INTRODUCTIONBeginning in the 1970s with research on alternative wasteforms to borosilicate glass (McCarthy 1977) and followedby the invention of Synroc—a synthetic rock made up ofstable titanate minerals (Ringwood et al. 1988)— consider-able effort has been dedicated to the development and sci-entific evaluation of ceramics designed for the safe disposalof nuclear wastes. These diverse wastes range from thereprocessed spent fuels from commercial power reactors tohigh-grade plutonium derived from decommissionednuclear weapons. Over the past 25 years or so, titanateceramics have evolved from the original polyphase assem-blages to specific compositions based largely on a singlephase. Advances in waste form development and testinghave been complemented by numerous mineralogicalinvestigations of the analogous crystalline phases in geo-logical environments (Lumpkin 2001; Lumpkin et al. 2004a).Information obtained from these studies has importantimplications for validation of the long-term performance ofnuclear waste forms for disposal in geological repositories.

One of the major goals of research on crystalline ceramicwaste forms is to provide a host matrix capable of providinga much higher level of chemical durability than borosilicateglass when placed in a geological repository. Tailoredceramics (Harker 1988), the Synroc titanate waste forms,and their special-purpose derivatives are reasonably welldeveloped and have been the subject of extensive dissolution

testing. The best of these wasteforms are superior to borosilicateglass in terms of aqueous dissolu-tion. However, a major concern isthat the actinide host phases willundergo a crystalline to amor-phous transformation due to alphadecay of the actinides. This maylead to volume expansion, cracking,and reduced chemical durability(Weber et al. 1998).

Radiation damage effects in thesematerials have been studied usingsynthetic samples doped withshort-lived 238Pu or 244Cm, naturalsamples (containing Th and U),

and various heavy ion irradiation techniques. Actinide dop-ing experiments are the most relevant to real waste forms,and natural analogue studies provide complementary infor-mation on the long-term behavior in geological environ-ments. However, many investigators have chosen to simu-late the damage produced by alpha-decay processes byusing ion irradiation techniques under controlled experi-mental conditions (Begg et al. 2001; Lian et al. 2003, 2006;Lumpkin et al. 2004b).

POLYPHASE WASTE FORMSIt is instructive to examine the polyphase waste forms first,as they have been studied more extensively and have them-selves spawned a number of the single-phase waste formsfor actinides (Stefanovsky et al. 2004). FIGURE 1A shows thetypical microstructure of a Synroc-C ceramic with numer-ous triple points and very low porosity. Other forms of Synroc,e.g. polyphase ceramics for disposal of partially reprocessedspent fuels, were designed around the use of pyrochlore asthe main actinide host phase (TABLE 1). Lumpkin et al.(1995) provided an extensive summary of the partitioningbehavior of lanthanides, actinides, and other elements inpolyphase Synroc samples. Zirconolite has a strong prefer-ence for the smaller lanthanide ions and tetravalentactinides (FIG. 2). Large lanthanide ions (e.g. Nd, Ce) andtrivalent actinides, however, prefer the larger Ca site of per-ovskite. In general, such partitioning may have a directimpact on both the chemical durability and the mechanicalproperties of any polyphase waste form. This may be espe-cially important for proposed glass-ceramic waste forms,wherein actinides partition strongly into the crystals as

E L E M E N T S , V O L . 2 , P P . 3 6 5 – 3 7 2 DECEMBER 2006

* Australian Nuclear Science and Technology Organisation PMB1, Menai, NSW 2234, AustraliaE-mail: grl@ansto.gov.au

365

The concept of nuclear waste forms based on minerals that containactinides has led to the development of polyphase and special-purposecrystalline ceramics. These ceramics are considered by many to be

attractive media for the long-term storage of actinides in geological repositories.The available data show that monazite, pyrochlore, zircon, and zirconoliteare all highly durable in both natural and synthetic aqueous systems at lowtemperatures. In comparison, perovskite is prone to dissolution and conversionto anatase and other secondary alteration products. The titanate and silicatephases of interest become metamict (amorphous) as a result of irradiation.Several compounds, including monazite, cubic zirconia, and the defect fluo-rite structure types with Zr on the B site, exhibit the attractive property ofradiation “resistance.” These results, together with other materials properties,are discussed briefly with respect to criteria for waste form performance.

KEYWORDS: ceramics, nuclear waste form, actinides,

radiation damage, aqueous dissolution

A natural example ofradiation damage,

mechanical behavior,and chemical alteration.

