The mechanism of the hydrothermal alteration of cerium- and plutonium-doped zirconolite

Journal of Nuclear Materials 410 (2011) 10–23

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The mechanism of the hydrothermal alteration of cerium- and plutonium-doped zirconolite P. Pöml a,b,⇑, T. Geisler a, J. Cobos-Sabaté b,1, T. Wiss b, P.E. Raison b, P. Schmid-Beurmann a, X. Deschanels c, C. Jégou d, J. Heimink e, A. Putnis a a

Institut für Mineralogie, Westfälische Wilhelms-Universität, Corrensstraße 24, 48149 Münster, Germany European Commission, Joint Research Centre, Institute for Transuranium Elements, P.O. Box 2340, 76125 Karlsruhe, Germany Institut de Chimie Séparative de Marcoule, UMR 5257, F-30207 Bagnols-sur-Cèze, France d CEA-DEN/DTCD/SECM, F-30207 Bagnols-sur-Cèze, France e Institut für Physikalische Chemie, Westfälische Wilhelms-Universität, Correnstraße 36, 48149 Münster, Germany b c

a r t i c l e

i n f o

Article history: Received 8 June 2010 Accepted 18 December 2010 Available online 30 December 2010

a b s t r a c t A comprehensive study on the aqueous stability of Ce- and Pu-doped zirconolite has been performed. Four series of hydrothermal experiments were carried out with Ce-doped zirconolite powders: (1) a solution series (1 M HCl, 2 M NaCl, 1 M NaOH, 1 M NH3, pure H2O), (2) a temperature series (T = 100–300 °C), (3) a surface area-to-fluid volume ratio series, and (4) a series using different reactor materials (TeflonÓ, Ni, and Ag). In addition, experiments on 238Pu- and 239Pu-doped zirconolite ceramics in a 1 M HCl solution have been performed. The 238Pu-doped zirconolite had already accumulated significant radiation damage and was X-ray amorphous, while the 239Pu-doped zirconolite was still well-crystalline. The results of the different experimental series can be summarized as follows: (1) After 14 days the degree of alteration is insignificant for all solutions other than 1 M HCl, which was therefore used for all other experimental series; (2) TiO2 and m-ZrO2 replaced the zirconolite grains to varying degrees in the 1 M HCl solution, i.e., zirconolite dissolution is incongruent; (3) the degree of alteration increases only slightly with increasing temperature; (4) the alteration rate is independent on the surface to volume ratio; (5) Ag dissolved from the silver reactors dramatically increases the reaction rate, while Ni from the Ni reactors reduces the solubility of Ti and Zr in the HCl solution, indicating that background electrolytes have a strong effect on the alteration rate. From the experiment with the Pu-doped samples at 200 °C in a 1 M HCl solution it was found that the amorphous 238Pu-doped zirconolite was altered to a significantly greater extent than the crystalline counterparts. The results suggest a coupled dissolution-reprecipitation mechanism, which is discussed in detail. Ó 2011 Elsevier B.V. All rights reserved.

1. Introduction The disposal of actinides (U, Np, Cm, Am, and also unrecyclable, so-called scrap Pu), generated during the burn-up of nuclear fuel or the dismantling of nuclear weapons, is of particular concern for the modern society. The actinides are highly radiotoxic and for their safe disposal special measures are needed. Three options are currently considered for excess weapons-grade Pu and minor actinides (i.e., Np, Cm, Am) recovered by chemical separation from spent nuclear fuel: The first option, transmutation by neutron irradiation in nuclear reactors would reduce the radiotoxic inventory. The second option would be the direct disposal of spent fuel while the third option would be to incorporate the separated actinides ⇑ Corresponding author. Tel.: +49 7247 951867; fax: +49 7247 95199867. E-mail address: [email protected] (P. Pöml). Present address: Centro Nacional de Aceleradores, Thomas Alva Edison No. 7, Parque Tecnolgico Cartuja 93, 41092 Seville, Spain. 1

0022-3115/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jnucmat.2010.12.218

into an inorganic matrix (waste form) to ensure the safe long-term sequestration of the waste over geological time scales (up to 105 years). A suitable waste form for the disposal of minor actinides has to fulfill many requirements: a high waste load, high mechanical strength, high chemical flexibility, high resistance to radiation damage and induced volume swelling from the incorporated waste load, and a high durability against aqueous solutions which may come in contact with the waste form in a geological repository. In this context, zirconolite (nominally CaZrTi2O7) is considered as a highly durable material that may be suitable for the immobilization of Pu and minor actinides. Zirconolite is an important phase in SynRoc-type titanate ceramics. SynRoc is a multi-phase mineral-analogue ceramic designed to immobilize high-level radioactive waste [1]. It consists of three main titanate minerals: zirconolite, perovskite (CaTiO3), and hollandite (BaAl2Ti6O16), in approximately equal proportions along with smaller quantities of rutile (TiO2), and metallic alloy phases. Waste elements are incorporated into each of the SynRoc

P. Pöml et al. / Journal of Nuclear Materials 410 (2011) 10–23

phases by a substitutional solid solution mechanism. The two Synroc phases capable of incorporating transuranic elements and thus the minor actinides are zirconolite and perovskite. The structure of pure end member zirconolite is an aniondeficient derivative fluorite structure having the C2/c space group [2]. Zirconolite solid solutions of the type CaZrxTi3xO7, where x ranges from 0.7 to 1.3, crystallize as the 2 M polytype. Numerous studies of the incorporation of lanthanide and actinide elements in zirconolite have shown that other polytypes such as 3T, 3O, and 4M occur with increasing levels of substitution on the Caand Zr-sites [3–5]. The appearance of these polytypes is dependent on the type of substitution and the temperature and oxygen fugacity during fabrication. The 3T and 3O polytypes were also found in nature [6]. Zirconolite consists of layers of corner-linked TiO6 octahedra separated by planes of Ca and Zr atoms. Titanium occupies three distinct lattice sites in zirconolite, two of which are octahedrally coordinated to oxygen, while the third is a fivefold-coordinated site that is only 50% occupied. The interlayer Ca and Zr atoms are eightfold and sevenfold coordinated to oxygen, respectively. As waste elements are incorporated into zirconolite via a substitutional solid solution mechanism, the direct substitution of a waste element for a host lattice element of a similar ionic size is required. To ensure that charge neutrality is maintained where the waste and host lattice elements have a different valence state, suitable charge compensation must be made. This most commonly takes the form of a second element (often Al3+ or Mg2+) of appropriate valence substituting on either the same or a different site within the zirconolite structure. The ionic size of the given waste ion therefore determines the possible sites within the zirconolite structure into which the waste ion may substitute, whilst the degree of valence mismatch, if any, between the waste and host ions determines the type and amount of charge compensation required. The incorporation of multivalent waste ions in zirconolite is more complex, since a change in valence state is associated with a change in the ionic size. Consequently, the range of host sites into which the multivalent ion is capable of substituting will vary depending on the prevailing valence state of the waste ion. The successful incorporation of multivalent ions in zirconolite and other nuclear waste ceramics therefore requires an intimate knowledge of their valence states under the given processing environment. In zirconolite large ions as for example Pu4+ (ionic radius in eightfold coordination 0.96 Å [7]) can be incorporated on either the Ca site (polyhedral volume 21.3 Å3) or the Zr position (polyhedral volume 15.0 Å3) [8]. Because experiments with nuclear waste ceramics doped with actinides are cumbersome, and only feasible under special conditions, often surrogates for the actinides are used that are much easier to handle, but show similar chemical properties. Cerium is often used as a surrogate for plutonium. Both elements are known to form tetravalent metal oxides and trivalent monazite structured phosphates when heated in air. Their ionic radii are very similar (Pu4+ 0.96 Å and Ce4+ 0.97 Å in eightfold coordination [7]) and they should thus occupy similar positions in a crystal lattice. But apart from the function as a surrogate for experiments involving Pudoped materials, the incorporation of Ce is also of interest in its own right, because rare-earth fission products are present in reprocessed waste. For the safe disposal of nuclear waste in a geological repository it is very important to assess the behavior of the waste form when it comes into contact with aqueous solutions. Of particular interest in this respect is the impact of self-irradiation damage on the stability of the waste form. Zirconolite has been the subject of numerous studies over the past decades, including irradiation studies, hydrothermal durability studies, and structural investigations, e.g. [2,9–11]. However, still very little is known about the processes or mechanisms involved in the hydrothermal alteration of

