4utpo3So UM-P-88/125
The Incorporation of Transuranic Elements in
Titanatc Nuclear Waste Ceramics
by
Hj. Matzke1, B.W. Seatonberry2, I.L.F. Ray1, H. Thiele1, H. Trisoglio1,
C.T. Walker1, and T.J. White3'4'5
1 Commission of the European Communities, Joint Research Centre, i Karlsruhe Establishment, ' \ 'I European Institute for Transuranium Elements, Postfach 2340, D-7500 Karlsruhe, Federal Republic of Germany.
2 Advanced Materials Program, Australian Nuclear Science and Technology Organization, Private Mail Bag No. 1, Menai, N.S.W., 2234, Australia.
3 National Advanced Materials Analytical Centre, School of Physics, The University of Melbourne, Parkville, Vic, 3052, Australia.
Supported by the Australian Natio-al Energy Research, Development and Demonstration Programme.
4 Member, The American Ceramic Society
5 Author to whom correspondence whould oe addressed 2
The incorporation of actinide elements and their rare earth element analogues in titanatc nuclear waste forms are reviewed. New partitioning data are presented for three waste forms contining Purex waste simulant in combination with either NpC^, PuC>2 or An^Oo. The greater proportion of transuranics partition between perovskitc and ztrconoiite, while some americium may enter loveringite. Autoradiography revealed clusters of plutonium atoms which have been interpreted as unrcacted dioxide or scsquioxide. It is concluded that the solid state behavior of transaranic elements in titanate waste forms is poorly understood; certainly inadequate to tailor a ceramic for the incorporation of fast breeder reactor wastes. A number of experiments are proposed that will provide an adequate, data base for the formulation and fabrication of transuranic-bearing jj [i waste forms. ' ' 1 ~>
I. Introduction
The potential of titanate-based ceramics as media for the solidification and stabilization of high level nuclear waste has been extensively documented. These studies have in the main been concerned with the formulation and preparation of the waste forms, optimization of their physical and chemical properties, and microstructural characterization. Significantly, the majority of this work has been conducted on material containing simulated (non-radioactive) waste.
Considering ceramic waste forms have, on occasion, been promoted as superior to vitreous waste forms for the disposal of transuranic (TRU) rich fast breeder reactor (FBR) wastes,1 surprisingly little effort has been made to verify this assumption. ,
For titanate waste forms, the available evidence suggests that all the 'actinide
(ACT) elements can be incorporated amongst perovskite and the zirconolite polytypes. However, there has been no rigorous investigation designed specifically to establish the solid solution limits of ACT elements in these phases nor their partitioning coefficients. In those cases where samples containing transuranic nuclides have been fabricated, the physical changes accompanying self-irradiation, rather than crystallochemical properties, have been emphasized.
This paucity of solid state chemical data is now limiting discussion of the merits of
TRU immobilization in titanate assemblages. In this paper, we review the literature describing the incorporation of ACT elements in titanate phases and present the results of a microstructural investigation of three titanate waste forms containing neptunium, plutonium and americium. Using this evidence we summarize the crystallochemical properties of the ACT elements and suggest a number of experiments which should be undertaken to optimize the design of a dedicated TRU waste form. 4
II. Previous Investigations
TRU elements are often regarded as comparable to rare earth (RE) elements of similar ionic radii and valence (Table I), and many workers have used these non-active analogues to evaluate the characteristics of ACT species.2-3'4 Although much useful data can be collected in this way, care is required when a mixture of
REs is involved to ensure that each is simultaneously in the correct oxidation state to simulate the TRU element of interest. For example, Ce + and Nd + could adequately simulate Np and Am respectively. However, under the reducing conditions used to fabricate titanate waste forms, cerium (as well as neodymium) is trivalent, thereby obviating the desired mimicry of solid state behavior.5»<>.7 An alternate experiment in which the waste form is fabricated under oxidizing conditions to stabilize tetravalent cerium is equally misleading, as trivalent titanium would not be formed. Since this species plays a key role in maintaining charge balance in many altervalent substitutions8 the partitioning of the TRU simulants would be different to that expected under actual fabrication conditions, and the proportions of radwaste bearing phases would alter.^ Therefore, in polyphase assemblages, where the simulation of two or more TRU elements is required, it is often impossible to design an appropriately controlled experiment.
