Zirconolite Phase Research Articles

Thermochemical Investigations of Zirconolite, Pyrochlore and Brannerite: Candidate Materials for the Immobilization of Plutonium

ABSTRACTStandard enthalpies of formation, ΔH°f (kJ/mol) at 298 K, were derived for three pyrochlore phases: Ca0.93Ce1.00Ti2.035O7.00 (−3656.0±5.6), Ca1.46U4+0.23U6+0.46Ti1.85O7.00 (−3610.6±4.1), Gd2Ti2O7 (−3822.5±4.9) and two zirconolite phases: CaZr1.03Ti1.97O7 (−3719.4±3.9) and CaHf1.02Ti1.98O7 (−3720.5±3.9). Enthalpies of formation with respect to an oxide phase assemblage, ΔH°f-ox (kJ/mol) at 298 K: CaO + MO2 + 2TiO2 = CaMTi2O7 or Gd2O3 + 2TiO2 = Gd2Ti2O7, and an oxide/perovskite phase assemblage: ΔH°f-pv+ox (kJ/mol) at 298 K: CaTiO3 + MO2 + TiO2 = CaMTi2O7, M = Ce, U, Hf, Zr were also calculated. All of the pyrochlore and zirconolite materials studied were stable in enthalpy with respect to their oxides. ΔH°f-ox: Ca0.93Ce1.00Ti2.035O7.00 (−54.1±5.2), Ca1.46U4+0.23U6+0.46Ti1.85O7.00 (−123.1±3.4), Gd2Ti2O7 (−113.4±2.8), CaZr1.03Ti1.97O7 (−89.6±2.8) and CaHf1.02Ti1.98O7 (−74.8±3.1). With respect to a perovskite plus oxide phase assemblage the hafnium zirconolite was marginally metastable in enthalpy (ΔH°f-pv+ox = +6.0±3.5 kJ/mol) while zirconolite was marginally stable in enthalpy (ΔH°f-pv+ox = −8.8±3.3 kJ/mol). The Ce-pyrochlore was not thermodynamically stable as measured by the enthalpy change relative to perovskite plus oxides: ΔH°f-pv+ox = Ca0.93Ce1.00Ti2.035O7.00 (+21.0±5.5). The U-pyrochlore sample was marginally stable in enthalpy relative to a perovskite plus oxide assemblage: ΔH°f-pv+ox = Ca1.46U4+0.23U6+0.46+Ti1.85O7.00 (−5.1±4.0). A significant field where the proposed waste form for excess weapons plutonium is not thermodynamically stable both at room temperature and at high temperature was defined.ΔH°f (kJ/mol) at 298 K were derived for three brannerite phases: CeTi2O6 (−2948.8±4.3), U0.97Ti2.03O6 (−2977.9±3.5) and ThTi2O6 (−3096.5±4.3). Enthalpies of formation with respect to an oxide phase assemblage, ΔH°f-ox (kJ/mol) at 298 K: MO2 + 2TiO2 = MTi2O6, M = Ce, U, Th were also calculated. ΔH°f-ox: CeTi2O6 (+29.4±3.6), U0.97Ti2.03O6 (−7.7±2.8) and ThTi2O6 (+19.4±1.6). Thus, Ce and Th-brannerite are entropy stabilized and thermodynamically stable only at high temperature.These thermochemical data are correlated to the observed trends in the radiation “susceptibility” of these phases. Thermodynamic data relevant to waste form processing and optimization are also discussed.

