77 (2014) 993-1010. 3

4. Experimental POCO grade ZXF-5Q – Particle size < 1 µm – Pore size < 0.3 µm – Density 1.78g/cm 3 (80% theoretical density) – Tensile Strength 79MPa – Anisotropy < 1.03 BAF – Young’s Modulus 14.5 GPa – Compressive Strength 175MPa – Thermal Conductivity 70 W/m/K – Green Pet Coke Filler, milled to size, isostatically molded* 4 POCO Grade ZXF-5Q Data sheet.

5. Experimental Methodology Irradiation creep experiments – utilize novel system designed to perform proton irradiation-induced creep experiments Post-irradiation Analysis – Crystal parameters – Analyze X-Ray Diffraction spectra with Williamson-Hall methodology 5

6. Irradiation Chamber 6 Campbell and Was, Journal of Nuclear Materials, 433 (2013) 86-94.

7. Irradiation Creep Experimental Conditions Applied tensile stress (1000ºC, 1.15x10 -6 dpa/s) – 5 MPa, 10 MPa, 20 MPa, 40 MPa Dose Rate (700ºC, 20MPa) – 2.95x10 -7 dpa/s to 5.51x10 -7 dpa/s Temperature (20MPa, variable dose rate) – 700ºC, 900ºC, 1000ºC, 1100ºC, 1200ºC Two samples used for each experiment, one with stress and one without stress – Residual stress from EDM machining resulted in curvature of the unstressed sample 7

8. Results 8

9. Irradiation Creep Example Data 9

10. Dose Rate & Temperature Control 10

11. Applied Stress Dependence (1000ºC, 1.15x10 -6 dpa/s) 11

12. Stress Dependence Comparison 12 AGOT H-337 and AXF-8QBGI from Gray, Carbon, 11, (1973) 183SM1-24 from Oku et al., JNM, 152, (1988) 225 IG-110 from Oku et al., JNM, 172, (1990) 77

13. Dose Rate Dependence (700ºC, 20MPa) 13

14. Dose Rate Comparison 14 Veringa from Veringa and Blackstone, Carbon, 14, (1976) 279.SM1-24 from Oku et al., JNM, 152, (1988) 225 IG-110 from Oku et al., JNM, 172, (1990) 77

15. Accumulated Dose Dependence (1000ºC, 1.15x10 -6 dpa/s, 20MPa) 15

16. Accumulated Dose Comparison 16 Neutron Data for H-451 from: Burchell, T.D., JNM, 381, (2008) 46.

17. Temperature Dependence (20MPa) 17 Temperature700ºC 900ºC1000ºC 1100ºC1200ºC Dose Rate2.95x10 -7 4.39x10 -7 8.10x10 -7 1.06x10 -6 1.17x10 -6 1.56x10 -6 2.04x10 -6

18. Temperature Comparison 18 Veringa and Dragon from Veringa and Blackstone, Carbon, 14, (1976) 279Burchell, T.D., JNM, 381, (2008) 46 Gray et al., Carbon, 5, (1967) 173Kelly and Burchell, Carbon, 32, (1994) 119 Mitchell et al., Nuc Energy, 41, (2002) 63SM1-24 from Oku et al., JNM, 152, (1988) 225 IG-110 from Oku et al., JNM, 172, (1990) 77 Perks PGA from Perks and Simmons, Carbon, 1, (1964) 441 Perks AGOT H-337 and AXF-8QBGI from Perks and Simmons, Carbon, 4, (1966) 85

19. Temperature Comparison 19 Veringa and Dragon from Veringa and Blackstone, Carbon, 14, (1976) 279SM1-24 from Oku et al., JNM, 152, (1988) 225 IG-110 from Oku et al., JNM, 172, (1990) 77

20. Difference Between Proton and Neutron Results Last year I presented these differences Showed work from Russia that found that the neutron to gamma flux ratio has significant effect on turn-around dose [1] Showed work from China that showed exposure to γ-rays at room temperature increased graphitization [2,3] I presented a hypothesis that γ-rays are annealing damage as it is being caused by neutrons in-reactor – Effectively reducing the number of defects available to assist with driving creep – For example, 900°C proton irradiation wouldn’t experience turn- around until 21dpa 20 [1] Nikolaenko et al., Atomic Energy, 87, (1999) 480. [2] Li, B. et al., Carbon, 60, (2013) 186. [3] Xu, Z. et al., Materials Letters, 63, (2009) 1814.

21. Creep Mechanism Comparison Experimental dependencies to compare with mechanisms: – Linear with Stress – Linear with Dose Rate – Arrhenius with Temperature 21

22. Basal Pinning-Unpinning High density of lightly pinned dislocation Irradiation produces and destroys pinning points From definition of mechanism creep rate should be: – Linear with stress – Not effected by dose rate – Increase with temperature 22

23. Stress-Induced Preferred Absorption (SIPA) Preferential absorption of defects at dislocations, strain occurs as dislocations climb – Linearly dependent on stress – Dose rate dependence arises in C i and C v – Temperature dependence arises in D i C i and D v C v – Should not be effected by accumulated dose if dislocation density does not change 23

