Ceramic breeder material development

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  • Fusion Engineering and Design 41 (1998) 3138

    Ceramic breeder material development

    N. Roux a,*, S. Tanaka b, C. Johnson c, R. Verrall d

    a Commissariat a` lEnergie Atomique DTA:CEREM:CE2M:LECMA C.E. Saclay, 91191 Gif sur Y6ette, Cedex, Franceb The Uni6ersity of Tokyo, Department of Quantum Engineering and Systems Science, 7-3-1 Hongo Bunkyo-ku, Tokyo 113, Japan

    c Argonne National Laboratory, Chemical Technology Di6ision, 9700 South Cass A6enue, Argonne, IL 60439-4837, USAd Atomic Energy of Canada, Chalk Ri6er Laboratories Fuel and Fuel cycle Di6ision, Chalk Ri6er, Ontario KOJ 1JO, Canada

    Abstract

    Lithium-based ceramics have long been recognized as promising tritium-breeding materials for fusion reactorblankets. In particular, their high thermal stability and chemical inertness are favorable safety characteristics. Themost important qualification for a candidate ceramic breeder material is most likely, the ability to withstand the rigorsof long-term irradiation at high temperature and under large temperature gradients. As a group, the lithium-basedceramics have shown good irradiation behaviour and excellent tritium release characteristics. Individual materialsperformance will depend upon the actual application, namely, pebble bed versus pellet concept, higher versus lowercooling temperature, etc. Recently Li2ZrO3 and Li2TiO3 were selected as the breeder material for the ITER breedingblanket due to their excellent tritium release behaviour at low temperature. Issues being addressed in support ofcurrent blanket design studies will be highlighted. 1998 Elsevier Science S.A. All rights reserved.

    1. Introduction

    Lithium containing ceramics are recognized asattractive tritium breeding materials for fusionreactor blankets. Indeed, their inherent thermalstability and chemical inertness are significantsafety advantages. When ceramic breeder researchwas initiated in the late 1970s, relevant propertiesdata were scarce or nonexistent for the lithium-based ceramics. Initial screening of candidateswas mainly based on examining physical andchemical characteristics and neutron activation.Li2O, LiAlO2, Li2ZrO3, and Li4SiO4 were retainedfor further investigation by research groups in theUSA, Japan, Canada, and the European Union

    (EU). An extensive R&D effort focused on deter-mining properties of unirradiated materials andon designing irradiation experiments in order tounderstand and quantify the effect of neutronirradiation on properties characteristics and onrecovery of generated tritium. With the publica-tion of these data, the relative merits of the candi-dates were shown and interest changedaccordingly. Less consideration is now given toLiAlO2 due its modest tritium release perfor-mance while Li2TiO3 has emerged as having sev-eral attractive characteristics.

    Several breeder blanket design options havebeen developed such as the high-temperature wa-ter-cooled or helium-cooled DEMO reactor con-cepts and the low-temperature water-cooled ITERdriver blanket concept. The ceramics under con-* Corresponding author.

    0920-3796:98:$19.00 1998 Elsevier Science S.A. All rights reserved.

    PII S0920-3796(97)00140-3

  • N. Roux et al. : Fusion Engineering and Design 41 (1998) 313832

    sideration exhibit different characteristics whichcan make one ceramic more adaptable to a spe-cific blanket concept. Currently, Li2O is consid-ered for the DEMO blanket being developed inJapan with Li2TiO3 as the alternative, a newformulation of Li4SiO4 (Li4SiO4SiO2TeO2)for the helium-cooled pebble bed (HCPB) DEMOblanket developed by the EU with either Li2ZrO3or Li2TiO3 as the alternative, and Li2ZrO3 andLi2TiO3 for the ITER driver blanket. The focus ofthe ceramic breeder research is now more orientedtoward design issues than it was in the past. Theobjective of this paper is to highlight the issuesbeing addressed in support of current blanketdesign studies rather than to review the extensivework and numerous results obtained in past years[1].

    2. Current R&D activity and main results

    The breeder blanket design program has severalcritical needs for the properties data base,including:

    Development of fabrication methods for theceramic breeder materials,Laboratory testing and evaluation of materialsperformance,Determination of irradiation behaviour of can-didate breeder materials.

    Moreover, fundamental studies are required tobetter understand and:or clarify several complexphenomena governing tritium release and breedermaterial behaviour. Finally, consideration shouldbe given to activation characteristics.

    2.1. Fabrication of ceramic breeders

    There is recognition that significant quantitiesof ceramics will be needed in the near future forthe fabrication of ITER test blanket modules andfor the ITER driver blanket. An effort has beeninitiated to evaluate fabrication process develop-ment. Among the fabrication issues is the hygro-scopic nature of several candidate lithiumceramics. Sensitivity to moisture increases as thelithium oxide content increases and as the specificsurface area of the ceramics increases. Due to the

    deleterious effects of moisture adsorption on ma-terials properties, precautions will have to betaken during fabrication, during storage of theceramics before loading in the reactor, and alsoduring loading to ensure material integrity.Avoiding moisture contamination of hundreds ofkilograms of materials is not a trivial problem. Inthis respect, sintered Li2TiO3 exhibits very littlehygroscopy, which is a significant advantage.

