1. Microstructural development of HOPG under ion-irradiation ○ Makoto Nonaka 1, Sosuke Kondo 2 and Tatsuya Hinoki 2 1 Graduate school of Energy Science, Kyoto University, Uji, Kyoto, Japan 2 Institute of Advanced Energy, Kyoto University, Uji, Kyoto, Japan

2. Outline Introduction Isotropic graphite for core of HTGR Irradiation induced dimensional change of graphite crystallite Comparison between neutron and ion-irradiation Experimental Ion-irradiation method How to evaluate the dimensional change Result and discussion Macroscopic dimensional change TEM microstructure

3. Isotropic Graphite for HTGR Isotropic graphite is promised as core structure material for High Temperature Gas-cooled Reactor (HTGR). The trend is dependent on material grade and irradiation temperature. Irradiation data of each material are essential. [1] T. Shibata et al. JNST, Vol. 47, No. 7, p. 591–598 (2010) Dimensional change ( % )Neutron irradiation ( 10 26 n/m 2 ) Dimensional change behavior of Isotropic graphite ( IG-110) under neutron irradiation at 600 °C [1] Expand Turnaround Shrink Turnaround point → Factor of material life-time

4. Dimensional change of graphite crystallite Displacement damage Inter layer → Interstitial (clusters) Intra layer → Vacancy (clusters) ( … discussed in [1][3][4] ) Dimensional change under neutron irradiation Shrinking in the basal plane Elongating in the c-axis direction → These show strong temperature and DPA dependence Issue Correlation between dimensional change and defect is not understood sufficiently Interstitial cluster Vacancy cluster [1] T. Shibata et al. JNST, Vol. 47, No. 7, p. 591–598 (2010) [2] B. T. Kelly et al. Carbon, 20, (1981), p3 [3]T. Tanabe et al. Appl. Phys. Lett., 61, (1992), p1638 [4]C. Karthik et al. J. Nucl. Mater., 412, (2011), p321 Dimensional change ( % ) Elongating in c-axis direction Shrinking in basal plane 150˚C 200˚C 450˚C 600˚C Dimensional change of crystallite under neutron-irradiation [1][2] Neutron Dose ( 10 26 n/m 2 )

5. Neutron-irradiation and ion-irradiation Neutron-irradiation Very low damage rate ( ~10 -7 dpa/sec ) [5] Radioactivation of specimen ⇒ Appropriated for performance test after irradiation Ion-irradiation Very high (+ controlable) damage rate ( 1.3×10 -4 dpa/sec in this work) Radioactivation free ⇒ Basic research of irradiation damage (DPA dependence, underlying mechanism etc… ) Objectives Approach Measure the dimensional change of HOPG TEM observation of irradiation-induced microstructural defects [5]George W. Hinman et al. Carbon 1970. Vol. 8, pp. M-351. Clarify the dependence of irradiation conditions on the dimensional change of each axis Clarify underlying mechanisms of dimensional change on microscopic scale

6. Ion-irradiation Irradiation conditions Temperature:200 °C, 600 °C Ion:5.1MeV Si 2+ Average dose:0.0077 dpa - 0.77 dpa (nominal) Damage rate:1.28 × 10 -4 dpa/sec Material Highly-Oriented Pyroritic Graphite( HOPG ) Density:2.22 mg/cm 3 Purity:> 99.999 ％ DuET, Kyoto University c-axis Basal plane HOPG All c-axes of each crystallite orient in the same direction.