Synthesis and crystallochemical interaction of titania and zirconia based ceramic precursors

Titania and zirconia based ceramic material useful in 'Synroc' (synthetic rock) technology of radioactive waste management contains hollandite [Ba(Al,Ti) 2 Ti 6 O 1 6 ], zirconolite (ZrCaTi 2 O 7 ) and perovskite (CaTiO 3 ). 9 0 Sr and 1 3 7 Cs arc the most heat and β-activity generating radioactive elements present in high level nuclear waste (HLW). Sr is crystallochemically incorporated via a simple subslitution for calcium in perovskite type structures while Cs is entraped in hollandite phase of the precursor. U is chemically fixed in zirconolite phase of the precursor. To simulate the crystallochemical mechanism, all three phases has been synthesized through oxide route by heating calculated quantities of respective oxides at 1150° for 8 h. 0.02 moles of strontium, cesium and uranium were added in perovskite, hollandite and zirconolite phase respectively to obtain the simulated wasteform. The sintered phases were pelletized and reheated three times to obtain the polycrystalline and densified material. Each of the titania and zirconia based synthetic phases have been characterized by X-ray powder diffraction, scanning electron microscopy and energy dispersive spectroscopy (SEM/EDS). Leaching studies on the ceramic phases show that the precursor material can crystallochemically incorporate Sr, Cs and U in the ceramic matrix.

Crystallization study of (TiO2, ZrO2)-rich SiO2-Al2O3-CaO glasses Part I Preparation and characterization of zirconolite-based glass-ceramics

Nuclear power reactors generate long-lived radionuclides such as minor actinides (Np, Am, Cm) which are mainly responsible for the long term radiotoxicity of high level nuclear wastes obtained after reprocessing of nuclear spent fuel. Specific highly durable matrices such as glass-ceramics appear as good candidates for the immobilization of minor actinides. This work concerns the synthesis and the characterization of zirconolite (CaZrTi2O7) based glass-ceramics prepared by controlled devitrification of (TiO2, ZrO2)-rich SiO2-Al2O3-CaO parent glasses for which neodymium was selected to simulate the radioactive trivalent minor actinides. The present study reports the effect of increasing TiO2, ZrO2 and CaO amounts in glass composition on the structure and the composition of the zirconolite crystals (formed as the only crystalline phase in the bulk of the glass), on their nucleation rate I(Z) and on the volume proportion of crystalline phase V of the glass-ceramics. It appears that I(Z) and V strongly increase when the parent glass composition changes. Neodymium electron spin resonance (ESR) shows that the total amount of Nd3+ ions incorporated in the zirconolite phase increases with TiO2, ZrO2 and CaO amounts in parent glass composition.

U- and Hf-Bearing Pyrochlore and Zirconolite and their Leached Layers Formed in Acidic Solution: Tem Investigation

ABSTRACTTransmission electron microscopy results from a sintered ceramics with stoichiometry of Ca(U0.5Ce0.25Hf0.25)Ti2O7 show the material contains both pyrochlore and zirconolite phases and structural intergrowth of zirconolite lamellae within pyrochlore. (001) plane of zirconolite is parallel to (111) plane of pyrochlore because of their structural similarities. The pyrochlore is relatively rich in U, Ce, and Ca with respect to the coexisting zirconolite. Average compositions for the coexisting pyrochlore and zirconolite at 1350 °C are Ca1.01(Ce3+0.13Ce4+0.19U0.52Hf0.18)(Ti1.95Hf0.05)O7 (with U/(U+Hf) = 0.72) and (Ca0.91Ce0.09)(Ce3+0.08U0.26Hf0.66Ti0.01)Ti2.00O7 (with U/(U+Hf) = 0.28) respectively. A single pyrochlore (Ca(U,Hf)Ti2O7) phase may be synthesized at 1350 °C if the ratio of U/(U+Hf) is greater than 0.72, and a single zirconolite (Ca(Hf,U)Ti2O7) phase may be synthesized at 1350 °C if the ratio of U/(U+Hf) is less than 0.28. An amorphous leached layer that is rich in Ti and Hf forms on the surface after the ceramics has been leached in pH 4 buffered solution. The thickness of the layer ranges from 5 nm to 15 nm. The leached layer functions as a protective layer and therefore reduces the leaching rate.

