Simulation experiments on neutron damage of tungsten

1. Simulation experiments on neutron damage of tungsten M. Fukumoto, H. Kashiwagi, Y. Ohtsuka, Y. Ueda Graduate School of Engineering, Osaka University M. Taniguchi, T. Inoue, K. Sakamoto, J. Yagyu, T. Arai Japan Atomic Energy Agency I. Takagi Graduate School of Engineering, Kyoto University T. Kawamura, N. Yoshida Interdisciplinary Graduate School of Engineering Sciences, Kyushu University

2. Purpose of this study & Outline of this talk • Purpose of this study • Investigation of hydrogen isotope behavior in damaged W • Outline of this talk • Blister formation • Effects of radiation damage on blister formation • Deuterium retention • D concentration in damaged W • Effects of annealing on D retention • TDS profiles as a function of fluence • Preliminary TMAP7 simulation

3. Experimental sequence • Damage Creation • Ion energy: 300 and 700 keV H- • Pulse duration: 1 s every 60 s (~1000 shots) • Temperature: below 473 K (to avoid recovery of defects) • W samples • Hot rolled and stress relived • Mirror-polished less than 0.01 mm roughness D depth distribution D desorption Blister formation • H-C irradiation • Ion energy: 1.0 keV (include H+, H2+, and H3+) • Fluence: 7.5 x 1024 H+/m2 • Carbon: ~0.8 % • Temperature: 473 K 1.5.Annealing • 673 K, 1 h • 1173 K, 1 h • D implantation • Ion energy: 1.0 keV (include D+, D2+, and D3+) • Fluence: 0.5 x 1024 ~ 8.0 x 1024 D+/m2 • Temperature: 473 K • SEM observation • SIMS/NRA measurements • NRA was used for absolute calibration • TDS measurements • 1 K/s, R.T. ~ 1100 K

4. 20mm 20mm 20mm Effects of radiation damage on blister formation • Fluence: 7.5 x 1024 H+/m2 • Temp.: 473 K • Carbon: ~0.8 % • The number of blisters was decreased with increasing radiation damage • The blisters with diameter of 20mm or less was decreased （c） 3.5dpa （b） 0.3dpa （a） 0dpa

5. Mechanism of blister formation trapped at grain boundaries →blister formation decrease of H trapped at grain boundaries • H was not accumulate at the grain boundaries within radiation damage • Small blisters (<20 mm) were decreased • Large blisters were formed since radiation damage was not produced ~1.5 mm damaged zone Undamaged W 700keV H- damaged W

6. D distribution as a function of fluence (473 K) • Fluence: 0.5 ~ 8.0 x 1024 D+/m2 • Temp.: 473 K • Damage: ~4.8 dpa • D conc. near surface was saturated at ~5.0x1023 D+/m2 • D conc.: ~0.9x1027 D/m3 • Trap density • 0.014 traps/W • Production rate • 0.014 traps/W·dpa • Similar to 800 MeV p damage* • ~0.01 traps/W·dpa • D conc. at ~1.0 µm was not saturated up to 8.0x1024 D+/m2 * B.M. Oliver et al., J. Nucl. Mater. 307-311 (2002) 1418.

7. Effects of 673 K annealing on D trapping • Fluence: 5.0 x 1024 D+/m2 • Temp.: 473 K • Damage: ~4.9 dpa • D concentration was decreased by annealing at 673 K for 1 h. • Change of surface density 0.8x1027 => 0.6x1027 D/m2 • ~20 % reduction • Most of self-interstitials could be eliminated*. • Vacancy type defects are still remained. * M. J. Attard et al., Phys. Rev. Lett., 19, (1967) 73.

8. Effects of 1173 K annealing on D trapping • Fluence: 5.0 x 1023 D+/m2 • Temp.: 473 K • Damage: ~4.4 dpa • D conc. was also decreased by annealing at 1173K for 1h. • Change of surface density 0.9x1027 => 0.2x1027 D+/m2 • ~80 % reduction (near surface) • Single vacancies could be annealed by this heat treatment* • Voids formation could be still take place** * D. Jeannotte et al., Phys. Rev. Lett., 19, (1967) 232. ** H. Eleveld et al., J.N.M., 212-215, (1994) 1421.

