Fusion Engineering and Design 41 (1998) 31–38

Ceramic breeder material development

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

a Commissariat a l’Energie 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) 31–3832

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 (Li4SiO4+SiO2+TeO2)for the helium-cooled pebble bed (HCPB) DEMOblanket developed by the EU with either Li2ZrO3

or 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 0.1–1.0 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 0.1–0.2 mm and 0.25–0.63 mmLi4SiO4 and Li4SiO4+SiO2 pebbles [2], and 0.25–0.63 mm Li4SiO4+SiO2+TeO2 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) 31–38 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 0.5–1.0 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 80–90% TD range. Good yieldand high production rates can be expected. Usingthe same process, preliminary trials were made atCEA, with Pechiney, to produce 1 mm Li2ZrO3

pebbles [8].(d) An agglomeration/sintering process has

been 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 Li4SiO4+SiO2 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 Li4SiO4

and Li2ZrO3 pebble beds.

N. Roux et al. / Fusion Engineering and Design 41 (1998) 31–3834

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 Li4SiO4+SiO2 pebbles, and for AECL Li2ZrO3 and Li2TiO3

pebbles were reviewed in [16]. Addition of TeO2

to the Li4SiO4+SiO2 ceramic shows significantimprovement in crush resistance [3]. Recent re-sults of crush load for CEA batches of Li2ZrO3

and 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 Li4SiO4

pebbles, and by AECL on Li2ZrO3 and Li2TiO3

pebbles. Results are summarized in [16,19]. Agood performance of FZK pebbles was observedup to temperature change rates of 50°C s−1 at400–500°C, compared to the maximum rate of20°C s−1 in the EU DEMO HCPB blanket. Acomparable behaviour was observed with

Li4SiO4+SiO2+TeO2 pebbles [3]. The thermalcycling behaviour of 1.2 mm Li2ZrO3 and Li2TiO3

pebbles 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 1–2 mm,as compared to 10–50 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 performance—neutronirradiation 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 wasfound at �400°C for 76% TD LiAlO2 pellets, at�350°C for 94% TD Li4SiO4 pebbles, and at�250°C for 73% TD Li2ZrO3 pellets. In theEXOTIC-7 irradiation test, 50% 6Li enrichedLi2ZrO3 and LiAlO2 pellets and Li2ZrO3 andLi4SiO4 pebbles were irradiated to 6–18% burnup[22,23]. Pellet stacks and pebble beds remainedessentially intact during irradiation. Postirradia-tion inventory measurements confirmed valuefrom previous EXOTIC tests. Tritium releasefrom Li2ZrO3 ceramic was excellent. Tritium resi-dence times were not affected by lithium burnup.Postirradiation examination of the mixed Be-

N. Roux et al. / Fusion Engineering and Design 41 (1998) 31–38 35

Li4SiO4 pebble bed proved to be very informative.As in earlier tests, implanted tritium was observedin beryllium when lithium ceramic and berylliumwere intimately mixed. The tritium inventory wasfound to be very high, with the smaller pebbleshaving the largest inventory. As a result, a mixedbed of beryllium pebbles and ceramic pebbles willnot be considered as a blanket design option. TheEXOTIC-8 experiment, which is now in the de-sign and fabrication stage, will focus on the needsof the EU helium-cooled pebble bed DEMO blan-ket in testing the tritium release from Li4SiO4+SiO2+TeO2 pebbles, Li2ZrO3 pebbles, andLi2TiO3 pebbles.

The CRITIC-II test focused on the irradiation ofLi2ZrO3 pebbles. The temperature gradient in thepebble bed ranged from 200°C at the outer edge to�1100°C at the center. The final burnup achieved0.5% total lithium after 272 full power days (FPD).Postirradiation examination of the pebbles indi-cated very low tritium inventory, except at thelowest temperature. Tritium inventory rangedfrom �5 to 0.01 wppm for the operational tem-perature range. Lifetime tritium release fromLi2ZrO3 pebbles showed very low tritium inventoryand excellent performance at the target burnup[24]. The CRITIC lIl test focuses on the continuingirradiation of Li2TiO3 pebbles. The tritium releasebehaviour of Li2TiO3 pebbles is comparable to thatof Li2ZrO3 pebbles in CRITIC II.

