"Joef Stefan" Institute,

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The Role of Hydrogen in Determination of
Deuterium Retention in Tungsten
Vincenc Nemanic, Bojan Zajec, Marko umer
"Joef Stefan" Institute, Dept. of Surface
Engineering and Optoelectronics
Ljubljana, Slovenia
1st Oct 2009
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Outline of the talk
1) Motivation for the work

• 2) Experimental methods
• general description
• selection and adaptation for our work.

3) Experimental results on D2 retention in
tungsten
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Motivation Tungsten is a serious candidate
for the first wall material in ITER Interaction
with deuterium/tritium at high fluences not well
known ? retention of fuel not predictable Better
prediction of tritium retention is needed!
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1) Experimental data on deuterium retention
obtained in tokamak experiments simulating and
approaching conditions in ITER ? post mortem
analysis
2) Refined classical experiments for more
accurate interaction data (equilibrium
kinetics) of gaseous hydrogen (H/D) with ITER
relevant metals our approach
An important fact Most of solubility,
diffusivity and permeability data in W obtained
decades ago using H2 or D2 or T2 using various
techniques.
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• EFDA Technology Work Programme
• TW6-TPP-RETMET
• The purpose determining deuterium retention in
  24 hour-expositions in D2 at p 0.1 mbar and
  below. Condition that may arise in ITER.
• ITER grade AISI316 at T 100, 250 and 400 C
• Nemanic
  V, Zajec B and Zumer M, 2008 Nucl Fus. 48,115009
• ITER-grade Be T 100 C and 250 C
• ITER-grade W T 250, 400 and 1000 C (this talk)
• Sample metals provided by
• EFDA Close Support Unit - Garching

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Experimental Basic interaction of hydrogen
(H/D/T) with bulk material is expressed by
diffusivity and solubility, experimentally
determined by
1) infusion/outgassing technique
or 2) membrane technique A careful selection of
all experimental details is needed to get
reliable results.
W. G. Perkins, J. Vac. Sci.
Technol. 10 (1973) 543 H/D/T hardly traced in the
bulk at low concentration.
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Prediction of metal hydrogen equilibrium states
using the Sievert law
Ks solubility constant The new equilibrium
state (p2, C2) from initial C1 (or p1) can be
calculated for V system volume, Vs sample
volume
Unfortunately, the values and thermal dependence
of solubility constant far from being useful!
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The retention at (p,T,t) is hardly predictable

• Scattering of published data on solubility and
  diffusivity disable accurate calculation assuming
  diffusion limited kinetics

• Surface limited kintics is in fact better
  description of the process, but no data available
  for recombination coefficients of W surface

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The principle of infusion/outgassing technique
Equilibrium between gas phase (H/D/T) and metal
sample achieved at specified conditions (high p,
high T) ? gas pumped off ? transient to a new
equilibrium observed (low p).


The principle of permeation technique Transient
flow observed from t 0 when pupstream is set ?
until steady downstream flow is achieved.
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For studying the retention in W at specified
conditions (p,T,t), the only choice is thus the
infusion / outgasing technique. The amount of
retained deuterium is determined from

• Small pressure drop (absorption by the sample)
  and changed composition of the remained gas
  corrected by the holder contribution

• Together with subsequent outgassing of D2 HD
• in vacuum after gas removal.

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Alumina almost ideal up to 1600 C, supposing
that by heating and simultaneous pumping, low
outgassing could be achieved.
The standard procedure blank run with D2 at
identical conditions with the empty sample
holder ? potential interaction can be revealed
and applied in experiment with the sample.
Isotope exchange interaction in W difficult to
quantify since it runs in the alumina sample
holder too.
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800C

Hydrogen solubility from 400 C to 1100 C
calculated from trusted (?) data. Alumina data
J Serra, J. Am.Ceram. Soc., 88 (2005)
15-18 Tungsten data R Frauenfelder, JVST, 6
(1969) 388-397 Silica data RW Lee, RC Frank, DE
Swets, J Chem.Phys., 36 (1962) 1062-1071
(diffusive H)
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800C

Hydrogen diffusivity from 400 C to 1100 C
calculated from the same references.
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Heat treatment intensity expressed in
dimensionless units for diffusion Fo D.t/d2 of
H in the bulk 2 mm thick plate, 24 h at 800 C
gives Fo 0.04 for alumina would not come
to equilibrium Fo 9.5 for silica (strongly
bound states neglected) Fo 130 for tungsten
? a new equilibrium state achieved in 30
minutes (Fo ? 3)

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The UHV system performance The achieved
detection limit for infusion or outgassing is
2?109 molecules/(cm2s) at A 76 cm2. Various
schedules used to convert QMS signals of H2, HD
and D2 into the absolute units by calibration
with H2/D2 mixtures.
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Experimental setup for infusion/outgassing
method D2
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D2 exposure (metering) section calibrated volume
cell, capacitance gauge and SRG gauge
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D2 exposure section alumina allows sliding of W
sample ar R.T.
Heated zone
Cold zone
Sample sliding
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Tungsten Plansee rod size O.D. 2.5 cm h
20 cm machined to a tube I.D. 2.1cm h 5
cm V 5.31 cm3, A 76.0 cm2
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The first approach using RF heating failed
Several attempts to get reliable results by RF
heating of the sample in silica tube failed due
to high hydrogen release and high isotope
exchange reaction in the silica holder Blank
runs could not be performed (RF). Novel
approach using alumnina tube in the oven caused
several month delay.

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Preparation steps Bake-out UHV system 4 h at
150 C, low outgassing achieved dp/dt 7 ?10-9
mbar/s

• 3 h to 800 C, followed by 48 h at 800 C, 150
  cm2
• of alumina released dN/dA 6.7?1015 H2/cm2
  dp/dt low

• ? W sample inserted ? intense outgasing followed
  
• in 45 h ?C 1.05?1018 H/cm3.

Residual outgassing at the end H2 80, CO 20
dN/dt ? 1.3?1010 H2/(s cm2), reasonably low ? 24
h deuterium exposures at 800 C started.
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Deuterium retention in ITER-grade tungsten
during 24 h exposure at 800 C in alumina

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Conclusions An UHV system with the ultimate
sensitivity of detecting flux 2?109
molecules/(cm2s) from (into) the sample (A 76
cm2) was built. High amount of H2 (?C 1.05?1018
H/cm3) had to be extracted in long-term heating
cycles at 800 C before D2 exposures were
possible and the retention became detectable.
The observed amount of retained D2 was low, but
consistent with the picture of very low
solubility and diffusivity. Clear evidence at
which surface (W, alumina or both) reaction
proceeded can not be given.
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The observed fact that the isotope exchange
reaction is the main mechanism for deuterium
retention in W may be compared to old papers on
H/D retention in silica A.Farkas, L.Farkas,
Trans.Farad. Soc. 31, 821 (1935) and quantified
in R.W.Lee, R.C.Frank, D.E.Swets, J.Chem.Phys.,
36, 4 (1962). Their system and sample geometry
allowed to apply the permeation method as well as
infusion / outgassing method. Only 1 of H was
diffusive.
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• The setup is prepared now for complementary
  testing
• permeation measurements on W discs or W films
  on metal discs for upstream H2/D2 from 1 bar to
  1 mbar
• permeation through W/Be alloys
• accurate post mortem analysis of suitably
  shaped D loaded samples.

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Acknowledgement This work was supported by MHEST
and SFA and by (EFDA), W6-TPP-RETMET. Thanks
for your attention.