Fusion Engineering and Design 58–59 (2001) 407–410

A high porous-matrix 2D-SiCf/SiC as permeator for theefficient recovery of tritium solved in Pb–17Li with compact

and efficient columns®

Luis A. Sedano *Uni�ersidad del Paıs Vasco/E.S.I.I.T, Alda. de Urquijo s/n, 48013 Bilbao, Spain

Abstract

The economical recovery of tritium from the breeding materials represents a major issue in the conception of futurefusion plants. The satisfactory structural compatibility of Pb–17Li with SiCf/SiC and the large hydrogen ratesthrough it, suggest the use of a high-porosity-matrix (HPM) 2D-SiCf/SiC as permeator membrane for the recovery oftritium dissolved in Pb–17Li. Uncertainties concerning Pb–17Li with SiCf/SiC interaction are identified in order toanalyze the main feasibility issues of SiCf/SiC extractors. For a wide range of operating modes the ideal recoveryefficiencies of a HPM 2D-SiCf/SiC column are obtained by the numerical implementation of a general dispersionmodel. The comparison of the obtained efficiencies with those of the actual bubbling columns demonstrates thepotential attractiveness of the proposed extractor concept. © 2001 Published by Elsevier Science B.V.

Keywords: High-porosity-matrix (HPM); 2D-SiCf/SiC; Pb–17Li; Tritium

www.elsevier.com/locate/fusengdes

1. Uncertainties concerning the Pb–17Li vs.SiCf/SiC interaction

The characteristics of 3D-SiCf/SiC enable itsconsideration as structural material for the designof reliable Pb–17Li blanket modules [1,2]. TheSiCf/SiC applications, its properties database, itsmanufacturing and the joining techniques of largemono-block structures are continuously expand-ing [3,4]. The main issues for the use of a high-porous-matrix (HPM) 2D-SiCf/SiC permeationmembrane (ex. Cerasep®N2-1) used in the presentwork (Table 1), are analyzed hereafter.

1.1. High-temperature SiCf/SiC corrosion by Pb–17Li under oxidizing/reducing conditions

The contiguous use of SiCf/SiC and Pb–17Licould be constrained by corrosion problems. Ex-perimental tests show that SiC decomposition inpure H2 is not significant below 1173 K. Thethermodynamic limit for such decomposition is�2000 K. Measurements performed with staticalloy for more than 1500 h at 1073 K show thechemical compatibility of the eutectic Pb–17Liwith SiC. These measurements also prove thecorrosion of SiC samples, by pure Li above 873 Kwith the clear formation of a Li2C2 brittle phase[5]. Pb–17Li attacks mainly the interfacial Cpresent between the fibers and matrix. The natural

* Tel.: +34-94-601-4277; fax: +34-94-601-4159.E-mail address: inpsemil@bi.ehu.es (L.A. Sedano).

0920-3796/01/$ - see front matter © 2001 Published by Elsevier Science B.V.

PII: S0920 -3796 (01 )00471 -9

L.A. Sedano / Fusion Engineering and Design 58–59 (2001) 407–410408

Table 12D-SiCf/SiC [7] (Cerasep® N2-1) data

Cerasep® N2-1Range, 293�T�1673 K

2DDimensionality9–5.7� (W m−1 K−1); �, �2400 kg m−3

�Cp (J kg−1 m−3) 620–1200Tensile stress in plane (MPa) 285–270Elongation (%) 0.65–0.78Young Mod. (x, y) (GPa) 230–170Fiber �40 vol.%. and micro-porosity

�10%Translaminar shear stress (MPa) 300–280

Fig. 1. Permeation rates through SiCf/SiC (p=1 kPa, �z=1mm) membranes (A, theoretical), differently coated Al–Si2D-SiCf/SiC membranes [9] (B, coated at 1223 K; C, at 1023K; D, at 993 K) and for a structural steel [11] (E).

self-healing at the SiCf/SiC surface is favored bythe formation of SiO2, under oxidizing conditions.CO and SiO mediate the active oxidation of SiC,especially at low oxidation potentials: PO2

�10−6

Pa and T�1273 K. Oxygen effects are directlyrelated to the bond breaking of fibers i.e. theceramic composite lifetime. The possiblereduction of the oxygen-derived aging effects onSiCf/SiC due to the in-contact presence ofPb–17Li needs the accurate experimentalquantification.

1.2. Tritium leak rates through Cerasep®N2-1

Primigenial studies on helium/tritium transportin the bulk of SiC, showed chemical vapor de-posited (CVD)-SiC to be quite impermeable tothese gases [6–8]. Such results are not applicableto chemical vapor infiltration (CVI)-SiC in 2D-SiCf/SiC Cerasep®N2-1. When measuring, by apermeation technique, the hydrogen leak ratethrough Cerasep®N2-1 the observed rates are sohigh that the pressure signal is quickly out of therange [9]. Large helium leak rates through HPMceramic composites have also been measured [10].Fig. 1 compares the hydrogen theoretical rates JP

through an helium saturated porous media of abare SiCf/SiC sample, the measurements of hydro-gen gas-phase permeation rates through differentcoated membranes [9], and measurements for astainless steel wall [11].