This complicates the correlation of properties of RE analogues with TRU elements and can limit the usefulness of this approach. It is, however, often practical to carry out simulation experiments using (nearly) single phase material and one RE analogue. A summary of relevant studies is given in Table II.
(1) RE ind ACT Incorporation in Single Phase Ceramics
Zirconolite Polytypes. Several studies have been made of the partitioning of trivalent REs into zirconolite polytypes.^ These phases which protypically have the formula CaZr^Oy,11 can employ two cation acceptor sites to accommodate REs;
3 3 8 a larger CaOg cube (volume = 21.3 A ) and smaller Zr07 polyhedron (15 A ).
Rosscll concluded that small REs partition onto both sites, whereas the larger, lighter REs (e.g. Nd ) enter the calcium-site only.12 However, at the time of this work the polytypic nature of zirconolite was not recognized. A more recent study by Fielding et al.13 has shown that when zirconolite-2M is doped beyond 5 wt% neodymium it is impossible to prepare this polytype as single phase. Rather, it coexists with zirconolite-3T and another polytype with as yet lmdctermined crystal structure. The 2M polytype has neodymium partitioned strongly into the calcium- site whilst the 3T form exhibited equal part;,ioning of neodymium over both cation acceptor sites. Small RE elements (e.g. Yb ) do not stabilize zirconolite-3T and reside almost exclusively in the zirconium-site.
Thorium, the largest ACT will partition into either the calcium- or zirconium- sites of the zirconolite polytypes in accordance with charge balance
1 A 4 + considerations.14 If no small ions suitable to replace titanium are present, Th will isomorphically replace Zr in zirconolite-2M. In the presence of Mg , Fe or Al thorium enters the calcium-site with the concommitant stabilization of zirconolite-3T. At higher levels, these coupled substitutions cause the formation of zirconolite-30.1^
Tetravalent uranium14 and plutonium16 have been examined individually as dopant species and both are reported as entering the zirconium-site. No data exists for neptunium, although it would be reasonable to assume that iis behavior would parallel that of uranium and plutonium. Of the trivalent ACTs, curium is believed to partition between the calcium- and zirconium-sites.17 The properties of trivalent plutonium and americium have not been examined. Pyrochlore. Unlike the zirconolite polytypes for which a rudimentary data base exists for the prediction of TRU partitioning, the other titanate phases which are usually considered as suitable immobilization matrices, viz. perovskite and pyrochlore, have received little attention. A recent examination of uranium incorporation in a calcium-rich betafite (pyrochlore) suggested that although the stoichiometry of this phase can be approximated to CaUTi20^, the Ca/U ratio is probably not unity.18 A plutonium analogue of nominal stoichiometry CaPuTi-O^ was prepared by Clinard et al.19 Similarity, cerium is reported as replacing zirconium to stabilize CaCeTi^O^ pyrochlore,20 although iti solid solubility with
CaZrTi20^ has not been determined. 6
Perovskite. The only data relating to ACT partitioning in CaTiO* perovskite was presented by Rossouw et al.21 These workers found that about S at% uranium could enter the structure replacing calcium; it is believed that charge balance was achieved by the reduction of tetravalent titanium. The existence of the end- member CaThCK has been reported, but there is no evidence for extensive miscibility with CaTiO,.22 An attempt to synthesise CaPuO-, has been unsuccessful.23 Evans and Marples24 reported the synthesis of [Ca Q^PU 04] lT*l Ov however no chemical data was presented in support of this stoichiometry.
(2) TRU Partitioning in Polyphase Assemblages
In addition to studies of (predominantly) single phase systems a number of experiments have been devised to determine the partitioning of one or more TRU !? ! elements in polyphase assemblages resembling those of| projected waste forms
(Table III). Angelini et al.25 prepared a four phase assemblage containing hollandite, perovskite, zirconolite (polytype unspecified) and pseudobrookite doped with plutonium, americium and curium. Using autoradiography they determined that all three TRU elements entered zirconolite and perovskite.
