Progress with tritium removal and mitigation 20056

1
Progress with tritium removal and mitigation
2005-6

• G Counsell1, D Borodin4, P Coad1, J Ferreira8, C
  Grisolia2, C Hopf3,
• W Jacob3, A Kirschner4, A Kreter4, K Krieger3, J.
  Likonen5, A. Litnovsky4,
• Markin3, V Philipps4, J Roth3, M Rubel6, E
  Salancon3, A. Semerok7,
• FL Tabares8, C. Tomastik9, A Widdowson1
• 1EURATOM/UKAEA Fusion Association, Culham Science
  Centre, Abingdon, OX14 3DB, UK
• 2Association EURATOM-CEA, CEA/DSM/DRFC Cadarache,
  13108 St.Paul lez Durance, France
• 3Max-Planck-Institut für Plasmaphysik, EURATOM
  Association, D-85748 Garching, Germany
• 4Institut für Plasmaphysik, Forschungszentrum
  Jülich, Association EURATOM-FZJ
• 5Association EURATOM-TEKES, VTT Processes, PO Box
  1608, 02044 VTT, Espoo, Finland
• 6Alfvén Laboratory, Royal Institute of Technology
  (KTH), Association EURATOM-VR, 100 44 Stockholm,
  Sweden
• 7CEA Saclay, DEN/DPC/SCP/LILM, Bat. 467,91191Gif
  sur Yvette, France
• 8Association Euratom/Ciemat. Laboratorio Nacional
  de Fusión. 28040 Madrid, Spain
• 9Institut für Allgemeine Physik, Vienna
  University of Technology, Wiedner Hauptstraße
  8-10, A-1040 Wien, Austria

2
Outline

• A reminder of the problem
• Advances in modelling tritium retention with
  mixed materials
• New challenges retention in gaps, under
  layers and in the bulk
• Progress with characterising de-tritiation
  techniques and associated problems
• Can we avoid retention in the first place?
• Future thoughts

3
Clear need for T removal schemes

• Acceptable ITER operation 2500 shots before
  maintenance period
• Long term T retention/shot must be
• lt 0.14g/400s shot

• Strategies for T removal essential if
• CFC targets in DT phase
• Removal efficiency must be 80 - 98

4
Be transport will impact C erosion

• gt80 of wall area in ITER is beryllium
• Eroded Be will transport to divertor (as ions)
• ? modify erosion and co-deposition

• Preliminary modelling using local erosion
  deposition model ERO (still many open questions)
• Model assumptions validated against TEXTOR C13
  injection experiments

• Be concentration in plasma plays key role
• Balance of inc. Be coverage on target (dec. C
  erosion) and inc. C erosion due to Be flux
• For a range of Be conc., T/C and T/Be ratios
• ? 0.5g 6.4gT/400s shot

5
Be2C formation potentially significant
ERO modelling of erosion mitigation with Be
seeding in PISCES
Assumptions ? Maximum possible stochiometric
combination of Be2C formed ? No chemical
erosion for Be2C Leads to non-linear evolution
of relaxation time for reduction in chemical
erosion - as observed
6
aCH co-deposits form in tile gaps

• All ITER plasma facing components will be
  castellated
• gt2,000,000 Gaps in ITER (typ. 0.5-1mm x 10mm)
• Increases plasma exposed areas by factor 2 - 5

ITER mock-up
CFC target (90,000 monoblocks) 50 m2 ? 215 m2 W
baffle dome (1.2M rods) 100 m2 ? 460 m2 Be
main wall (300,000 tiles) 680 m2 ? 1290 m2

• CxHy molecules and radicals form aCH
  co-deposits deep in gaps how much and how deep
  is on-going research

CFC tile segments from JET Mk1 divertor, 6mm
gaps Retention in gaps twice that on
plasma-facing surfaces (protected from re-erosion)
7
Potential for significant T inventory

• TZM castellated monoblocks exposed to plasma for
  200s in TEXTOR
• D retention fall-off dependent on GD and Ttile
• Dgap 0.4 - 4 of GD, between low and high
  GD at Ttile 200- 260?C
• Factor 10 decrease in Dgap, 30 ? Ttile ? 200?C