The SEM image showsallanite from Amelia,

Virginia, with high (green) and low (white) levels ofTh. Radiation damage in the main part of the crystal

has compressed the crystalline, low-Th zone, resultingin brittle failure and the development of numeroustension cracks. Late aqueous fluids have migrated

through the cracks and caused a preferentialalteration (blue areas) of the amorphous allanite. The

zoned allanite behaves as a composite material.

Gregory R. Lumpkin*

Ceramic Waste Forms for Actinides

opposed to the glass matrix. Actinide doping experimentsare required in order to demonstrate the performance ofthese materials.

Most of the basic dissolution tests on polyphase materialshave been conducted in the temperature range of25–200°C, with periodic replacement of the aqueous fluid.Results of these earlier studies revealed that the dissolutionrates of the most soluble elements, like Al, Ca, Sr, Mo, Cs,and Ba, exhibit a non-linear decrease with time, typically

approaching 0.001 g/m2/d or lower after about 90 days attemperatures of 70–100°C. Dissolution rates of the rareearth elements are about an order of magnitude lower thanthose of the soluble elements, and Ti and Zr rates are lowerstill, often reaching detection limits within a few days.Ringwood et al. (1988) suggested that the dissolution ratesof the actinides (Np, Pu, Am, Cm) are approximately twoorders of magnitude lower than those of the more solubleelements and exhibit typical values of less than 10-4 g/m2/din short-term tests (less than 60 days). Solubility calcula-tions indicated that the solubility of Pu controls the disso-lution behavior in all experiments except for those con-ducted under the most acidic conditions. The solubilitylimits of Np and Am were not exceeded under any of theexperimental conditions.

Alteration depths for certain phases have also been reported(Smith et al. 1992; Lumpkin et al. 2004a). After one year at150°C in pure water, the average alteration depths are about350 nm for Al-rich oxides, 160 nm for perovskite and inter-metallic phases, and 15 nm for hollandite. Similar experi-ments conducted at 70°C gave values of about 80 nm for Al-rich oxides, 65 nm for the intermetallic phases, 60 nm forperovskite, and 5 nm for hollandite. These estimates are ingood agreement with surface SEM and cross-sectional TEMobservations. The microscopy work also shows that, at 70°C,the major alteration product consists of an amorphous orpoorly crystalline Ti–O–H film derived mainly from the dis-solution of perovskite. At 150°C the alteration of perovskiteto anatase + brookite is the dominant surface feature. Othersecondary phases include monazite, Al–O–H phases, andpoorly crystalline Fe–O–H material. Zirconolite appears tobe resistant to chemical alteration under these conditions.

Measurements of accelerated radiation damage and dissolu-tion tests on polyphase Synroc samples (both Na-free andNa-rich types) doped with 244Cm have been reported byMitamura et al. (1992, 1994). Up to the maximum doseachieved during this study (1.3 × 1015 α/mg), the Na-freesamples showed a consistent decrease in density with

366E L E M E N T S 366 DECEMBER 2006

Analytical electron microscope data showing howactinides and lanthanides (indicated as a function of their

ionic radii) partition between zirconolite and perovskite in Synroc. Thedark line is the average for Na-free Purex waste (PW). The actinides fol-low a different trend line (green). Increased Na, as in Japanese waste(JW), lowers the partition coefficient D, pushing more lanthanides andactinides into perovskite (orange region). This can be reversed by sub-stitution of K for Na (blue region).

FIGURE 2

A

(A) Transmission electron microscope image showing themicrostructure of Synroc-C prepared by hot pressing

under reducing conditions. Note the lack of porosity, the lamellar struc-ture in zirconolite (Z), and the twin domains in perovskite (P). Otherphases are hollandite (H) and rutile (R). (B) High-resolution transmissionelectron microscope image showing the microstructure of a pyrochlore(Py) waste form containing plutonium, prepared by high-temperaturesintering. Note the structural intergrowth of zirconolite and pyrochlore.Inserts are electron diffraction patterns of the two phases.

FIGURE 1

B

367E L E M E N T S DECEMBER 2006

increasing dose. A Na-rich specimen, however, showed asignificant change in the dose–density behavior at a dose of8.5 × 1014 α/mg. This change was possibly related to theonset of cracking (which was not a major problem in theNa-free samples). For the longest dissolution period, therelease rates of Ca and Sr in the Na-free samples were withinerror of one another and increased from approximately0.0003 g/m2/d to 0.002 g/m2/d. Perovskite is clearly themajor source of these elements in solution, even thoughthe damage rate of zirconolite appears to be much higheraccording to the X-ray diffraction results.