However, they were unable to examine the valence of higher TRU cations, and the relevance of this experiment is unclear since the ceramic was prepared from the melt to promote grain growth. Consequently, elemental partitioning is likely to be different from that encountered in the low temperature, reactive hot pressing routes now favored. Van Konynenburg and Guinan26 studied the partitioning of 5 wt% PuCU in an assemblage consisting of perovskite, zirconolite, nephcline and two spinels. In these experiments care was taken to emulate the redox conditions recommended for successful fabrication of a waste form. Although this was not entirely successful, it is believed that an oxygen potential close to the Fe-FeO buffer7 was achieved ensuring that the plutonium existed in the trivalent state. Scanning electron microscopy combined with energy dispersive X-ray spectroscopy showed that plutonium was incorporated in pcrovskitc and zirconolite, with preferential partitioning into the former phase. 7
Evans and Marples24 prepared two waste forms, doped with either 2 wt% or Tig
5 wt% Pu02. and designed to consist primarily of hollandite, perovskite, ziiconolitc and pyrochlore. X-ray diffraction patterns recorded from the latter sample after three years storage reportedly consisted only of hollandite. This was interpreted as indicating that a-recoil damage had rendered zirconolite and perovskite metamict (X-ray amorphous), thereby supporting the notion that these phases (but not hollandite) contained the majority, of plutonium. However, inspection of the traces presented by Evans and Marples shows that hollandite reflections are depressed considerably in the 2 wt% doped ceramic when compared with the 5 wt% doped ceramic, suggesting that fabrication was poorly controlled.
Furthermore, the same workers also prepared single phase perovskite and zirconolite containing S wt% • PuC^. but after a storage time equivalent to the polyphase ceramic, they were able to measure X-ray reflections with sufficient accuracy to determine the lattice parameter dilation. Thus, the results as documented are anomalous and cannot be used to support the notion cf plutonium partitioning into zirconolite and perovskite.
Solomah et al.27 made a preliminary investigation of a titanate ceramic containing ~1 wt% mixed oxide (MOX) FBR waste of unspecified composition.
However, this study was primarily concerned with an evaluation of waste form dissolution characteristics and no information regarding the phase assemblage or
TRU partitioning was collected. Furthermore, the waste loading employed was at least ten times lower than that required in a commercially viable disposal scheme, and indeed any reasonably inert encapsulation matrix (e.g. rutile or alumina) would have adequately immobilized the quantity of TRU species present.
Finally, Matzkc et al.28 prepared an assemblage consisting of a hollandite- type, perovskite, zirconolite and rutile and doped with 10 wt% NpC^- Again, the material was synthesized under reducing conditions to simulate preferred fabrication conditions (although the sintering temperature of 1230°C is about 100-
150°C higher than now proposed). Using electron microprobc analysis, these workers found neptunium predominantly entered pcrovskitc and zirconolitc (the 8 latter phase being favored slightly) with minor partitioiing to hollandite and rutile.
I. III. Experimental Methods
(1) Materials Preparation
Four titanate waste forms were synthesized using a 10 wt% loading of PW-4b-
D radwaste simulant plus actinide addition (Table IV). The resultant ceramics
238 237 239 contained either 0.5 wt% U02, 2.0 wt% Np02. 0.6 wt% Pu02 or 0.005 wt%
241 238 Am203 and 0.5 wt% U02. With the exception of Am, the quantity of ACT incorporated was equal to or greater than that expected in a waste form with a 10 wt% waste loading (Table V). Nitrate solutions of the active and non-active waste solutions were added to the slurried precursor powder (Table VI), then flash dried
and calcined for 1.5 hours at 750°C under 3.5% H2/N2 gas. Titanium metal (2 wt%) was mixed with the calcine using a rotary blender. This powder was hot-pressed at > * 15 MPa for 2 h in a graphite die at either 1150°C (uranium and neptunium samples) or 1280°C (plutonium and americium samples). Details of the fabrication method are given elsewhere.^ 9
(2) Characterization
Scanning electron microscopy (SEM) using backscattered electron imaging
(BEI) were used to investigate the homogeneity of the specimens. The phase assemblage was established by powder X-ray diffraction (XRD), analytical electron microscopy (AEM) and selected area electron diffraction (SAD). An extended description of our experimental techniques is given in an earlier report.•* ®
Alpha autoradiography was used to determine the distribution of plutonium
in the waste form. This technique involves placing the material in contact with a
strip of plastic detector* for periods of 5, 10 and 30 seconds. The plastic was then etched in 6.25 M NaOH solution at 75°C for six h. The alpha particle tracks were examined under an optical microscope at 250x.29
* Type CR39, Pershorc Mouldings, U.K. 10
IV. Results
(1) Phase Assemblages
A su.nmary of the observed phase assemblages is given in Table VII. XRD
shows that the hollandite-type, perovskite and zirconolite polytypes are the
dominant waste-bearing phases in all cases (Fig. 1) with lesser quantities of other
aluminotitanates and intcrmetallic alloys identified by SAD and autoradiography.
