Journal of Nuclear Materials 19 [-194 (1992) 199-203North-Holland

journal ofnuclear

materials

Management options for used lithium ceramic breeder materials

J.H. Miles nand G.J. ButtelWorth b

1/ AEA Fuel Semices, Harwell LaboratOlY, axon., OX! lORA, UKI> AEA Fusion, Culham Laboratory, Abingdon, OXOIl., OX143DB, UK

After use in a reactor, the tritium breeder material will require management as radioactive waste. Since the fractionalburnup of"Li may be quite low, there could be a strong incentive to recover the remaining 6Li for reuse. The options forpostservice management of Ll 20, LiAI0 2 , LizSiO" Li 2 ZrO, and LiVO, are considered. Ion exchange is suggested forrecovery and decontamination of the Li component. Reclamation and decontamination of anionic material can be readilyaccomplished only with the oxide, silicate and vanadate. If direct disposal is the preferred option an environment free ofground water would be advantageous on account of the solubility and reactivity of the materials. Alternatively, vitrificationcould be employed to increase their chemical stability.

1. Introduction

Refractory lithium compounds such as the oxide,aluminate, Silicate, vanadate and zirconate are underinvestigation as tritium breeders for fusion reactors.These materials are likely to be utilised on a batchbasis and replaced when either their breeding effi­ciency or the integrity of their containing structuresreaches predetermined limits. Of the two isotopes oflithium, 6Li is the one mainly responsible for tritiumgeneration and, since this isotope constitutes only 7.5%of natural lithium, the attainment of an adequatebreeding ratio in some designs may demand the use oflithium highly enriched, up to perhaps 90%, in 6Li.The fractional burn-up of 6Li during service is likely tobe low, hence breeder material removed from a reactorcould still contain a substantial proportion of its initial6Li content. Thus, to save on enrichment costs and toconserve lithium resources, there could be strong in­centives to recover remaining 6Li, as an alternative toonce-through use followed by disposal. The presentpaper offers a broad assessment of the practicableoptions.

2. Waste management choices

Consideration of the possibilities for chemical treat­ment of expired breeder material indicates the follow­ing main options:(1) Direct disposal to a suitable environment.(2) Conversion to a more stable chemical form, fol­

lowed by disposal.(3) Reclamation of lithium without removal of activa­

tion products.

(4) Recycle of whole breeder without removal of acti­vation products.

(5) Reclamation of lithium with decontamination fromactivation products.

(6) Recycle of whole breeder after decontaminationfrom activation products.

The first two options may be attractive when thespent breeder no longer contains valuable quantities of6Li. After removal of residual tritium, the used materi­als fall within the categories of low level or intermedi­ate level wastes [1] (UK classification) and any form oftreatment is liable to increase their mass or volume.The simplest management option is therefore directdisposal to a suitable repository. Suitable clay depositsor disused salt mines, for example, might offer theadvantages of an environment free from mobile groundwater which could leach the ceramic or exchange withany residual tritium. In most cases the untreatedbreeder is partially soluble in water and is liable todecompose and release radionuclides to the near-fieldenvironment. The second option thus comprises dis­posal following appropriate treatment to improve theintegrity of the waste material in the ground waterenvironment.

When useful amounts of 6Li are present it may beworthwhile recovering the lithium component for reuse.Since remote handling techniques will be employed forremoving breeder components from the reactor, simi­lar techniques could be used to fabricate and installnew ones and the need for separation of the radioac­tive species present in the lithium could be avoided.This represents option (3) and entails the addition ofnew anionic material to the recovered lithium.

Since the anionic part of the used breeder willconstitute a radioactive waste requiring disposal it could

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200 J.H. Miles, G.J. Butterworth / Management options for used Libreeder materials

in principle be advantageous to return this too to thereactor and thus avoid the need to dispose of thiswaste (option (4)). The argument that there is no needto decontaminate material that is being returned to thereactor again applies. If completely recovered, therewill be sufficient anionic material not only for thereclaimed 6Li but also for the replacement 6Li needed.

