Investigation of Pyrochlore-Based U-Bearing Ceramic Nuclear Waste

Jan 28, 2004 - A durable titanate ceramic waste form (Synroc) with pyrochlore (Ca(U,Pu)Ti2O7) and zirconolite (CaZrTi2O7) as major crystalline phases ...
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Environ. Sci. Technol. 2004, 38, 1480-1486

Investigation of Pyrochlore-Based U-Bearing Ceramic Nuclear Waste: Uranium Leaching Test and TEM Observation H U I F A N G X U , * ,† Y I F E N G W A N G , †,‡ PIHONG ZHAO,§ WILLIAM L. BOURCIER,§ RICHARD VAN KONYNENBURG,§ AND HENRY F. SHAW§ Transmission Electron Microscopy Laboratory, Department of Earth and Planetary Sciences, The University of New Mexico, Albuquerque, New Mexico 87131, Sandia National Laboratories, Albuquerque, New Mexico 87185, and Lawrence Livermore National Laboratory, L-219, Livermore, California 94550

A durable titanate ceramic waste form (Synroc) with pyrochlore (Ca(U,Pu)Ti2O7) and zirconolite (CaZrTi2O7) as major crystalline phases has been considered to be a candidate for immobilizing various high-level wastes containing fissile elements (239Pu and 235U). Transmission electron microscopy study of a sintered ceramic with stoichiometry of Ca(U0.5Ce0.25Hf0.25)Ti2O7 shows the material contains both pyrochlore and zirconolite phases and structural intergrowth of zirconolite lamellae within pyrochlore. The (001) plane of zirconolite is parallel to the (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. The synthesized products were used for dissolution tests. The single-pass flow-through dissolution tests show that the dissolution of the U-bearing pyrochlore is incongruent. All the elements are released at differing rates. The dissolution data also show a decrease in rate with run time. The results indicate that a diffusion-controlled process may play a key role during the release of U. TEM observation of the leached pyrochlore directly proves that an amorphous leached layer that is rich in Ti and Hf formed on the surface after the ceramic was leached in pH 4 buffered solution for 835 days. The thickness of the layer ranges from 6 to 10 nm. A nanocrystalline TiO2 phase also forms in the leached layer. The U leaching rate (g/ (m2 day)) in acidic solutions can be expressed as log(NR) ) -5.36-0.20pH, where NR is the normalized rate. * Corresponding author phone: 505-277-7536; fax: 505-277-8843; e-mail: [email protected]. † The University of New Mexico. ‡ Sandia National Laboratories. § Lawrence Livermore National Laboratory. 1480

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Conservative leaching rates of uranium [log(NR)] for the U-bearing ceramic at pH 2 and pH 4 solutions are -5.76 and -6.16 g/(m2 day), respectively. The results show that the U release rate of the ceramic waste is 10 times slower than that of defense high-level waste glass and about 1000 times slower than that of spent fuel. The pyrochlore-based ceramic is an ideal waste form for immobilizing longlived radionuclides of 239Pu and 235U due to the Ti- and Hfrich leached layer that forms on the ceramic surface. The leached layer functions as a protective layer and therefore reduces the leaching rate as thickness of the leached layer increases.

