"Jožef Stefan" Institute, Dept. of Surface Engineering and Optoelectronics Slovenian Fusion Association (SFA). MHEST Deuterium retention in ITER -grade: stainless steel, Be and W Vincenc Nemanič, Bojan Zajec, Marko Žumer Ljubljana, Slovenia Cadarache, 15 th June 2009

2) Experimental methods: general description selection and adaptation for our work. 3) Results on ITER - grade stainless steel, Be and W 1) Motivation for the work Outline of the talk:

Motivation: tritium retention prediction Basic concepts to predict tritium retention data for metals applied in future fusion reactors: 1) Deuterium data obtained in 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 obtained decades ago.

EFDA Technology Work Programme: TW6-TPP-RETMET The purpose of our study was to determine deuterium retention in 24 hour-expositions in D 2 at p = 0.1 mbar and below ITER grade AISI316 at T = 100, 250 and 400 °C ITER-grade Be T = 100 °C and 250 °C ITER-grade W T = 250, 400 and 1000 °C 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

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

Hydrogen detection mode applied in any of both techniques: 1)Dynamic method, ion current of characteristic mass number applying mass spectrometer is recorded at constant pumping speed or 2) Static method (gas accumulation), pressure recorded by non-ionizing gauges in valved-off system followed by mass spectrometry, instrument located in a separate UHV system

Both techniques types require low hydrogen background since it influences the sensitivity (and discrimination limit when deuterium is applied). The most troublesome is outgassing of the sample holder and its potential simultaneous interaction with hydrogen (H/D). Isotope exchange interaction difficult to distinguish since it runs: in the sample and in the sample holder. Best option for H.metal interaction is applying both techniques ifusion/ourgassing and permeation, since they give complementary data.

Materials most suitable for sample holder: Kovar glass: almost ideal up to 450 °C, no detectable interaction with H/D. (used in our lab for ITER-grade stainless steel and Be) Silica: wide range of T, thermal shock resistance, used for RF heating, but exhibits anomally, noticed in: 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). A minor part of hydrogen is diffusive, isotope exchange in silica or quartz using D 2 unpredictable, quantified work with metal samples troublesome or impossible.

Materials most suitable for sample holder: Pure alumina: at present, the only candidate for W sample holder from 500 °C to 1000 °C. Several exposures of empty thimble to deuterium showed some isotope exchange, too. Resolving the difference when the sample in hot zone or cool zone, still troublesome.

Experimental setup (3 UHV chambers) for infusion/outgassing or for permeation method using H 2 or D 2

Exposure section with calibrated cell, CM and SRG gauges

Exposure section – thimbles: glass or alumina

The ultimate sensitivity determined by the background outgassing rate of H 2 and small volume (~1.3 L). Inner sources of H 2 are: UHV system walls (at R.T.) metal sample (elevated T) sample holder i.e. extension tube (elevated T) QMS (ionization cell itself) The achieved detection limit ~ molecules/(cm 2 s) Various schedules used to convert QMS signals of H 2, HD and D 2 into the absolute units by calibration H 2 /D 2 mixtures. Equal procedure steps applied for all investigated metals

Results: Stainless steel ITER grade (AISI 316, Co (<0.05 wt. %); Nb (<0.01 wt. %) 25 mm O.D tube, 50 mm high A = 74.6 cm 2 V = 4.66 cm 3 wet cleaning, drying 1)sample preparation 2)cut from a massive 45 kg block cm 3

Experimental steps applied for stainless steel (similar for Be and W (glass replaced by alumina)) "blank run" steps (sample at R.T.): UHV system after bake-out: dp/dt= mbar/s UHV system + hot tubular extension exposure (0.1 mbar D 2, 400°C, 24 h) no observable isotope exchange detected sample in a tubular extension moved into the oven and heated to 400°C for 8 days outgassing rate (H 2 ) below dp/dt = mbar/s (i.e. 9.2×10 10 molec. H 2 /(cm 2 s)), registered C 0 = /cm 3

Pressure vs. time curve composed from several cycles, the importance of low outgassing is evident. The observed kinetics is governed by the RLM rather than by the DLM Devation from the RLM noticed after 20 h when hydrogen from strogly bound sites became prevalent

Deuterium retention in ITER-grade stainless steel during 24 h exposure at 400 °C No detectable level of HDO was formed.