There must, on the other hand, be advantages forthe refabrieation process in being able to operate with­out the constraints associated with the handling ofradioactive materials. The anions concerned are rela­tively cheap and plentiful and it may therefore bepreferable to use fresh, inactive anionic material tocombine with the fully decontaminated lithium (option(5)). More detailed consideration indicates this as aspecially attractive option since the anionic componentis more difficult to decontaminate than the lithium.

Option (6) would be favoured if the volumes ofwaste for disposal were of primary concern, such thatthe anionic part of the ceramic should also be decon­taminated and reused. Since the contaminants repre­sent only a minute fraction of the total ceramic mass, itshould be possible to reduce them to a relatively smallvolume. This route would reclaim an amount of an­ionic material sufficient not only for the reclaimedlithium but also for the replacement 6Li.

3. Chemical forms of the transmutation products

The principal transmutation nuclides generatedduring irradiation of the selected breeders are identi-

fied in ref. [1]. Although the chemical forms in whichthese transmutation products will exist in the usedbreeder are not well defined at present, table 1 repre­sents an attempt to predict the likely states. Radionu­clides might also arise from impurity elements in theceramic and from corrosion or spallation of construc­tional materials. In the absence of information onlikely impurity levels, any effects of impurities havebeen excluded.

In general, the nuclides may be present as gases oras cations or anions. Alkali and alkaline earth elementswill be present as cations even in alkaline solution if atlow concentration and can therefore be removed on acation exchange resin. Trivalent metal ions can also bekept in the cationic form if the solution is acidified.Thus it appears that treatment with cation exchangeresin would suffice to remove the above types of ra­dionuclide from solution. There is an analogy here withthe chemistry of water softening, however, where thestronger absorption of the Caz+ ion is reversed in thepresence of very high NaCI concentrations in the re­gencration process, and the Ca2+ is released. Similarly,if the Li concentration is too high then the absorptionof the transmutation products may be prevented. Theallowable Li + concentration would need to be deter­mined empirically.

A small minority of elements, exemplified by Tc,form anions. These will not adhere to a cation ex­changer but could be absorbed on an anion exchangeresin. In the present case Tc is the only elementforming an anion and this arises only with the zir­conate.

Table 1Probable chemical form of transmutation nuclides after dissolution of breeder in acid or water

Breeder Nuclide Likely chemical form

H 20Uncertain, but possibly CH 4, CO2 , COBeZ+

Li 2Si03

Li zZr03

2(,AI Al 3+22Na Na+Material also includes H, C and Be nuclides as above

H, C, Be, AI and Na nuclides as above

91y y3+

93mNb 94Nb 9sNb Hydrolysis polymers93Zr,9SZr ' ZrO z+, but -1% hydrolysis polymers89Sr, 90SI' Sr Z+

99Tc TcO.!Material also includes H, C and Be nuclides as above

49y y3+, YOHSICr Cr3+4SCa Ca2+46SC Sc3+Materials also contains H, C and Be nuclides as above

J.H. Miles, G.J. Butcc/worth / Management optiollS for used Li breeder materials 201

Elements forming hydrolysis polymers are repre­sented by Nb. These polymers have a negative chargein alkaline solution and a positivc charge in acidicsolution; the pH value at the transition is known as theisoelcctdc point and is different for each polymer. Thecolloids will absorb on cation exchangers from solu­tions of low pH and on anion exchangers at high pH.Unfortunately, for strong acid solutions such as mightbe used to dissolve the breeder ceramics, the absorp­tion ceases [2] and in this case there is little prospect ofachieving substantial decontamination from material ofthis type by ion cxchange. It may be feasible to removeNb and Zr by a solvent extraction process, though thispossibility is not very attractive on account of the extrawaste streams introduced.

