"jožef stefan" institute, dept. of surface engineering and optoelectronics the role of...
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"Jožef Stefan" Institute,Dept. of Surface Engineering and Optoelectronics
The Role of Hydrogen in Determination
of Deuterium Retention in TungstenVincenc Nemanič, Bojan Zajec, Marko Žumer
Ljubljana, Slovenia
1st Oct 2009
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2) Experimental methods:
• general description • selection and adaptation for our work.
3) Experimental results on D2 retention in tungsten
1) Motivation for the work
Outline of the talk:
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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: kTVpCVCV ss /.221
1
2
C
Cu
012 uAu
pKC s
21
.. ss KTk
C
V
VA
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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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)
0.0007 0.0008 0.0009 0.0010 0.0011 0.0012 0.0013 0.0014 0.0015
1013
1014
1015
1016
1017
C /
H/c
m3
1/T / 1/K
alumina W silica
800°C
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0.0007 0.0008 0.0009 0.0010 0.0011 0.0012 0.0013 0.0014 0.00151E-11
1E-10
1E-9
1E-8
1E-7
1E-6
1E-5
1E-4
D /
cm2 /s
1/T / 1/K
alumina W silica
Hydrogen diffusivity from 400 °C to 1100 °C calculated from the
same references.
800°C
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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
~ 2109 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 zoneCold 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 = 6.71015 H2/cm2; dp/dt low
W sample inserted intense outgasing followed:
in 45 h C ~ 1.051018 H/cm3.
Residual outgassing at the end: H2 80%, CO 20%
dN/dt 1.31010 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
1 2 3 4 5 6 7 8 9
p m/e m/e m/e m/e dN/dA C dN/dA
mbar 2 3 4 28 (CO) 1015D/cm2 1016D/cm3 1014D/cm2 1 0.0110 0.26 0.49 0.25 <0.01 3.0 4.3 3.8 2 0.0095 0.20 0.48 0.33 <0.01 2.9 4.2 3.6 3 0.0094 0.14 0.44 0.42 <0.01 3.3 4.7 3.9 4* 0.0102 0.64 0.32 0.05 <0.01 1.1* 1.6* 2.2 5 0.0098 0.45 0.44 0.12 0.26 4.1 5.9 4.2
Table 3: Retention of deuterium in tungsten at p = 0.01 mbar and T = 800 °C in 24 hours. The amount is reconstructed from the total pressure change and gas composition at the end of exposure. The number in the first column is exposure number. Exposure No.4 with * was done in hydrogen, but the released deuterium comes from the sample and the thimble loaded previously. Retention of deuterium is expressed as number of atoms per unit area in column 7 and as number of atoms per unit volume in column 8. The amount of deuterium released in the first 24 h after exposure is given in column 9.
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Conclusions
An UHV system with the ultimate sensitivity of detecting flux
~ 2109 molecules/(cm2s) from (into) the sample (A ~ 76 cm2)
was built.
High amount of H2 (C ~ 1.051018 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.