Selection of Matrices for Immobilization of Actinide Fraction of HLW

ABSTRACTHLW after the separation of U, Pu and Np contains two major radionuclide groups: fission products and TRU elements. The later ones are the most dangerous and should be separated in the special actinide fraction. This process has been realized at PA “Mayak”.The composition of the actinide fraction depends on a separation method:A. Radionuclides of Am and Cm with minor amounts of U, Np and Pu.B. Radionuclides of Am and Cm with minor amounts of U, Np and Pu and also REEs.C. Radionuclides of Am and Cm with minor amounts of U, Np and Pu and also REEs and perhaps some amount of Zr.The fraction with composition A can be burned off in fast reactors whereas the fractions with compositions B and C should be immobilized in stable matrices. Alpha-Activity of actinide fraction is subsequently determined by 244Cm, 241Am, 244Am, 239Pu and 242Pu during a long-term storage. The storage time depends on conditions and can reach several hundreds of thousands years. The main requirements to matrices are chemical and radiation resistances. The fraction with composition B can be incorporated in perovskite ceramics whereas in case of composition C the additional zirconolite and/or pyrochlore phases are necessary.

Crystallization sequence and microstructure evolution of Synroc samples crystallized from CaZrTi 2O 7 melts

Scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM) and analytical electron microscopy (AEM) studies have been conducted on samples crystallized from melts with a composition of zirconolite {(Ca 0.9Gd 0.1)Zr(Ti 1.9Al 0.1) 2O 7}. The formation of a whole suite of Synroc phases (zirconia, ZrTiO 4, zirconolite, perovskite and rutile) has been observed. In the CaZrTi 2O 7 system, the formation of these phases follows the crystallization sequence of Ti-bearing zirconia→ ZrTiO 4 phase → Zr-rich zirconolite → Zr-poor zirconolite → rutile/ perovskite . This sequence is induced by a fractional crystallization process, in which Zr-rich phases tend to crystallize first, resulting in continuous depletion of Zr in melt. Consistent with this melt compositional evolution, the Zr content in the zirconolite decreases from the area next to the ZrTiO 4 phase to areas next to rutile or perovskite. High-resolution TEM images show that there are no glassy phases at the grain boundary between zirconolite and perovskite. The fractional crystallization-induced textural heterogeneity may have a significant impact on the incorporation of radionuclides into crystalline phases and the resistance of radionuclides to leaching processes. Exsolution lamellae and multiple twinning resulting from the phase transition from tetragonal zirconia to monoclinic zirconia may decrease durability of the Synroc. Fast cooling of the melt may produce more zirconolite phase and relatively uniform texture. In general, however, a Synroc prepared by melting is less uniform in texture than that prepared by a sol–gel method.

Use of linear free energy relationship to predict Gibbs free energies of formation of zirconolite phases (MZrTi 2O 7 and MHfTi 2O 7)

In this letter, the Sverjensky–Molling equation derived from a linear free energy relationship is used to calculate the Gibbs free energies of formation of zirconolite crystalline phases (MZrTi 2O 7 and MHfTi 2O 7) from the known thermodynamic properties of the corresponding aqueous divalent cations (M 2+). Sverjensky–Molling equation is expressed as ΔG 0 f,M vX = a M vX ΔG 0 n,M 2+ + b M vX + β M vX r M 2+ , where the coefficients a M vX , b M vX , and β M vX characterize a particular structural family of M v X, r M 2+ is the ionic radius of M 2+ cation, ΔG f,M vX 0 is the standard Gibbs free energy of formation of M vX, and ΔG 0 n,M 2+ is the standard non-solvation energy of cation M 2+. This relationship can be used to predict the Gibbs free energies of formation of various fictive phases (such as BaZrTi 2O 7, SrZrTi 2O 7, PbZrTi 2O 7, etc.) that may form solid solution with CaZrTi 2O 7 in actual Synroc-based nuclear waste forms. Based on obtained linear free energy relationships, it is predicted that large cations (e.g., Ba and Ra) prefer to be in perovskite structure, and small cations (e.g., Ca, Zn, and Cd) prefer to be in zirconolite structure.