SINGLE-PHASE WASTE FORMSThese crystalline ceramics are primarily designed for theencapsulation of actinides and other elements (e.g. short-livedSr and Cs) that would arise from the possible separation ofhigh-level wastes (HLW) into various fractions; single-phasewaste forms could also host Pu from dismantled nuclearweapons (TABLE 1). Technically, they are not always single-phase materials, as demonstrated by the important class ofpyrochlore-based ceramics designed for Pu immobilization.As shown in FIGURE 1B, these ceramics consist of pyrochlorewith some zirconolite, often in layered intergrowths. Depend-ing upon the bulk composition, the pyrochlore ceramicsmay also contain some perovskite and brannerite. It is alsocommon practice to include an excess component, e.g.TiO2, in the formulation in order to improve the processingflexibility or to eliminate an unwanted (e.g. soluble) phase.

Pyrochlore Cubic pyrochlore is a derivative of the fluorite structure andcorresponds to the general formula A2B2X6Y, in which Aand B are 8- and 6-coordinated cation sites and X and Y are4-coordinated anions sites. Typically, A = Na, Ca, Y, lan-thanides, and actinides, and B = Ti, Zr, Nb, Hf, Ta, Sn, andW. Many more elements are found in natural samples, andhundreds of different pyrochlore compositions have beensynthesized. Prototype waste form compositions are usuallybased on the CaUTi2O7 end-member. Other actinides sub-stitute directly for U in this compound. The neutronabsorber Hf substitutes directly for Ti on the B site, whereasGd occupies the A site and substitutes for Ca and U.

Weber et al. (1986) were among the first investigators toconduct dissolution tests on single-phase pyrochlore sam-ples (Gd2Ti2O7) doped with 244Cm. The dissolution testswere limited to annealed, fully crystalline samples and fullyamorphous samples, and were exercised at 90°C in purewater for 14 days. Weight losses of 0.02% and 0.05% for thecrystalline and amorphous pyrochlore samples, respectively,were measured. The dissolution rate of Cm increased by afactor of 17 as a consequence of amorphization.

Significant new results are now available (Strachan et al.2005; Icenhower et al. 2006) for synthetic pyrochlore sam-ples doped with Ce or Pu–U and for single-phase Gd2Ti2O7

for comparison. The Pu–U pyrochlores were fabricated with238Pu or 239Pu in order to compare samples with high andlow alpha-decay doses for a range of flow rate to surfacearea ratios, while at the same time minimizing artifacts (e.g.effect of F from teflon, effect of atmospheric CO2 on pH,and effect of radiolysis at low flow rates). Results for solutionsbuffered at pH = 2 and temperatures of 85–90°C reveal that asteady-state, forward reaction rate can be determined forhigh values of the flow rate to surface area ratio. Under theseconditions, release rates of 0.0013 to 0.0002 g/m2/d wereobtained for the different pyrochlore compositions. However,the most important result of this work is that the dissolutionrates of the crystalline and X-ray amorphous (238Pu-doped)samples are the same, within experimental error.

Dissolution studies on ion-irradiated titanate samples gen-erally indicate that maximum Y and lanthanide releaserates may be anywhere between no change and 20 timeshigher as a result of amorphization (Begg et al. 2001). Sim-ilar pyrochlore samples with Zr on the B site have beenstudied, and these samples retained crystallinity at a dosethat renders the titanate amorphous. After a maximumtime of 28 days, the dissolution rates of the irradiated andunirradiated zirconate samples were the same, droppingfrom 0.1–1 g/m2/d to about 0.05–0.09 g/m2/d for Ca andfrom 0.01–0.1 g/m2/d to 0.002–0.006 g/m2/d for Gd. Thesevalues are similar to those of the unirradiated titanatepyrochlore. The variability of results in the studies notedabove probably relates to the experimental details, includ-ing temperature and redox state, nature of the apparatusused, and quality of the samples (e.g. porosity, surface area,tendency to crack, soluble impurity phases).

