"Jožef Stefan" Institute, Dept. of Surface Engineering and Optoelectronics The Role of Hydrogen in Determination of Deuterium Retention in Tungsten Vincenc Nemanič, Bojan Zajec, Marko Žumer Ljubljana, Slovenia 1 st Oct 2009

2) Experimental methods: general description selection and adaptation for our work. 3) Experimental results on D 2 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 H 2 or D 2 or T 2 using various techniques.

EFDA Technology Work Programme: TW6-TPP-RETMET The purpose: determining deuterium retention in 24 hour- expositions in D 2 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, 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: K s – solubility constant The new equilibrium state (p 2, C 2 ) from initial C 1 (or p 1 ) can be calculated for V – system volume, V s – sample volume: 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 p upstream 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 D 2 & 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 D 2 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) Tungsten data: R Frauenfelder, JVST, 6 (1969) Silica data: RW Lee, RC Frank, DE Swets, J Chem.Phys., 36 (1962) (diffusive H) 800°C

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/d 2 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 ~ molecules/(cm 2 s) at A ~ 76 cm 2. Various schedules used to convert QMS signals of H 2, HD and D 2 into the absolute units by calibration with H 2 /D 2 mixtures.

Experimental setup for infusion/outgassing method D 2

D 2 exposure (metering) section: calibrated volume cell, capacitance gauge and SRG gauge

D 2 exposure section: alumina allows sliding of W sample ar R.T. Heated zone Cold 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 cm 3, A = 76.0 cm 2

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 = mbar/s 3 h to 800 °C, followed by 48 h at 800 °C, 150 cm 2 of alumina released dN/dA = H 2 /cm 2 ; dp/dt low W sample inserted intense outgasing followed: in 45 h C ~ H/cm 3. Residual outgassing at the end: H 2 80%, CO 20% dN/dt H 2 /(s cm 2 ), 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

Conclusions An UHV system with the ultimate sensitivity of detecting flux ~ molecules/(cm 2 s) from (into) the sample (A ~ 76 cm 2 ) was built. High amount of H 2 ( C ~ H/cm 3 ) had to be extracted in long-term heating cycles at 800 °C before D 2 exposures were possible and the retention became detectable. The observed amount of retained D 2 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 H 2 /D 2 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.