1. Introduction A durable titanate ceramic waste form (Synroc) with pyrochlore (Ca(U,Pu)Ti2O7) and zirconolite (CaZrTi2O7), a derivative structure of pyrochlore, as major crystalline phases has been shown to be particularly promising for immobilizing various high-level wastes containing fissile elements (239Pu and 235U) (1-12). General stoichiometry for zirconolite and pyrochlore can be expressed as ABTi2O7. Tetravalent actinides generally occupy site B positions with a coordination number of 7 or 8. Thermodynamic study indicates that U- and Pupyrochlore phases are stable with respect to rutile, perovskite, and their oxides UO2 and PuO2 (12). The Synroc ceramic waste form will be thermodynamically stable in aqueous silica-depleted and aqueous carbonate-depleted groundwater environments (such as the Waste Isolation Pilot Plant (WIPP) site in Carlsbad, NM) (13). The concept of Synroc was originally proposed by Ringwood et al., because natural minerals of pyrochlore and zirconolite are very resistant to chemical weathering and low-temperature alteration (2). The first Synroc fabrication technology was developed by Dosch et al. at Sandia National Laboratories (1). During the past 2 decades, Synroc has been subjected to extensive studies because of its chemical durability (1-14). Synroc immobilizes radionuclides by incorporating them into appropriate phases and forming solid solutions. With large polyhedra (with coordination numbers ranging from 7 to 8) in the structures, Synroc is able to accommodate a wide range of radionuclides (e.g., actinides, Pu, U, Ba, Sr, Cs, Rb, and Tc, etc.) as well as neutron absorbers (e.g., Gd and Hf) (10). U- and Pu-loaded Synroc generally contains phases of pyrochlore and zirconolite (1-3, 7, 8, 15). Various Synroc formulations (e.g., Synroc-C, Synroc-D, Synroc-E, and Synroc-F, etc.) have been developed for specific high-level wastes (1, 15, 16). Homogeneity of sintered ceramics is an important factor in long-term chemical durability. Ce3+, Ce4+, U4+, and U6+ in the studied crystalline phases may be used as chemical analogues for Pu3+, Pu4+, and Pu6+, respectively. It is necessary to test chemical durability of the ceramic phase at low temperature in order to predict long-term behavior of the ceramic waste in an actual geological repository. Because the ceramic phases of pyrochlore and zirconolite have very low solubility (17), they are chemically durable in aqueous solution. Therefore, it is difficult to measure leaching rates at room temperature. It has been proposed that there is a thin leached layer (or alteration layer) ranging from a few nanometers to a few tens of nanometers on the leached (or altered) ceramic phase (17-19). However, there is no direct evidence to prove existence of the leached layer. In this article, we address the leached layer formed on 10.1021/es030582v CCC: $27.50

 2004 American Chemical Society Published on Web 01/28/2004

FIGURE 1. SAED pattern (A) and bright-field TEM image (B) from an area containing intergrown zirconolite and pyrochlore. (001) of zirconolite is parallel to (111) of pyrochlore.

FIGURE 2. EDS spectra from zirconolite (A) and pyrochlore (B); EELS spectra from zirconolite (C) and pyrochlore (D). Weak postedge peaks of M5 and M4 edges characterize Ce4+ (25, 26). the ceramic surface and its role on the long-term release of uranium and other radionuclides in geological repositories.

2. Samples and Experiments The starting material with stoichiometry of Ca(U0.5Ce0.25Hf0.25)Ti2O7 was prepared by mixing and grinding powders of TiO2 (anatase), UO2, Ca(OH)2, HfO2, and CeO2 and mixing with deionized water. The mixed material was dried overnight in air at about 90 °C. The dried material was pressed and then sintered at 1350 °C for 5 h in an environment of slowly

flowing Ar gas at 1 atm to avoid oxidation of uranium. Part of the sintered sample was crushed in acetone. Drops of acetone suspension containing the crushed particles were dropped on holey C-coated nylon grids for transmission electron microscopy (TEM) observation in order to avoid peak overlaps from Hf and the conventional TEM Cu grid. All TEM and energy-dispersive spectroscopy (EDS) results were carried out with JEOL 2010F FEG-TEM and associated Gatan GIF system and 2010 high-resolution TEM and Oxford Link ISIS EDS system. Point-to-point resolution of the highVOL. 38, NO. 5, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. Chemical Formulas of the Pyrochlore and Zirconolitea pyrochlore

zirconolite

Ca0.99(Ce3+0.12Ce4+0.18U0.52Hf0.21)(Ti1.95Hf0.05)O7 Ca1.02(Ce3+0.13Ce4+0.19U0.54Hf0.16)(Ti1.96Hf0.04)O7* Ca1.01(Ce3+0.13Ce4+0.19U0.55Hf0.15)(Ti1.95Hf0.05)O7 Ca1.00(Ce3+0.13Ce4+0.18U0.52Hf0.19)(Ti1.96Hf0.04)O7 Ca1.00(Ce3+0.13Ce4+0.19U0.49Hf0.21)(Ti1.94Hf0.06)O7 Ca1.02(Ce3+0.12Ce4+0.18U0.54Hf0.18)(Ti1.94Hf0.06)O7*

(Ca0.91Ce0.09)(Ce3+0.10U0.25Hf0.66)(Ti1.99Hf0.01)O7 (Ca0.94Ce0.06)(Ce3+0.09U0.25Hf0.65Ti0.01)Ti2.00O7* (Ca0.90Ce0.10)(Ce3+0.06U0.25Hf0.67)Ti2.00O7 (Ca0.92Ce0.08)(Ce3+0.09U0.26Hf0.65)Ti2.00O7 (Ca0.93Ce0.07)(Ce3+0.11U0.26Hf0.64)Ti2.00O7 (Ca0.88Ce0.12)(Ce3+0.05U0.26Hf0.66Ti0.02)Ti2.00O7* Average

