The effect of displacement damage on deuterium retention in plasmaexposed tungsten

1
The effect of displacement damageon deuterium
retention in plasma-exposed tungsten
W.R.Wampler, Sandia National Laboratories,
Albuquerque, NM R. Doerner University of
California, San Diego CA Guang-Nan Luo, Institute
of Plasma Physics, Hefei, China 9th
International Workshop on Hydrogen Isotopes in
Fusion Reactor Materials Salamanca, Spain June
2-3, 2008

• Questions
• How much deuterium is retained in tungsten from
  exposure to plasma?
• Does displacement damage increase D retention?
• How much displacement damage in ITER how will
  this impact T retention?

2
Retention of deuterium in tungsten exposedto
PISCES plasma with displacement damage

• Tungsten samples were irradiated with 12 MeV
  Silicon ions to simulate displacement damage by
  fusion neutrons to depth of 2 microns.
• Three damage levels, 0.01, 0.1 and 1 dpa, from
  Si ion fluences of 5E12, 5E13, 5E14/cm2.
• Implanted Si is unlikely to change D
  retention,104 displacements/Si, max Si
  concentration Si as received).
• Expose to PISCES D plasma at T45, 200, 500 C
  to fluence of 1022 (and 1021 D/cm2)
• Measure D concentration vs depth to 3 µm by
  D(3He,p)a NRAin damaged and undamaged regions.
• Two types of tungsten- Plansee 99.97- Vacuum
  Plasma Sprayed (VPS) from ASIPP,

ITER lifetime neutron fluence (0.3 MWa/m2) is
estimated to produce 0.1 dpa. dpa from Si
ion irradiation was calculated using SRIM
2006. Vacancies in tungsten anneal at 500C
but not at 200C. Eleveld and Van Veen, JNM
212-215 (1994) 1421
3
Retention of D in tungstenwith displacement
damage

• T40C
• Plansee
• - D mainly
• - Little increase with damage
• - 2x1016 D/cm2
• VPS
• D retention similar to Planseeup to 0.1 dpa,
• 2x greater at 1 dpa

4
Retention of D in tungstenwith displacement
damage

• T200C, 1022 D/cm2
• Plansee
• Below 0.1 dpa, 2x more D retainedthan at 40C
  but nowextends to 2.5 µm.
• 3x increase from damage.
• VPS
• D retention similar to Plansee

5
D retention is insensitive to D fluence
Plansee at T200C D retention decreased
0.73x by 10x lower D fluence.
6
D retention is much lower at 500 C

• Plansee
• D retention much lower thanat 40 200C.
• D retention increased at 1dpaand follows damage
  profile
• VPS
• D retention 5x higher than Planseebut with
  similar depth profile.
• Vacancies are mobile at 500C,
• D traps are now probably less numerous but
  thermally more stable vacancy clusters.

7
Blisters on Plansee W exposed at 200C.
Few or no blisters seen on VPS W or on Plansee W
exposed at 40 or 500C
8
Summary of D retention in damaged tungsten
Areal density D retained (1016/cm2) within
damaged region (
Concentration of D retained (appm) at damage peak
(2µm)
9
Deuterium trapping - Energetics
Trapping D bound to vacancy Precipitation
formation of gas from solution Enthalpies HM
0.39 eV migration Hs 1.04 eV solution Frauenfeld
er JVST 6 (1969) 388 Hv 1.43 eV dissociation
from trap Ht Hv HM 1.04 eV binding to trap
(vacancy) Eleveld and Van Veen, JNM 191-194
(1992) 433
vacuum surface solution vacancy
Equilibrium condition
Chemical potential
and
L0 0.01 D/W/atm1/2
Ideal gas
assuming Ss St
solution
traps
For traps 50 occupied (s 0.5) the equilibrium
gas pressure is Pt104 atm, almost independent of
T since Hs Ht
Traps are strong relative to solution but weak
relative to gas phase. Precipitation is favored
over trapping.
10
Deuterium trapping from plasma - Kinetics
High Ps means D is likelyto precipitate
(blisters)before it reaches the traps when
T
With strong traps and fast surface recombination,
flux of D into trapsFt Fp xp/xt (2x1018
/cm2s) (3 nm/2000 nm) 3x1015 /cm2s Time to fill
traps Nt/ Ft 30 seconds for Nt1017/cm2
However, concentration c and corresponding
chemical potential or equilibrium gas pressure Ps
depend strongly on temperature Fout D(T)
c/xp and Psc/L(T)2 (for ideal gas)
11

• Conclusions
• D retention in plasma-exposed tungsten is mainly
  near the surface for Tdamage below 0.1 dpa,corresponding to 1
  milligram/m2 of tritium.
• D retention is similar in Plansee and VPS
  tungsten.
• Displacement damage increases D retention,but
  the effect is modest and significant only above
  40C and 0.1 dpa.The concentration of retained D
  is much lower than the concentration of
  displacements produced.
• D retention within 3 microns is much smaller at
  500C than at 40 or 200C,(in Plansee W), due to annealing of damage.
• High chemical potential of D injected from plasma
  leads to gas precipitation. Plansee tungsten
  exposed at 200C has D blisters from precipitation
  of D2 gas.
• Bubbles or blisters near the surface will
  intercept mobile D preventing it from reaching
  greater depths. However, blisters will also
  impede heat transport, potentially increasing W
  erosion in ITER by melting of blister caps.

12
Extra slides
13
Retention of deuterium in VPS tungsten exposed to
PISCES plasma (without displacement damage)

• Vacuum plasma sprayed (VPS) tungsten from ASIPP
  was exposed to deuterium plasma in PISCES.
• Three 1 diameter disks were exposed at
  temperatures of 50, 200 500 C to a fluence of
  1022 D/cm2.
• D retention was measured by nuclear reaction
  analysis (NRA) and thermal desorption
  spectrometry (TDS).

• Conclusions
• D retention is below 1 milligram/m2.
• RT 200 C samples Areal densities of D
  measured by TDS NRA are similar. Most of the D
  is within 3 µm.
• 500 C sample NRA sees less D but TDS sees more
  D than in the other two samples. Most of the D is
  deeper than 3 µm.
• The lower temperature TDS peak in the RT 200 C
  samples must be from the near-surface D.The
  higher temperature TDS peak in the 500 C sample
  must be from D at depths greater than 3 µm.