Lumpkin and Ewing (1988) have shown that naturalpyrochlores with Nb, Ta, Ti, and minor Sn and Zr on the Bsite are subject to amorphization at a critical dose of about1016 α/mg. This dose is about 2–3 times higher than that forCaPuTi2O7 (Clinard et al. 1984a) and is consistent with con-ditions encountered during storage at elevated tempera-tures in the Earth’s crust (Lumpkin 2001). This dose, how-ever, increases with the geological age of the sample forunmetamorphosed samples; this points to long-termannealing via atomic diffusion. This study also provided asystematic evaluation of strain, crystalline domain size, andmicrostructural details as a function of dose. An example ofnatural pyrochlore, associated with zirconolite, from Ti-richveins of the Adamello massif in northern Italy is shown inFIGURE 3. These actinide-rich, radiation-damaged, and com-positionally zoned crystals have coexisted for forty million

Waste form Main phases Application/waste loading

Synroc-C Zirconolite, perovskite,hollandite, rutile

HLW from reprocessing, up to 20 wt%

Synroc-D Zirconolite, perovskite, spinel, nepheline US defense wastes, 60–70 wt%

Synroc-F Pyrochlore, perovskite, uraninite Conversion of spent fuel,approximately 50 wt%

Tailored ceramics

Magnetoplumbite, zirconolite,spinel, uraninite, nepheline

US defense wastes, to 60 wt%or higher

Pyrochlore Pyrochlore, zirconolite-4M,brannerite, rutile

Separated actinides or Pu, up to about 35 wt%

Zirconolite Zirconolite, rutile Separated actinides, up to about 25 wt%

Monazite Monazite Actinide–lanthanide wastes, up to about 25 wt% actinides

Zircon Zircon Pu-rich wastes from dismantlednuclear weapons

Glass-ceramicsTitanite, zirconolite, pyrochlore,perovskite, nepheline, sodalite,alumino-silicate glass

Canadian wastes (low actinidecontent), complex legacywastes, intermediate level wastes

Others Britholite, kosnarite, murataite,crichtonite

Proposed as host phases foractinides and lanthanides

SUMMARY OF WASTE FORMS FOR HIGH-LEVEL WASTES FROM REPROCESSING AND SPECIAL-PURPOSE FORMULATIONS FOR PU AND ACTINIDES

TABLE 1

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years without cracking or significant loss of U and Th. Thisprovides a nice illustration of the long-term performance ofthe pyrochlore waste form for Pu shown in FIGURE 1B.

The radiation damage properties of many synthetic III-IVpyrochlore and defect fluorite materials containing Na, Ca,Cd, and lanthanides on the A site and Ti, Zr, Sn, Hf, Nb, andTa on the B site have now been studied in detail using Krion irradiation. As noted previously (see Ewing et al. 2004),the incorporation of Zr at the B site of the pyrochlore struc-ture promotes retention of the crystalline structure to veryhigh ion fluences. By radiation “resistance” or “tolerance,”it is understood that the heavy ion or alpha-recoil collisioncascade still forms but anneals back to the original crys-talline configuration on a very short timescale (e.g. picosec-onds). The effect of composition on the radiation toleranceof pyrochlore is shown in FIGURE 4A. Here, we have plottedthe critical temperature above which the material remainscrystalline (Tc) versus the metal–oxygen radius ratio (rM/rx,in which rM is the average of the A and B cation radii). Thisis a convenient way of plotting, as it reflects Pauling’s Rulesand also effectively separates the data into the main groupsconsisting of the titanate and stannate pyrochlores. Withinthese two groups, Tc increases as rM/rX increases due to theeffect of increasing A-site cation size. These results arerelated in part to classical radius ratio arguments and to thex(48f) anion positional parameter. In general, the radiationtolerance of pyrochlore correlates with the gradual struc-tural transformation toward the fluorite-like geometry,energetic considerations, and increased ionic bonding (e.g.Minervini et al. 2000; Lian et al. 2003, 2006; Ewing et al.2004; Lumpkin et al. 2004b; Trachenko et al. 2005). Theseeffects can be modelled empirically, as shown in FIGURE 4B,and used to predict the critical amorphization temperatureof compounds in the III-IV pyrochlore composition space(Lumpkin et al. 2006).

Cubic ZirconiaThese compounds are essentially fluorite–defect fluoritestructures based on the general formula MO2-x with M = Ca,lanthanides, actinides, and other elements simulating

impurities. All are resistant to amorphization under mostconditions. However, these materials also require higherprocessing temperatures, and they exhibit reduced chemi-cal flexibility. The solubility of zirconia in pure water at lowtemperatures and near-neutral pH values is quite low,prompting proposals to employ such materials directly asinert matrix fuels and nuclear waste forms that can be usedin nuclear reactors and then placed in geological reposito-ries. Dissolution tests generally indicate elemental releaserates between 10-4 g/m2/d and 10-7 g/m2/d, depending onthe experimental conditions.