In an irradiation test at the FFTF fast reactor,burnups in excess of 5% were achieved. The ce-ramics operated at a centerline temperature of�1000°C and edge temperature of �400°C. Inspite of these extreme conditions, Li2O andLi2ZrO3 performed very well; only Li2O exhibiteda very small loss of lithium during irradiation.For Li2ZrO3 at this burnup level, tritium releasewas constant with no indication that second phaseformation degrades tritium release behaviour.Material fracture was not observed in the ceramicbreeder material. Lifetime tritium release ofLi2ZrO3 pebbles showed very low tritium inven-tory and excellent performance of pebbles to1100°C for �200 FPD [25].

Ongoing laboratory tests are in general agree-ment with the above in-situ tritium release tests.The laboratory isothermal tests indicate very good

tritium release down to 200–250°C for Li2ZrO3

and Li2TiO3 pebbles [8,10]. Improved tritium re-lease for pebbles of Li4SiO4 with addition of TeO2

is reported [3].

2.4. Fundamental studies

For the effective operation of a ceramic breederblanket, it is important to understand tritiumtransport and release characteristics, and the rolethat hydrogen plays in this process. There areindications that grain size largely determineswhether tritium release is limited by diffusion ordesorption [26]. In other words, the larger thegrain size, the higher the probability that bulkdiffusion will determine the release rate. Forsmaller grain size, the reactions taking place onthe grain surface become extremely important,especially regarding the role that hydrogen playsin the overall process. The presence of 0.1% H2 inthe helium purge gas enhances the release oftritium from the lithium ceramic. The releasedtritium has been found in the form of both HTand HTO.

Present understanding of tritium release wouldsuggest that the release rate cannot be enhancedarbitrarily by simply increasing the hydrogen par-tial pressure in the helium purge stream. This canbe understood from the Hartree-Fock theoreticalcalculations [27–29] of dissociative hydrogenchemisorption on the (110) and (111) surfaces ofLi2O. These calculations indicate that a majorityof the surface sites (i.e. the terrace sites) are notavailable for hydrogen chemisorption. Only a mi-nority of sites, such as step ledges and pointdefects, are favorable. Once those sites are occu-pied, further increases of the hydrogen partialpressure would not be useful.

The ab-initio calculations on the dissociativehydrogen chemisorption on lithium oxide surfacesprovide one component of the quantitative basisfor an understanding of the role of hydrogen inaffecting the release of tritium from lithium ce-ramic breeders. These calculations suggest het-erolytic adsorption of hydrogen onto the ceramicsurface. The presence of hydrogen in the purgegas stream provides a very different environment.The hydrogen is chemisorbed onto the lithiumceramic surfaces, forming OH− and Li+H−Li+.

N. Roux et al. / Fusion Engineering and Design 41 (1998) 31–3836

There are two possible reactions with the T+,namely:

Li+H−Li+ +T+ =2 Li+solid+HT or OH− +T+

=HTO+O2−vacancy

In both tritium release processes, the rate scales asthe product of the surface coverage of thechemisorbed hydrogen species and the tritiumconcentration. However, the rate is not well char-acterized, resulting in some doubt remaining as tothe details of the surface interactions in the tri-tium release process. As the hydrogen coverage isusually much larger than the tritium concentra-tion, the tritium release rate is proportional to thefirst power in the tritium concentration and not toits square, as is the case in the absence of hydro-gen from the purge gas.

Complementary to the above, are the studies ofthe tritium release process through various analyt-ical means using FTIR, work function measure-ments, and deuterium as tracer material. In theFTIR study, the deuteroxyl group, OD, was di-rectly observed on the Li2O surface at high tem-perature and under controlled atmosphere.Multiple peaks were observed in the O–D stretch-ing vibration region, exhibiting varied dependenceon temperature and oxygen potential of the sur-face [30]. Also examined were mass spectrometricdata of D2 and D2O interactions with lithiumceramics, vapour pressure data available for usein materials studies, characterization of surfacesites on Li2ZrO3 as a function of drying andsintering conditions, and the effect of oxygenpotential on tritium release [31]. Work functiontechniques were used to examine the defect struc-ture of Li4SiO4 [32]. Also, work function resultswere compared with vapour pressuremeasurements.