Deuterium retention in ITER-grade stainless steel during 24 h exposure at 250 °C More details: V Nemanič, M Žumer, B Zajec, Nucl. Fus., 48, 11, (2008)

Beryllium Brush Wellman (S-65C VHP, Ti film on one side) tile size: cm 3 A = cm 2 V = 3.84 cm 3 Ti film removal, wet grinding, cleaning, SEM, EDXS, XPS

X-ray photoelectron analysis -XPS XPS: very surface sensitive technique XPS depth profiling (by Ar ion sputtering) => in-depth distribution of elements Be covered by Be-oxide BeO film thickness ~ (3 ± 1) nm

Beryllium – hydrogen (H,D,T) interaction published data on diffusivity and solubility very scattered and almost useless for prediction of results (A.A. Pisarev, Fusion Techn., 28, (1995) 1262) no data about hydrogen amount in our sample available a few reports on the same Be quality found as a rough guidance for scheduled measurements

Be "Sample 1" investigated for hundreds of hours by the same procedure as well proved on Stainless steel The amount of hydrogen extracted at 250°C in 72 h was low, C ~ H/cm 3. No clear evidence of interaction with D 2 at 250°C in 24 h and 0.1 mbar Temperature increased to 400 °C for 420 hours resulting in C ~ /cm 3 (6.5 appm) of hydrogen Kinetics perturbed presumably by traces of Ti film deuterium retention data could be innacurate Further precautions introduced for "Sample 2"

Some results taken on Be "Sample 2" at 400°C for 570 h 1)The amount of hydrogen extracted C ~ H/cm 3 (~ 4.9 appm) i.e. ~ H/cm 2 2) Recombination limited kinetics – 2 types of sites present a minor part ~ H/cm 3 released in the first 20 h (fast) could be analog to diffusive H in silica? the major part ~ H/cm 3 released 550 h (slow) could be analog to slowly releasing H in silica at high T 3) retention reconstructed from QMS analysis

24h, 0.01 mbar48h, 0.05 mbar their ratio determined by QMS at the end of cycle

The amount of retained deuterium at specified exposures What could be the amount of H(D) still contained in the sample that makes isotope exchange possible?

A slow decreasing in H 2 (HD, D 2 ) kinetics and intense isotope exchange could be only explained when C in 560 h represents a minor part (35%?) of all H(D) assuming j = K L C 2. determined C ~ H/cm 3

Tungsten Plansee rod size: O.D.= 2.5 cm h = 20 cm machined to a tube: I.D. 2.2cm h = 5 cm V = 5.31 cm 3, A = 76.0 cm 2

Initial experiments in silica using RF heating gave unreliable results due to: simultaneous outgassing of hydrogen and isotope exchange during deuterium exposure, manifested in high HD ratio HD could not be attributed to W only

Hydrogen solubility from 400 °C to 1100 °C calculated from trusted (?) data. Alumina data: Serra, J. Am.Ceram. Soc., 88 (2005) Tungsten data: 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 trusted reported data. 800°C

For ~ 2 mm thick materials, 24 h at 800 °C means: Fo = 0.04 for alumina – will not come to equilibrium Fo = 9.5 for silica Fo = 130 for tungsten In 48 h at 800 °C, alumina released dN/dA = H 2 /cm 2 leading to dN/dt H 2 /(s cm 2 ), low W sample inserted intense outgasing in 45 h C = H/cm 3. Residual outgassing at the end: H 2 80%, CO 20% deuterium exposures

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 ~ molecules/(cm 2 s) from (into) the sample (A~30 cm 2 ) was built to measure deuterium retention during the low pressure isothermal exposure of ITER grade stainless steel, beryllium and tungsten. High amount of H 2 extracted in long term extractions from all three metals prior exposures were feasible.

The setup is prepared now also for: complementary permeation measurements (Stainless steel, Be, W) or tritium permeation barrier films post mortem analysis of suitable shaped D loaded samples. We are interested for cooperation.....

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