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zirconolite and, additionally, systematic studies under comparable conditions providing information about the effect of structure, composition, temperature, and solution composition on the alteration behavior are very rare or not existent. The present study focuses on a series of experiments conducted on Ce-doped zirconolite ceramics that were synthesized by putting special care on phase purity and homogeneity. The synthesized zirconolite ceramics were then analyzed for their chemical composition, their structure, and for details of their cerium load. Their stability in hydrothermal solutions was then tested in several series of hydrothermal experiments focusing on the dependence of alteration on temperature, solid-to-solution volume ratio, and solution composition. Additionally to the experiments on Cedoped zirconolite, we present results from experiments with two Pu-doped zirconolite samples. One sample was doped with fastdecaying 238Pu (half life of 87.7 years) and thus became X-ray amorphous within laboratory time-scales, while the second sample was doped with 239Pu and was thus still crystalline at the time of the experiment. To compare the Pu-doped samples with the Ce-doped samples, a Ce-doped zirconolite with otherwise same composition as the Pu-doped samples was synthesized. It was therefore possible to directly compare (1) the behavior of 238Puand 239Pu-doped zirconolite, i.e., of radiation damaged and crystalline zirconolite, and (2) the behavior of Pu- and Ce-doped zirconolites treated under the same conditions. The experiment with these samples allowed us to comment on the important question whether the dissolution or alteration rate of zirconolite depends on the degree of radiation damage that accumulates in actinide-bearing zirconolite with time. 2. Analytical and experimental methods 2.1. Analytical methods 2.1.1. BET-measurements Nitrogen sorption measurements were carried out on a Micromeritics ASAP 2010 sorption analyzer at liquid nitrogen temperature (77 K) at the Department of Physical-Chemistry of the Westfälische Wilhelms-Universität Münster, Germany. All samples were degassed at 423 K under vacuum for 24 h before analysis. The data were collected and automatically processed using the software ASAP 2010. For all samples the complete adsorption-desorption isotherm was measured, as plots of Vad (volume adsorbed cm3/ g) against the relative gas pressure p/p0 reveal the pore structure of the adsorbing material by its shape. As all samples were non-porous the specific surface area is given by the single-point specific surface area at a relative gas pressure of about 0.2. The surface area was used to calculate the surface-to-solution volume ratio (S/V), which is needed to calculate the normalized elemental loss rate of an element i(ri) according to 1

ri ðgm2 d Þ ¼

102 C i ; X i  S=V  Dt

ð1Þ

where Ci is the concentration (lg/g) of element i in solution, Xi is the mass fraction of element i in the zirconolite, and Dt the duration of the experiment in days. 2.1.2. Inductively-coupled plasma optical emission spectrometry (ICPOES) The experimental solutions were analyzed by inductivelycoupled plasma optical emission spectrometry (ICP-OES) using a Horiba Jobin Yvon Ultima 2 spectrometer at the Institute for Transuranium Elements (ITU) in Karlsruhe, Germany. The samples were fed to the Meinhard nebulizer fitted to a conical spray chamber via a peristaltic pump, then transferred to argon plasma where the

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sample is decomposed, atomized, and ionized. During the analysis three replicas per line and element were performed for each sample. The samples were centrifuged, decanted, and then diluted using specially distilled, ultra-pure 2% HNO3 acid. The dilution factors of the samples were around 10. Appropriate blanks following the same scheme of sample preparation were used for the analysis. External calibration was performed using a series of dilutions of certified standard solutions (Alfa Aesar Specpure) in the concentration range between 10 and 3000 ppb. Since the calibration is undertaken in 1% HNO3 and the samples have a different composition, a high generator power was used to minimize any matrix effects. The relative standard deviations obtained from the three replicate measurements were in the order of ±4.5%, ±3.5%, ±4.1%, ±3.6%, ±5.6%, and ±5.0% for Zr, Ti, Ce, Pu, Ca, and Al, respectively. 2.1.3. Powder X-ray diffractometry (XRD) The X-ray diffraction data were collected at room temperature using a high resolution Bruker D8 X-ray diffractometer mounted in a Bragg-Brentano configuration at ITU in Karlsruhe, Germany. The instrument was equipped with a curved Ge monochromator (1 1 1), a ceramic Cu X-ray tube (40 kV, 40 mA) and a Vantec position sensitive detector covering 6° in 2h. The scan was collected from 13° to 150° in 2h using 0.0086° step-intervals with counting steps of 14 s. Structural refinement was performed using the FullProf software [12]. The shape of the peaks was described by the Pseudo-Voigt function and the background was fitted based on linear interpolation between a set of about 50 background points with refinable heights. The scattering factors of Ca2+, Zr4+, Ti4+, O2, Al3+, and Ce3+ were used. 2.1.4. Scanning electron microscopy (SEM) Backscattered electron (BSE) images and energy-dispersive Xray (EDX) semi-quantitative analyses were acquired with a JEOL 840 scanning electron microscope at 20 kV acceleration voltage at the Institute for Mineralogy of the Westfälische Wilhelms-Universität Münster, Germany. For both BSE images and EDX analysis the beam current was adjusted in order to get the best possible contrast and counting statistics. High-resolution secondary electron (SE) images of the sample surfaces were obtained using a JEOL JSM 6300F equipped with a field emission gun and operated at 5 kV acceleration voltage. For EDX semi-quantitative analyses of crystals found at the sample surface after the experiments the acceleration voltage was increased to 10 kV. BSE and SE images and EDX semi-quantitative analyses for the Pu-doped samples were acquired using a Philips XL40 SEM mounted inside a glove-box at the ITU in Karlsruhe, Germany. 2.1.5. Electron probe micro-analysis (EPMA) The Ce-doped starting materials were quantitatively analyzed by electron probe micro-analysis (EPMA) using a JEOL JXA 8900 Superprobe operated at 20 kV acceleration voltage and 15 nA beam current at the Institute for Mineralogy of the Westfälische Wilhelms-Universität Münster, Germany. These conditions result in a lateral resolution of 2–4 lm, depending on the element analyzed [13]. Counting times were 20 s on the peak and 10 s each on the negative and positive backgrounds. As reference materials natural minerals and synthetic compounds were used: a natural diopside and kyanite for Ca and Al, and synthetic zirconia, rutile, and a Ce phosphate for Zr, Ti, and Ce, respectively. The PAP correction procedure was applied to correct for matrix effects [14]. 2.2. Fabrication and characterization of the starting material 2.2.1. Fabrication For this study two Ce-doped zirconolite ceramics were synthesized. Cerium was used as a surrogate for a real waste load, i.e.,

