Breeder foam: an innovative low porosity solid breeder material

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<ul><li><p>Fusion Engineering and Design 81 (2006) 455460</p><p>Breeder foam: an innovative low porositb, A</p><p>za, LosWI 537A, USA</p><p>June 2y 2006</p><p>Abstract</p><p>Ceramic foam or cellular ceramics are proposed as a new solid breeder material configuration. Such cellular breeder materialswould have an open cell structure consisting of a network of three-dimensional interconnected ligaments. Ceramic breederfoams could address some of the challenges facing packed breeder beds and potentially enhance thermal performance, increasebreeding ratconductivitiand slowerthe stand-alohighlight the 2005 Else</p><p>Keywords: So</p><p>1. Introdu</p><p>Solid bron pebblewhich servmaterial, reand intercoitate releasT). Howev</p><p> Corresponfax: +1 310 2</p><p>E-mail ad</p><p>0920-3796/$doi:10.1016/jio, and improve structural reliability. Foam densities are not limited to those of mono-sized pebble beds, thermales are higher compared with similarly dense pebble beds; and morphology changes are expected to be much smallerthan in pebble beds. Heat transfer between breeder and coolant walls can be enhanced in principal, by bondingne breeder foam to the structure. Correlations of thermo-mechanical properties of ceramic foams are reviewed topotential advantages of a foam configuration for solid breeders.</p><p>vier B.V. All rights reserved.</p><p>lid breeder; Ceramic foam; Cellular ceramic; Foam properties; Foam processing; Foam applications</p><p>ction</p><p>eeder blanket concepts are typically basedbeds of beryllium and lithium ceramics,</p><p>e as neutron multiplier and tritium breedingspectively. The large internal surface areannected open porosity of pebble beds facil-e and removal of transmutation gases (He,er, long term reliable performance of solid</p><p>ding author. Tel.: +1 310 794 5990;06 4830.dress: shahrams@ucla.edu (S. Sharafat).</p><p>breeder pebble beds remains a field of intense R&amp;Dfor fusion power reactor development [14]. The lowthermal conductivity and wall-interface conductance ofpacked bed configurations result in characteristicallythin bed thickness of a few centimeters. Furthermore,thermo-mechanical performance due to pebble move-ment, sintering, and pebble fracture or disintegrationcontinue to pose significant challenges [4].</p><p>Ceramic breeder foams could address a numberof solid breeder thermo-mechanical performancechallenges, because foams can be tailored to havehigher thermal conductivity, higher heat convection,improved wall-bed thermal conductivity, and stand-</p><p> see front matter 2005 Elsevier B.V. All rights reserved..fusengdes.2005.06.374S. Sharafat a,, N. Ghoniem a, M. Sawana University of California Los Angeles, 420 Westwood Pla</p><p>b University of Wisconsin, Madison,c Ultramet, Inc., Pacoima, C</p><p>Received 8 February 2005; received in revised form 13Available online 18 January solid breeder material. Ying a, B. Williams c</p><p>Angeles, CA 90095-1597, USA06, USA</p><p>005; accepted 13 June 2005</p></li><li><p>456 S. Sharafat et al. / Fusion Engineering and Design 81 (2006) 455460</p><p>alone structural rigidity. Furthermore, sintering inceramic foams is not a major concern, because of theabsence of high stress contact points. These featurescould reduca breeder bfoams havfusion applfeasibilityapplicationtechniquesreferred toand mechaalong withmance of cmaterials idiscussed,tial advantblanket app</p><p>2. Ceramiapplication</p><p>EngineecategorizedFoam strucprismatic owith solid eEngineeredmers, metaporous britconnected,turing techcategories:or space holier [6]. Figthese techn</p><p>Commenation of parea to volow thermlocalized shigh thermthem uniqucoolants unrange of apexchange firefractory</p><p>soot traps, flame rectifiers, and solar radiation collec-tors [6].