In addition to these phases, minor unidentified phases were sometimes observed
by XRD and AEM. In particular, the assignment of a strong reflection in the XRD trace of the UO2 doped material to Magnlli phases is tentative, and based upon
earlier observations that the diffracted intensity from these phases is variable,7 I ! I possibly as a result of preferred orientation effects.
(2) Homogeneity
Low magnification (250x) backscattered electron images of the four waste
forms are shown in Fig. 2. At this level the microstructures are dissimilar, and
with the exception of the americium doped material, inhomogeneous at the tens of
microns scale. The plutonium and neptunium samples are mottled and contain
swirls of contrast as remnants of segregation which occurred during calcination.
These waste forms also contain large metal relics and clusters of small metal
particles (white areas) indicative of contamination due to blender blade wear and
poor mixing respectively. All other microstructural differences are due to
variations in precursor and calcine homogeneity.31
Transmission electron microscopy (TEM) revealed fine grained (£. 1 um )
microstructures (Figs. 3(a)-(c)), similar to those observed previously for non-
active samples.3° The uranium-bearing waste form was examined as an ion beam
thinned sample, whilst the remaining materials were crushed under cthanol and
deposited on holey carbon support films. This latter technique occasioned non-
random sampling of crystal fragments, as evidenced by preferential selection of
low density, alumina-rich fragments This non-random sampling is reflected in
the schematics of the plutonium- and neptunium-bearing waste forms. 11
The distribution of plutonium was examined by autoradiography. In general, the dispersion of this element is homogeneous, but on occasion clusters of alpha particle tracks were observed which ve suggest are due to unreacted PuO™ (Fig. 4).
In a related study of neptunium, americium and curium doped waste forms, similar alpha par'icle clusters were observed.2 ^
(3) Solid State Chemistry
EDS showed that uranium, neptunium and plutonium partitioned into perovskite (Fig. S) and zirconnlite (Fig. 6). However, as no effort was made to avoid strong electron channelling effects whilst collecting these spectra, quantification was unwarranted. Furthermore, variation in the composition of different grains prevented generalizations to be drawn regarding preferential partitioning of }l I, these elements. The distribution of TRU-bearing phases is I not contiguous (Figs.
3(d)-(f)). Rather, they exist as islands within a matrix of hollandite, rutile a.id alumina.
The valence of neptunium is necessarily 4+. X-ray photoelectron spectroscopy has determined that uranium is also tetravalent,6 but plutonium may be tri- or tetravalent. The absence of obvious differences in the partitioning of plutonium as compared with uranium and neptunium suggest that the former element may also be tetravalent, although in either valence state plutonium could enter both perovskite and zirconolite. Tetravalent TRU elements are incorporated into perovskite (CaTiCK) by replacing calcium. Charge balance is maintained by reduction of titanium or insertion of other trivalent metals (eg. Al +, Fc ),
according to the following substitution.21
CaCa TiTi TiTi Z TOlta
In zirconolite (CaZrTi20y), tetravalcnt TRUs isomorphically replace zirconium,
with minor partitioning onto the calcium site.'4 12 *lsZ™ir (2)
In addition to TRU elements, neodymium and other trivalent REEs also partition into perovskite and zirconolite vi? mechanisms described earlier.** The result suggests that the trivalent TRU species, amercium and curium, will behave similarly.