plutonium, and was designated to substitute for Ca in the zirconolite structure. Since Ce can be either IV+ or III+, but Ca is II+, charge balance in the zirconolite formula was achieved by substituting Ti4+ by Al3+. The intended composition of the two zirconolite ceramics was Ca1xCexZrTi22xAl2xO7, where x was 0.13 and 0.15. The ceramics were synthesized by a solid-state reaction from a mixture of oxide powders, i.e., CaO, CeO2, ZrO2, TiO2, and Al2O3. Sintering was performed in air and Ce should thus be mostly in the IV+ valence state. The powders were dried before use at 150 °C in order to remove any adsorbed water. The CaO was calcined at 900 °C to remove any CaCO3 that is often present in CaO due to absorbed CO2. The powders were mixed in appropriate amounts and then ball milled for 45 min. The resulting powders were pressed into tablets of a diameter of 13 mm using a hydraulic press at 300 MPa. After overnight calcination at 900 °C, the tablets were twice re-ground, re-compacted and sintered in air at 1400 °C for two, and then for 14 days. After the last sintering step, the tablets were examined by SEM for compactness and for any secondary phases such as perovskite (CaTiO3). Perovskite is less stable than zirconolite in aqueous solutions, and could thus have an undesirably influence on the results of this study. If perovskite or other secondary phases, such as ZrO2 or TiO2, were detected their proportions were estimated from BSE images and the respective oxide precursors were added to the material in order to remove these phases. The preparation steps were repeated until secondary phases were either eliminated or only present in small quantities. It was found that it is difficult to synthesize pure Ce-doped zirconolite. Some secondary phases were always present, no matter how much of the precursor powders was added to the mixture to adjust the stoichiometry of the target zirconolite. One reason for this could be the unexpected reduction of Ce IV+ to Ce III+ at high temperatures that was revealed by EELS measurements on the samples (not presented here). However, in this study, we found that the best solution for synthesizing pure samples was to prepare samples with a slight ZrO2 excess in order to remove any perovskite from the ceramic. ZrO2 is a very stable material under a wide range of oxidation/reduction environments and is very durable in aqueous solutions and has thus been considered a candidate material for Pu disposal and for the use as an inert matrix for Pu burn-up in a reactor or accelerator-based neutron source [15]. Although repeated synthesis attempts revealed that the formation of perovskite cannot always be avoided, it was undetectable from the samples used in the present study. 2.2.2. Characterization From SEM-based BSE imaging and EPMA analyses it was found that the ceramics are very homogeneous. BSE images of the two Ce-doped ceramics used in this study are shown in Fig. 1. Fig. 1a shows the ceramic PP27 (XCe = 0.13) that was used as an analogue ceramic to the Pu-doped ceramics. The middle grey matrix is the zirconolite, the black areas are porosity. No secondary phases could be detected here. In Fig. 1b the ceramic PP17 is shown (XCe = 0.15). Again the zirconolite has a grey BSE intensity, while the porosity appears black in the BSE images. The white spots in (b) represent excess ZrO2. The chemical compositions for the ceramics used in this study are shown in Table 1. The EPMA analyses revealed that the ceramics are very close to the nominal composition. The Ce contents of the ceramics are as expected and correlate with the Al contents (Al was used for charge balance). However, some irregularities are seen in the data. The Zr content of the ceramics varies between 0.97 and 1.08. Together with the rest of the cation values this shows that the ceramics are not completely ideal and that some excess Zr might be located at the Ti position (e.g., PP17) and vice versa (PP27).

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Fig. 1. BSE images of the two Ce-doped ceramics used in this study. (a) Sample PP27 doped with 0.13 Ce. The zirconolite has a grey BSE intensity, the black areas are porosity. No additional phases were recognized by BSE imaging. (b) Sample PP17 (XCe = 0.15). Some excess ZrO2 was found in this sample (white BSE contrast).

Table 1 Characteristics of the samples used in this study. Characteristics

PP17

PP27

238

239

Target doping level

Ce = 0.15

Ce = 0.13

Pu = 0.13

Pu = 0.13

Ca0.85Ce0.15ZrTi1.7Al0.3O7 Ca0.82Ce0.15Zr1.08Ti1.65Al0.29O7

Ca0.87Ce0.13ZrTi1.74Al0.26O7 Ca0.82Ce0.13Zr0.97Ti1.78Al0.28O7

Ca0.87Pu0.13ZrTi1.74Al0.26O7 Ca0.92Pu0.13Zr1.01Ti1.72Al0.25O7

Ca0.87Pu0.13ZrTi1.74Al0.26O7 Ca0.83Pu0.15Zr1.03Ti1.71Al0.26O7

13.06 37.46 37.48 4.26 1.43 5.46

13.17 34.25 40.70 4.08 1.31 4.98

14.37 34.58 38.03 3.58

12.70 34.74 37.51 3.70

99.15

98.48

9.72 100.27

11.09 99.75

Ce and Pu valence Ce3+ / Ce4+ (EELS) Pu (XANES)

80 / 20%

80 / 20%

Surface area BET surface (m2 g1) Geometrical surface (m2 g1)

0.0905(91)

Sample composition Target composition Actual composition Oxides (wt.%) CaO ZrO2 TiO2 Al2O3 CeO2 Ce2O3 PuO2 Total

Crystal parameters a (Å) b (Å) c (Å) b (°) V (Å3) Density (g cm3) Crystallinity at time of experiments Integrated dose

Pu zirconolitea

Pu zirconolitea

100% Pu4+

12.44538(5) 7.25524(3) 11.35802(5) 100.6409(3) 1007.928(7) 4.63 (from XRD) Crystalline

0.00056

0.00097

0.00090

12.44607(7) 7,25086(4) 11,32298(7) 100,68603(47) 1004.119(10) 4.56 (from XRD) Crystalline

12.472(3) 7.248(2) 11.426(4) 100.62(1) 1015(1) 4.45 Amorphous

12.431(7) 7.248(4) 11.33(3) 100.57(3) 1003(2) 4.39 Crystalline

7  1018 a-decays/g

The numbers in brackets represent the 2r standard error and refer to the last digits. a Data from [17–19].

In general, it seems that the Zr contents of the ceramic increases with the number of sintering steps. These steps were done to eliminate any secondary phases. Ceramics that experienced less sintering cycles have Zr values close to one (ideal), while ceramics that experienced more sintering cycles have values up to 1.08 for Zr. The valence state of cerium was measured by electron energy loss spectroscopy (EELS) and X-ray photoelectron spectroscopy (XPS). From these measurements it was found that Ce is not completely in the IV+ valence state, but that 80% or more of the cerium occurs in the trivalent state, which is in agreement with previous studies on the incorporation of cerium into zirconolite [16]. The results of these measurements will be published elsewhere. The results from the XRD measurements are listed in Table 1. The zirconolites were found to be of the 2 M polytype. Theoretical

densities of the zirconolites were calculated from the XRD data as 4.63 g/cm3 for the sample PP17 and 4.56 g/cm3 for the sample PP27. From the XRD results and SEM investigations on the zirconolites synthesized for this study, it can be concluded that the available samples are suitable for this type of experiment. Details about the fabrication and the characterization of the Pu-doped samples are published elsewhere [17,18]. At the time of the hydrothermal experiments the 238Pu-doped sample was X-ray amorphous and had accumulated a dose of 7  1018 a-decays/g. 2.3. Hydrothermal experiments The hydrothermal experiments were carried out with powder that was produced from the synthesized ceramic pellets by