4. Behaviour of 3H and 14C in wet processing

Although the bulk of the tritium generated wouldbe extracted from the breeder during reactor opera­tion, the fraction remaining at shutdown may still besignificant in radiological terms. Even if only 0.1 to 1%of the tritium generated is retained, it may dominatethe activity and ingestion and inhalation doses for mostof the materials in the 1-100 yr time interval [1]. Muchof the tritium remaining after reactor service could beremoved by specific detritiation treatments such asheating in vacuum after pulverisation. The remaindershould be liberated during dissolution in water or acidor dissolution in a glass matrix in preparation forpermanent disposal and the proportion released canprobably be increased by sparging with helium and/orhydrogen.

In all the ceramics except the zirconate the totalactivity excluding tritium is dominated by 14C at longcooling times; for the oxide and silicate this is so evenat short cooling times [1]. The importance of 3H and14C in controlling the environmental hazards from thebreeder materials is clear. The behaviour of 14C inthese systems is, however, rather uncertain. In theconditions of high temperature and alkalinity duringservice the formation of carbide is likely, the presenceof which will lead to generation of methane duringprocessing in acid solution. It is also possible, if theoxygen potential is not too low, to form basic carbonatewhich· will yield CO 2 in acidic aqueous solution. Car­bon monoxide might also be formed and removed inthe purge gas during operation. Gases containing sig­nificant amounts of 14C would need to be containedand trapped in solids suitable for disposal.

5. Available treatments

5.1. Permanent disposal

Because of their solubility and chemical reactivity,disposal of the used breeder materials in a water-free

environment or after conversion to a more stable formis indicated. Repositories in salt formations or certainclay deposits may in principle bc suitable for directdisposal, the rules varying from country to country.Incorporation in a cementitious matrix might not pro­vide sufficient leach resistance in a ground water envi­ronment and in this case incorporation in a vitreousmatrix such as that used for fission reactor wastesshould be suitable. The glass mix used by British Nu­clear Fuels Limited (BNFL), for example, comprises25% waste solids to which arc added 46% Si0 2 , 16%B20 3, 8.3% Na 20 and 4.0% Li 20. The lithium ceram­ics could be added up to about 25% to this mixture.

5.2. Lithium extraction and purification

Two general methods, solvent extraction and ionexchange, are considered briefly as potential routes forseparation of the Li component of used breeders.

5.2.1. Solvent extractionOf several possible extractants one of the most

promising appears to be a variant of the multidentate"crown ether" compounds in which a long chain car­boxylic acid group is attached to the crown ether as aside chain [3-5]. For Li the best extractant is a 14­member ring with four ether oxygens and a 4-n-decylphthallate side chain. Extraction is low for other alkalimetal ions but the rejection of other metal ions is notalways very high. The solvent extraction process wouldrequire considerable development, without guaranteeof success, and entail high reagent costs.

5.2.2. Ion exchangeIon exchange resin systems are usually simpler and

cheaper than solvent extraction systems. With resins offine particle size and operation at low flow rates theyallow separation of ions with very similar properties.The order of affinity of exchange resins for hydratedions at equilibrium lies in the order of increasing ioniccharge and, for a given charge, the large ions of higheratomic mass are most strongly absorbed. Thus Li + isthe most weakly absorbed of all the cations.

To treat Li 20 by this method, for example, it wouldfirst be reacted with water to give LiOH. Passage ofthe solution through a small cation exchange columnthat had been converted to the Li form would removethe cationic trace contaminants, LiOH passing throughunchanged. Some of the activation products may be inanionic form and these could be retained on an anionexchange column in the hydroxide form.

If a solution of a salt is passed successively througha cation exchanger in the H + form and an anionexchanger in the OIr form usually only water re­mains. In the case of Li salts, however, because of thelow Li+ retention the solution emerging will consist ofLiOH, though of lower concentration than the feed

202 J.H. Miles, G.J. Buuerworth / Management options for used Libreeder materials

solution due to retention of some Li. Thus successivecolumns of H+-form catiun and OH--form anion ex­changers could be used for all the water soluble ceram­ics. Further details specific to the individual breedersare given in a later section.

6. Options for the treatment of each material

This section considers the applicability of the sixmanagement options identified in section 1 to each ofthe selected breeder materials.