In general, its low concentration precluded the detection of amercium, and its neptunium daughter, by EDS. However, in some background stripped spectra of loveringite there was the suggestion that these elements were indeed present.
Certainly, the complex chemistrv of the phase, best expressed as i
Ca Na RE M M l l-x-y-Z x y°zUl f tl4y+2z 10-x+y-22Jx21 03g. should be amenable to the insertion of trivalent TRU elements.32 In nature, this mineral can incorporate significant quantities of RE and occur as radiation-damaged varieties - the remnants of the decay of uranium and thorium isotopes.3 3
Nonetheless, the partitioning of amercium in loveringite is conjectural and should be the subject of further investigation. 9 As reported previously, hexavrlent uranium was a minor constitutent of hollandite. 13
V. Discussion
There are two cryst'.llocheniical requirements for the successful incorporation of TRU elements in ceramic waste forms. First, a crystallographic site of appropriate size should be available to accommodate each transuranic element. As enunciated by Pennemann and Eller,34 it is not possible to determine the partitioning of TRU elements by merely comparing ionic radii, but rather judgements should be based upon bond length - bond strength calculations.
However, this approach carries simplifications which may preclude accurate predictions of interphase partitioning of actinide species. For example, calculations by Pennemann and El'er suggest that Am will preferentially enter the calcium-site of zirconolite-2M. However, polytypism was not taken into account. Nd which might be expected to have crystallochemically-similar traits to Am + (Table I) partitions onto either the calcium- or zirconium-sites to differing degrees depending on which polytype is stabilized. Using a similar analysis, Pennemann and Eller concluded that trivalent americium was too large to enter the CaOg site in perovskite, but trivalent REs of similar size are known to replace calcium. Clearly, more experimental and theoretical work is required to investigate the usefulness of bond length-bond strength calculations in assessing differences in the solid state behavior of rare earth and transuranic species.
Second, the TRU valence states should be stable to oxidation and reduction in the repository environment, and as far as possible resist radiation-induced changes of valence. In practise, TRU valencies greater than four should be avoided as these will enhance TRU dissolution.35 In this regard, it appears that with possible exception of minor uranium, the present fabrication conditions achieve this aim by reducing all TRU elements to the trivalent or tetravalcnt state; however it must be emphasized that little data regarding Am and Cm is available.
In addition, essentially no information has been collected on the effects of crystal field stabilization of TRU elements in ceramic waste forms. Such data is essential if their resistance to radiation-induced (P-emission) redox changes is to be assessed. 14
Autoradiography described in this study and related work,29 clearly shows that a portion of the TRU elements are not incorporated in the titanate phases, but exist di- and si quioxides. The reasons for this cannot be stated with any certainty, but it is unlikely that the solid solution limit for TRU species in the various phases has been reached. Rather, we believe that elemental segregation has taken place during calcination. This effect is tolerable, however, because the small quantity of actinides not incorporated in radwaste phases will be effectively immobilized, except at the surface, by encapsulation within a matrix of resistate phases. It is important to note that this behavior is not paralleled by REs. No RE oxide precipitates have ever been observed - a result which is an instructive demonstration that care is required in the use of chemical analogues. I , li .1 15
VI. Recommendations
At present, our understanding of the solid state properties of TRU elements in titanate ceramics used for waste encapsulation is far from comprehensive. Most of the experiments which have been undertaken were in the nature of reconaissance studies, utilized samples that were prepared with unrealistic fabrication conditions, or were unexpectedly abandoned. Although incomplete, the evidence which has been assembled suggests that TRU elements can be successfully incorporated in titanate waste forms. However, there are two major areas requiring investigation: the construction of ternary and quaternary oxide phase diagrams and the characterization of polyphase was'e forms prepared by accepted procedures. j
The first area requires the solid solution limits of individual TRU elements in the principal host phases, viz. perovskite, zirconolite and possibly loveringite, to be established under near equilibrium conditions. Initially, control experiments should be carried out using long-lived isotopes to minimize effects due to radiation damage and transmutation.37 Once the crystal chemistry is well defined, the propensity of ACTs to change valence during storage should be investigated using shorter-lived isotopes. The construction of phase diagrams is essential, since without this data the formulation of waste forms must remain speculative.