ZirconoliteZirconolite is a derivative of the fluorite structure type andcan be considered as a condensed version of pyrochlore.The prototypical end-member composition is CaZrTi2O7 forthe 2M polytype. Different polytypes (e.g. 3T, 3O, 4M) arisethrough solid solution toward lanthanide and actinide end-members. Actinide and lanthanide elements may substituteon both the Ca and Zr sites, with the appropriate charge-balancing mechanisms. Charge compensation may beachieved in some cases via substitution of elements like Mg,Al, Fe, Nb, Ta, and W for Ti. The observed natural composi-tions are complex, illustrating a high level of chemical flex-ibility, and this has been confirmed to a large extentthrough experimental work. Zirconolite becomes amor-phous when doped with 238Pu or 244Cm or when subjectedto heavy ion irradiation (Clinard et al. 1984b; Weber et al.1986; Wang et al. 2000). The critical amorphization doseand total volume swelling are nearly identical to those ofpyrochlore. Both natural and synthetic zirconolites exhibita high level of resistance to aqueous dissolution, and evenamorphous samples with accumulated doses up to ~2 dis-placements per atom retain U and Th and their daughterproducts in the structure. Zirconolite also exhibits excep-tional corrosion resistance below 250°C in various acidicand basic fluids over a wide range of pH. Between 250 and500°C at elevated pressure (50 MPa), zirconolite is subject topartial corrosion in acidic and basic media, but significantcorrosion occurs only above 500°C (Lumpkin et al. 2004a).

The dissolution kinetics of zirconolite have been deter-mined as a function of pH using pure water in single-passflow-through tests at temperatures of 75°C and lower(Zhang et al. 2001). These authors studied a Ce-, Gd-, andHf-doped zirconolite containing about 16 wt% UO2, andthe measured release rates of Ti and U indicate that zir-conolite dissolves congruently after about 20 days follow-ing an initial period during which U is released at a some-what faster rate than Ti. The limiting rate constants areequivalent to U release rates of 6.4 × 10-7 to 1.3 × 10-5

g/m2/d for zirconolite over the entire pH range of 2–12 andtemperature range of 25–75°C. The dissolution rate of zir-conolite is characterized by a shallow v-shaped pattern witha minimum near pH = 8, similar to the results obtained forpyrochlore.

Ion irradiation studies have been performed on syntheticsamples using 1.0 MeV Kr or 1.5 MeV Xe ions (Wang et al.2000). Results showed that the radiation response of end-member zirconolite-2M, CaZrTi2O7, varies with ion massand energy. In this example, the heavier Xe ions resulted ina higher Tc value (654K for Kr and 710K for Xe). Additionalresults for Kr ions showed that Tc ranges from 550 to 1020K,with the highest values obtained from samples with thehighest Nd and Ce contents (substituting for Ca and Zr),regardless of the polytype structure. Relative to the pureend-member, two samples with Nb and Fe substituting forTi had the lowest Tc values. These results indicate that thereis a significant relationship between radiation tolerance andcation mass in the zirconolite system.

Backscattered SEM image (colorized) showing inter-growth of natural pyrochlore (Py, gray) and zirconolite

(Z, blue, green, yellow) from Ti-rich veins, Adamello, Italy. Pyrochlore(containing about 30 wt% UO2) occurs as overgrowths on zirconolite, isamorphous due to alpha-decay damage, and exhibits a darker rim dueto minor hydration. Zirconolite is chemically zoned, with up to about25 wt% Th + U oxide, and its structure ranges from crystalline to amor-phous. Note the absence of cracking from differential swelling.

FIGURE 3

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PerovskitePerovskite is an ABX3 structure type based on a frameworkof octahedral B-site cations and large A-site cations. Themajor A-site cations include Na, Ca, Sr, lanthanides, andminor actinides. The B site is occupied predominantly by Tiand minor Al in nuclear waste form compositions. Ion irra-diation and natural analogue studies indicate that the criti-cal amorphization dose of perovskite is higher than that ofpyrochlore and zirconolite by a factor of about 2–5. Disso-lution studies, however, demonstrate that perovskite is theleast durable phase present in polyphase waste forms(Lumpkin et al. 2004a). Perovskite reacts quickly with aque-ous fluids to form an amorphous Ti–O–H film at tempera-tures below about 100°C or crystalline TiO2 polymorphs athigher temperatures. Using cross-sectional electronmicroscopy and atomic force microscopy, Zhang et al.(2005) illustrated the epitaxial nature of anatase growth onsingle-phase perovskite specimens in aqueous dissolutionexperiments conducted at 150°C. Thermodynamic calcula-tions show that perovskite is unstable with respect to titan-ite, titanite + quartz, rutile, and rutile + calcite (Nesbitt et al.1981). The range of groundwater compositions used in thisstudy was representative of groundwaters emanating fromdunites, peridotites, serpentinites, rhyolites, granites, andlimestones. Therefore, from a natural systems and experi-mental viewpoint, perovskite is not the best host phase foractinides (or radioactive Sr) in waste forms destined for ageological repository.