6. 1. Lithium oxide Li2°Contaminants: 3H, 14C, lOBe(1) In this material the residual activity is very low

and it could be suitable for near-surface disposal. Thedisposal option is therefore most attractive for materialnot enriched in 6U. It will leach out of cement and thedry environment of a salt deposit may be preferable inview of its solubility and reactivity.

(2) Vitrification appears to bc the best stabilisationtreatment; about 6% LizO could bc incorporated inthe glass mixture currently used by BNFL.

(3) The LizO could be recycled directly or, alterna­tively, dissolved in water as UOH and used as feed­stock to the fabrication process. The latter usuallyentails precipitation of the peroxide followed by itsthermal decomposition to oxide.

(4) In this case there is no added anion to berecovered.

(5) Of the activation products listed, dissolution inwater should remove 3H predominantly as gas but witha small proportion exchanged with H in the water. TheJ4C will probably be liberated as CH 4 but in acidsolution as COo or retained in alkaline solution ascarbonate. If ne~essary, lOBe could be removed by ionexchange.

(6) In the present case this option is identical tooption (5).

Summary: dispose in dry repository, vitrify or recy­cle directly.

6.2. Lithium aluminate LiAl02

Contaminants: 3H, zZNa, 14C, 26AI, lOBe(1) The residual activity is low and the material is

not very soluble in water. Disposal in clay may beattractive since 2%1 will adsorb on it and low rates ofgroundwater movement in deep clay will ensure 3Hdecay before emergence into the biosphere.

(2) LiAIOz would be acceptable up to 25% in glass.(3) Reflux with water and filtet off the A1 20 3. The

LiOH solution will be suitable as a feedstock forpreparation of new ceramic.

(4) Reflux with water and filter. The AI 20 3.xHzOshould be suitable for refabrieation with the LiOHdescribed in option (3).

(5) Apart from any 3H, 2zNa dominates the activityat cooling times below 4 yr. Separation of Na from thebulk Li bv cation exchange would be very difficult andprobably ~ot worthwhile in view of- its low activity.

(6) There is no simple way of separating Z6AI fromthe bulk AI and option (6) is thus, strictly speaking,impossible.

Summary: disposal in clay or salt or vitrify, refluxwith water, filter off Al z0 3, no method for completedecontamination.

6.3. Lithium metasilicate Li2Si03

Contaminants: 3H, 14C, zZNa, lOBe, 26AI(1) Clay formations could offer suitable disposal

sites since lOBe, 26AI and 22Na probably adsorb on clay.Salt formations would also be suitable because of theabsence of ground water.

(2) The silicate is expected to dissolve to an extentof 15-20% in the BNFL glass mix and is thus one ofthe most suitable materials for disposal in this way.

(3) Even if not water soluble, the material willdissolve in nitric acid. The colloid chemistry of H zSi03is well known and it may bc preferable to precipitate itand filter off the LiNO, solution.

(4) Acid dissolution may be used and, with carefulchoice of conditions, the silicate may behave as asimple anion. Anion exchange would then allow sepa­rate recovery of the silicate and adjustments to the Licontent could be made before Li 2Si03 refabrication.

(5) Dissolutiun in water or acid should result indecontamination from 3H and 14C by separation of3H z and 14CH4, though there is some question as towhether 14C will exist entirely in the carbide form and,if it docs, whether it will react to produce exclusivelyCH 4. The silicate may be removable by anion exchangeand the 26AI and lOBe by cation exchange. The zZNawill be difficult to segregate by cation exchange but itsactivity is fairly insignificant.

(6) The Li can be purified as in option (5) and thesilicate as in option (4). There does not appear to beany anionic contaminant requiring removal from thesilicate component.

Summary: dispose in clay or salt formations or vit­rify, dissolve in water, filter off the silica, decontami­nate Li by cation exchange (can ignore 22Na).