7. Neutron-irradiationIon-irradiation Displacement damage of irradiation Depth from irradiated surface ( μm ) Dose ( DPA ) Ion accumulation ( at% ) TRIM 2008 Profile ( nominal 0.77dpa ) Damage depth 2.75μm Conceptual diagrams of displacement damage No damage gradient Damage gradient in a depth direction

8. How to evaluate the dimensional change 200±5μm-thickness c-axis Strips for evaluating shrinkage in the basal plane 800±5μm-thickness c-axis Strips for evaluating elongation in the c-axis c-axis Basal plane

9. How to evaluate the dimensional change Strips were sandwiched between fixtures. Strips were irradiated with Si 2+ ion. The region from the irradiated surface to 2.75 μm were damaged. Dimensional change was induced only the very surface. Strips were curved due to strain mismatch between irradiated region and unirradiated. Dimensional change of long axis were evaluated from the curvature. HOPG Fixture ~2 mm

10. How to evaluate the dimensional change Strips were sandwiched between fixtures. Strips were irradiated with Si 2+ ion. The region from the irradiated surface to 2.75 μm were damaged. Dimensional change was induced only the very surface. Strips were curved due to strain mismatch between irradiated region and unirradiated. Dimensional change of long axis were evaluated from the curvature. HOPG Fixture ~2 mm Si 2+

11. How to evaluate the dimensional change Strips were sandwiched between fixtures. Strips were irradiated with Si 2+ ion. The region from the irradiated surface to 2.75 μm were damaged. Dimensional change was induced only the very surface. Strips were curved due to strain mismatch between irradiated region and unirradiated. Dimensional change of long axis were evaluated from the curvature. HOPG Fixture 2.75μm irradiated unirradiated ~2 mm Si 2+

12. How to evaluate the dimensional change Strips were sandwiched between fixtures. Strips were irradiated with Si 2+ ion. The region from the irradiated surface to 2.75 μm were damaged. Dimensional change was induced only the very surface. Strips were curved due to strain mismatch between irradiated region and unirradiated. Dimensional change of long axis were evaluated from the curvature. HOPG Fixture 2.75μm irradiated unirradiated Elongate or Shrink ~2 mm Si 2+

13. How to evaluate the dimensional change Strips were sandwiched between fixtures. Strips were irradiated with Si 2+ ion. The region from the irradiated surface to 2.75 μm were damaged. Dimensional change was induced only the very surface. Strips were curved due to strain mismatch between irradiated region and unirradiated. Dimensional change of long axis were evaluated from the curvature. HOPG Fixture 2.75μm irradiated unirradiated Elongate or Shrink Shrinkage cause upper Elongation cause downer ~2 mm Si 2+

14. HOPG Fixture How to evaluate the dimensional change Strips were sandwiched between fixtures. Strips were irradiated with Si 2+ ion. The region from the irradiated surface to 2.75 μm were damaged. Dimensional change was induced only the very surface. Strips were curved due to strain mismatch between irradiated region and unirradiated. Dimensional change of long axis were evaluated from the curvature. 2.75μm irradiated unirradiated Elongate or Shrink Shrinkage cause upper Elongation cause downer Evaluate curvature ~2 mm Si 2+

15. HOPG Fixture How to evaluate the dimensional change Strips were sandwiched between fixtures. Strips were irradiated with Si 2+ ion. The region from the irradiated surface to 2.75 μm were damaged. Dimensional change was induced only the very surface. Strips were curved due to strain mismatch between irradiated region and unirradiated. Dimensional change of long axis were evaluated from the curvature. 2.75μm irradiated unirradiated Elongate or Shrink ~2 mm Si 2+ If shrink in basal plane → Upward curve If elongate in c-axis → Downward curve Elongate c-axis Shrinkage c-axis Shrink

16. Dimensional change direction 500μm Si 2+ unirradiated c-axis 1 mm Si 2+ Upward curve→Shrank in the basal plane Downward curve→Elongated in the c-axis direction Strip images from parallel to short axis Shrinking in the basal plane 200˚C, 0.23dpa Elongating in the c-axis 200˚C, 0.23dpa

17. Dimensional change direction 500μm Si 2+ unirradiated c-axis 1 mm Si 2+ Upward curve→Shrank in the basal plane Downward curve→Elongated in the c-axis direction Strip images from parallel to short axis Shrinking in the basal plane 200˚C, 0.23dpa Elongating in the c-axis 200˚C, 0.23dpa