9. TDS spectra of two samples • Fluence: 5.0 x 1024 D+/m2 • Temp.: 473 K • Fitted by Gaussian functions. • Peak 1: ~770 K • Peak 2: ~860 K • Peak 3: ~920 K • Damaged W has much higher D desorption Undamaged W ~4.8 dpa damaged W

10. Fluence dependence of each peak • Fluence: 0.5 ~ 8.0 x 1024 D+/m2 • Temp.: 473 K • Damage: ~4.8 dpa • Damaged samples • Peak 1 (~770 K) • one order of magnitude higher than undamaged sample • increased with fluence • Peak 2 (~860 K) • same as undamaged sample • constant with fluence • Peak 3 (~920 K) • only damaged samples • increased with fluence • D was trapped at the vacancies (Peak 1) and voids (Peak 3)

11. D distribution as a function of fluence (673 K) • Fluence: 0.5 ~ 5.0 x 1024 D+/m2 • Temp.: 673 K • Damage: ~3.2 dpa • In the case of 673 K implantation, trapping and annealing of damage were simultaneously took place • Radiation damage around ~1.1 mm could be annealed during implantation

12. D Concentration (x1027 D/m3) D distribution simulated by TMAP7 • Simulation conditions • Trap energy: 1.34eV(vacancies)* 2.1eV (voids)* • Diffusion coeff.: Fraunfelder’s • Trapping rate: • De-trapping rate: • Distribution: TRIM-88 • Trap density: 0.014 traps/W·dpa • Other conditions: same as exp. • D trapping proceeds from surface trapping sites • All trap sites were filled less than 6.0 x 1022 D+/m2 • Much lower than exp. results (8.0 x 1024 D+/m2) • TMAP7 results did not agree with exp. results *M. Poon et al., JNM, 374 (2008) 390.

13. Conclusion • Blister Formation • Hydrogen isotopes were not accumulate at the grain boundaries within damaged zone • Deuterium depth profiles • D conc. near surface was saturated at the fluence of 5.0 x 1023 D+/m2 • D conc. near surface was 0.9 x 1027 D/m3 • Damage production rate was similar to 800 MeV p irradiated W • D conc. at ~1.0 mm was increased but not saturated up to the fluece of 8.0 x 1024 D+/m2 • Preliminary TMAP7 simulation did not reproduce exp. Results • TDS measurements • D was trapped at the vacancies and voids produced with high-energy ion irradiation

14. Experimental sequence • Damage Creation • Ion energy: 300 and 700 keV H- • Pulse duration: 1 s every 60 s (~1000 shots) • Temperature: below 473 K (to avoid recovery of defects) • H-C irradiation • Ion energy: 1.0 keV (include H+, H2+, and H3+) • Fluence: 7.5 x 1024 H+/m2 • Carbon: ~0.8 % • Temperature: 473 K • SEM observation • W samples • Hot rolled and stress relived • 99.99 at% • Mirror-polished less than 0.01 mm roughness • D implantation • Ion energy: 1.0 keV (include D+, D2+, and D3+) • Fluence: 0.5 x 1024 ~ 8.0 x 1024 D+/m2 • Temperature: 473 K • SIMS/NRA measurements • NRA was used for absolute calibration • TDS measurements • 1 K/s, R.T. ~ 1100 K

15. Outline of this talk • Background and Purpose of this study • Experimental sequence • Experimental results • Blister formation • Effect of radiation damage on blister formation • Deuterium retention • D concentration in damaged W • Effects of annealing on D retention • TDS profiles as a function of incident fluence • Preliminary TMAP7 simulation • Conclusion

16. Background and Purpose of this study • Background of this study • In ITER, W is a candidate PFM for diverter region • Extensive studies have been made for “undamaged” W • In DT fusion phase, fast neutrons are generated • W is simultaneously irradiated by hydrogen isotopes and neutrons • Interaction between radiation-induced defects and hydrogen isotope in W materials is very important • Trapping, release, and diffusion in damaged W are not clear • Purpose of this study • Investigation of surface morphology and deuterium behavior in damaged W • Blistering, D depth distribution and desorption characteristics

17. Redeposition layer of W Blister Gap Surface sputtered by FIB m 4 m 10 m m Fabrication and measurement were made by FIB (30 keV Ga) Relationship between blister diameter and depth of blister gaps A blister with diameter of 25 mm had a blister gap at 5 mm in depth. 5 mm 25mm 100mm A large blister with diameter of approximately 100 mm had a blister gap at 10 mm in depth. A.A. Haaszet al.: The effect of ion damage on deuterium trapping in tungsten, J. Nucl Mat., 266-269, pp.520-525(1999). 10 mm

18. ブリスタの直径と亀裂深さの関係 ~ 5.5 mm 照射損傷により減少するブリスタ直径 照射損傷でも減少しないブリスタ直径 0dpa • 300keV,700keV H-による照射損傷で、深さ1.5mm付近の亀裂が減少 • 300keV H-: 損傷の範囲より深い • 700keV H-: 損傷の範囲内 ~ 1.5 mm ~ 1.0 mm 300keV, 3.7dpa

19. 20mm 20mm 20mm Effect of damaged zone on blister formation • Fluence: 7.5 x 1024 H+/m2 • Temp.: 473 K • Carbon: ~0.8 % • The blisters less than 20 mm in diameter were decreased with an increase in damaged zone • 300keV H-：decrease of small blisters was low • 700keV H-：small blisters were suppressed （c） 700 keV, 3.5dpa （b） 300 keV, 3.7 dpa （a） 0dpa