Microstructure Evolution and Weathering Reactions of Synroc Samples Crystallized from CaZrTi2O7 melts: TEM/AEM investigation and geochemical modeling

AbstractScanning electron microscopy (SEM), transmission electron microscopy (TEM), and analytical electron microscopy (AEM) studies have been conducted on samples crystallized from a melt with a composition of zirconolite {(Ca0.9Gd0.1)Zr(Ti1.9Al0.1)2, O7}. The formationof a whole suite of Synroc minerals (zirconia, ZrTiO4, zirconolite, perovskite, and rutile) has been observed. The formation of these minerals follows the crystallization sequence of Ti-bearing zirconia → ZrTiO4 phase → Zr-rich zirconolite → Zr-poor zirconolite → rutile/perovskite. This sequence is induced by a fractional crystallization process, in which Zr-rich mineral phases tend to crystallize first, resulting in continuous depletion of Zr in melt. Consistent with this melt compositional evolution, Zr content in the zirconolite decreases from the area next to ZrTiO4 phase to the area next to rutile or perovskite. High-resolution TEM images show that there are no glassy phases at the grain boundary between zirconolite and perovskite. The fractional crystallization-induced textural heterogeneity may have a significant impact on the incorporation of radionuclides into crystalline phases and the resistance of radioniclides to leaching processes. Exsolution lamellae and multiple twinning result from the phase transition from tetragonal zirconia to monoclinic zirconia may decrease durability of the Synroc. Fast cooling of melt may produce more zirconolite phase and relatively uniform texture. In general, however, a Synroc prepared by a through-melt method is less uniform in texture than that prepared by a through-sol-gel method. The reaction path calculation for the alteration of U-bearing zirconolite in an oxidizing fluid shows that zirconolite is first altered into a perovskite-like phase (CaZrO3), followed by rutile (TiO2 ), and U6+ -bearing phases of soddyite [(UO2)2SiO42H20] and haiweeite [Ca(U02)2Si6O15·5H2O].

Leaching Behavior of Zirconolite-Rich Synroc Used to Immobilize High-Fired Plutonium Oxide

ABSTRACTThis study reports on the use of zirconolite-rich Synroc to demonstrate the safe immobilisation of ‘high-fired’ Pu02. The zirconolite-rich Synroc used in this study was prepared by adding 13 wt% Pu with equimolar amounts of Gd and Hf, relative to Pu, as neutron absorbers. The incorporation of the Pu and neutron absorbers has been studied microstructurally as well as by longer-term leach testing. This work has shown that the sintered ceramic can immobilise 13 wt% of Pu with almost complete incorporation of the Pu (≃ 98%) into the zirconolite phase. Durability studies have shown that under a wide range of leaching conditions there is no major separation of the Pu and neutron absorbers, with the majority of these elements either remaining in the matrix or leaching at low (< 10−4 g m−2 d−1) and comparable rates from the waste form.

Characterization of a Plutonium-Bearing Zirconolite-Rich Synroc

ABSTRACTA titanate-based ceramic waste form, rich in phases structurally related to zirconolite (CaZrTi2O7), is being developed as a possible method for immobilizing excess plutonium from dismantled nuclear weapons. As part of this program, Lawrence Livermore National Laboratory (LLNL) produced several ceramics that were then characterized at Argonne National Laboratory (ANL). The plutonium-loaded ceramic was found to contain a Pu-Gd zirconolite phase but also contained plutonium titanates, Gd-polymignyte, and a series of other phases. In addition, much of the Pu was remained as PuO2-x. The Pu oxidation state in the zirconolite was determined to be mainly Pu4+, although some Pu3+was believed to be present.