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crushing the ceramic pellets in an agate mortar and then sieving the resulting powder in several steps. The sieves were of a mesh size of 400, 200, and 100 lm. The resulting grain fraction smaller 100 lm was then washed using ethanol through a sieve tissue with a mesh size of 40 lm in order to remove any particles smaller than 40 lm. The surface area of the resulting powder was 0.0905(91) m2 g1. The experiments were run using cold-sealed reactors (3 cm3 volume) of different materials. For temperatures below 200 °C Teflon reactors were used. These reactors were cleaned thoroughly before the experiments by several cycles of boiling in distilled ultrapure HNO3, HF, and HCl. This was done to remove any unwanted species present in the Teflon that could potentially enter the experimental fluids. The cleaning was also intended as a preleaching of fluoride from the Teflon reactors to reduce potential fluoride concentrations in the experimental solutions. Nickel reactors were used for temperatures higher than 200 °C, because Teflon becomes soft at higher temperatures. Additionally, some experiments were carried out in silver reactors, to asses the influence of nickel ions on the reaction. Silver was thought to be more inert than nickel. The experiments were performed at different temperatures, with different solid surface-to-fluid volume ratios, and solution compositions. One experiment with a 1 M HCl solution was repeated in order to test for the reproducibility of the experiments. The results from the duplicate Teflon experiments show that the experiment could be well reproduced within the error. Also, the solid surface-to-solution volume ratio (S/V) series was duplicated to control the reproducibility. The S/V value was changed by varying the amount of solution between 2 and 0.5 ml. All

experiments were carried out with about 0.1 g of sample PP17 (XCe = 0.15) and a 1 M HCl solution for 14 days at 200 °C. We decided to run the experiments with the Pu-doped samples in a 1 M HCl solution for 3 days only since we knew that this duration is sufficient to give detectable amounts of material in solution while minimizing any potential effects of radiolysis. A complete list of all experiments is given in Table 2. The experiments on the Pu-doped zirconolites were performed in a glove box at ITU in Karlsruhe, using a similar furnace and reactor set-up to the inactive experiments. Discs cut from a pellet were used for the experiments (Fig. 4a). As it was not possible to determine the exact surface of these samples using BET, samples with a well-defined geometric form were preferred in order to estimate the surface by the geometry and the porosity of the sample. For direct comparison a cuboid was prepared from the Ce-doped zirconolite with the appropriate composition. The estimated surface areas of the Pu-zirconolite discs and the Ce-zirconolite cube are given in Table 1. These values should be taken as rough estimates.

3. Results 3.1. Scanning electron microscopic observations 3.1.1. Ce-doped zirconolite ceramics All experimental run products were mounted and then polished until cross-sections of some grains could be examined by SEM for alteration features. In none of the run products any alteration features could be detected in cross-sections, with the exception of the

Table 2 Complete list of the experimental details of all experiments that have been performed in this study. Note that some experiments were repeated in order to test for reproducibility. Sample

Solution

Amount (ml)

Temperature (°C)

Duration (d)

Reactor Type

Fluid–solid-ratio series PP17 0.150

1 M HCl

2 1.5 1 0.5

200

14

Teflon

Temperature series PP17

1 M HCl

2

100 110 120 130 140 150 160 170 180 190 200 210 230 250 270 290

14

Teflon

14

Solution series PP17

Ce/Pu-content

0.150

0.150

Reactor comparison PP17 0.150 Experiments with Pu doped zirconolite Pu 0.13 Pu 0.13 PP27 0.13 238 239

Nickel

1 M HCl 2 M NaCl 1 M NaOH 1 M NH3 H2O

2

200

Teflon

1

300

1 M HCl

2

300

14

Silver Nickel

1 M HCl

2

200

3 3 14

Teflon

Nickel

P. Pöml et al. / Journal of Nuclear Materials 410 (2011) 10–23

experiment run in a silver reactor described below. However, SEM investigations of the grains’ surfaces revealed newly grown crystals at the surface in all experiments. Interestingly, in all but the silver reactor experiments grains with and without new crystals at the surface were found. It was thus not possible to make any conclusions about the degree of alteration of the zirconolites based on the SEM observations only. Fig. 2a shows the surface of a typical grain that is covered with precipitated crystals. The crystals were identified as TiO2. Their bipyramidal shape suggests that the crystals are anatase. However, also tabular crystals of rutile were found as shown in the right inset of Fig. 2b. Based on their crystal shape and EDX measurements, the needle like crystals could be identified as baddeleyite (monoclinic ZrO2). Some areas of the grain were covered by very small needle-like crystals only a few nm in size. The inlay in Fig. 2a shows a high magnification SE image of these tiny crystals which reveals that they are arranged parallel to two directions. Because of their very small size these crystals could not be analyzed by EDX. However, they are probably also baddeleyite. Other grains were homogeneously covered by alteration products, as for example the grain shown in (Fig. 2b). This grain is evenly covered by ZrO2 needles and TiO2 crystals. The right inlay in Fig. 2b shows a magnification of these crystals. Sometimes special features like the one in the left inlay in Fig. 2b were found, where there is extensive growth of ZrO2 crystals around a hole in the zirconolite grain. In turn, other grains were found to be almost completely covered by a layer of new TiO2 crystals, as in the case of the grain shown in Fig. 2c. It is very likely that this grain would be fully covered by a layer of alteration products after an extended experimental run time.

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Fig. 2d shows the final state after a complete reaction of a zirconolite grain. The grain shown here was a relict of the experiment run in the silver reactor that resulted in the complete dissolution and replacement of the zirconolite powder. Again, the relatively small crystals could be identified as ZrO2 and the bigger bipyramids were TiO2. The inlay in Fig. 2d shows a backscattered electron (BSE) image of a cross-section through a zirconolite cuboid altered under the same conditions. This zirconolite was completely replaced by ZrO2 and TiO2. No remnants of the original zirconolite could be found. The BSE image shows a very complex intergrowth of ZrO2 (lighter BSE contrast) and TiO2 (darker BSE contrast). Fig. 3 presents the results from the SEM investigation of the reference experiment with the Ce-doped zirconolite cuboid carried out at 200 °C in a 1 M HCl solution in a Teflon reactor for 14 days. This experiment yielded a surprising result. While five of the cuboid surfaces were almost unaltered and showed only a very minor degree of crystal growth on only a few areas of the cube’s surface (Fig. 3b), one side of the cube was completely covered by a thick layer of crystals (Fig. 3c). This crystal carpet turned out to be an irregular arrangement of ZrO2 crystals (Fig. 3d). 3.1.2. Pu-doped zirconolite ceramics The optical micrograph of the altered 239Pu-doped zirconolite disc (Fig. 4a) shows no difference to the unaltered disc (not presented here). The vertical scratches from the cutting of the disc are still visible. Also SEM analysis of the surface of this sample revealed neither changes of the surface such as dissolution features nor new crystal growth. In contrast, optical investigations of the radiation damaged 238Pu-doped zirconolite after the experiments

Fig. 2. SE images of the Ce-doped zirconolites after experimental alteration in (a) 1 M HCl, 7 days, 300 °C, Ni reactor, (b) and (c) 1 M HCl, 14 days, 200 °C, Teflon reactor, and (d) 1 M HCl, 7 days, 300 °C, Ag reactor. The inlay in (a) shows the alignment of tiny needle-like crystals identified by EDX analyses as ZrO2 (likely baddeleyite) while those in (b) also reveal bipyramidal crystals that were identified as TiO2 (likely anatase). The inlay in (d) shows a BSE image of a completely reacted zirconolite from the experiment with a silver reactor. It reveals a complex intergrowth of the alteration products ZrO2 (white) and TiO2 (grey).