TZM monoblocks from TEXTOR, 0.5mm gaps

• Extrapolation to ITER based on GD from
  B2-EIRENE modelling (Kukushkin, 2005)
• ? 0.5 5gT/400s shot
• Maybe other factors, however
• strong function of gap width
• carbon source (local or remote)
• period of exposure

8
Shaped castellations may help

• Planar and imbricated castellated tungsten blocks
  exposed in TEXTOR
• Co-deposition significantly reduced for
  imbricated blocks
• Also of note -
• Metal intermixing noted in deposits
• Some deposits buried under W layer

9
D/T retention in CFC bulk

• ToreSupra
• 75-85 D retention in short shots (lt30s)
• Up to 100 D retention in long shots (gt100s)
• Retention in short shots easily recovered by He
  glow
• Measurements of C erosion suggest co-deposition
  alone may not explain retention
• ? more than 1 mechanism?

• Retention in bulk CFC being considered for
  high fluence conditions
• Lab studies indicate D retention to several mm
  in bulk
• D inventory ? fluence0.5
• Calcs. suggest this may initially exceed
  co-deposition in Tore Supra

• could affect choice of CFC for ITER
• Flux and time dependence needs more study

10
T-removal through oxidation

• Tritium trapped in aCD/T co-deposits ?
• Oxidation an obvious candidate for detritiation
  through the reaction
• aCD/T O ? COx DTOD2OT2O
• In-situ no need for vessel entry
• Volatile products pumped from vessel
• Several schemes under investigation
• Baking in O2
• ECR or ICR m-wave plasma in O2 or He/O2 mix
• DC Glow discharge cleaning in He/O2 mix
• Studies on-going in both laboratory and tokamak
  environments and both laboratory produced and
  tokamak co-deposited films

11
O2 baking efficient, but at high Twall
RF Antenna protection tile from TEXTOR 180 mm
thick aCD co-deposit

• Molecular chemistry O2 penetrates all regions
  of deposition but .
• Low D removal efficiency below 300?C (cf ITER
  wall bakeout temp 240?C)
• High O2 pressure needed for high removal
  efficiency
• Co-deposit not fully removed becomes flaky
  and peels off
• ? Inhibited O penetration and release of
  volatiles due to carbide formation with
  impurities? WC and BeC may form in ITER
• O3/O2 mix effective at lt200?C and low pressure
  but damage to bulk CFC seems to be too high

12
O-Plasma effective at room temp

• ECR plasma in 100 O2
• Products CO, CO2, H2, H2O

• Erosion rate, nE
• increases with Tsurf ? chemical reactions
• and with bias volts ? collisions
• ? 2 step process surface damage by ion
  bombardment then chemical erosion

• He/O2 mixture
• nE limited by He ion flux at high O2
• ? nE saturates above few O2

13
He/O GDC in the tokamak environment
Cleaning not uniform on all surfaces
200nm and 350nm soft aCH deposit on Si coupons
in Asdex Upgrade

• Asdex Upgrade
• 49h, 25g removed, 7x1018 C-at/s
• ? nE 1.4x1017 C-at m-2s-1
• TEXTOR
• 3h, 5.2g removed, 2x1019 C-at/s
• nE 5.7x1017 C-at m-2s-1
• i.e. 0.075 - 0.3g T/h over 150m2

• CO and CO2 dominant
• T2O 30 times higher than He GDC
• Production saturates at low O

Arcing and pitting occurred on boron dominated
surface layers

• No removal in boronised regions
• B-coated sample coupons and boron coated
  co-deposited tiles unaffected
• - Impact of WC, BeC in ITER?
• or from shadowed areas
• aCH coated samples behind first wall, deep in
  divertor untouched

• Tokamak and Lab studies less clear on removal
  from tile gaps

14
Cleaning in shadowed areas

• Hydrocarbon coated elements at base of
  castellated structure exposed to He/O2 discharge
• 2 orientations
• Directly exposed
• Facing bottom of the chamber (shadowed from
  plasma)
• Cleaning rate nearly identical in both
  orientations
• Suggests atomic oxygen survives several
  collisions with the walls.

