Update on Helium Retention Behavior in Tungsten

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OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY

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, NC2 Oak Ridge National Laboratory, Oak Ridge, TN

3 National Institute of Standards and Technology, Gaithersburg, MD

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As-rolled Powder Metallurgy W

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Powder Met W annealed at 1000°C for 1 hr

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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

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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


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Summary of Powder metallurgy W annealing results

Grain number, NA (mm-2)

Ave. Grain Area, A (m2)

Ave. Grain diameter, d (m)

ASTM Grain Size #, G

Annealed at 1200°C for 1 hr 11911 84 10.6 10.6

Annealed at 1200°C for 2 hrs 6222 161 12.7 9.6

Annealed at 1200°C for 5 hrs 4088 245 15.6 9.0

Annealed at 1300°C for 1 hr 8336 119 11.0 10.1

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Recrystallization in Powder Metallurgy W

0

10

20

30

1200°C, 1hr 1200°C, 2hrs 1200°C, 5hrs 1300°C, 1hr

Annealing Condition

0

100

200

300

1200°C, 1hr 1200°C, 2hrs 1200°C, 5hrs 1300°C, 1hr

Annealing Condition

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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 .

MeV Helium

MeV Helium

First Wall Armor

200

600

1000

1400

1800

2200

2600

3000

Surface

1 micron

5 microns

10 microns

100 microns

Time (s)

3-mm Tungsten slab

Density = 19350 kg/m3

Coolant Temp. = 500°C

h =10 kW/m2-K154 MJ DD Target Spectra

vacancy

0 1 2 3 4 5 6 7 8 9 10

Time of microseconds

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AFM of blistering

Topographical AFM image of surface blisters on polycrystalline tungsten

Blister caps are ~1.9 m tall comparable to helium implant depth

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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.)

Total Dose 1E19 3E19 5E19 1E20

Dose/Step 100% (1) 100% (1) 100% (1) 100% (1)

1E18 85% (10)

1E17 65% (100) 70% (300) *70% (500) 77% (1000)

1E16 5% (1000)

Total Dose → 1 19E 3 19E 5 19E 1 20EDose/Ste p ↓ 100% (1) 100% (1) 100% (1)1 18E 100% (10)1 17E 75% (100) 75% (100) *75% (500)1 16E 30% (1000)

Polycrystalline W

Single Crystal W

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0

0.2

0.4

0.6

0.8

1

1.2

1016 1017 1018 1019 1020

Relative He Retention

Integrated He Dose (ions/m2)

1000 steps100 steps10 steps 1 step

Step = Implantation + Anneal

1019 in

1020 in 1000 steps 1 step

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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.

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Neutron Depth Profiling 3He(n, p)T used 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

0.0E+00

4.0E+19

8.0E+19

1.2E+20

1.6E+20

2.0E+20

0 0.5 1 1.5 2Depth (microns)

Concentration (3He/m

3) 1 step

1000 steps

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

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Producing IFE helium ion spectrum

0

50

100

150

200

250

800 1000 1200 1400 1600

Energy (keV)

Backscatter Counts

no foil

foil

tilted foil

• 1.6 MeV 3He degraded by 1.37 m C foil, backscattered from Au film

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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

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0

10

20

30

40

50

12.8 12.9 13.0 13.1 13.2 13.3 13.4 13.5

Energy (MeV)

1 step asimplanted

100 steps

1000 steps

0

20

40

60

80

100

120

140

160

12.800 12.900 13.000 13.100 13.200 13.300 13.400 13.500

Energy (MeV)

1 step as-implanted

100 steps

1000 steps

Cavity distribution in He-implanted and annealed W

Single crystal W implanted with1019 He/m2 followed by annealing at 2000°C

Polycrystalline 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 the 1000 step annealed (33 min.) single crystal W

* 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.

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Observed Area

Specimen

Specimen

1m

Over Focus Image

100nm

Under Focus Image

100nm

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

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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

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Helium TDS on Single Crystal W (5 x 10 20 He/m 2 )

0.0E+00

5.0E-11

1.0E-10

1.5E-10

2.0E-10

2.5E-10

3.0E-10

3.5E-10

4.0E-10

4.5E-10

0 200 400 600 800 1000 1200

Heating Time (s)

3 He Partial Pressure (Torr)

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).

620˚C

730˚C

900˚C

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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

Update on Helium Retention Behavior in Tungsten

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