</p><p>erami</p><p>pen c0%)d to eam isre perbetw</p><p>eramiPI [9]oamse, Sv</p><p>00 PP 104</p><p>sph0.34 m</p><p>Mecha</p><p>am prof thety ofolid fative densityics. Tof un</p><p>h andble 1or thes, suchthe pland t</p><p>ariatiove denw dencompgth an</p><p>Therm</p><p>e pried solvity ce structural and Be material requirements inlanket. However, high density Li-ceramice not been fabricated or researched forications. This paper is meant to address theof foam or cellular Li-ceramics for fusions. Several ceramic foam manufacturingalong with commercial applications are</p><p>. Density based correlations for thermalnical properties of ceramic foams are givenexamples of high temperature creep perfor-eramic foams [5]. Stability of breeder foamn a neutron irradiation environment is notdue to lack of any experimental data. Poten-ages of ceramic breeder foams for fusionlications are highlighted in the conclusions.</p><p>c foam manufacturing ands</p><p>red foams have cellular structures which areas either open cell or closed cell foams.</p><p>tures are an assembly of irregularly shapedr polyhedral cells connected to each otherdges (open cell) or with faces (closed cell).foams have been manufactures from poly-</p><p>ls, glasses, and ceramics. Ceramic foams aretle materials with fully open, partially inter-or closed porosity. Ceramic foam manufac-niques can be classified into three generalsponge-replication, foaming agent based,lder method, which have been detailed ear-. 1 shows examples of foams produced byiques.rcial ceramic foams offer a unique combi-roperties, such as low density, high surfacelume ratio, high stiffness to weight ratio,al and electrical conductivity, and highlytrain and fracture characteristics [7,8]. Theal shock resistance of ceramic foams makesely suitable for spreading flames, fuels, oriformly. Ceramic foams are use for a diverseplications, such as metal melt filtration, ion-ltration, heat exchangers, catalyst support,</p><p>linings, thermal protection systems, diesel</p><p>3. C</p><p>O(709nectethe foor porangecial c100 Psity fvolum8011.76a 60%0.05</p><p>3.1.</p><p>Fotiondensithe sa rellow-dceram</p><p>termslengt</p><p>Tasity ffoam(el),(f ),the vrelatiof loofferstren</p><p>3.2.</p><p>Thpackductic foam structures and properties</p><p>ell ceramic foams exhibit high porositieswith non-uniform spherical-like cells con-ach other by ligaments. The tortuosity ofcharacterized in terms of the pore diameter,inch (PPI) density. Typical pore diameters</p><p>een a few microns to 2 mm and in commer-c foams pore densities range between 10 and. The interconnecting ligaments of low den-provide an enormous surface area per unit. The exposed surface area of a 12% denseI ceramic foam varies from 12.3 104 tom2/m(solid)3, which is equivalent to that oferical packed beds with diameters ranging</p><p>m [10].</p><p>nical and thermal properties</p><p>operty correlations are expressed as a func-relative density (*/s); where * is the</p><p>the cellular solid and s is the density ofrom which the foam is made. In general,ensity of 0.3 is the cut-off value betweenfoams and higher density micro-cellular</p><p>he relative density of foams is expressed init cell geometric features, such as ligamentthickness.lists correlations based on the relative den-rmal and mechanical properties of open cell</p><p>as stiffness (E*), the elastic collapse stressastic collapse stress (pl), the crush strengthhe fracture toughness (KIC). Fig. 2 showsn in relative crush strength as a function ofsity for ceramic foams [11]. Foams made</p><p>sity engineering ceramics such as aluminaaratively high strengths up to 80 MPa crushd 25 MPa modulus of rupture [12].</p><p>al conductivity</p><p>mary operating limitations of a sphereid breeders are due to low thermal con-aused by small pebble-to-pebble contact</p></li><li><p>S. Sharafat et al. / Fusion Engineering and Design 81 (2006) 455460 457</p><p>Fig. 1. Ceramic foam examples; (a) TiO2 foam: positive replicated of polyurethane foam [19]; (b) pyrolyzed ceramic SiCSi3N4 compositefoam [7]; (c) micro-cellular SiOC foam using a foaming agent and pre-ceramic polymer [9]; Placeholder techniques, SEM of macro-porousceramic material prepared using suspended ceramic nano-particles with polymer spheres: (d) -Al2O3, (e) TiO2, (f) 3Y-TZP [20].