• O/O may penetrate several mm into
  sufficiently wide gaps
• ? Castellation and tile gap design may be
  important for ITER

15
Impurities in aCH reduce efficiency

• Tokamak produced aCH co-deposits on W
  substrate efficiently cleaned in He/O discharge

• Similar nE to tokamak GDC
• nE for tokamak co-deposits up to factor 10
  less than for laboratory produced
• 80 co-deposit eroded during first 20 of
  plasma exposure

Fully removed with 6.25 hours lab GDC Gi2.5x1018
m-2s-1, 8mbar, 20 O2 in He

• effect of impurities in co-deposit building up
  at surface?
• W, Be will mix with aCH in ITER

16
Collateral damage and recovery OK
Not all injected O2 is pumped out of the vessel
during GDC

• Some O retained in metal oxides

• O retention higher at larger Vbias
• ? opportunity for optimisation
• H2 discharge effective at removing oxides

• TEXTOR Recovery 66h H2 GDC, 0.5h He GDC
  boronisation
• Asdex Upgrade Recovery 72h baking at 150?C,
  10h He GDC boronisation)
• Will recovery extrapolate to BeO?

17
BeO formation a challenge to oxidation

• Beryllium samples pre-oxidised to variety of
  oxide thicknesses (20-100 nm)
• Samples exposed to hydrogen plasma for 5 - 6
  hours at 300 C - 600 C

• Residual oxide layer thickness always 25 nm
• Thick layers are reduced but thin layers
  actually increase
• Development of an equilibrium state with the
  plasma
• Oxide thickness depends on the plasma
  conditions, especially oxygen impurities

• Complex (yet to be determined) consequences for
  formation and impact of oxygen based
  de-tritiation schemes

18
Alternative chemistry may have a role

• N2 injection into Asdex Upgrade sub-divertor
• Factor 5 reduction in aCH net co-deposition
  rate
• No significant N retention
• Effect not seen with Ar (laboaratory studies)
• Scavenging proposed as one mechanism
• moping-up of reactive radical pre-cursors
• But also alternative explanations -

• Synergistic interaction of H and N at surface
  peaks at 7525
• Erosion rates high in H2/N2 plasmas
• nE up to 1mm/hour for lab deposits
• in ECR plasma
• less than O, but not optimised

19
Alternative chemistry may have a role

• Cryo-trapping assisted mass spectroscopy
• Quantitative analysis of compounds formed
  during discharge cleaning
• Predeposited hydrocarbon films exposed to
  N2/H2 and CH4/N2/H2 plasma glow

• Without CH4
• Chemical sputtering dominates
• HCN molecules are formed and released
• With CH4
• N ions release nitrogenated compounds from
  surface react with cracked CH4
• ? C2H2 dominates, little HCN
• Role of atomic N as yet unknown

20
T-removal through photonic cleaning

• aCH co-deposits have poor thermal conductivity
  compared to substrates (CFC, Be, W)
• Surface heat flux leads to rapid temperature rise
  in co-deposit ? ablation or chemical
  bond-breaking
• Two photonic cleaning schemes under
  investigation
• LASER
• Flash-lamp
• Requires vessel access, but can operate in high
  magnetic fields and in vacuuo, inert gas or
  atmospheric conditions
• Studies on-going in both laboratory and tokamak
  environments and both laboratory produced and
  tokamak co-deposited films

21
Laser cleaning of TEXTOR tile

• Energy density threshold for removal
• Threshold factor 5 lower for co-deposit
  compared to graphite
• ? selective removal

• No difference between active and inert gas
  environment

100mm2 in 2s with 20W Ytterbium fibre laser
(1060nm, 120ns, 20kHz), 2J/cm2 on f250mm spot _at_
40cm ? 0.03 - 0.3g T/h over 150m2
22
Laser cleaning in JET BeHF

• gt200 mm of co-deposit easily removed by single
  scan at low scan rate (40s) and pitch
• At fastest rate (4s), not all deposit removed
  even with 4 passes

Galvo-scanning fibre laser developed for use in
JET Be handling facility
Tritium release only accounts for 10 of tritium
in cleaned region ? micro-particulate?
23
Flash-lamp cleaning of tritiated aCH