Ca1.01(Ce3+0.13Ce4+0.19U0.52Hf0.18)(Ti1.95Hf0.05)O7 U/(U + Hf) ) 0.72

(Ca0.91Ce0.09)(Ce3+0.08U0.26Hf0.66Ti0.01)Ti2.00O7 U/(U + Hf) ) 0.28

a Note: The chemical formulas are normalized to 7-oxygen. The formulas with * are from intergrown pyrochlore and zirconolite. The ratio of U/(U + Hf) only considers U and Hf in site B positions of zirconolite and pyrochlore with the stoichiometry of ABTi2O7. The U/ (U + Hf) ratio for the pyrochlore is about 0.69 if all Hf atoms are at B sites. Because a small amount of U may replace Ti in octahedral sites (32), the proposed cation distribution among difference sites of the formula may be considered as an approximation if U5+ exists in the pyrochlore.

FIGURE 3. HRTEM image showing a coherent boundary between the pyrochlore and zirconolite (left). A structural model shows that (001) of zirconolite is parallel to (111) of the pyrochlore (right). Inserts A and B are Fourier transform patterns from the areas of zirconolite and pyrochlore, respectively. Inserts C and D are noise-filtered HRTEM images from the areas of zirconolite and pyrochlore, respectively.

resolution TEM is 0.19 nm. Mineral standards were used for quantification of the collected EDS data as described in ref 20. A theoretical kCe value was used, because no ceriumsilicate standard was available for Ce. Electron energy-loss spectroscopy (EELS) studies of Ce were carried out with a JEOL 2010F HRTEM with a GIF system. Dissolution tests were carried out by using a method of single-pass flow-through (21). The buffered leach solutions of pH 2 (HCl, 0.01 m), pH 4 (potassium acid phthalate/HCl, 0.005 m), and pH 6 (potassium acid phthalate/NaOH, 0.005 m) were used for the dissolution tests. Single-pass flowthrough (SPFT) tests measure the release rates of elements dissolved from powdered ceramic into flowing pH-buffered solutions. They are designed to measure the dissolution rate far from equilibrium where rates are not affected by saturation effects. This is done by adjusting the flow rate and surface area of the sample. They are not designed to mimic repository conditions; they are instead designed to extract model parameters for a kinetic model of ceramic dissolution. A complete discussion of the SPFT method can be found in refs 22-24. The plots are of normalized release rate (NR) vs time. The normalized release rate was obtained according 1482

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to the relationship of

NR )

(couti - cini)q fiA

where cini is the blank concentration of element i in the buffered leach solution, coutI is the concentration of element i in the effluent solution, q is the solution flow rate, fi is the weight fraction of element i in the ceramic, and A is the BET-measured surface area of the ceramic. Normalized rates are given in units of g/(m2 day). The TEM sample of the leached pyrochlore was prepared by directly depositing fine grain powders on a holey C-coated nylon grid for transmission electron microscopy observation in order to avoid any possible damage and artifacts on the ceramic surface.

3. Results and Discussions TEM and EDS results show that the sample contains both pyrochlore and zirconolite phases. Figure 1 shows a selectedarea electron diffraction (SAED) pattern and bright-field TEM image of the pyrochlore intergrown with zirconolite. EDS

