Fusion Engineering and Design 49–50 (2000) 157–161

Novel designs of the ITER gas box liner aimed at reducingtritium codeposition

C. Cazzola, J. Boscary, R. Matera *European Commission, JRC, IHCP, Via E. Fermi 1, 21020 Ispra, VA, Italy

Abstract

Carbon impurities create carbon films saturated by tritium and deuterium onto the gas box liner. The reference gasbox liner (GBL) formed by chevron shaped tungsten tiles is capable of depositing and/or recombining only 5% of thecarbon impurities. The innovative solution presented here increases within the range 102–103 the surface interactingwith the pumped gas by using an array of filaments, either disordered (felt) or ordered (spiral). This paper describesthe experiments carried out to prepare felt/spiral samples and indicates possible ways of fabricating a representativegas box liner mock-up. © 2000 Published by Elsevier Science B.V.

Keywords: International thermonuclear experimental reactor; Gas box; Tritium codeposition

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1. Introduction

Carbon impurities, in the form of carbonatoms, ions and hydrocarbon radicals, are pro-duced in international thermonuclear experimen-tal reactor (ITER) during normal and off-normaloperation as a consequence of physical and chem-ical sputtering and sublimation of the carbonbased armour of the lower vertical target. Eventu-ally, the impurities are transported to the gas boxliner (GBL), where they might form carbon filmssaturated by tritium and deuterium on the cold

surfaces (TB500°C) of the pumping duct and ofthe liner itself. The carbon films are likely to formin a recessed position, where they cannot be easilyremoved. The large tritium inventory thus createdis unacceptable for a safe and economical opera-tion of the reactor.

Experimentally, this phenomenon has been ob-served in the Mark IIa divertor of joint Europeantorus (JET), where carbon flakes saturated withdeuterium were predominantly found on the coldlouvers behind the pumping slot [1]. This zone ischaracterised by a strong flow of gas through aregion of high impurity fluxes, a situation verysimilar to the working conditions expected in theITER divertor.

Tritium codeposition is strongly temperaturedependent and can be largely reduced if carbon

* Corresponding author. Tel.: +39-332-786294; fax: +39-332-789434.

E-mail address: materar@netscape.net (R. Matera).

0920-3796/00/$ - see front matter © 2000 Published by Elsevier Science B.V.

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deposition occurs at high temperature. This is theapproach of the reference GBL design, where thetungsten armour is shaped in such a way to protectthe supporting structure from the surface heat fluxand to condense the carbon impurities by lettingthem interact with the hot tungsten surface at atemperature corresponding to a low tritium code-position rate (T\500°C). The upper temperaturelimit is �1200°C to avoid recrystallisation oftungsten and the formation of brittle tungstencarbides. By adapting the cooling rate and thearmour thickness to the local heat flux, whichvaries in the different liner locations from 0.1 to0.8 MW/m2, it should be possible to get the rightarmour temperature. According to recent esti-mates [2], the reference GBL operates in the righttemperature range; however, the tungsten tiles areainteracting with the exhaust gases is limited andcan deposit a mere 5% of the carbon impurities.An increase by at least a factor of 20 of the armoursurface in the favourable temperature range isneeded in order to allow the hydrocarbon radicalsto undergo enough collisions for deposition.

This paper analyses possible ways to increase thesurface interacting with the pumped gas by using,instead of solid tungsten tiles, an array offilaments, either disordered (felt) or ordered (spi-ral) in order to increase the surface interacting withthe carbon species by orders of magnitude withrespect to the geometrical cross section of the liner.The experiments carried out to prepare felt/spiralsamples with varying armour thickness andfilaments volume fraction and diameter are de-scribed, and possible ways of fabricating a repre-sentative GBL mock-up are schematicallyindicated.

2. Analysis of the reference design and ofalternative solutions

The reference GBL design along with somealternative solutions is schematically sketched inFig. 1. The performance of the different ge-ometries is evaluated by a simple parameter, thesurface multiplication factor (SMF) defined as theratio between the effective area exposed to theexhaust gases and the geometrical cross section ofthe component. An attempt to model the thermalbehaviour of the proposed structures by FEManalysis was unsuccessful, due to the complexityof the geometry and the large number of arbitraryassumptions for the input parameters. Dependingon the local heat flux, neutron heat depositionand cooling pattern, only a part of the exposedsurface would be in the 500BTB1200°C temper-ature range. As the heat transfer regime is domi-nated by thermal radiation, due to the lowthrough-thickness thermal conductivity of the felt,the temperature distribution across the armour isexpected to be rather flat. Assuming conserva-tively that only 20% of the exposed surface workproperly, a SMF above �100 is actually needed.