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Fig. 3. SE images of (a) an unaltered surface of the Ce-doped zirconolite cube used as a reference for the experiments on the Pu-doped zirconolite discs, (b) the surface of the zirconolite cuboid which shows the beginning of crystal growth on the surface of the cube, and (c) and (d) of a thick layer of crystals that was only found on one side of the cuboid. The needles shown in (d) were identified as ZrO2. Experimental conditions were: 1 M HCl, 14 days, 200 °C, Teflon reactor.

revealed the occurrence of different zones at the surface that are distinguishable by their different color (Fig. 4a). Furthermore, the optical investigations of the broken surface of a cross-section of the sample revealed an approximately 20–30 lm thick outer rim that is characterized by a white color and apparently surrounds the complete disc (Fig. 4b). The white color of this outer layer indicates the occurrence of porosity, causing scattering of light. The individual zones on the top of the disc are also visible in the BSE image of the surface (Fig. 4d). The different zones, distinguishable by a darker and a brighter BSE contrast, cover the same areas as on the optical micrograph. SE images of these zones revealed that the zones with a darker BSE contrast correspond to zones covered by a thick layer of TiO2 crystals (Fig. 4e and inlay), similar to the crystal carpet of ZrO2 on the cuboid of the Ce-doped zirconolite (Fig. 3c and d). The areas in Fig. 4d emitting a higher BSE intensity correspond to areas showing no new crystal growth (Fig. 4d, left side). However, even though these areas look rather unaltered, EDX analyses of these surface areas revealed that Ca, Pu, and Al were absent and that the Ti–Zr ratio of the surface is about 1:1 (Fig. 4c), corresponding to a zirconium titanate phase like ZrTiO4 rather than to a zirconolite phase (Zr/Ti = 0.5): This observation is in agreement with the white rim visible in the optical micrograph (Fig. 4b) that reveals the occurrence of a 30 lm thick outer reaction rim around the whole disc. 3.2. Solution data 3.2.1. Solution series The ceramic PP17 (XCe = 0.15) used in this study was altered using different solutions while keeping temperature, surface to

volume ratio, duration, and the type of reactor (in this case Teflon) constant. The concentrations of the elements in the experimental solution after the experiments are shown in Fig. 5. For all the solutions other than HCl (displayed in the middle box in the plots of Fig. 5), the concentration of all elements in the experimental solution was an order of magnitude lower than for the HCl experiments, indicating, as expected, a significantly lower alteration rate in these solutions. The Ti, Zr, and Ce concentrations in solution were in the order of 0.1, 0.2, and 0.5 lg/g, respectively (Fig. 5a–c). The Ca concentrations in solution show a greater scatter (Fig. 5e). Ca was detected in low concentrations in the two experiments with the alkaline solutions 1 M NaOH and 1 M NH3 (0.2 and 1 lg/g, respectively). On the other hand, Ca concentrations in the 2 M NaCl and H2O experiments reach concentration levels (8 and 5 lg/g, respectively) close to those measured in the HCl experiments. However, we cannot rule out that such high Ca concentrations are the result of contamination, since Ca is a very abundant element and thus contamination with Ca is very often a problem. Al was not detected in the experiments with 2 M NaCl and H2O (Fig. 5f). In the alkaline solutions 1 M NaOH and 1 M NH3 Al concentrations of 0.7 and 1.5 lg/g, respectively, were found, which are only slightly lower than the concentrations in the HCl experiments (2 lg/g). Since Teflon can only be used at temperatures below 200 °C, silver and nickel reactors were used for experiments at higher temperature. Silver was thought to be more inert than nickel. The results from this study revealed a dramatic influence of the reactor material on the alteration process (see also Section 3.2.2). When nickel reactors were used instead of Teflon the concentrations of Ti, Zr, and Ce in solution were significantly lower. This effect is

P. Pöml et al. / Journal of Nuclear Materials 410 (2011) 10–23

17

Fig. 4. (a) Optical micrograph of the 238Pu-(left) and the 239Pu-doped (right) zirconolite ceramic after experimental alteration. Note that the surface of the amorphous 238Pudoped sample (left) shows areas with different colors. (b) Optical micrograph of the cut surface. Note the existence of a continuous white appearing rim that is marked by arrows. (c) EDX spectra of the unaltered (left) and the altered (right) 238Pu-doped zirconolite. (d) BSE image of the surface of the metamict 238Pu-doped zirconolite. Parts of the ceramic were covered by a thick crystal layer, while other areas do not show any crystal growth. (e) SE image of a magnified section of an area that is covered by crystals. The inlay shows a further magnification of the crystal layer. The crystals were identified as TiO2. Experimental conditions were: 1 M HCl, 3 days, 200 °C, Teflon reactor.

displayed in the left boxes of Fig. 5a–c, where the results of experiments using the different reactor materials are shown. Surprisingly, using silver reactors had a very different effect. Silver reactors led to a dramatic increase of the zirconolite alteration rate, reflected by extremely high concentrations of Ce, Ca, and Al in solution (Fig. 5c, e, and f). The results from SEM investigations described above support these measurements. They reveal that the complete zirconolite powder was replaced by a mixture of ZrO2 and TiO2 precipitates. The results from the experiments with the 238Pu-and 239Pudoped zirconolite indicate a different behavior of the crystalline 239 Pu-doped zirconolite and the severely radiation-damaged 238 Pu-doped sample. Based on the solution data the crystalline zirconolite was hardly attacked, while under the same conditions the X-ray amorphous zirconolite released very high concentrations of Pu, Ca, and Al into solution. This finding is also supported by the SEM results shown above. No alteration features were found on the crystalline sample, whereas the surface of the amorphous sample was partly covered by a thick layer of TiO2 crystals. 3.2.2. Temperature series Fig. 6 shows the logarithm of the normalized elemental loss rates as a function of the inverse temperature (Arrhenius diagram). The most striking and unexpected feature of these data is the large break or non-continuous evolution of the Ti, Zr, and Ce concentration in solution between the experiments run in Teflon and nickel reactors. In the case of the experiments in nickel reactors, the concentration of these elements in the experimental solutions drop dramatically to significantly lower values and remain constant

even up to temperatures of 300 °C. However, the Al and Ca concentrations slightly increase with increasing temperature throughout the whole temperature series. A linear fit to the Ca rate data yielded an apparent activation energy of only 8 ± 4 kJ/mol, reflecting a very weak temperature-dependence of the reaction (Fig. 6). The behavior of Ti and Ce shows a significant increase in the normalized elemental loss rates within the temperature range from 140 °C to 200 °C reflecting an apparent activation energy in the order of 15–20 kJ/mol, while the Zr concentrations decrease slightly within the temperature range between 100 and 200 °C. 3.2.3. Solid surface-to-fluid volume ratio (S/V) series The solution results of the S/V series are shown in Fig. 7. The Al (Fig. 7a), Ca (Fig. 7b), and Ce (Fig. 7e) concentrations in solution show a linear increase with an increasing S/V value. This linear correlation corresponds to a doubling of the element concentrations in the solution if doubling the S/V value (dashed lines in Fig. 7). For Zr and Ti no linear increase is observed. The Ti concentration increases from 17.3 ± 0.8 lg/g to a maximum concentration of 26.5 ± 2.3 lg/g at a S/V of 9.33  108 m1 before decreasing to 15.5 ± 1.4 lg/g. Although at a much lower concentration level, Zr shows a similar behavior. 3.2.4. Interelement diagrams Fig. 8 shows a selection of interelement diagrams with the solution data of all experiments. Those diagrams involving Ti and Zr (relatively low solubility) on one axis and Ca, Al, and Ce (relatively high solubility) on the other (Fig. 8a–c) clearly show that the dissolution of zirconolite in all solutions is incongruent. Especially in the

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P. Pöml et al. / Journal of Nuclear Materials 410 (2011) 10–23

(a)

(b)

(c)

(d)

(e)

(f)

Fig. 5. Diagrams showing the element concentration in the experimental solutions. Here the results of experiments that were performed (i) in Teflon (T), nickel (Ni), and silver (Ag) reactors (left box), (ii) with different types of experimental solution (2 M NaCl, 1 M NaOH, 35% H2O2, 1 M NH3, and H2O) (middle box), and (iii) with Ce-, 239Pu-, and 238 Pu-doped zirconolite are compared (right box). Note that the two experiments on the left represent duplicated experiments demonstrating the experimental reproducibility. The middle line of each bar represents the measured concentration of the element, while the size of the bar represents the standard error (2r).