Furthermore, if titanate waste forms prove to be superior for the immobilization of
FBR waste,1 such knowledge will be required to determine if different ceramic
formulations are required for each stage MOX fuel burn-up, as the TRU content of waste will increase with each reprocessing cycle.
The second area concerns the partitioning and immobilization of TkU species
in polyphase assemblages. We have already alluded to the fact that, even at low levels, some portion of the TRU waste does not enter the expected phases, but precipitates as oxides. Although this is not severely deliterious to the performance of the waste form, the precise reasons for this phenomenon require clarification.
As noted above, these experiments should be conducted using short- and long- 16 lived isotopes to deconvolute crystallochemical properties and radiation damage effects.
In all cases, characterization will be incomplete unless the full compliment of SEM,38 AEM,30 MOssbauer spectroscopy,39,4® and surface analytical techniques41*42 are used. Surface analysis is particularly important during dissolution experiments because many TRU species strongly adsorb onto the walls of (sub)hydrothermal reaction vessels or may reprecipitate onto the surface of the waste form.^ 9
Acknowledgements: T.J. White would like to thank Dr Van Geel, Director of the
European Institute for Transuranium Elements (Karlsruhe) for permission to use electron microscope facilities, and acknowledges the help and support extended to him by many scientists during two periods of attachment. The manuscript was critically reviewed by K. Hawkins and K. Smith. Ion-beam-thinned sections were prepared by Mr R. Warren. 17
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The Chemistry of Actinides, Pergamon Press, Oxford, 1975.
36 R.D. Shannon, "Revised Effective Ionic Radii and iSystematic Studies of
Interatomic Distance in Halides and Chalcogenides," Acta Crystallogr., A3_2_, 751-767
(1976).
37 S.E. Kesson and C.J. Ball, "A Review of Radiation Effects in Synroc and Related
Titanate Phases." J. Aust. Ceram. Soc., 24 [1] 89-99 (1988).
38 J.A. Cooper, D.R. Cousens, R.A. Lewis, S Myhra, R.L. Segall, R.St.C. Smart, P.S.
Turner, and T.J. White, "Microstructural Characterization of Synroc C and E by
Electron Microscopy," J. Am. Ceram. Soc, 6JL [2] 64-70 ('985).
39 J.M. Friedt, R. Poinsot, J. Rebizant, and W. Muller, "237Np MOssbauer Study of the
After Effects of the Am a-Decay of Some Metallic and Insulating Hosts," J. Phys.
Chem. Solids, 40., 279-287 (1979).
40 L.A. Boatner, G.W. Beall, M.M. Abraham, C.B. Finch, P.G. Huray, and M. Rappaz,
"Monazite and Other Lanthanide Orthophosphates as Alternate Actinidc Waste
Forms"; pp.289-296 in Scientific Basis for Nuclear Waste Management Vol.2. Edited by G.J. McCpthy, Plenum, New York, 1980.
41 S. Myhra, H.E. Bishop, and J.C. Riviere, "Surface Analysis Features of Synroc B and C," Surf. Tcchnol., 19_, 145-160 (1983).