Other factors to be taken into account in thechoice of the GBL design are the vacuum conduc-tance, the ease of fabrication, the resistance tonormal and off-normal heat loads and the effectof neutron irradiation on the armour material.

2.1. Reference armour [3]

The liner is clad with chevron shaped W ar-mour blocks on stalks set in a cast pure Cumatrix, which is subsequently EB welded to a

Fig. 1. Design geometry of the liner elements, (a) reference designs; (b) radiative tiles; (c) felt tile; (d) double spiral tile. SMF is theratio between the effective area exposed to the exhaust gases and the geometrical cross section of the component.

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Fig. 2. SEM microstructure of a felt tile. The felt parametersare, d=25 mm; Vf=15%; and t=13 mm.

favour of radiative cooling with the followingadvantages, a more uniform tile temperature anda greater hot area; the tile does not have to berigidly fixed to the support structure, therebyseparating the high temperature tile from thecooled structure; if the tile overheats due to dis-ruption loads, then there is no braze joint todamage or Cu interlayer that may creep; there-radiated load to the heat sink can be lowenough to eliminate the copper heat sink leavingonly the steel support structure thus simplifyingthe design and minimising the cost. However, theSMF of �3 is largely insufficient for the com-plete deposition of carbon impurities.

2.3. Felt geometry

Tungsten felt is easily commercially available atlow cost, since it is a by-product of the electric-light bulb fabrication. Tungsten fibres are firstcleaned with caustic soda, then, rinsed in waterand, then, in ethanol with ultrasonic and dried.The felt is formed by compressing the fibres ofdiameter d to the required volume fraction Vf andthickness t. The required tile shape is stabilised bya thermal treatment. The felt tile is brazed to thecopper heat sink as in the reference ITER design.A schematic drawing of the proposed solution isshown in Fig. 1c. Fig. 2 shows the SEM mi-crostructure of a tungsten felt with a tungstenwire diameter d=25 mm, a volume fraction Vf=15% and an armour thickness t=13 mm. Bysimple geometrical considerations, it turns outthat

SMF=4tVf

d

The SMF of the tungsten felt shown in Fig. 2 is�310.

Fig. 3 reports the SMF of a felt armour as afunction of the armour thickness, for Vf=15%and d ranging from 0.01 to 1 mm. The linersurface can be increased by orders of magnitude(10BSMFB100 000) by varying the parameterst, Vf, and d in the range, 1×10−2B tB1×10−1

m, 10BVfB30%, 1×10−6BdB1×10−3 m.The limit is given by the required gas conduc-tance, which scales inversely with SMF.

CuCrZr heat sink, which incorporates the coolantchannel (Fig. 1a). Apart from its poloidal geome-try, this plasma facing component differs from thevertical target in that 8 mm poloidal slots oropenings are incorporated in the upper section ofthe liner, through which the gas from the divertorchannel is exhausted into the pumping ducts inthe cassette body. Chevron shaped W armourblocks on stalks replace the macro-brush of thevertical target. The chevron blocks are approxi-mately 50×35×6 mm thick, the reduced cross-section stalks to provide a reduced thermalconductance. In this way, the stalks allow theplasma facing surface of the liner to be operatedat between 500 and 1200°C over the most likelyheat flux ranging from 0.3 to 0.8 MW/m2, whileminimising trapping of tritium through codeposi-tion with carbon. The chevron avoids line of sightthrough the liner to the cold surfaces of thesupport structure. The SMF of the chevron tiles is�5, largely insufficient to deposit the total ofcarbon impurities content. However, the design issuitable thermal-hydraulically and mechanicallyfor heat flux up to and beyond 1 MW/m2.

2.2. Radiati6e W-tiles

An alternative design consisting in tungstentiles of a simple geometric form (Fig. 1b) has beenproposed by [2]. The tiles are loose fastened to thecooled structure to minimise conductive cooling in

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Fig. 3. SMF of the felt liner as a function of the tile thicknessfor different fibre diameters (in mm) and Vf=15%.

2.4. Double spiral geometry

Tungsten double spirals are commonly used inmany lighting applications. The first spiral iswound round a molybdenum core and then thesecond spiral is formed around a molybdenumcore of larger diameter, as shown in the SEMpicture of Fig. 4. The diameter of the large spiralis 0.386 mm, the wire diameter d=20.34 mm. Atthe end of the forming process, the molybdenumcore is removed by chemical etching. The produc-tion technology of the double spirals is well estab-lished and a large variety of tungsten wirediameters and spiral dimensions can be producedat relatively low cost. The armour is assembled toform armour by interweaving the spirals eachother and inserting a small portion of the spiralsinto copper by the well developed active metalcasting process. The armour is finally brazed orwelded to the water-cooled heat sink. Fig. 1dshows a schematic drawing of the double spiralarmour. Fig. 5 shows a set of spirals placed sideby side and not interwoven before the chemicaletching to dissolve the molybdenum core.