</p><p>areas (Fig. 3a). At 25 C bulk Li4SiO4 has a thermalconductivity of about 2.8 W/m K, which decreases toabout 0.25packed conTable 1, 62conductivitfactor of 2.bed.</p><p>Table 1Correlations fopen cell foam</p><p>Property</p><p>Density</p><p>Stiffness</p><p>Elastic collap</p><p>Plastic collap</p><p>Crushing stre</p><p>Fracture toug</p><p>Creep</p><p>Thermal cond</p><p>C1 (1) is a cmodulus, l theflow parametestrength, y the</p><p>The packing fraction of a monosphere sized bed islimited to</p><p>ently ldensiom tea therabout</p><p>ed bedW/m K in a 62% volume fraction spherefiguration. Based on the correlation given in% dense Li4SiO4 foam could have a thermaly as high as 0.63 W/m K, which is almost a5 times higher than that of a sphere packed</p><p>inherwithAt rohavetor ofpackor mechanical and thermal properties of low densitys [6,8]</p><p>Formulas</p><p>= C1(</p><p>tl</p><p>)2</p><p>EEs</p><p> 1.0(</p><p>s</p><p>)2</p><p>se stress</p><p>elEs</p><p> 0.05(</p><p>s</p><p>)2</p><p>se stressply</p><p> 0.3(</p><p>s</p><p>)3/2</p><p>ngth f</p><p>fs 0.2</p><p>(s</p><p>)3/2</p><p>hness KIC</p><p>fs 0.65l</p><p>(s</p><p>)3/2</p><p>f0</p><p> 0.6n+2</p><p>( 1.7(2n+1)n</p><p>0</p><p>)(s)(3n+1)/2</p><p>uctivity s</p><p> 0.35(</p><p>s</p><p>)</p><p>onstant which depends on cell geometry, E the Youngsstrut length and t the strut thickness, n the diffusionalr; subscripts: f is the foam, s the solid, fs the failureyield.</p><p>Fig. 2. Variatceramic foamstrength) [11]64%. However, foam structures are notimited in density and it is feasible that foamsties above 80% can be developed [13,14].mperature, 80% dense Li4SiO4 foam couldmal conductivity of 0.78 W/m K, or a fac-three times higher than that of a 62% sphere.ion of relative crushing strength as a function of relativedensity (dotted line is based on alumina solid fracture.</p></li><li><p>458 S. Sharafat et al. / Fusion Engineering and Design 81 (2006) 455460</p><p>Fig. 3. Samp[1,21].</p><p>3.3. Breed</p><p>One ofconductioners and acstanding stof being boicantly impand coolanlenges havceramic fobeen bondesition to de[15].</p><p>3.4. Thermal creep</p><p>reactor creep models of sphere packed solider bed</p><p>e), the(c),</p><p>. Reimrience0 C atempely relec operseparales [1]breed</p><p>vior, f, suchInbreedtic (creepnentsexpeat 85highuous</p><p>cycliwallpebb</p><p>Abehamentles of (a) Li3SiO4 pebbles and (b) Al2O3 foam</p><p>er wall interface thermal conductivity</p><p>the critical blanket design issues is heatacross the interface between solid breed-</p><p>tively cooled structures. Foams are free-ructures and therefore offer the potentialnded to structures. Bonding could signif-rove thermal conductance between breedert wall, however significant material chal-e to be overcome in developing reliableam-to-metallic bonds. Tungsten foam hasd to structures using chemical vapor depo-posit a tungsten face sheets onto the foam</p><p>Furthermorwas shownmuch lowedense opento occur by108 and 1The activatwas typicacreep strailess than 6behavior b[5], at 900Li4SiO4 fo0.02% com</p><p>4. Structu</p><p>DellOrpebble bedlarge elasterable bedCyclic thein pebble fbetween peduring thernot inherenand fragmeized crackistrengths oas high ass are complex, in that creep depends on elas-rmal expansion (th), swelling (sw), thermaland on time-dependent plastic (pl) compo-ann and Worner have shown that Li4SiO4</p><p>s thermal creep strains of the order of 3%fter 100 h at 2.2 MPa uniaxial load [16]. Atratures thermally induced loads are contin-ased by creep of a packed bed. However,ation can result in partial sintering, breeder-tion/contact cycles, and fragmentation of.</p><p>er-foam would have an entirely differentoremost due to absence of material realign-as caused by initial loading of pebble beds.e, the creep behavior of the ceramic foamsto follow that of bulk ceramics except at</p><p>r stresses [5]. Compressive creep of 30%-cell Al2O3 1200 and 1500 C was shown</p><p>diffusional flow for strain rates between06 s1 for stresses in the range 20100 kPa.ion energy for steady state creep of foam</p><p>l of that for dense Al2O3 [5]. At 2 MPa then rate of 80% dense Li4SiO4 at 900 C is 1010 s1 [17]. Based on similar creepetween foams and bulk ceramic materialsC and 2 MPa loading a 80% relative denseam would have a creep strain of less thanpared with 3% for a sphere packed bed.