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Update on Helium Retention Behavior in Tungsten

Explore the impact of thermal processes on helium retention in tungsten materials, crucial for IFE power plant applications. Research focuses on annealing effects, grain boundaries, and cavity formation in single crystal and polycrystalline tungsten. New techniques using neutron depth profiling and thermal desorption spectroscopy are employed for detailed analysis.

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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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