"Jožef Stefan" Institute, Dept. of Surface Engineering and Optoelectronics Deuterium retention and release from ITER-grade: stainless steel, Be and W Vincenc Nemanič Ljubljana, Slovenia

Tritium retention data rarely obtained in fusion reactors (so far, only TFTR and JET used DT fuel)  most of missing data needed for accurate modeling has to be taken in small experimental setups using T at conditions similar to those in tokamaks (high sensitivity, strict safety precautions, expensive) using D instead of T (including this study) (various methods combined to compensate lower sensitivity, additional data could be taken, costs greatly reduced)

EFDA Technology Work Programme: TW6-TPP-RETMET The purpose of our study was to determine deuterium retention 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

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 ~ 2  10 9 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

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 6  23  40 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= 9  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 = 9  mbar/s (i.e. 9.2×10 10 molec. H 2 /(cm 2 s)), registered  C 0 = 1.76  /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

K L (400 °C) = 1.8  cm 4 /s. The recombination coefficient that governs the outgassing at 400 °C for the first 20 hours:

a small difference in dp/dt detected during the exposure  deuterium retention determined from isotope exchange reaction. Pure D 2 "converted" in 24 h into a gas mixture: 0.1 mbar0.01 mbar

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

Beryllium Brush Wellman (S-65C VHP, Ti film on one side) tile size: 2.4  2.0  0.4 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

Fig.1 a. from R.P. Doerner et al. Fus. Eng. & Des 49–50 (2000) 183 The same quality Be (S-65C): "650°C is sufficient to remove any retained H in Be"(?!) small TDS H 2 peaks

Fig.2 a. from R.P. Doerner et al. Fus. Eng. & Des 49–50 (2000) 183 TDS of the same Be sample charged by D ions HD peak 1.2  D/cm 2 retained as subsequently reconstructed by TDS of HD & D 2

Macaulay-Newcombe RG, Thompson DA, JNM (1994) 942 TDS of Be HIP sample loaded by D 2 (24h, 500°C, 0.13bar) 3  D/cm 2 released as HD & D 2 400°C NOTE: rates at 400°C almost at the detection limit

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 ~ 2  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 ~ 7.3  /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 ~ 5.5  H/cm 3 (~ 4.9 appm) i.e. ~ 8  H/cm 2 2) Recombination limited kinetics – 2 types of sites present a minor part ~1.1  H/cm 3 released in the first 20 h (fast) the major part ~4.4  H/cm 3 released 550 h (slow) 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 the observed  C represents a minor part (35%?) of all H(D) assuming j = K L C 2 determined  C ~ 5.5  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

Deuterium retention in ITER-grade tungsten during 24 h exposure at 800 °C (Data incomplete, long-term heat treatment to decrease alumina and sample outgassing, measurements in run)

Conclusions An UHV system with the ultimate sensitivity of detecting ~ 2  10 9 molecules/(cm 2 s) from (into) the sample (~30cm 2 ) was used to measure deuterium retention during the low pressure isothermal exposure of ITER grade stainless steel, beryllium and tungsten. The achieved values are low, but in a reasonable good agreement with published data obtained by deuterium or tritium.

Acknowledgement This work was supported by: Slovenian Ministry of Education, Science and Sport and (EFDA), W6-TPP-RETMET