• Photon flux from 500J, 140ms flash-lamp
• ?3.6MW
• Rep. rate 5Hz
• Focused using semi-elliptical cavity
• Footprint 30cm2 _at_ 30mm
• ?375MWm-2, 6J/cm2

• Trials now conducted using flash-lamp in JET
  berylium handling facility
• Aim to clean thick, tritiated co-deposit from
  inner divertor CFC tile

• Three positions treated (with varying
  co-deposit thickness and tritiation)
• Tritium release monitored and tile sent for
  SIMS/IBA/SEM analysis

JET 2004 trial showed engineering feasibility of
flash-lamp technology
24
1st demonstration of T-removal

• Total T release 9mg.
• Decreasing efficiency with number of pulses
• 40 of T inventory 70-90 mm co-deposit,
  removed (off gas SEM)
• ? 0.075g T/h over 150m2

• 7mm de-tritiation at surface of treated zone
• ? Consistent with FE calcs of bulk heating
  above 700K
• Build-up of Ni at surface ? explanation for
  roll-over of tritium release/pulse? (similar
  results for Be on other treated tiles)

25
Could prevention be the best cure?

• Hot liner added to PSI-2 linear plasma device
• CH4/H2 added to H2 plasma column

• Cold liner CH4 decomposes to form
  co-deposted layers on liner walls
• Hot liner co-deposit re-eroded to form CH4,
  C2H2 and heavier species
• - 100 C pumped through duct or returned to
  main chamber
• Relies on presence of sufficient atomic
  hydrogen (C2H2 production saturates at high CH4
  flows)

26
The year that was .

• Significant advances during 2005-6 but also
  growing recognition of new challenges -
• Improvements in modelling of retention and
  co-deposition, including with beryllium fluxes
• Importance of retention in gaps, castellations
  and bulk CFC now better understood and
  characterised
• More machines prepared to run with oxidising
  plasmas but difficulties of shadowed regions,
  mixed materials and oxide formation now clear
• Further studies into alternative chemistry
  (apart from oxygen)
• Technological approaches continue to develop
  and first trials with tritiated samples
• A growing understanding that ITER may require a
  combination of techniques to operate within the
  inventory limit

27
Big toolbox needed for ITER

• No single T-removal scheme likely to be
  sufficient lets not close any doors
• Integration of different schemes on different
  timescales will probably be required the good
  housekeeping approach

• Example of T-removal integrated into ITER
  operating schedule
• Extrapolated from predicted/measured T-removal
  rates allowing for future optimisation

28
Key issues for the near future

• Characterise the impact of Beryllium Carbide
  formation in ITER relevant conditions
• CFC bulk retention is it real? Need for
  confirmation of the effect and improved
  characterisation (impact of surface temperature,
  morphology etc.)
• Tile gaps how does deposition in gaps scale
  the ITER? Resolve issue of shadowing on plasma
  removal schemes (e.g. how many wall collisions
  can the chemically active atoms survive)
• Demonstrate impact of repetitive oxidising
  plasmas and plasma recovery on Beryllium
  surfaces
• Explore further the impact of surface
  temperature on deposition in ITER relevant
  conditions
• Begin to develop a serious scheme for operating
  ITER with a CFC/Be/W materials mix in the DT
  phase
• Start to characterise retention in an all metal
  ITER is it a problem or not?

How can the EU-PWI and the SEWG help?
29
Many presentations this year ..
Not a comprehensive list! - G Counsell et
al 33rd EPS, 11th PFMC D Borodin et al 11th
PFMC P Coad et al 17th PSI J Ferreira et
al 11th PFMC C Grisolia, et al 24th SOFT, 17th
PSI C Hopf, et al 17th PSI W Jacob, et al 17th
PSI A Kirschner, et al 17th PSI A Kreter, et
al 17th PSI K Krieger, et al 17th PSI J.
Likonen, et al 17th PSI A. Litnovsky, et al 11th
PFMC A Markin, et al 11th PFMC V Philipps, et
al 17th PSI J Roth, et al 17th PSI M Rubel, et
al 17th PSI, 21st IAEA E Salancon, et al 17th
PSI A. Semerok, et al 21st IAEA FL Tabares et
al 17th PSI, 21st IAEA C. Tomastik, et al 11th
PFMC A Widdowson, et al 17th PSI