The SMF of the double spiral geometry cannotbe derived by the expression (1), because Vf is nota simple function of the spiral geometry andlayout. However, it can be obtained by measuringthe weight of a single double spiral. For an ar-mour where the spirals are placed side by side andnot interwoven with d=0.02 mm, t=30.6 mm,the weight is 7×10−3 g, Vf=10% and SMF�500.

2.5. Comparison between the two armours

Attempts are at present carried out to preparefelt and spiral armour samples and thereafter arepresentative GBL mock-up with the aim ofoptimising the armour design and validating itscarbon filtering efficiency as a function of thenumber of normal cycles and plasma disruptions.

The analyses and the limited number of testscarried out so far, indicate that both geometriescan easily fulfil the requirement of reaching aSMF\100. The optimisation of the geometryand the choice of the most suited armour for theGBL can only be made after a deeper characteri-

Fig. 4. Fabrication process of the double tungsten spiral. TheSEM picture shows the fine spiral wound around the molybde-num core.

Fig. 5. Set of spirals placed side by side and not interwovenbefore the chemical etching to dissolve the molybdenum core.

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Fig. 6. Current production of metallic felt. The picture showsa steel wool sample.

The choice of the fibre diameter is dictated bythe effect of plasma disruptions. During plasmadisruptions, the re-radiation due to the vapourshielding effect on the vertical target causes anenergy deposition of up to 10 MJ/m2 onto theGBL. The erosion of tungsten is determined bythe loss of melted layer, which is estimated to beup to 100 mm under this energy density. In orderto extend the GBL lifetime it is advisable to usefibres with such a diameter.

3. Summary and conclusions

The presence of carbon films saturated by tri-tium and deuterium onto the cool parts of theGBL is unacceptable for a safe and economicaloperation of ITER. The reference GBL is de-signed with chevron shaped tungsten tiles, whichare capable of depositing and/or recombining amere 5% of the carbon impurities. The conceptpresented here aims at increasing the surface in-teracting with the pumped gas. The solid tungstentiles of the reference design are replaced by anarray of filaments, either disordered (felt) or or-dered (spiral). The performance of the two op-tions is evaluated by the SMF, defined as the ratiobetween the effective area interacting with theexhaust gases and the geometrical cross section ofthe component.

Both felt and spiral armours appear to be veryefficient since the SMF can be orders of magni-tude higher than the SMF of the reference GBLand thus should allow a complete removal ofcarbon impurities without tritium codeposition.

References

[1] J.P. Coad, M. Rubel, C.H. Wu, The amount and distribu-tion of deuterium retained in the JET divertor after the Cand Be phases in 1994–1995, J. Nucl. Mater. 242–243 (1997)408.

[2] I. Archipov, A. Gorodetsky, A. Makhankov, A. Markin, I.Mazul, V. Odintsov, D. Serebrennikov, A. Zakharov, R.Zalavutdinov, Operational Condition Analysis and PossibleApproaches to Liner Design, Final Report Task RS10/98,Russian Federation Home Team, 21st July 1998, p. 117.

[3] ITER Final Design Report, DDD 1.7, Paragraph 2.1.2.[4] C. Ibbott, ISFNT-5, private communication.

sation of both armours, including the measure ofthe through thickness thermal conductivity, car-bon deposition rate, hydrogen inventory in theco-deposited film, fibre loss due to carbide forma-tion and/or recrystallisation, behaviour underplasma disruption (Q up to 10 MJ/m2), vacuumconductance, thermal fatigue behaviour undernormal conditions for the design number of cyclesand reactivity with steam as a function of temper-ature. This work is planned as far as the carbondeposition rate and hydrogen inventory is con-cerned [4].

From the technological point of view, the felt iseasier to be fabricated and its fabrication cost islikely to be similar, if not cheaper, than the costof the reference design.

An example of a felt commonly fabricated isthe steel wool (Fig. 6) present in every kitchen.Transferring this technology to tungsten shouldnot represent a major problem.

The regular pattern of the spiral armour holdspromise of a much higher vacuum conductanceand better surface related properties, since itachieves a larger SMF, Vf being equal.

Both felt and spiral tiles are certainly moreresistant than the chevron tiles under thermalfatigue because of the loose character of the tile.The latter advantage is even more important,when considering the embrittlement, which al-ready occurs in tungsten at the level of 0.01 dpa.