Solid solution of metal oxides in the zirconolite phase CaZrTi 2O 7. I. Heterotype solid solutions

Structure refinement based on powder X-ray diffraction intensities has been carried out on zirconolite phases containing ∼10 mole% of various oxides in solid solution. Unambiguous crystallographic location of solute cations was not always possible, but it is shown that the large cations Th iv, Nd iii, Gd iii, Yb iii, and Sc iii substitute for Ca ii and Zr iv in the structure, and that the smaller Fe iii, Co ii, Mg ii, Ta v, and Sn iv substitute for Ti iv, while some exchange of Ti iv and Zr iv also occurs. There is evidence that both cation and anion arrays are intact in these solid solutions, so that the overall stoichiometry is always M 4O 7, i.e., the formal charges of substituting cations are mutually dependent. It is shown that the structure will tolerate substitution of Nd iii or Th iv for Ca ii only if Zr iv is simultaneously substituted by cations of lower charge, e.g., Sc iii or Mg ii: if these are not available, an appropriate amount of Ti iii becomes stabilized in the structure, and this may be supplied from a suitable excess of TiO 2 in the reaction mixture. The extent of the heterotype solid solutions has not been explored: however, in the case of Sn iv, substitution for Ti iv can be complete. Zirconolite-related phases with rhombohedral substructure that form at high solute concentrations are discussed.

Solid solution of metal oxides in the zirconolite phase CaZrTi 2O 7. II: The ternary phase CaZr xTi 3− xO 7

The extent of solid solution of CaO, ZrO 2, and TiO 2 in the CaZrTi 2O 7 phase has been determined using powder X-ray diffraction methods. The CaO content always lies between 24.0 and 25.0 mole%, while the atomic ratio[Ti]/[Zr] may vary from 1.4 to 2.6 at 1300°C and from 1.2 to 2.1 at 1480°C. The relevant portion of the ternary phase diagram CaO ZrO 2 TiO 2 has been drawn. Structure refinement based on powder X-ray diffraction intensities has been carried out on zirconolite phases CaZr x Ti 3− x O 7 with x = 0.9, 1.0,and1.2: the resultant cation distributions agree with those from a previous study.

Neptunium Doping of the Crystalline Ceramic Waste Form Synroc B

The crystalline ceramic waste form synroc B was produced with a nominal loading of 10 wt% NpO{sub 2} by soaking green pellets with Np solution followed by sintering in reducing atmosphere (Ar/8% H{sub 2}). Electron microprobe analysis showed that Np was partitioned between the zirconolite (10.7 wt% Np) and perovskite phases (9.5 wt%), whereas the remaining two phases of hollandite and rutile contained {le} 0.2 wt% Np. The partitioning of Np was not significantly changed by an oxidizing anneal. Leach tests (190{degree}C, 7d, deionized water) showed the leach rate for Np to be <5 {times} 10{sup {minus}3} g/(m{sup 2}{center dot}d).

Scanning and high-resolution Auger analysis of zirconolite/perovskite surfaces following hydrothermal treatment

Two titanate minerals, zirconolite with perovskite minor phase inclusions, have been subjected to hydrothermal chemical attack. The effects of the treatment have been investigated with scanning and high-resolution Auger analysis. The results suggest that both mineral surfaces act as substrates for carbonate precipitation. Dissolution was found to be the main mechanism of attack for perovskite, followed by precipitation from solution and growth of TiO2 on the perovskite phase regions. There was also precipitation of calcium as fluoride and phosphate, and growth of these crystallites at random locations on the surface of the two-phase ensemble. The zirconolite phase regions were not affected by the hydrothermal treatment, within the resolution of the present experiment.

X-ray photoelectron spectroscopy studies of valence states of cerium and uranium in SYNROC C

Small quantities of lanthanides and actinides have been added to the SYNROC precursor slurry as part-simulation of high level nuclear waste (HLW) species. There are two main parameters which influence the crystallographic destination of the HLW species — the cation size and its valence state. Lanthanides and actinides are predicted to incorporate as a trivalent species into the zirconolite phase and/or as a trivalent or tetravalent species into perovskite. X-ray photoelectron spectroscopy (XPS) was used to investigate the valence states of cerium and uranium in SYNROC C. The analysis of the characteristic satellite structures, binding energies and peak shapes in the XPS spectra showed that cerium and uranium in SYNROC C adopt trivalent and tetravalent states respectively, in accord with crystallochemical prediction.