18. DPA Shrinking 200˚C 600˚C 200˚C 600˚C The curvature increased with increasing DPA. The curvature observed at 200˚C was greater than that of 600˚C. Curvature ( mm -1 ) Shrinkage in the basal plane Elongation in the c-axis DPA Elongating Curvature ( mm -1 ) Temperature and DPA dependence

19. Dimensional change under ion-irradiation DirectionShrank in the basal plane, elongated in the c-axis DPA dependence Increased with increasing DPA Temperature dependence Higher temperature reduce dimensional change Ion -irradiation Dimensional change ( % ) Elongating in the c-axis direction Shrinking in the basal plane 150˚C 200˚C 450˚C 600˚C Dimensional change of crystallite under neutron-irradiation [1][2] Neutron Dose ( 10 26 n/m 2 ) → Consistent with the neutron-irradiation result qualitatively Dimensional change behavior under neutron-irradiation irradiation Basal plane elongating Shrinking [1] T. Shibata et al. JNST, Vol. 47, No. 7, p. 591–598 (2010) [2] B. T. Kelly et al. Carbon, 20, (1981), p3

20. Microstructure after irradiated at 200 ᵒC Irradiated surface 1μm Selected area diffraction pattern 0.45 dpa The diffraction pattern changed caused by irradiation → The deterioration of the crystallizability There are no other microstructural change which could be recognized using conventional TEM. Before irradiation After irradiation Cross section TEM image after irradiated Observation from direction Damaged region

21. Microstructure after irradiated at 600 ᵒC (1) 1μm 0.27 dpa Irradiated surface Damaged region Cross section TEM image after irradiated Observation from direction

22. Microstructure after irradiated at 600 ᵒC (2) 600 ᵒC, 0.27dpa Defect size (nm) Average size 5 nm Defect density 9.8×10 22 /m -3 Defect size distribution after irradiated at 600 ᵒC Number of defect Dark field image unirradiated Elongation in the c-axis under neutron-irradiation ( same condition ) [1] ≒ 0.51 % [1]T. Shibata et al. JNST, Vol. 47, No. 7, p. 591–598 (2010) Implying that smaller defect which can not be observed give the dominant effect to elongation 50nm Irradiation defects recognized

23. Microstructure after irradiated at 600 ᵒC (3) Irradiation temperature ( ⁰C ) Defect density ( m -3 )Average defect size ( nm ) Defect density v.s. irradiation temperature Average defect size v.s. irradiation temperature [6] P. Honer and G. K. Williamson et al. Carbon (1966) Vol. 4, pp. 353-363. [7] Ya. I. Shtrombakh et al. Jun Nuk Mater 225 (1995) 273-307 ※ The figure beside the marker shows DPA. Defect density→ Appropriate for neutron-irradiation data Defect size→ Difficult to compare to neutron-irradiation result Ion-irradiation may be good simulation of neutron-irradiation. Irradiation temperature ( ᵒC ) This work Horner Shtrombakh This work Shtrombakh ( Interstitial ) Shtrombakh ( Vacancy )

24. Summary Evaluation of dimensional change to clarify dependence of irradiation conditions on the dimensional change of each axis Shrank in the basal plane, elongated in the c-axis Increased with increasing DPA Higher temperature reduce dimensional change → Consist with the neutron-irradiation result qualitatively TEM observation to clarify the mechanism of dimensional change on microscopic scale The crystallizability deterioration at 200 ᵒC irradiation Irradiation defects formed at 600 ᵒC irradiation → Appropriate defect density for neutron-irradiation data Future work TEM observation of samples irradiated at various condition Analyzing the correlation between defects and dimensional change ( HRTEM, combine with low mag image, Raman spectroscopy, etc… )

25. International Nuclear Graphite Specialists Meeting (INGSM) September 15-18, 2013 Seattle, WA

26. Thermal diffusion of defects in graphite

27. Basal plane LOW HIGH Vacancy Vacancy cluster