BranneriteBrannerite, ideally UTi2O6, consists of layers of Ti octahedraconnected by columns of U octahedra. It is a minor butactinide-rich phase in some of the pyrochlore-based com-positions designed for the disposal of Pu. Brannerite mayaccount for up to 20 percent of the U and 15 percent of thePu in these waste forms (Thomas and Zhang 2003). Naturaland synthetic brannerites can incorporate substantial amountsof Ca, REE, Th, and other elements. In both cases, the incor-poration of lower-valence cations on the A site may be chargebalanced by oxidation of some U4+ to U5+ and/or U6+ ions.Synthetic samples are easily amorphized by ion irradiation.Electron microscopy studies show that most natural bran-nerites with ages greater than about 10 Ma are fully amor-phous due to alpha-decay damage and are commonlyaltered by natural aqueous fluids. Altered natural branner-ite typically loses U, and the concentration may decrease toapproximately 1 wt% UO2 in the most heavily altered areas.

The most important findings to emerge from experimentalstudies (e.g. Zhang et al. 2003) are that certain species, e.g.phthalate, will increase the solubility of titanium. In bicar-bonate solutions, however, the uranium release rate isstrongly dependent on bicarbonate concentration. In acidicsolutions, the dissolution of brannerite involves a preferen-tial release of uranium. TEM results have shown that bran-nerite exposed to a solution with a pH = 2 at 90°C for fourweeks produces a relatively small amount of reaction prod-uct, which consists mainly of polycrystalline anatase.When exposed to a pH 11 solution at 90°C for four weeks,a fibrous secondary phase is formed on the surface. Thisphase is amorphous and contains a significant proportionof U6+, as indicated by surface analytical studies. Thomasand Zhang (2003) have shown that the aqueous dissolutionof brannerite in a open atmosphere can be modelled as afunction of pH using two reaction steps: oxidation of U4+ atthe surface followed by release of U6+ into solution, whichis catalyzed by protons under acidic conditions or carbon-ate species under alkaline conditions. This is an importantadvance, as the release of uranium at 40°C can be predictedquantitatively from this model.

ZirconZircon, ZrSiO4, is a common accessory mineral found in avariety of geological environments. The major elementalimpurity in natural zircon is Hf, which substitutes for Zr.Trace to minor amounts (generally 5000 ppm or less) ofother elements may be present, including Ca, lanthanides,and actinides on the Zr site and P on the Si site. Higher con-centrations have been reported but are exceptional. Amor-phization, with a critical dose of about 4 × 1015 α/mg in nat-ural and actinide-doped samples, and total volume swellingof up to 18 percent are well-known characteristics of zircon

(A) Critical amorphization temperature of pyrochlore anddefect fluorite compounds versus the metal to oxygen

ionic radius ratio. Trends for samples with Ti and Sn on the B site areshown in red and blue, respectively. Radiation tolerance is promoted bysmaller A cations in each series. For a given A cation, the critical tem-perature is lowered dramatically by substitution of Hf and especially Zron the B site. (B) Three-dimensional surface of the predicted criticaltemperature for amorphization of III-IV pyrochlore and defect fluoritecompounds versus the individual A-site and B-site cation radii. This sur-face was calculated from an empirical model based on structural, ener-getic, and bonding terms.

FIGURE 4

A

B

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(e.g. Holland and Gottfried 1955; Weber et al. 1998). Themineral is highly durable in most environments and com-monly survives weathering to be recycled in the Earth’s crust.

To investigate the effect of radiation damage on dissolutionrates, Ewing et al. (1982) conducted experiments on naturalzircons at 87°C in an aqueous solution containing 5 wt%KHCO3. The results showed that the dissolution rateincreases by nearly two orders of magnitude, from 2 × 10-4

g/m2/d to 2 × 10-2 g/m2/d, as the alpha-decay dose increasesfrom 6 × 1013 α/mg to greater than 1 × 1016 α/mg, spanningthe crystalline–amorphous transformation. Hydrothermalexperiments using chloride fluids containing H, Al, and Caat temperatures of 175 to 650°C have produced reactionrims on zircon grains, with rim thickness dependent onalpha-decay dose. Experiments on heavily damaged naturalzircons also showed severe loss of Si, U, Th, and Pb and gainof H2O species, Al, and Ca (Geisler et al. 2001). Studies usingnatural samples have also dramatically illustrated theeffects of alpha-decay damage on physical properties (Chak-oumakos et al. 1987, 1991; Zhang and Salje 2001) andchemical behavior, with loss of up to 40–50% of the U due toaqueous alteration in certain zircon crystals (Lumpkin 2001).