FIGURE 4. Normalized release rates [g/(m2 day)] of U and other elements, and flow rates (mL/day) of the pyrochlore-based ceramic in pH 4 buffer solution. spectra show that the pyrochlore is relatively rich in U, Ce, and Ca with respect to zirconolite (Figure 2). EELS data indicate that Ce in zirconolite is in the form of Ce3+, Ce in the pyrochlore is in the forms of Ce3+ and Ce4+, and the ratio of Ce4+/(Ce4+ + Ce3+) is about 0.6 according to the intensity ratio of M4 and M5 edges from Ce3+ and Ce4+ standards and methods used in refs 25 and 26 (Figure 2). Ce4+ is sensitive to high-energy electron beams. A low-dose and spread beam was used for collecting the EELS spectra to avoid a reduction reaction of Ce4+. The composition of the pyrochlore and the zirconolite are listed in Table 1 according to the obtained EDS spectra and EELS data. The chances for finding pyrochlore and zirconolite grains is about 10 to 1. In some pyrochlore grains, there are zirconolite lamellae within pyrochlore. (001) of zirconolite is parallel to (111) of pyrochlore. A similar kind of phenomenon was also reported by Buck et al. (8). The cell parameters measured from the SAED patterns are a ) 10.5 Å for the pyrochlore (cubic) and a ) 12.5 Å, b ) 7.4 Å, c ) 11.6 Å, and β ) 100° for zirconolite (monoclinic), respectively. High-resolution TEM image shows the interface between pyrochlore and zirconlite is straight and coherent (Figure 3). The structural relationship between the two phases is illustrated in Figure 3. The value of d111 (6.1 Å) of the pyrochlore is slightly larger than d002 (5.7 Å) of the zirconolite, because the pyrochlore contains more U (large ionic radius) than the zirconolite does (Table 1). The compositions of the zirconolite lamellae and its neighboring pyrochlore are also listed in Table 1. Such lamella result from epitaxial growth during the crystallization of both pyrochlore and zirconolite. The (001) plane of zirconolite is very similar to the (111) plane of pyrochlore (27, 28). In some areas, the zirconolite lamellae display disordered structure that results from nonperiodic stacking of two-layer and three-layer zirconolite polytypes as occurs in Zr-zirconolite (20).

According to the obtained EDS and TEM data, it can be proposed that there is an immiscibility gap in a solid solution between U-pyrochlore (CaUTi2O7) and Hf-zirconolite (CaHfTi2O7) at 1350 °C. The ratios of U/(U + Hf) in the B site of ABTi2O7 are 0.72 for pyrochlore and 0.28 for the zirconolite (Table 1). According to the composition of the zirconolite and pyrochlore, it can be estimated that there are about 11 mol % of zirconolite (including isolated zirconolite grains and zirconolite lamellae within pyrochlore) in the mixture of pyrochlore and zirconolite with bulk stoichiometry of Ca(U0.5Ce0.25Hf0.25)Ti2O7. It is proposed that a single-phase pyrochlore (Ca(U,Hf)Ti2O7) may be synthesized at 1350 °C if the ratio of U/(U + Hf) is larger than 0.72, and a singlephase zirconolite (Ca(Hf,U)Ti2O7) may be synthesized at 1350 °C if the ratio of U/(U + Hf) is smaller than 0.28. Any mixtures with the U/(U + Hf) ratio between 0.72 and 0.28 will form a mixture of pyrochlore and zirconolite. The ionic radius difference between Pu4+ and Hf4+ is smaller than that between U4+ and Hf4+. However, chemical (Gibbs free energy of formation) difference between Pu4+ and Hf4+ is larger than that between U4+ and Hf4+ (29). It can be inferred that an immiscibility gap between CaPuTi2O7 and CaHfTi2O7 will be similar to that of CaUTi2O7 and CaHfTi2O7. The coexisting phase compositions may provide a guide for preparing homogeneous Hf-bearing pyrochlore for hosting U and Pu. Recent results by Helean et al. (30) also support the possibility of immiscibility gaps among different pyrochlore phases. The sample was used for leaching experiments, and the results show that dissolution is incongruent. All the elements are released at differing rates (Figure 4). Initial release rates for Ca, U, and Ce are high, but the initial release rate for Ti is low. In many cases, elemental concentrations are below detection, especially Ti, Hf, and Ce. These data suggest that the elements that are released more slowly, or are left behind VOL. 38, NO. 5, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 5. HRTEM image showing the leached amorphous layer that is rich in Ti and Hf. Lattice fringes in the crystalline core are not so obvious because of the amorphous leached layer coating (top). An HRTEM image of unleached pyrochlore clearly showing crystalline surface (bottom) is also displayed for comparison. in a leached layer, probably remain as a Ti- and Hf-enriched surface layer. The data for Ca were not plotted because of possible contamination from water. Many Ca analyses had background corrections comparable to the measured values. The release data for U appear to be the best in terms of reproducibility, consistent trends, and signal-to-noise ratio. U is also generally one of the most rapidly released elements. For this reason we have used the U release rates to calculate overall dissolution rates for the U-bearing pyrochlore ceramic. The dissolution data also show a decrease in rate with run time (Figure 4). This trend continues throughout the duration of the tests. Short-term dissolution results from pH 2 and pH 6 solutions also show that the dissolution rates are related to pH values of the buffered leach solutions. However, the rate dependence on pH is not strong. Average dissolution rates {log(NR)} of U between 300 and 350 days in pH 2, pH 4, and pH 6 leaching solutions are -5.15, -5.55, and -5.83 g/(m2 day). A pH difference of 4 units (pH 2-6) only results in less than a factor of 10 change in the dissolution rates. By using average dissolution rates of U between 300 and 350 days in pH 2, pH 4, and pH 6 leaching solutions, it is possible 1484