42 S. Myhra, A. Atkinson, J.C. Riviere, and D. Savage, "Surface Analytical Study of
Synroc Subjected to Hydrothcrmal Attack," J. Am. Ceram. Soc, 6J7_(3] 223-227 (1984). 21
Table I. Ionic Radii* of Actinide Ions and Rare Earth Analogues
Co-ordination No. Co-ordination No. VI VIII VI VIII Actinidc Rare Earth
Th4+ 0.94 1.05
U4+ 0.89 1.00
Np4+ 0.87 0.98 Ce4* 0.87 0.97
Pu4+ 0.86 0.96 Pr4 + 0.85 0.96
4+ J Am 0.85 0.95 II ' Cm3+ 0.85 0.95
Pu3+ 1.00 Ce3+ 1.01 1.14
Am3+ 0.98 1.09 Nd3+ 0.98 1.11
Cm3+ 0.97
* Taken from Shannon (Ref. 36) Table TI. Actinide Bearing Phases
Phase* REE simulant/ Preparative Additional Phases Stoichiometry Reference
ACT ion Conditions in the Assemblage
4+ Zirconolite-2M Th hot press, Pt capsule Th02 + unidentified [Ca][Zri_xThx][Ti2] O7,x<0.2 [14],[15] phase(s)*^*
4+ 2+ it •• Zirconolite-3r Th ,Mg (Ca1.xThx][Zr][Ti2.xMgx]O7,x~0.2 [14),[15]
4+ 2+ II it Zirconolite-30 Th ,Mg [Ca1.xThx][Zr][Ti2.xMgx]O7,x~0.3 [14].[15]
4+ it Zirconolite-2M u - *[Ca][Zr g3U n] Ti20? [14].[15]
4+ Zirconolite-2M(?) Pu air sinter oxides - [Ca][Zr 8QPu 20] Ti20? [16]
3+ Zirconolite-2M(?) Cm air sinter oxides/1325°C/40h - [Ca98Cm Q4Zr 9g][Ti2] 0? [17]
3+ Zirconolite-2M Nd air sinter oxides/1425°C/40h - *[Ca g9Nd 13][Zr 94Nd 05][Ti2] 0? [13]
3+ ti Zirconolite-3T Nd - [Ca g3Nd n][Zr 82Nd 18][Ti2] 0? [13]
4+ Pyrochlore U hot press/1100°C/graphite die perovskite, tCa][U][Ti2] 0? [18] uraninits
4+ Pyrochlore Pu air sinter/1350°C/24h Ti02JPu02^r [Ca][Pu][Ti2] 0? [19]
4 Pyrochlore Ce * air sinter <1300°C - [Ca][Ce][Ti2] 0? [20]
4+ Ca U sint«r/Ar-3%H2/1300°C/24hr CaU0 Percvskite U 4 *t .95 .05HTi] 03 [21]
+ protypical stoichiometries are CaTi03 (perovskite) and CaZrTi2^7 (zirconolite and pyrochlore)
* solid solution limit Table III. Titanate Waste Forms Containing TRU Elements
Phase Assemblage TRU Dopant Loading (wt%) Preparative Conditions* Partitioning/ Reference Analytical Procedure
H, P, Z, PB Pu, Am, Cm 0.1 Oxides melted under oxidizing Pu, Am, Cm detected in P & Z [25] conditions. using SEM/EDS and alpha autoradiography.
P, Z, N, Spl, Sp2 Pu 5 Spray dray a slurry of nitrates, Pu in P & Z, with partitioning [26] oxides and one sulphate. Air into the former phase probably calcine 650°C/64hr. Preheat favored. The sample was 900°C/2hr in graphite die. Hot characterized by SEM/EDS. press 1100°C/lhr.
H, P, Z, R** Pu 2 and 5 Matrix oxides slurried with Not determined. [24] + Magnox PuOo and Magnox waste. waste Hot-press/13506C/graphite die.
H, P, Z* MOX Wastet -1 Finely ground oxides and Not determined. [27] carbonates calcined at 800°C/ air to produce porous pellets. (includes all waste These were impregnated with species) waste by soaking. Final sinter
at 1240°C/Ar -4% H2/3hr.