24. Glide with SIPA enhanced climb (PAG) Additive to SIPA, but strain arises from dislocation glide rather than climb – Squared stress dependence – Dose rate dependence arises in C i (linear) – Temperature dependence arises in D i C i (Arrhenius) – Should not be effected by accumulated dose if dislocation density does not change 24

25. Climb and Glide from Dislocation Bias Similar to PAG but interstitials are absorbed at dislocations and vacancies are absorbed at voids – Squared stress dependence – Dose rate dependence arises in C i (linear) – Temperature dependence arises in D i C i (Arrhenius) – Should not be effected by accumulated dose if dislocation density does not change 25

26. Creep Rate Comparison Experimental Creep Rate Dependencies Basal Pinning- Unpinning SIPAPAGCGDB Linear with StressAgree Disagree Linear with Dose RateDisagreeAgree Arrhenius with TemperatureAgree Constant with DoseDisagreeAgree 26 Mechanism Creep Rate Dependencies SIPA – only mechanism that had significant agreement of experimentally-determined and mechanism-predicted creep rate dependencies If creep is driven by a mechanism dependent on defects for creep to occur (climb driven) then the effect of applied stress should be observed in the microstructure changes

27. Dose and Stress Affects on Crystallite Dimensions 27

28. Dose and Stress Affects on Lattice Parameters 28

29. Sources of Lattice Spacing Changes C-spacing – single interstitials, interstitial clusters, interstitial loops – Single interstitials not stable at these temperatures – Loops observed only cause increase around the loop edge [1] – Primary source must have [1]: Stable configuration, low diffusibility, no tendency to grow, subject to radiation annealing Six atom hexagonal clusters – where density is dependent on dose rate and temperature Interatomic spacing – Poisson’s ratio effect, vacancies – Poisson’s does not account for all the contraction – Single and di-vacancies cause rearrangement of covalent bonds, but above 500ºC vacancies are mobile – Vacancy lines form at high dose (low temperature) and with onset of irradiation at high temperature Average number of vacancies in a line increases with temperature Atoms around uncollapsed lines will have rearranged covalent bonds [2] Concentration of uncollapsed lines will saturate with dose, and saturation density should decrease for higher temperatures [3] 29 [1] Reynolds and Thrower, Philosophical Magazine, 12, (1965) 573-593.[2] Kelly et al., Journal of Nuclear Materials, 20, (1966) 195-209. [3] Henson et al., Carbon, 6, (1968) 789-806.

30. Sources of Crystal Parameter Changes C-spacing variation decreases with increasing dose – Seems counter intuitive – In immediate vicinity of cluster, increase is greater than average [1] – Distribution of clusters is fairly uniform [1] Crystallite size – measure of size of regions in graphite with perfect structure 30 [1] Bacon and Warren, Acta Crystallographica, 9, (1956) 1029-1035.

31. Stress Effects on Crystal Parameters Samples only received a dose of 0.25dpa – Below dose to reach the saturation values In neutron irradiations, the lattice parameter change of a crept sample is less than uncrept samples [1] – Smaller interstitial concentrations due to stress-enhanced recombination or stress-enhanced interstitial mobility – Concentration remains constant but cluster density decreases due to clusters being swept together by gliding dislocations 31 [1] Richards and Kellett, Journal of Nuclear Materials, 25, (1968) 45-57.

32. Discussion Arrhenius temperature dependence gives an irradiation creep activation energy of ~0.3eV – Same order of magnitude of interstitial migration energy Results from microstructure analysis show the crept samples had less lattice and crystallite change than the uncrept sample – Suggests lower interstitial concentration in the crept samples due to interstitials driving creep Best-agreement between experimental results and creep mechanisms suggests a mechanism similar to SIPA is the controlling mechanism Results from Karthik showed positive climb of partial basal dislocations – Climb occurs when interstitials are absorbed at the edge of the defect 32

33. Radiation Damage in Graphite 33 Karthik et al., Journal of Nuclear Materials, 412 (2011) 321–326 "Carbon Materials for Advanced Technologies", ed. Timothy D. Burchell, 1999.

34. Is there a Glide Path for Dislocations? 34 Images courtesy of Helen Freeman – work to be published

35. Proposed Mechanism Stress-Induced Climb of Basal Plane Dislocations – Dislocation climb driven mechanism, rather than glide driven (agreement with linear stress dependence) – ~0.3eV activation energy suggests interstitial migration is the rate-limiting mechanism – In-situ TEM from Karthik observed basal plane climb – Same behavior as SIPA, but ignore term due to vacancy absorption at dislocations 35

36. Conclusions Irradiation-induced creep rate dependencies found to be: – Linear dependence on applied stress and dose rate – Arrhenius dependence on irradiation temperature (approximately linear in this temperature range) – No dependence on accumulated dose Creep rate dependencies on experimental conditions mostly agree with the dependencies observed for neutron irradiation creep of graphite Proposed mechanism is modified SIPA to only depend on interstitial absorption at dislocations 36

37. Mechanism of Proton Irradiation-Induced Creep of Ultra-Fine Grain Graphite ZXF-5Q A.A. Campbell & G.S. Was INGSM-15 September 18, 2014 Research Supported by: US DOE under NERI Contract # FC07-06ID14732 INL under Contract # DE-AC07-05ID14517