The influence of irradiation defects on tritiumrelease is receiving some attention with studies onselected ceramics. These irradiation defects maybecome important for low temperature operationof the breeder blanket [33,34].

2.5. Acti6ation considerations

An attractive feature of fusion energy is that itcan be an environmentally benign energy form.

To ensure that this advantage is not lost in thedevelopment of this energy producing technology,every effort must be made to ensure that allmaterials conform to this goal. An acceptableguideline could be that all materials should meetthe requirements for near surface burial as ra-dioactive waste. Long-lived nuclides and path-ways to the biosphere are certainly among theprimary considerations for the evaluation of ac-ceptable blanket and structural materials. For ex-ample, from a ceramic breeder perspective, Li2O,Li2TiO3, and Li4SiO4 are more attractive thanLi2ZrO3 due to its long-term radioactivecharacteristics.

3. Discussion

Ceramic breeder research is focused on thedevelopment of several blanket concepts forDEMO reactors and for ITER. The forthcomingconstruction of test blanket modules in ITER andof the ITER driver blanket requires base engineer-ing data. Thus, in addition to ongoing genericwork and fundamental studies, emphasis is placedon engineering tests of blanket submodules bothout-of-pile and in-pile. Issues of current interestare reactor-relevant-scale fabrication of ceramics,thermal mechanical and thermal hydraulic be-haviour of blanket submodules, tritium release,tritium inventory, tritium transport/release model-ing, and irradiation behaviour to end-of-life burn-up and dpa conditions. International plans forlong-term irradiations have been delayed by thesuccessive shutdown of several fast breederreactors.

Four ceramics are still being developed by thefusion-blanket research groups: Li2O, Li2ZrO3, anew formulation of Li4SiO4, and Li2TiO3. Li2Oand Li4SiO4 exhibit the highest lithium densitybut also the highest lithium vaporization rate andgreatest sensitivity to moisture. Prospects for massproduction of pebbles are good. Irradiation be-haviour and tritium release are adequate for cur-rent DEMO blanket concepts. Neutron activationis not a concern for Li2O and of little concern forLi4SiO4.

N. Roux et al. / Fusion Engineering and Design 41 (1998) 31–38 37

Li2ZrO3 exhibits greater thermal stability and isless sensitive to moisture. Even though mass pro-duction of Li2ZrO3 pebbles is not yet demon-strated, its feasibility is expected. Excellentirradiation behaviour and excellent tritium releasewere observed in several worldwide irradiationtests to lithium burnups up to 10%. Activation ofzirconium is a concern, though it is small incomparison to that from the blanket structuralmaterials.

The expanding data base for Li2TiO3 affirms itsattractiveness. Negative features are not evident inthe reported data. Lithium density, thermal stabil-ity, thermal conductivity, and mechanical strengthare good, and its insensitivity to moisture is asignificant advantage. Pebble fabrication is beinginvestigated by several groups. Tritium releaseannealing tests show Li2TiO3 to exhibit excellentrelease even at low temperatures. In-situ tritiumrelease experiments are in progress at NRU inChalk River and at HFR in Petten for confirma-tion and for investigation of irradiation be-haviour. There is little concern that neutronactivation of Li2TiO3 will be a problem.

Owing to their overall favorable properties andgood tritium release behaviour at low tempera-tures, �225°C, Li2ZrO3 and Li2TiO3 were se-lected for the ITER driver blanket.

4. Conclusions

The development of the properties data basefor lithium containing ceramics has yet to identifya critical issue that would negate their use astritium breeding materials in a fusion reactor.Several of the candidate materials have performedwell to burnups in excess of 10% under some verydemanding in-reactor conditions.

From current blanket design perspectives, itwould appear that lithium ceramic pebbles arelikely to be the choice for future breeder blanketdesigns. From among the several candidate mate-rials, Li2O, Li4SiO4, Li2TiO3, and Li2ZrO3, haveemerged as offering the greatest potential for useas a tritium breeder material in a fusion reactor.A detailed knowledge of ceramic breeder operat-ing conditions is needed from blanket design stud-

ies so as to ensure that ceramic breeder data baseis appropriately covered. Only with equivalentdata bases of requisite properties can all materialsbe fairly evaluated and a sound selection be made.

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