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Update on Helium Retention Behavior in Tungsten

Update on Helium Retention Behavior in Tungsten. D. Forsythe, 1 S. Gidcumb, 1 S. Gilliam, 1 N. Hashimoto 2 , J. D. Hunn, 2 G. Lamaze, 3 N. Parikh, 1 S. J. Zinkle 2 , L. Snead 2 1 Dept. of Physics and Astronomy, UNC-Chapel Hill, Chapel Hill, NC

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Update on Helium Retention Behavior in Tungsten

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  1. Update on Helium Retention Behavior in Tungsten D. Forsythe,1 S. Gidcumb,1 S. Gilliam,1 N. Hashimoto 2, J. D. Hunn,2 G. Lamaze, 3 N. Parikh,1 S. J. Zinkle2, L. Snead2 1 Dept. of Physics and Astronomy, UNC-Chapel Hill, Chapel Hill, NC 2 Oak Ridge National Laboratory, Oak Ridge, TN 3National Institute of Standards and Technology, Gaithersburg, MD

  2. As-rolled Powder Metallurgy W

  3. Powder Met W annealed at 1000°C for 1 hr

  4. Powder Met W annealed at 1200°C for 1 hr Planimetric procedure (ASTM Designation: E112-96) Number of Grains, NA (/mm2) = 11911 Average Grain Area, A = 1/ NA= 84 (m2) Average Diameter, d = √(1/ NA) = 9.2 (m) ASTM Grain Size #, G = (3.321928 log10 NA) - 2.954 = 10.6

  5. Powder Met W annealed at 1300°C for 1 hr Planimetric procedure (ASTM Designation: E112-96) Number of Grains, NA (/mm2) = 8336 Average Grain Area, A = 1/ NA= 119 (m2) Average Diameter, d = √(1/ NA) = 11.0 (m) ASTM Grain Size #, G = (3.321928 log10 NA) - 2.954 = 10.1

  6. Summary of Powder metallurgy W annealing results

  7. Recrystallization in Powder Metallurgy W

  8. At room temp. growth of He bubbles beneath the surface causes blistering at ~3 x 1021/m2 and surface exfoliation at ~1022/m2. For IFE power plant, MeV He dose >>> 1022/m2 . First Wall Armor MeV Helium vacancy MeV Helium 0 1 2 3 4 5 6 7 8 9 10 Time of microseconds

  9. AFM of blistering • Topographical AFM image of surface blisters on polycrystalline tungsten • Blister caps are ~1.9 m tall comparable to helium implant depth

  10. Direction of Research Over Past Year • Complete study of stepwise dose annealing. Automate system for very large dose(>1019 n/m2) and higher (>2000°C.) Single Crystal W Polycrystalline W

  11. Where We are Going • It is now clear that : - Helium retention is a function of material and a combination of implanted dose and annealing temperature - For IFE-relevant levels of implanted helium and peak annealing temperatures we are near a limit below which helium may not accumulate • The direction we are moving : - More refined experiments designed to give a) More precise measurement of low level accumulation b) Better understanding of the kinetics - More detailed and experimentally coupled modeling.

  12. Neutron Depth Profiling • 3He(n, p)Tused to obtain absolute helium depth profile • Used to profile monoenergetic 1.3 MeV 3He implanted in tungsten • Ratio of areal densities determined by NDP agreed with ratio of proton yields resulting from NRA Single crystal W implanted with monoenergetic 1.3 MeV 3He at 850°C and flash-annealed at 2000°C to a dose of 1020 He/m2

  13. Producing IFE helium ion spectrum • 1.6 MeV 3He degraded by 1.37 m C foil, backscattered from Au film

  14. Variable energy helium implantation • 1.6 MeV 3He beam degraded by carbon foils • Foil thickness: 1.37, 2.00, 2.55, 3.55 m • Approx. 10 different tilt angles (~0 – 40°) for each foil • 43 degraded energy profiles weighted appropriately • Implanted two single crystal samples with 1020 He/m2 at room temp. • One sample flash annealed to 2000°C after implant • Both samples to be analyzed by NDP

  15. Cavity distribution in He-implanted and annealed W Single crystal W implanted with1019 He/m2 followed by annealing at 2000°C * Single step annealing (2 sec.) resulted in the formation of a large number of tiny cavities. * No visible cavities were observed in the1000 step annealed (33 min.) single crystal W Polycrystalline W implanted with1019 He/m2 followed by annealing at 2000°C * The presence of grain boundaries led to significant cavity formation and greater cavity growth than in single crystal tungsten. * Annealing in 1000 steps resulted in no visible cavity formation even though the NRA results found polycrystalline tungsten had more He retention than single crystal tungsten.

  16. Specimen Specimen 1m Under Focus Image Observed Area 100nm 100nm Over Focus Image • Cavity Distribution of Helium-implanted Single Crystal W • Implanted at RT to 2 x 1017 m-2 and annealed at 2000°C for 5 sec. • and repeated this 50 times for a total dose of 1 x 1019 m-2

  17. Thermal Desorption Spectroscopy • Implant single crystal and polycrystalline tungsten with 3He • Mass spectrometer monitors 3He partial pressure while sample temperature is ramped from room temperature to 2400°C • Goal is to determine differences in helium trapping/detrapping mechanisms for single crystal and polycrystalline tungsten under different implantation conditions

  18. 730˚C 900˚C 620˚C TDS data for a single crystal W sample implanted with 5 x 1020 He/m2 at 850°C. The temperature was ramped from room temperature to 2400°C at ~2°C/s. Well defined desorption peaks were observed at 620, 730, and 900°C. The “plateau” between 1000 and 1200 s occurred while the sample was held at 2400°C (temperature ramp stopped due to furnace limitations).

  19. Summary • At IFE relevant conditions, variables affecting retention and eventual spalling include: - amount of helium implanted for each fusion event 1016 ions/m2 (~ IFE) packet, 2000°C has limited retention - annealing temperature following event currently limited to 2000°C due to specimen fatigue issues in ion beam chamber (specimen holder redesign needed) - microstructure as expected, helium retention at grain boundaries is an important factor • Issues: - current experiment limited in total dose and annealing temperature - more IFE-relevant irradiations should include: shorter pulse, higher temperature annealing (requires laser) - need to define defect energies by using recently developed TDS system

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