Ion irradiation studies have been published for the orthosil-icates (A = Zr, Hf, Th) using 0.8 MeV Kr or Xe ions. Onlyminor differences in the dose–temperature response ofZrSiO4 were observed when irradiated with the two differ-ent ions. Even the critical temperatures of these silicatecompounds indicate a similar behavior under Kr ion irradi-ation (Tc ~900–1100K). Under certain conditions, silicatezircons may decompose into the component oxides, tetragonalZrO2 or HfO2 and amorphous SiO2 (Meldrum et al. 1999).

MonaziteLike zircon and thorite, monazite also has ABO4 stoichiom-etry, but the crystal structure is monoclinic and consists ofchains of alternating BO4 tetrahedra and AO9 polyhedralsites. These chains are cross linked by edge sharing with theAO9 polyhedra, effectively closing off open tunnels and cre-ating a structure that is approximately 10% denser than thezircon structure type. Natural monazite contains up to27 wt% UO2 + ThO2 and remains crystalline in spite of highaccumulated alpha-decay doses; however, monazite can beamorphized by heavy ion irradiation, and the differentcompounds have been studied in some detail as a functionof temperature. Irradiated, fully amorphous synthetic mon-azite has excellent aqueous durability and is roughly equiv-alent to the corresponding crystalline samples.

Monazite is highly insoluble in most hydrothermal andlow-temperature fluids; however, solubility may beenhanced in aqueous fluids with low pH, low phosphatecontent, or high F concentrations, which can lead to theformation of REE-fluoride complexes. At temperaturesbelow 250°C, the solubility of monazite in aqueous solu-tions decreases with increasing temperature (see Boatnerand Sales 1988; Lumpkin et al. 2004a). Experiments recentlyreported by Oelkers and Poitrasson (2002) have providedresults on the steady-state dissolution rates of monazite attemperatures of 50–230 ºC, pH ranging from 1.5 to 10, andvariable flow rate and surface area. Using a natural sampleas the starting material, these authors showed that therelease rates of the REEs and U are essentially congruent forall experimental conditions.

Systematic ion irradiation studies of the orthophosphates(Meldrum et al. 1997) have been conducted for six mon-azite-structure compounds (A = La, Pr, Nd, Sm, Eu, Gd) andsix zircon-structure compounds (A = Sc, Y, Tb, Tm, Yb, Lu).The critical amorphization temperatures of these materialswere found to increase systematically with cation mass

from about 330 to 490K for the monazites and from about470 to 580K for the zircon-structure orthophosphates (com-pare this with the high values found for silicate zircons).

OTHER COMPOUNDSA number of materials based on compounds such as britho-lite, crichtonite, garnet, kosnarite, murataite, and titanite(sphene) have been proposed as waste forms for actinides,lanthanides, and other elements (e.g. radioactive Sr and I,and a range of impurities such as transition metals). Withthe possible exception of britholite and titanite, none ofthese compounds have been studied to the same extent asthose described above. All have natural analogues, althoughthe contents of Th and U can be quite low in many naturalsamples. Stefanovsky et al. (2004) summarized the work onthese phases, and they will not be dealt with further in thisarticle (apart from the comparison shown in TABLE 2).

DISCUSSION AND CONCLUSIONS Generally speaking, nuclear waste forms will be required toconform to a number of performance criteria. Althoughthere are at present no universal criteria for acceptance,numerous scientific articles have stated that waste formsshould exhibit low elemental release rates if and when theyare exposed to water in the geological repository, crystalchemical flexibility, potential for high waste loadings, pro-cessing efficiency (both from an engineering and cost per-spective), and suitable physical properties. The latter itemincludes mechanical properties, thermal behavior, andresponse to alpha-decay damage. Note that radiation damagemay dramatically affect the physical properties of materialsthat become amorphous due to such damage. Hence, inrecent years we have seen the emergence of radiation toleranceas an important research topic. The performance of potentialwaste form phases for actinides is summarized in TABLE 2.