    Among the options for the tritium breedingblanket design, both pebble and pellet configura-tions have been considered. Presently, the pebbleconfiguration is the preferred option in most blan-ket designs due to the potential advantages in theassembly of blankets with complex geometry andin the relief of thermal stress and irradiationcracking. Among characteristics which governseveral relevant ceramic pebbles properties areshape, size, and density. Though spherical shapeis desired, there is no experimental evidence thatslight deviation in spherical form is critical. Peb-ble size is dictated by both design (pressure drop,heat transfer, packing fraction) and material char-acteristics (thermal stress and irradiation crackingresistance). Thus, the desired pebble diameter is inthe 0.11.0 mm range, with those ceramics ex-hibiting poorer thermal, mechanical, and irradia-tion behaviour being limited to the smaller size.For tritium breeding ratio (TBR) considerations,the density of the pebbles should be near theoret-ical to ensure a maximum smear density for thepebble bed.

    A number of methods are available to producepebbles, however, few can simultaneously meetcurrent shape, size, density, purity, yield, andproduction rate requirements. Processes being ex-plored or developed include:

    (a) A melting:spraying process was used atFZK, in collaboration with Schott Glaswerke, forthe production of 0.10.2 mm and 0.250.63 mmLi4SiO4 and Li4SiO4SiO2 pebbles [2], and 0.250.63 mm Li4SiO4SiO2TeO2 pebbles [3]. Afterannealing, spherical pebbles of 98% TD exhibitingsatisfactory mechanical strength were obtained. Amelting:dropping process was used by JAERI incollaboration with Mitsubishi to produce 1 mmLi2O spheres [4].

  • N. Roux et al. : Fusion Engineering and Design 41 (1998) 3138 33

    (b) Sol-gel type processes are being investigatedat JAERI, with NFI, to produce 1 mm Li2O and1.6 mm Li2TiO3 pebbles [5]. They are also beingemployed at ECN to produce 0.51.0 mmLi2TiO3 pebbles [6]. In all cases, the pebble densi-ties were less than 80% TD.

    (c) A process consisting of extrusion,spheronization, and sintering has, for severalyears, been used by AECL to produce 1.2 mmLiAlO2, Li2ZrO3, and Li2TiO3 pebbles in collabo-ration with Ceramics Kingston [7]. Material den-sities are in the 8090% TD range. Good yieldand high production rates can be expected. Usingthe same process, preliminary trials were made atCEA, with Pechiney, to produce 1 mm Li2ZrO3pebbles [8].

    (d) An agglomeration:sintering process hasbeen used by JAERI, in collaboration withKawasaki, for producing 1 mm Li2O, Li4SiO4,and Li2ZrO3 pebbles. Pebble densities in the 90%TD range were obtained [9]. This process is alsobeing investigated at CEA for producing 1 mmLi2TiO3 pebbles. Pebble density of 90% TD andgood mechanical strength were obtained [8].

    Lithium ceramics in pellet configuration remainan option even though it is now not given asmuch consideration as in the past. Pellet fabrica-tion makes use of proven technologies in theceramic industry. Pressing and sintering of ce-ramic powders is an easy, inexpensive process thathas already been demonstrated on the industrialscale. Pellets and rectangular blocks can be easilyobtained to a few centimeters in size with excel-lent material homogeneity and controlled density.Thus, LiAlO2, Li2ZrO3, and Li2TiO3 pellets meet-ing dimensional, microstructural, and purity char-acteristics were successfully produced by Pechineyin collaboration with CEA [10].

    2.2. Laboratory testing and materials performance

    2.2.1. Lithium transportVaporization behaviour of candidate breeder

    ceramics is one of the most important propertiesconsidered in material selection and in the designof the tritium-breeding blanket. Limitation of theceramic breeder operating temperature may be

    dictated by the potential for lithium loss duringtritium recovery by the purge gas. To evaluate themaximum allowable temperature for the ceramicbreeder, vapour pressures over the lithium ceram-ics were measured as a function of temperature, invacuum, and in the presence of D2 and or D2O.

    Measurements were made by Knudsen effusionmass spectrometry [11,12]. Candidate ceramicsshow the following ranking for increasing lithiumoxide vapour pressures: LiAlO2, Li2TiO3,Li2ZrO3, Li4SiO4, and Li2O [12]. This ranking wasconfirmed by results reported in [13], which showthat for a D2 partial pressure of 100 Pa (currentH2 pressure in the helium purge gas), Li2TiO3 andLiAlO2 should be comparable with respect tolithium loss and better than Li4SiO4.