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to obtain a linear relationship between log(NR) and solution pH in acidic solutions:

log(NR) ) -4.83 - 0.17 pH Using two available average dissolution rates of U between 700 and 800 days in pH 2 [log(NR) ) -5.76 g/(m2 day)] and pH 4 {log(NR) ) -6.16 g/(m2 day)} leaching solutions, the above linear relationship between log(NR) and solution pH in acidic solutions becomes

log(NR) ) -5.36 - 0.20 pH The ceramic releases actinides at a considerably slower rate than other waste forms of borosilicate glass and spent fuel. The measured dissolution rate of the pyrochlore/zirconolite ceramics is about 10 times slower than that of defense highlevel-waste (HLW) glass and about 1000 times slower than that of spent fuel (31). The dissolution rates of oxides for which the rate-limiting step for dissolution involves a surface reaction typically show a much greater dependence on pH.

FIGURE 6. Bright-field TEM image showing the leached amorphous layer on the grain surface that is rich in Ti and Hf. The thickness of the leached layer at the grain boundary is also similar to the other areas.

FIGURE 7. HRTEM image showing a nanocrystalline TiO2 phase in the leached amorphous layer that is rich in Ti and Hf. For example, the HLW borosilicate glass dissolution rate changes by 3 orders of magnitude over the same pH range (21). Some ceramics, such as albite, show about the same

dependence of dissolution rate on pH as the pyrochlorebased ceramic (22). The results indicate that a diffusioncontrolled process may play a key role during the release of VOL. 38, NO. 5, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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U. A leached layer will limit the release rate of U. One would expect to see a Ti- and Hf-rich surface alteration layer on the altered ceramic or precipitation of titanium oxide and/or hafnium oxide phases in the system if this case is true. A sample that was reacted at pH 4 for 835 days at room temperature was selected for TEM investigation. TEM images show an amorphous layer on the grain surface. The thickness of the layer ranges from 6 nm to about 10 nm (Figures 5 and 6). EDS results from the leached sample indicate that the leached layer is rich in Ti and Hf and poor in Ca and U. The formation of such a stable layer will function as a protective layer and reduce the dissolution rate of the ceramics. The results are also consistent with the observation from naturally altered pyrochlore (26). In some area of the leached layer, nanometer-size TiO2 crystal recrystallized from the amorphous layer (Figure 7). It is expected that a more crystalline phase that is rich in TiO2 and HfO2 will form in the leached layer in an actual geological repository environment. Further release of U will be dominated by diffusion-controlled U release because of the formation of the leached layer. The results from TEM observations are also consistent with the observed results from dissolution tests (Figure 4). In summary, both pyrochlore and zirconolite phases could form in the targeted ceramic waste form. There are no amorphous phases at the grain boundaries. The flow-through dissolution tests show that the dissolution of the U-bearing pyrochlore is incongruent, and the dissolution data also show a decrease in rate with run time. The results indicate that a diffusion-controlled process may play a key role during the release of U. It is proposed that long-term leaching of U and Pu will be controlled by the Ti- and Hf-rich layer. The leaching rate decreases as the thickness of the leached layer increases. The proposed pyrochlore-based ceramic waste is chemically durable for hosting U, Pu, and neutron absorbers in accordance with the obtained results.

Acknowledgments This work is based upon research conducted at the Transmission Electron Microscopy Laboratory in the Department of Earth and Planetary Sciences of the University of New Mexico, which is partially supported by the National Science Foundation (NSF; Grant CTS98-71292) and the State of New Mexico. This work was partially performed under the auspices of the U.S. Department of Energy via University of California Lawrence Livermore National Laboratory under Contract No. W-7405-Eng-48.

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Received for review August 6, 2003. Revised manuscript received December 12, 2003. Accepted December 19, 2003. ES030582V