H, P, Z, R* Np 10 Green pellets produced from Zirconolite (10.7 wt%) [28] oxide powders. Soaked in Perovskite (9.5 wt%) HNOj solution of NpC^. Hollandite (<.2 wt%)
Sintered 1230°C/Ar -8% H2/ Rutile (<.2 wt%) 15hr. Analyses by EMPA t Mixed Oxide Waste of unspecified composition. * Idealized assemblage not established by XRD or EMPA. * Phase analysis based on optical microscopy. • Precurser powder compositions given in original papers. ** Rutile identified by present workers. H - hollandite, P - perovskite, Z - zirconolite, PB - pseudobrookite, Sp - spinel, R - rutile 24
Ti ble IV. Specification of PW-4b-D Simulated Waste Plus Actinide Additive
Waste No.l Waste No.2 Waste No.3 Waste No.4 wt% wt% wt% wt%
Actinide U02 5.16 Np02 20.0 Pu02 6.0 Am203 0.05
U025.16
Fission Products M0O3 13.12 11.07 13.01 13.12
Ru02 7.54 6.36 7.48 7.54
Rh203 1.24 1.05 1.23 1.24
PdO 3.72 3.14 3.69 3.76
Ag20 0.21 0.18 0.21 0.21
1 cao 0.16 0.14* 0.16 0.16 1.86 1.57 1.84 1.86 Te02
C^O 8.26 . 6.97 8.19 8.26
BaO 3.93 3.32 3.90 3.93
SrO 2.68 2.26 2.66 2.68
Zr02 12.50 10.55 12.40 12.50
Y2°3 1.55 1.31 1 54 1.55 12.19 10.29 12.09 12.19 Ce02
Nd203 15.50 13.08 15.37 15.50
GdjOj 3.72 3.14 3.69 3.72
Processing Contaminants
Cr2°3 0.83 0.70 0.82 0.83
Fe2°3 3.82 3.22 3.79 3.82
NiO 0.31 0.26 0.31 0.31
P2°5 1.65 1.39 1.64 1.65 99.5 100.0 100.0 100.0 25
Table V. Comparison ol ACT Levels
ACT Oxide wt% Achieved wt% Expected in Purex Waste
uo2 0.5 0.5
Np02 2.0 0.2
Pu02 0.6 0.002
An^O-i 0.005 0.04 26
Table VII. Comparison of Phase Assemblages
(JO^-doped NpCK-doped Pu02"doped An^O^-doped
Hollandite Hollandite Hollandite Hollandite
Zirconolite-Poly types Zirconolite-Poly types Zirconolite-Poly types Zirconolite-Poly types
Magneli Phases Magn61i Phases Magneli Phases Magneli Phases
Perovskite Perovskite Perovskite Perovskite
Loveringite Pseudobrookite Alumina Loveringite
Alloy Alumina Alloy Alloy
Alloy Pu02
I It II 27
Table VI. Approximate Composition of Precursor Powder
Component Content (wt %)
Ti02 71.2
CaO 11.1
Zr02 6.8
BaO 5.5
A1203 5.4
I! 'I 28
Figure Captions XRD traces for waste forms doped with (a) UC^, (b) NpC^, (c) PuC^ and
(d) Ani20o. H - hollandite, Z - zirconolite, P - perovskite, M - Magndli phases. BSE images of waste forms doped with (a) UC^, (b) Np02, (c) PUO2 and
(d) An^Og.
Schematic representation of phase distribution in (a) UC^, (b) Np02
and (c) Pu02 doped waste form. (d), (e), (f) elemental partitioning
(shaded areas) of uranium, neptunium and plutonium respectively. H - hollandite, Z - zirconolite, P - perovskite, R - rutile, A - alloy, L - 'I loveringite, PB - pseudobrookite, Al - alumina, X - unknown. Alpha track etch pattern produced by P11O2 segregation.
EDS spectra for perovskite containing (a) no waste elements (b) uranium, (c) neptunium, and (d) plutonium.
EDS spectra for zirconolite containing (a) no waste elements, (b) uranium, (c) neptunium, and (d) plutonium. ^;^5^$?j?§§§m§§^^
29
28 30 32 28 30 32
Figure 1 30
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31
^ 50 nm sTjj 50 nm MM
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Figure 3 [ISSSSSSSSSSS*^?^^^ '"::"V
32
;- • * V irf ^ ^
50 pm
Figure 4 33
I 1 1 1 r T, a Co 1 Al Sf i1 u
FT-
Co
Al Nd
Pu
J _ _L_ !_..._._ i J 23456 KeV
Figure 5 34
-i —i i 1 T. a
Co Zr A A \
i U 1 u — i Al 1 ' b Co T I Nd
1 1 I
Co
/ Nd V l/\ Nd Nd •
i i • 1 If d 1V Co 1
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• < 1 1 2 3 4 5 6 KeV
Figure 6