Structural damage in a self-irradiated zirconolite-based Ceramic

The zirconolite phase of SYNROC nuclear waste was fabricated with 5 mol% 238PuO 2 substituted for a like amount of ZrO 2, in order to induce self-irradiation damage. The resulting product exhibited a matrix of monoclinic zirconolite containing ∼~ 3.8 mol% PuO 2 along with roughly 20 vol.% of the cubic polymorph with approximately twice the PuO 2 content of the matrix. After a dose of 2.1 × 10 25 α decays/m 3 at room temperature (800 d storage) swelling reached 5.5 vol.% and neared saturation. The monoclinic phase became X-ray metamict at ∼~1.0 × 10 25 α/m 3 after slight atomic rearrangement within the crystalline material. Periodic TEM examination revealed a gradual evolution from the crystalline state to an amorphous condition with residual crystallites, consistent with a model involving conversion by alpha recoil tracks. Optical metallography showed extensive microcracking, attributed to differences in swelling rates of the two zirconolite polymorphs.

Surface analysis features of Synroc B and C

Surface analysis features of Synroc B and C, proposed high level nuclear waste forms without (B), and with (C), incorporated simulated waste, were studied. It was shown that all species of the solid matrix as well as some important waste species can be detected reliably with X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy. Fully quantified depth profiles for all major species were obtained by XPS analysis and ion beam sputtering; the inferred abundances are in good agreement with the nominal stoichiometry. Information about the chemical states of all major species and some waste species was also inferred from details of XPS peak positions and envelopes; there is good correlation, in general, with the valence states predicted for the phase assemblage. However, there were non-tetravalent contributions to the Ti 2p spectral envelope, some anomalies related to the Ca 2p envelope and molybdenum was found mainly as a hexavalent species. The effect of ion bombardment (for profiling purposes) was to reduce most species to states of lower valency. Hollandite and zirconolite phase regions were particularly susceptible to ion beam modification. The Ti 2p envelope was most severely affected, but molybdenum and iron XPS peaks also exhibited shifts with increasing ion dose.

Ion Implantation Studies of Nuclear Waste Forms

ABSTRACTThe first observations of physical and chemical changes induced by lead implantation damage and leaching are reported for two proposed U.S. nuclear waste forms (PNL 76–68 borosilicate glass and Sandia titanate ceramics) for commercial wastes. To simulate the effects of recoil nucleii due to alpha decay, the materials were implanted with lead ions at equivalent doses up to approximately 1 × 1019 a decays/cm3 . In the titanate waste form, the zirconolite, perovskite, hollandite, and rutile phases all exhibited a mottled appearance in the transmission electron microscope (TEM) typical of defect clusters in radiation damaged, crystalline solids. One titanate phase containing uranium was found by TEM to be amorphous after implantation at the highest dose. No enhanced leaching (deionized water, room temperature, 24 hours) of the irradiated titanate waste form, including the amorphous phase, was detected by TEM, but Rutherford backscattering (RBS) suggested a loss of cesium and calcium after 21 hours of leaching. The RBS spectra also indicated enhanced leaching from the PNL 76–68 borosilicate glass after implantation with lead ions, in general agreement with the observations of Dran, et al. [6,7] on other irradiated materials. Elastic recoil detection spectroscopy (ERD), used to profile hydrogen after leaching, showed penetration of the hydrogen to several thousand angstroms for both the implanted and unimplanted materials. These basic studies identified techniques to follow the changes that occur on implantation and leaching of complex amorphous and crystalline waste forms. These studies were not designed to produce comparisons between waste forms of gross leach rates.