In the blanket channels the tritium is mainlytransported in solution in the Pb–17Li or‘trapped’ within helium cavitation bubbles bred

under irradiation. Once crossed the segment head-ers, the alloy arrives to the tritium recovery unitswith homogenized tritium radial concentrations.The diffusion step in the bulk of the alloy channelcould be then, in a first approximation, neglected.The tritium self-diffusion in helium saturated con-nected pores is a very fast process [12]. Thus, at agiven temperature and depending on concentra-tions, the diffusion through the alloy boundarylayer or the molecular recombination at the largealloy/composite specific interfacial area will deter-mine the kinetics of the recovery of dissolvedtritium.

1.3. Hermetic characteristics of Cerasep®N2-1 toPb–17Li

A (CVI)-SiCf/SiC presents micro-porosity be-tween fibres and macro-porosity between threadsmade up of hundreds of fibres. The low Pb–17Liviscosity and its very large surface tension inducesthe liquid metal percolation through the porousmembrane by viscous drag flow (VDF) and/or byintrabundle (ib) or extrabundle (eb) capillaryeffects.

The details of the theoretical evaluation of theVDF and ib/eb Pb–17Li rates, for 600�T (K)�1050 and loading pressures up to 10 MPa, are

L.A. Sedano / Fusion Engineering and Design 58–59 (2001) 407–410 409

provided in a previous study [13]. The maximumvalue obtained �10 cm3 per year per m2 corre-sponds to (eb) capillary flows. Such value meansvery long membrane degradation times by thealloy intake and then potentially long operationtimes. Such theoretical values need to be confi-rmed experimentally with parametrical tests.

2. Sizing and performances of anextractor/permeator Cerasep®N2-1 column

A bubbled extractor consists of a cylindricalchannel in where the Pb–17Li flows from thetop to the bottom and of a nozzle at the bot-tom of the column and produces a flux of risingbubbles to a free surface. The flux of dissolvedtritium atoms to the ascending bubbles reducesprogressively the axial tritium concentration inthe column [14]. Irrespective of mixing effectsbetween phases and dilution factors, the efficien-cies (�) of a bubbled extractor are intrinsicallyconstrained by the low product of the bubbleinterfacial area per unit of volume, ALB (m−1)[15] and the low mass-transfer coefficient to theextracting bubbles, KD (m s−1) [16,17].

In the proposed concept the tritium dissolvedin Pb–17Li is allowed to permeate through theCerasep®N2-1 membrane. The permeating tri-tium is recovered by a fast sweeping flow ofhelium at the external face of the wall. A nu-merical procedure has been developed to obtain� values without dilution restrictions using dis-persion models [18] for a bubbling/permeatorcolumn [19].

Fig. 2 shows the attractiveness of the pro-posed concept by comparing the iso-efficiencyand the sizing of the proposed concept at refer-ence working conditions with those of a purebubbling column.

3. Discussion and conclusions

The construction of compact and high effi-cient HPM 2D-SiCf/SiC tritium recoverycolumns does not have primary drawbacks.Nevertheless, experimental tests are needed in

order to qualify the aging effects, the Pb–17Liintake flow rates and the HPM 2D-SiCf/SiCpermeability to the tritium dissolved in contigu-ous Pb–17Li. Merit iso-efficiency lines for sizinghave assumed perfect contactor transfer rates2DM/R�z (DM, inter-pore tritium diffusivity inthe membrane of radius R and thickness �z)that might be modified if other interfacial mech-anisms act upon the permeation rates.

Other than the notorious increase of recoveryefficiencies, the potential advantages of the pro-posed concept are significant. Complex geometryfor the increase of the total bubble residencetime in bubbled column is avoided. The veryfast tritium permeation through the walls re-duces the radioactive inventories improving thesafety of the fuel recovery unit. With a compactefficient extractor series of extractors, in practiceequivalent to large extractors, are not required.For the increase of the transfer rates to bubblesthe actual solution consisting in the heating ofthe outgoing Pb–17Li flow consuming �0.11MW K−1 [20] would be avoided. Then, theeconomy of tritium recovery system is improved.

A compact prototype is at present being builtto experimentally confirm the reported capabil-ities.

Fig. 2. Extractor iso-efficiency and sizing for conventional [20](left) and the proposed extractor–permeator column made ofSiCf/SiC (right). (T=623 K; gas pressure in the injectionnozzle, P0=0.5 MPa; gas flow rate, �g0=10 mol s−1; alloyflow rate, �l=10 l s−1; inlet tritium concentration in the alloy,cT(H)=10−2 mol m−3; wall thickness, �z=1 cm).

L.A. Sedano / Fusion Engineering and Design 58–59 (2001) 407–410410

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