diagram Zr versus Al (Fig. 8c) the solution data of all experiments plot clearly away from the stoichiometric line. However, in the diagrams where two rather soluble elements are plotted against each other, for instance Ca versus Al (Fig. 8d), the solution data of nearly all experiments plot close to the stoichiometric line. Note also the nice alignment of the four experiments of the (S/V) series along this line (grey circles, Fig. 8d and f). Exceptions are the solution data from the experiments with alkaline solutions. The solution data from the experiment with the Ce-doped zirconolite cuboid agree nicely with the data obtained from the experiments that were carried out with the zirconolite powder (Fig. 8, empty rhombus). This demonstrates that the experiments performed at the University of Münster and those at ITU in Karlsruhe yielded reproducible results. The direct comparison of the experiments with the Pu-and Ce-doped crystalline ceramic

shows that the Ce-doped zirconolite behaved similar to the crystalline 239Pu-doped zirconolite. However, all element concentrations in the solution of the Ce-doped zirconolite experiment are slightly higher than those of the corresponding experiment with the 239Pudoped zirconolite, which most likely reflects the longer duration of the experiments with the Ce-doped sample (14 days compared to 3 days, Table 2). 4. Discussion 4.1. The mechanism of the hydrothermal alteration of crystalline zirconolite The solution data presented in Fig. 8 reveal that the dissolution of crystalline zirconolite is incongruent under all physico-chemical

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the surface of the zirconolite grain is covered only with relatively few needle-like ZrO2 and subordinate, bipyramidal TiO2 (anatase) crystals, while Fig. 2c shows a grain that is almost completely covered by TiO2 and ZrO2 crystals. Fig. 2d shows the final stage. Here, the original zirconolite is completely replaced by a mixture of ZrO2 and TiO2 crystals. For zirconolite the reaction can be written as:

ðCa1x Cex ÞZrðTi2y Aly ÞO7 þ Hþ ! ZrO2 # þð2  yÞTiO2 # 3þ

4þ þ ð1  xÞCa2þ aq þ xCeaq þ yAlaq

þ ð3 þ yÞH2 O:

Fig. 6. The normalized elemental loss rates (in g m1d1) of the temperature series plotted in an Arrhenius diagram. The solid line represents a linear fit to the Ca data, yielding an apparent activation energy of about 8 kJ/mol, whereas stippled lines are fit by eye. Their slope corresponds to an activation energy of 20 kJ/mol. Note the good reproducibility shown by the duplicated experiment at 200 °C.

Fig. 7. Elemental concentrations in solution plotted against the solid surfaceto-solution volume ratio (S/V). The dashed lines represent the expected dilution of the Ca, Ce, and Al concentrations, respectively, by changing the S/V value at a constant reaction rate. Experimental conditions were: 1 M HCl, 200 °C, 14 days.

conditions used in the present study, which is consistent with the occurrence of ZrO2 (likely monoclinic baddeleyite) and TiO2 (rutile) crystals at the surface of the treated zirconolite ceramics. Incongruent dissolution of zirconolite has also been observed in a number of other studies (e.g., [20,21]). Moreover, hydrothermal experiments with a polycrystalline titanate pyrochlore ceramic under very similar conditions have revealed that the pyrochlore grains were partly replaced by rutile and subordinate anatase [22]. The alteration was described to start with the congruent dissolution of the pyrochlore until a certain supersaturation of TiO2 phases at the pyrochlore-solution interface is reached, leading to the precipitation of TiO2 at the surface of the pyrochlore. At this stage, the rate of dissolution is coupled to the rate of precipitation of the TiO2 phases. Such a process can be represented by the following reaction:

A2 Ti2 O7 þ 6Hþ ! 2TiO2 # þ2A3þ aq þ 3H2 O:

ð2Þ

In the series of images presented in Fig. 2 this process can be recognized also in the present study. From Fig. 2a and b it is apparent that

ð3Þ

The replacement does not start before the solution at the zirconolite–solution interface is supersaturated with either ZrO2 or TiO2. The driving force for such a replacement reaction is given by the solubility difference between zirconolite and the product phases TiO2 and ZrO2. The precipitation of both product phases at the reaction interface shifts the reaction (3) to the right hand side, i.e., allows more zirconolite to dissolve in the near vicinity of the precipitated product phases (principle of LeChatelier). The results from the experiment series varying S/V lead to a linear trend between the Ca, Al, and Ce concentrations and S/V, which shows that the degree of alteration was independent of the S/V value (Fig. 7). Such a behavior is in agreement with the proposed process. Based on the solubility model, one would expect the dissolution rate of a phase to be controlled only by the onset of saturation in solution. If saturation is reached, the dissolution rate should drop to zero. The concentrations of the elements in solution after a sufficiently long time should therefore be constant regardless of the S/V ratio. However, during the hydrothermal alteration of zirconolite, zirconolite saturation in solution will never be reached as zirconolite dissolution and TiO2 and ZrO2 precipitation are coupled with the slowest rate determining the overall reaction rate. The dissolution can thus potentially proceed as long as the solution can reach the reaction interface. Since there exists a large difference in the molar volume as well as in the solubility in solution between zirconolite and both reaction products and the product phases cover the volume of the original zirconolite, as in pseudomorphism, the product layer should show a large porosity, which is indeed observed (Fig. 3, Fig. 4e). This porosity allows the solution to access the reaction interface at any one time. Porosity in the product phase is generated whenever there is a volume deficit reaction. This volume deficit does not only refer to the change in the molar volumes of the original zirconolite and the product solids, but also to their relative solubilities, which will determine how much of the original zirconolite is dissolved and how much of the product phase(s) is precipitated. The importance of porosity formation in determining the long-term alteration or replacement rate during such an interface-coupled dissolution-reprecipitation reaction has been demonstrated and discussed for simple salt systems [23,24], but also for silicates [25], including silicate glass [26], and titanates [22]. Even though the structure of the surface of zirconolite grains changed during the experiments, it is apparent from Fig. 2 that even fine morphological features (e.g., holes) are retained during the replacement process, demonstrating the close coupling between the dissolution and precipitation reaction. In general, the lower the solubility of the product phase the better the preservation of morphological features will be. However, a close coupling between dissolution and precipitation can only be achieved when the rate-controlling mechanism is dissolution, and there is a low activation energy barrier for nucleation of the product phases at the interface. Nevertheless, the irregular behavior of Ti and Zr seen, e.g., in Fig. 7 indicates that a kinetic barrier towards nucleation and growth of ZrO2 and TiO2 must exist, an effect that has also been observed in corrosion experiments with Tibearing borosilicate glass [26]. If the nucleation is the rate controlling step, implying that dissolution is fast and nucleation

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(a)

(b)

(c)

(d)

(e)

(f)

Fig. 8. Selected interelement diagrams showing the solution data of all experiments performed in this study. The solid line in each plot represents the stoichiometric element ratio in the solid. See text for details.

is slower, the coupling may not be so closely related in space and time. Such a situation has been explored by Xia et al. [27], who observed a change in the rate controlling step during the replacement of pentlandite by violarite as the pH changes from acidic to neutral. In the case of zirconolite the reaction layer consists of two (ZrO2 and TiO2) or better three (baddeleyite, rutile, and anatase) different

reaction products that compete with each other during nucleation and growth. Such competition results in an irregular arrangement of larger areas consisting of only one phase, as seen in the inlet diagram of Fig. 2d. In a 1 M HCl solution ZrO2 seems to have a lower solubility than TiO2, as evidenced by our solution data (Fig. 8e) and SEM images