</p><p>ral integrity and fragmentation</p><p>co et al. [1] have demonstrated that breeders exposed to several MPa of loading undergooplastic deformations along with consid-height reduction due to bed compaction.</p><p>rmal loading of the pebble beds resultedragmentation. The point-to-point contactbbles introduced large stress concentrationsmal loading. Such stress concentrations aret to foam structures. For pebbles sinteringntation is a concern, while in foams local-</p><p>ng of individual ligaments may occur. Crushf open cell 30% dense Al2O3 foams are80 MPa [12]. The crush strength of foams</p></li><li><p>S. Sharafat et al. / Fusion Engineering and Design 81 (2006) 455460 459</p><p>increases with relative density (see Fig. 2) [11]. Toavoid foam fracture, the combined thermal expansionplus swelling induced loads would have to be keptbelow the f</p><p>As longof the breetry of a foawith minim</p><p>5. Tritium</p><p>Tritiumthe availabrial. The g12.6% relameasuredwhich is c0.582 10having a reto maintaindense foambled to aboabout 0.1 mthicknessesmaximumfactor of 3 frates in eqsignificantllengths.</p><p>6. Swellin</p><p>At 3 at(V/V0) of2.7, and 2%Helium retwith increahelium reteand 900 Cin breederbecause difthose in peretention inever, becaustructureshave to be</p><p>7. Conclusions</p><p>A number of solid breeder packed bed challengesbe a</p><p>er cod netwcontial suty cewide</p><p>s. Theretentwere bead i</p><p>otentiaresul</p><p>that bfor fu</p><p>owled</p><p>e authurgen), Pro, Dr.</p><p>chen,gh Usugg</p><p>cation</p><p>rence</p><p>. DellOhermo-ebble b2004) 1. Reimahanics oes. 61. Tanak</p><p>n Japan2000) 2. Piazzulewiczelium c2002) 8.C. Gorellanooam crush strength.as burn-up does not result in disintegration</p><p>der material itself, the macroscopic geome-m breeder structure should be maintainableal internal damage.</p><p>release</p><p>release depends among other things onle open surface area of the breeder mate-eometric surface areas, Sv, of commercialtive dense foams with 30 PPI have been</p><p>to be of the order of 0.423 104 m2/m3,lose to that of a spherical packed bed,</p><p>4 m2/m3 made of 0.5 mm diameter pebbleslative density of about 60% [18]. In ordersuch large surface areas in a 60% relativethe pore density would have to be dou-</p><p>ut 60 PPI and the pore diameter reduced tom [6]. Such foams would have ligamentof about 0.13 mm, which means that the</p><p>tritium diffusion path length is reduced by arom that of 0.5 mm pebbles. Tritium releaseuivalently dense foam structures would</p><p>y increase due to shorter diffusion path</p><p>g and helium-retention</p><p>.% 6Li-burnup the volumetric swellingLi4SiO4 has been measured to be about 0.4,at 500, 700, and 900 C, respectively [17].</p><p>ention of Li4SiO4 was reported to decreasesing temperatures. At 1 at.% burn-up of 6Li,ntion was 0.7, 0.6, and 0.06% at 500, 700,, respectively [17]. Helium release ratesfoam structures are expected to be higher,fusion path lengths in foams are smaller thanbbles beds. Thus swelling, due to heliumbreeder foam ligaments could be less. How-se of lack of any data on ceramic foam</p><p>in an irradiation environment, this wouldverified experimentally.</p><p>couldbreednectevidesinterndensifor amenttiumrialslie ahthe pcouldgestsficial</p><p>Ackn</p><p>ThDr. JmanyItaly)Munboroumanyappli</p><p>Refe</p><p>[1] GTp(</p><p>[2] Jc</p><p>D[3] S</p><p>i(</p><p>[4] GLh(</p><p>[5] KAddressed by developing a foam or cellularnfiguration. The existence of an intercon-</p><p>ork of open porosities and ligaments pro-nuous thermal conduction paths and largerfaces for tritium and helium release. Lowramic foams are produced commerciallyvariety of applications in severe environ-rmal, mechanical, creep, swelling, and tri-ion relate...</p></li></ul>

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