6.4. Lithium metavanadate LiV03

Contaminants: 3H, 49V, 45Ca, 14C, 47Se, lOBe(1) Solubility considerations would indicate disposal

in a salt formation with suitable containment.(2) The behaviour of vanadates in the vitrification

process is not known but may well be satisfactory.

J.H. Miles, OJ. Butterworth / Management options for used Li breeder materials 2m

(3) The ceramic may be dissolved in water and thevanadate ion removed by an anion exchange resin,previously converted to the OH - form. Thc resultingLiOH may be usable as feed to the vanadate fabrica­tion process or, if not, the oxide prepared by the usualcarbonate-peroxide route and used to reconstitute thevanadate by reaction with Y20,.

(4) Dissolve the ceramic in nitric acid and absorbthe vanadate on a nitrate form cation exchanger. Thiswill provide a LiNO] solution and it should be possibleto obtain vanadic acid solution by washing the columnwith dilute HN0 3.

(5) Any yH, YOH ions formed (also Cr H , Ca2+,Sc3 + and Be2+) should be absorbable on a cationexchange resin. Passage of the solution successivelythrough a cation exchanger (small quantity) and anionexchanger (largc quantity) yields a decontaminated Listream.

(6) For some time after irradiation, 49y (t l / 2 = 0.9yr) dominates the activity. This cannot be separatcdfrom the bulk vanadium but extcnded cooling wouldallow essentially complete decay.

Summary: dispose in salt formation or vitrify, dis­solve in water, remove Y by anion exchange, use cationand anion exchange to decontaminate Li, allow suffi­cient time for decay of 49y.

6.5. Lithium zirconate Li]Zr03

Contaminants: 3H, 95Nb, 9SZr, 91y, K9Sr, SKy, <lOSr,14C, 93Zr, 94Nb, 93mNb, lOBe, 99Tc

(l) Since this ceramic is slightly water-soluble, dis­posal in salt seems preferable to clay.

(2) The zirconate is probably soluble only to about13% in glass but the relatively high long-lived activitymay justify the extra cost of vitrification.

(3) Dissolution in water should yield LiOH andfinely divided Zr02, which can be filtered off. If thezirconate ion is stable, absorption on an anion ex­changer would offer an alternative.

(4) Proceed as in option (3) but recover the zir­conate by strong HN03 wash of the anion exchangecolumn. Finely divided Zr02 can then be recovered bythermal decomposition of the eluate.

(5) Use of a preliminary cation exchange bed (lowvolume) will remove yH and Sr2+ contaminants. Re­moval of Zr, Nb, Tc is more difficult. If the material issoluble in water it may be possible to prepare a feed at

pH"" 2 from which hydrolysis polymers of Nb willabsorb on the surface of a cation exchanger, but thiswill not be possible if aeid dissolution is needed. In thecase of Tc the ion TeO; will not absorb on an anionexchange resin in the presence of bulk ZrO;, thoughremoval of Zr as ZrO:>. by filtration could make thispossible. This route depends on the ability to dissolvethe material in water and to absorb the Nb at pH"" 2and the Tc on an anion exchanger.

(6) The discussion under option (5) again appliesbut, in addition, decontamination from 9"Zr, 95Zr wouldin principle be needed. Overall, there seems littleprospect of success for this option.

Summary: dispose in salt or vitrify because of activ­ity level, dissolve in water, filter off zirconia, no methodfor complete decontamination.

7. Conclusions

Conceptual chemical techniques exist for recoveryand decontamination of lithium from ceramic breedermaterials. The incentive for recycling Li will stronglydepend on the costs of recovery in comparison with thecosts of 6Li isotopic enrichment.

Reclamation and decontamination of anionic mate­rial, which would greatly reduce the volume of wastefor disposal is possible with the oxide, silicate andvanadate. With appropriate handling precautions itmay be feasible to refabricate breeders for reuse with­out decontamination. However, since the anions in­volved are cheap and are more difficult to decontami­nate than the lithium, it may be preferahle to use freshanionic material combined with lithium which has beendecontaminated and replenished with 6Li. This willavoid the constraints of fabricating radioactive materi­als.

References

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