    2.2.2. Thermal performance of ceramic breedersSystem thermal conductivity is an important

    issue with respect to breeder temperature control.Thermal conductivity of bulk Li2O, LiAlO2,Li4SiO4, and Li2ZrO3 was studied and correla-tions were derived as a function of porosityand temperature [14]. For materials of thesame density, thermal conductivity ranks as fol-lows: Li2O\LiAlO2\Li2ZrO3\Li4SiO4. Ther-mal conductivity values for Li2TiO3 areintermediate between those for LiAlO2 andLi2ZrO3 [10,15]. Thermal conductivity measure-ments at FZK on Li4SiO4SiO2 show an in-crease when compared to pure Li4SiO4. Thermalconductivity of a pebble bed is controlled by thethermal conductivity of the gas phase and itspressure. It is expected that the thermal conduc-tivity of pebble beds of these materials will followthe ranking as given above, provided the samepacking fraction, pebble size, and pebble densityare maintained. Indeed, this was confirmed inmeasurements on pebble beds of Li2O, Li2ZrO3,and Li4SiO4 [16,17]. Experimental values are ingood agreement with theoretical model predic-tions, which can be used to estimate the thermalconductivity while awaiting specific data. Heattransfer coefficients for the wall:pebble bed arealso needed; some data are available for Li4SiO4and Li2ZrO3 pebble beds.

  • N. Roux et al. : Fusion Engineering and Design 41 (1998) 313834

    2.2.3. Mechanical properties and thermalmechanical beha6iour

    Mechanical property characteristics of bulkLi2O, LiAlO2, Li4SiO4, and Li2ZrO3 ceramicsalong with properties correlations as a function ofporosity, grain size, and temperature were re-ported [14]. Mechanical properties of Li2TiO3 andLi2ZrO3 were compared in [10]. A convenient wayto evaluate mechanical strength of a ceramic inpebble form is to measure its crush load. Crushload depends on pebble nature, pebble diameter,pebble microstructure, and on the fabrication pro-cess. Results for JAERI Li2O, Li2ZrO3, andLi4SiO4 pebbles, for FZK Li4SiO4 and Li4SiO4SiO2 pebbles, and for AECL Li2ZrO3 and Li2TiO3pebbles were reviewed in [16]. Addition of TeO2to the Li4SiO4SiO2 ceramic shows significantimprovement in crush resistance [3]. Recent re-sults of crush load for CEA batches of Li2ZrO3and Li2TiO3 pebbles, for AECL Li2ZrO3 andLi2TiO3 pebbles, and for JAERI Li2O andLi2TiO3 pebbles are reported in [5,8,16], respec-tively. The broad range of values observed reflectsthe effect of the above mentioned factors.

    Thermomechanical testing involves integratedtests that incorporate relevant blanket geometryand operating conditions. During a blanket oper-ation, the ceramic breeder will be subjected to anumber of stresses induced by thermal expansion,thermal gradients, thermal shocks, and thermalcycling, which may cause fracture of the ceramic.Fracture has to be limited to avoid purge gaspressure drops and downstream particulate trans-port. Thermal cycling tests were conducted atENEA on LiAlO2, Li2ZrO3, and Li2TiO3 pelletsunder conditions representative of an operatingreactor. An overall good behaviour of the pelletswas observed under DEMO conditions [18]. Simi-larly, several thermal cycling tests were made byFZK on pebbles of various formulations ofLi4SiO4, by JAERI on Li2O, Li2TiO3, and Li4SiO4pebbles, and by AECL on Li2ZrO3 and Li2TiO3pebbles. Results are summarized in [16,19]. Agood performance of FZK pebbles was observedup to temperature change rates of 50C s1 at400500C, compared to the maximum rate of20C s1 in the EU DEMO HCPB blanket. Acomparable behaviour was observed with

    Li4SiO4SiO2TeO2 pebbles [3]. The thermalcycling behaviour of 1.2 mm Li2ZrO3 and Li2TiO3pebbles from early AECL developmental produc-tion runs showed a decrease in pebble strengthwith an increasing number of cycles. Such be-haviour was thought to be due to the anisotropicthermal expansion of both ceramics combinedwith the large grain size of the materials tested[16,19]. Utilization of smaller grain size materialsshould improve the Li2ZrO3 and Li2TiO3 be-haviour. This will be checked through testingCEA material with a typical grain size of 12 mm,as compared to 1050 mm for the AECL material.An extended test campaign of 1000 cycles, wasperformed on a water-cooled BIT ITER blanketmock-up (1 pin) with AECL Li2ZrO3 pebbles totest (both functional and endurance testing) theirthermal hydraulic and thermomechanical perfor-mance. The test confirmed the capability of theproposed blanket design in terms of functionality,thermal hydraulic response, and temperature con-trol [20].

    2.3. In-pile testing and performanceneutronirradiation beha6iour

    The requisite data base covering irradiationperformance and tritium release characteristics iscritical to the evaluation and selection of a lithiumceramic for the tritium breeding blanket. Relevantdata are obtained from laboratory and in-reactortests. The EXOTIC-6 irradiation test at HFRPetten focused on tritium release studies on candi-date ceramics in pellet and pebble configuration[21]. A tritium residence time of one day wasfo...

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