P. Pöml et al. / Journal of Nuclear Materials 410 (2011) 10–23

which mainly revealed the occurrence of ZrO2 needles at the surface (Fig. 2). The observation that only one side of the Ce-doped zirconolite cuboid shows new crystal growth, which has to be the side that stayed in contact with the bottom of the Teflon reactor, indicates that supersaturation of ZrO2 (and TiO2) was reached in the solution located between the zirconolite cuboid and the bottom of the Teflon reactor. This can be explained by a slow chemical exchange (with respect to zirconolite dissolution) between the solution located between the zirconolite cuboid and the bottom of the reactor and the surrounding solution reservoir so that locally the supersaturation of TiO2 or ZrO2 could be reached. This demonstrates the importance of a solution boundary layer at the surface of the reactant or parent phase. Even a monolayer of dissolved original phase can supersaturate this boundary layer with respect to the product phase(s). The existence of such a solution boundary layer at the surface of a dissolving solid has also been suggested by other studies [24,26]. All experiments using solutions other than HCl yielded only small amounts of zirconolite-formular elements in solution and no visible signs of alteration on the grain scale. Even those experiments made in strong alkaline solutions (1 M NaOH, 1 M NH3) did not reveal a significant degree of alteration, indicating that zirconolite is kinetically more stable against alkaline than against acidic solutions. However, it surprisingly was found that the use of reactors made from different materials can have a significant effect on the kinetics of the alteration reactions. Since nickel and silver, in contrast to Teflon, are not stable in HCl solutions, the use of such reactor materials will always lead to an enrichment of Ni and Ag in solution, respectively. Since the Ca concentrations in solution increase with increasing temperature between 200 and 300 °C in Ni reactors, indicating a slightly increasing reaction rate with increasing temperature, the presence of Ni ions in solution does not seem to have a strong effect on the overall reaction kinetics. However, the low concentrations of Ti, Zr, and Ce in solution indicate that more rutile/anatase and baddeleyite precipitated, while Ce is either incorporated in these phases or precipitated as an undetected new phase. One explanation for this behavior could be that these elements cannot form soluble chlorine complexes in presence of Ni. The use of silver reactors had a different effect. In the presence of Ag ions in solution a relatively large piece (1.5  1.5  1 mm, 9 mg) of the Ce-doped zirconolite ceramic was completely replaced by TiO2 and ZrO2, demonstrating that Ag ions in solution dramatically increase the dissolution rate. Such an effect is used in the nuclear industry to rapidly remove PuO2 from used reactor parts in only weakly acidic solutions [28]. The Ag ions apparently have a similar effect on the rate of zirconolite dissolution in acidic solutions and thus on the overall reaction rate. Our data point to a dramatic effect of background electrolytes on the replacement rate, which has not yet been observed and needs to be further investigated by systematic studies. The results of the temperature series are difficult to interpret, as the presence of Ni ions in the solution had an unexpected effect on the replacement reaction (Fig. 6). However, as we did not observe a Ca phase as reaction product, Ca is unlikely to be incorporated in TiO2 [29], and EDX analysis showed no significant Ca concentrations in the alteration products, the increasing concentration of Ca in solution indicates an increasing degree of alteration, or better replacement, with increasing temperature. Nevertheless, the effect of temperature is surprisingly weak with an apparent activation energy of the reaction between about 8 and 20 kJ/mol. Fillet et al. [30] also reported a low activation energy for the forward dissolution of Nd-zirconolite in pure water, ranging between 15 and 30 kJ/ mol. Such magnitudes of the activation energy are similar to activation energies reported for diffusion-controlled reactions in solution media such as transport away from solid surfaces and far

21

lower than those for breaking bonds in crystals, which are usually larger than 80 kJ/mol [31]. Although they are lower than activation energies reported for a wide variety of mineral–solution alteration reactions involving surface-controlled dissolution and reprecipitation, which lie in the range between 40 and 80 kJ/mol [31], the low activation energy is rather consistent with a surface-controlled process than with a solid diffusion-controlled process. It is also important to recall that the Zr loss rates show a negative trend with increasing temperature. This again indicates a kinetic barrier for the precipitation of ZrO2 at low temperatures, resulting in a high supersaturation of ZrO2 in solution. However, how this affects the overall kinetics of the alteration process is currently not known. 4.2. The hydrothermal alteration of radiation-damaged (238Pu-doped) zirconolite One of the most important questions for the assessment of the suitability of zirconolite as waste form is whether its dissolution or alteration rate depends on the degree of radiation damage that accumulates over time. An experimental study with 16 differently radiation-damaged zircon grains, for instance, has clearly demonstrated that for this material radiation-damage dramatically affects its stability in aqueous solutions [32]. However, macroscopic volume swelling in self-irradiated zircon can reach 18%, which is much higher than the total volume expansion of radiationdamaged zirconolite of only 5–6% at saturation that is reached at a dose of approximately 0.3–0.5  1018 a-decays/g [9,11,33]. Nevertheless, our solution data as well as the SEM observations revealed a significant faster alteration of the X-ray amorphous 238 Pu-doped zirconolite. Furthermore, the crystalline zirconolite showed no dissolution features or crystal growth under the SEM and the concentrations of all elements in the solution were relatively low. In contrast, the treatment of the 238Pu-doped zirconolite under the same conditions resulted in very high concentrations of Ca, Al, and Pu in the solution, which is in agreement with the occurrence of TiO2 crystals at the surface. In fact, the Ca release rate from the 238Pu-doped sample is 155 times higher than that of the 239 Pu-doped zirconolite. These observations are clear evidence for a much higher degree of alteration of the radiation-damaged 238Pudoped zirconolite than of the undamaged zirconolite, an observation that has also been made in other experimental studies as summarized in Table 3. Weber et al. [11], for instance, observed a 8.3fold increase in the Ca release rate from X-ray amorphous Cmdoped zirconolite in pure water at 90 °C when compared to the release rate observed for the same sample after it had been annealed at high temperature. However, Strachan et al. [9] reported that they did not observe a difference in the alteration or dissolution kinetics between synthetic crystalline and X-ray amorphous zirconolite. It seems to be conceivable, at first glance, that the higher degree of alteration of the 238Pu-doped zirconolite is due to micro-cracks that formed as a result of the self-irradiation of the sample. Any micro-cracking would have allowed the solution to penetrate into the sample, which, in turn, would have increased the available surface and thus also the overall alteration kinetics of the zirconolite. However, our results from the Ce-doped zirconolite powders indicate that this cannot be the main reason, since the powders have the highest surface area of all samples studied while the degree of alteration is similar to that of the crystalline 239Pu-doped zirconolite. Another possibility is that water hydrolysis, resulting from the radiation of the sample, is responsible for the higher degree of alteration of the 238Pu-doped zirconolite, as suggested by Tribet et al. [35]. However, the experiments were carried out only for 3 days to minimize any radiolysis effects and a 1 M HCl solution was used that is already highly aggressive and oxidizing. It thus

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Table 3 Compilation of measured ratios between the dissolution or alteration rate of X-ray amorphous (radiation-damaged) and crystalline zirconolite (rCa(am)/rCa(cr)). Radiation dose

Structural state

Mode

T (°C)

Solution

rCa(am)/rCa(cr)a

Reference

(Ca, Pu)Zr(Ti,Al)2O7 (Ca, 238Pu)Zr(Ti,Al)2O7 (Ca,Cm)(Zr,Cm)Ti2O7

Insignificant 7  1018 a-decays/g 2.2  1025 a-decays/g

Static

200

This study

Static

90

8.3

[11]

Non-irradiated 3 1014 Pb3+ ions/cm2 1.3  1018 a-decays/g

Dynamic

100

2.2

[30]

Static

95

1 M HCl (pH  0) pure H2O pure H2O pure H2O

155

(Ca,Nd)Zr(Ti,Al)2O7

Crystalline X-ray amorphous Annealed, crystalline X-ray amorphous Crystalline Amorphous Slightly damaged

14.3

[34]

8.0  1019 a-decays/g

X-ray amorphous

Insignificant >2.6  1018 a-decays/g

Crystalline X-ray amorphous Annealed, crystalline

Dynamic

90

1

[9]

Composition 239

Natural zirconolite (Kaiserstuhl, Germany) Natural zirconolite (Sri Lanka) Ca(Al,Gd,Hf, 239Pu,U)(Hf,Ti)2O7 Ca(Al,Gd,Hf, 238PuU)(Hf,Ti)2O7