Starting with the requirement of aqueous durability, it isclear that perovskite is the least durable phase present inthe polyphase waste forms. This is shown diagrammaticallyin FIGURE 5, where dissolution rates are compared for differentphases as a function of solution pH. Alternatives to perovskiteas hosts for radioactive Sr include the magnetoplumbitestructure type and monazite. Among the titanates,pyrochlore and zirconolite are the currently preferredactinide host phases. They exhibit dissolution rates withminimal variation as a function of pH, incorporate a largerange of impurities, are well established with regard to pro-cessing, and are capable of high waste loadings. Althoughtitanate pyrochlore and zirconolite are rendered amorphousby alpha-decay processes, the total volume expansion is low(nearly the same for both phases) and the effect of amor-phization on aqueous durability appears to be withinacceptable limits.

Crystalline zircon is generally considered to be highlydurable in aqueous systems. However, there is a clear indi-cation of a difference in the behavior of crystalline andamorphous zircon in aqueous fluids, and this is supportedby microscopic and micro-analytical studies of natural sam-ples. The use of zircon as a waste form may be compromisedby the unusually large volume swelling and anisotropicunit cell expansion, leading to cracking and increased dis-solution rates. Monazite has the particular advantage ofremaining crystalline to alpha-decay doses well above thosethat render zircon, brannerite, pyrochlore, and zirconoliteamorphous. Monazite also exhibits low dissolution ratesand appears to have the unusual property of decreasing sol-ubility up to about 250°C. The crystal chemistry of monazite

371E L E M E N T S DECEMBER 2006

is reasonably flexible, but does not allow the large range ofsubstitutions possible in the titanate pyrochlore and zir-conolite structure types.

How do the other radiation-resistant materials perform?The existing experimental data indicate that cubic zirconiaand the zirconate pyrochlore–fluorite phases have chemicaldurability comparable to or slightly better than that of thetitanate pyrochlores and zirconolite. However, these phaseslack the crystal-chemical flexibility of the titanates and gen-erally require significantly higher processing temperatures.Furthermore, they have no direct natural analogues, theclosest minerals being baddeleyite and uraninite. Thus, wecannot confirm the long-term behavior of these zirconium-based materials to the same extent as successfully studiednatural pyrochlores, zirconolite, monazite, and zircon.

ACKNOWLEDGMENTSI am grateful to numerous colleagues in Australia, France,Germany, Switzerland, the USA, and the UK for their guid-ance, teamwork, and support over the years. Thanks to R.C.Ewing and an anonymous reviewer for their assistance inimproving this manuscript. �

Aqueous dissolution data for U release from pyrochlore,zirconolite, and brannerite. Results are also shown for Ca

release from perovskite (green line). This figure shows the excellent per-formance of pyrochlore and zirconolite in aqueous fluids. Although Carelease from perovskite is shown, it is clear that this compound shouldbe avoided as a major actinide host phase.

FIGURE 5

Aqueousdurability

Chemical flexibility

Waste loading

Radiation tolerance

Volume swelling

Natural analogues

Perovskite(Ca,Sr)TiO3

Low Medium Low Medium High Yes

PyrochloreGd2(Ti,Hf)2O7

High High High Low-high Medium Yes

ZirconoliteCaZrTi2O7

High High Medium Low-medium Medium Yes

ZirconZrSiO4

High Medium Low (?) Low High Yes

MonaziteLnPO4

High Medium High High Low Yes

ZirconatesGd2(Zr,Hf)2O7

High Medium Medium High Low No

Zirconia(Zr,Ln,Act)O2-x

High Medium Medium High Low No

BranneriteUTi2O6

Medium Medium High Low ? Yes

CrichtoniteCa(Ti,Fe,Cr,Mg)21O38

? High Medium Low (?) ? Yes

MurataiteZr(Ca,Mn)2(Fe,Al)4Ti3O16

High High Medium Medium ? Rare

GarnetCa3Zr2(Al,Si,Fe)3O12

? High Medium Low ? Yes*

TitaniteCaTiSiO5

Medium Medium Low Low Medium Yes

Minerals of the apatitegroup (e.g. britholite)

Medium Medium Low Low Medium Yes

KosnariteNaZr2(PO4)3

Medium Medium Medium Low ? Yes

* Natural garnets typically do not contain substantial amounts of Th or U.

SUMMARY OF THE PERFORMANCE CHARACTERISTICS OF INDIVIDUAL PHASES MEASURED AGAINST POTENTIAL SELECTION CRITERIA AND OTHER MEANS OF EVALUATION (E.G. NATURAL SAMPLES)

TABLE 2

372E L E M E N T S DECEMBER 2006

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