"Jožef Stefan" Institute, Dept. of Surface Engineering and Optoelectronics The Role of Hydrogen in Determination of Deuterium Retention in Tungsten Vincenc.

"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

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:

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!

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.

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

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.

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!

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

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.

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.

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.

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

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

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)

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.

Experimental setup

for infusion/outgassing

method D2

D2 exposure (metering) section: calibrated volume cell,

capacitance gauge and SRG gauge

D2 exposure section: alumina allows sliding of W sample ar R.T.

Heated zoneCold zone

Sample sliding

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

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.

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.

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.

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.

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”.

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.

Acknowledgement

This work was supported by MHEST and SFA and by

(EFDA), W6-TPP-RETMET.

Thanks for your attention.