0.01 M HNO3 (pH  2)

a rCa(am) and rCa(cr) represent the Ca release rate obtained from the amorphous (or radiation-damaged) and the crystalline zirconolite sample, respectively. A value of >1 thus means that the damaged sample altered or dissolved faster than its crystalline counterpart.

seems to be unlikely that the production of radiolysis products such as H2O2 would have increased the alteration rate of the 238 Pu-doped sample in our experiment by the observed factor of 155. Icenhower et al. [36] also observed higher elemental release rates from 238Pu-bearing titanate ceramics compared to those from 239 Pu-bearing specimens but concluded that the higher elemental release rates from the 238Pu-doped titanate ceramics were an artifact that resulted from the presence of up to 100 ppm of fluorine in solution released from the Teflon due to irradiation. Their conclusion was based on the observation that annealed 238Pu-bearing titanate ceramics gave the same elemental release rates than the unannealed, X-ray amorphous counterpart. In this respect, we recall that we observed a dramatic enhanced alteration rate in Agbearing solutions, indicating a strong effect of Ag ions on the dissolution-reprecipitation rate. However, the Teflon reactors were cleaned thoroughly before the experiments by several cycles of boiling in distilled ultrapure HNO3, HF, and HCl in order to remove any unwanted species present in the Teflon. Furthermore, since 100 ppm F in a 1 M HCl solution would not significantly change the solution pH, it seems unlikely that fluorine in solution was responsible for the observed significantly faster alteration of the 238 Pu-bearing specimen. We also note that experiments with natural, radiation-damaged or externally irradiated samples also yielded higher alteration rates than experiments with crystalline or less damaged samples (Table 3). As discussed above, the observations made on crystalline zirconolite are fully consistent with an interface-coupled dissolution-reprecipitation process that is based on the congruent dissolution of the zirconolite accompanied by the reprecipitation of ZrO2 and TiO2 at an inwardly moving reaction front. However, the observation that Ca, Al, and Pu were absent from surface areas of the 238Pu-doped zirconolite disc that are not covered by newly grown TiO2 crystals suggests these elements were selectively removed from the near surface areas. EDX measurements detected Zr and Ti at a ratio of 1:1, rather than 1:2 as in the original zirconolite, indicating that also some Ti was selectively ‘‘leached’’ from these areas. Such an interpretation is in agreement with the observation that the alteration products found on the surface of the Ce-doped samples consist of ZrO2 and TiO2, while we have only identified TiO2 on the surface of the 238Pu-doped sample. However, more detailed studies are needed to investigate whether the surface areas were indeed selectively leached, possibly by a diffusion-controlled process similar to that described for the alteration of radiation-damaged zircon [25], or were replaced by a new crystalline phase such as ZrTiO4. In any case, all observations made in this study strongly suggest that self-irradiation structural damage in zirconolite, as in pyrochlore [22], does have a great

effect on the stability of zirconolite in aqueous solutions, at least in highly acidic ones.

4.3. Implications for the assessment of the long-term performance of a zirconolite waste form We would like to address two issues that are related to the relevance of our study for the evaluation of the long-term performance of a potential zirconolite waste form in a geological repository. First, when analyzing the results from this work, the question that immediately arises is whether we can compare the experimental results obtained from three compositionally and texturally different samples? In this study we used (1) Ce-doped zirconolite powders, (2) Ce-doped zirconolite monoliths, and (3) pieces of Pu-doped zirconolite pellets. For the powders, the BET surface area could be determined, however, we could only roughly estimate the geometrical surface area of the monoliths. Both estimates are hardly comparable. Ideally, if comparisons are to be made, tests should be carried out on ‘‘identical’’ samples with the same history and origin, but this is not typically feasible in practice, especially when highly radioactive materials are involved. However, by carrying out the additional experiment on the Cemonolith we successfully closed the gap between the Ce-doped zirconolite powders and the Pu-doped zirconolite pellet pieces, as our results show that experiments run with these texturally different samples under the same conditions yielded consistent results. The second obvious question is what kind of conclusions we can draw from the results from this study, which were mainly derived from batch experiments run with a strong acidic solution that only rarely, if at all, occurs in nature, but certainly not in a geological repository. In any case, the temperature range from 100 to 300 °C is potentially relevant to deep-borehole repositories. The issue of using strong acidic (or basic) conditions in laboratory experiments to enhance the reaction kinetics has been discussed in detail by Pöml et al. [22] with respect to the alteration of pyrochlore. In their experimental study the pyrochlore samples were found to be replaced by anatase and rutile. The authors pointed out that evidence from studies on natural samples supports the formation of TiO2 polymorphs as alteration products. Furthermore, Zhang et al. [37] reported the formation of TiO2 crystals on the surface of Nd-doped zirconolite altered under more moderate conditions (7d, 150 °C, H2O) and Pan [38] has described the breakdown of natural zirconolite to a new mineral assemblage consisting of zircon, titanite, and rutile in metamorphosed ferromagnesian silicate rocks at Manitouwadge, Canada. This reaction can be expressed as follows (modified slightly from [38]):

P. Pöml et al. / Journal of Nuclear Materials 410 (2011) 10–23

CaZrTi2 O7 þ 2SiO2 ! ZrSiO4 þ CaTiSiO5 þ TiO2 :

ð4Þ

This illustrates the potential instability of zirconolite at high temperature and pressure. The formation of zircon and titanite has not been observed in the present study, since the experiments were carried out in silica-undersaturated solutions, in which ZrO2 has the lowest solubility of all potential phases. The agreement of nature and laboratory experiments in terms of alteration mechanism, features, and products thus strongly supports the relevance of batch experiments under high proton activity for assessing the alteration mechanism and the long-term behavior of nuclear waste ceramics. We also note that the alteration of zirconolite may be different in neutral media and groundwater under low temperatures. Under these conditions the precipitation of hydroxides, e.g., Ti(OH)4, and amorphous phases has been observed, which form a thin layer at the surface, as reported by Leturcq and coworkers [39,40]. The formation of the amorphous Ti-OH surface layer at low temperatures may be due to kinetic factors inhibiting the nucleation of anatase or rutile. The sole occurrence of ZrO2 in the experiments with the Ce-doped zirconolite cuboid may indicate that under these conditions the formation of Ti-OH was favored. This suggests that other factors may control the nature of the phases formed under natural conditions, which in turn seem to depend on the alteration conditions. These effects have to be investigated in future experimental studies. Considering the issues discussed above, we conclude that our results are indeed of major importance for the assessment of the long-term performance of a zirconolite waste form. Apart from the introduction of a new mechanistic model for the alteration of crystalline zirconolite in aqueous solutions of which all details that determine the overall reaction kinetics are not yet fully understood, the experimental observation of a significantly increased rate of alteration of heavily radiation damaged zirconolite is a key point to be considered in the development of ceramic nuclear waste forms. In this respect it is also important to recall that we have presented evidence that with increasing degree of self-irradiation damage, i.e., with increasing storage time the alteration mechanism may change from an interface-controlled to a diffusion-controlled process. Such change in the mechanism seems to be accompanied by an increase of the alteration rate. Acknowledgements We would like to thank Jonathan P. Icenhower, M. Walter, and M. Ayranov for their help and fruitful discussions. The Deutsche Forschungsgemeinschaft (Grant Ge 1094/5) and the 6th Framework Programme of the European Commission through the ACTINET Network of Excellence (Joint Research Proposal 03-02) are acknowledged for financial support. References [1] A.E. Ringwood, S.E. Kesson, N.G. Ware, W. Hibberson, A. Major, Nature 278 (1979) 219. [2] B.M. Gatehouse, I.E. Grey, R.J. Hill, H.J. Rossell, Acta Cryst. B37 (1981) 306.

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