Formation of Studtite during the Oxidative Dissolution of UO2 by

Manresa, Universitat Polite`cnica Catalunya, Manresa, Spain, and Laboratory of Electrochemistry and Materials,. Universitat de Barcelona, Barcelona, S...
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Environ. Sci. Technol. 2004, 38, 6656-6661

Formation of Studtite during the Oxidative Dissolution of UO2 by Hydrogen Peroxide: A SFM Study F . C L A R E N S , † J . D E P A B L O , †,‡ I . D ´ı E Z - P E ´ REZ,§ I. CASAS,† J. GIME ´ N E Z , * ,† A N D M . R O V I R A ‡ Chemical Engineering Department, ETSEIB, Universitat Polite`cnica Catalunya, Barcelona, Spain, Environmental Technology Area, Centre Tecnolo`gic de Manresa, Universitat Polite`cnica Catalunya, Manresa, Spain, and Laboratory of Electrochemistry and Materials, Universitat de Barcelona, Barcelona, Spain

Understanding the formation of alteration phases on the surface of spent nuclear fuel, such as those observed during leaching experiments, is necessary in order to predict the concentration of radionuclides in the near-field of a final repository. Hydrogen peroxide has been identified as one of the oxidants formed by the radiolysis of water in the presence of spent nuclear fuel; especially due to R activity. The presence of this species in solution can contribute to the formation of uranium peroxide secondary phases. In this work, we have studied the oxidative dissolution of synthetic UO2 disks in hydrogen peroxide solutions of two different concentrations (5 × 10-4 and 5 × 10-6 mol dm-3), both at pH 5.8 ( 0.1. The solid surface evolution of the disks has been followed by means of ex-situ scanning force microscope (SFM) measurements, and uranium concentration in solution has been determined by inductively coupled plasma mass spectrometry. During the first stage of the experiment, SFM images indicate that only UO2 dissolution is occurring. After 142 h, a secondary phase is observed on the surface of the solid at 5 × 10-4 mol dm-3 hydrogen peroxide concentration. This secondary phase has been identified by X-ray diffraction as studtite (UO4‚ 4H2O). From the analysis of SFM topographic profiles at different elapsed times, a precipitation rate for the studtite has been estimated to be in the range of (8-32) × 10-10 mol m-2 s-1.

Introduction Understanding the formation of uranium secondary phases during the alteration/dissolution of spent nuclear fuel is critical for predicting the concentration of uranium and other radionuclides in the near-field of a final repository. The formation of a particular uranium phase will depend on its solubility limit and precipitation kinetics, both of which are related to the chemistry of the solution. Although reducing conditions are expected in most geological repositories, oxidizing species can be formed due the radiolysis of water induced by the activity of the spent nuclear fuel. * Corresponding author present address: Department Enginyeria Quı´mica H4, ETSEIB-UPC, Avda. Diagonal 647, 08028-Barcelona, Spain; phone: +34 93 4017388; fax: +34 93 4015814; e-mail address: [email protected]. † ETSEIB, Universitat Polite ` cnica Catalunya. ‡ Centre Tecnolo ` gic de Manresa, Universitat Polite`cnica Catalunya. § Universitat de Barcelona. 6656

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Natural analogue studies on uranium ore deposits have shown that the uraninite alteration reaction path begins with the formation of uranyl oxide hydrates such as schoepite (UO3‚2H2O), dehydrated schoepite (UO3‚(0.8-1)H2O), and becquerelite (Ca[(UO2)6O4(OH)6]‚8H2O). In the presence of silica-rich groundwater, uranyl silicate minerals are formed (e.g., soddyte ((UO2)2(SiO4)‚2H2O), and uranophane (Ca(H3O)2[(UO2)(SiO4)]2‚2H2O)) (1, 2). This reaction path is similar to that observed in laboratory studies performed with UO2 with a high surface/volume (S/V) ratio (3, 4). Uranium oxides and silicates have also been identified in spent fuel leaching studies (5-7). Another uranyl secondary phase found in nature is studtite (UO4‚4H2O), and the properties of this mineral are described elsewhere (8, 9). The formation of this uranium peroxide in nature has recently been discussed (10), and the authors pointed out that studtite is unstable in systems where hydrogen peroxide is not present. On spent nuclear fuel surfaces, however, hydrogen peroxide is one of the oxidants formed by water radiolysis, specifically due to R activity (11). Uranium peroxides (studtite and meta-studtite (UO4‚2H2O)) have been observed in long-term leaching experiments on spent fuel (12), in UO2 leaching experiments exposed to 4He2+ radiation (13), during the corrosion of UO pellets 2 containing plutonium (14), and on the Chernobyl ‘lavas’ (15). These phases have been also identified in unirradiated UO2 leaching experiments at different hydrogen peroxide concentrations (16-18). Recently, the oxidative dissolution of UO2 at different hydrogen peroxide concentrations has been studied in detail by measuring the evolution of the uranium concentration in solution by inductively coupled plasma mass spectrometry (ICP-MS) (19). In these experiments, an initial uranium release, followed by a decrease in uranium concentration was observed. The decrease was attributed to the possible precipitation of a uranium-bearing secondary phase. To study the initial dissolution of UO2 and the subsequent precipitation of a new phase in more detail, we have performed a study using scanning force microscopy (SFM) in order to analyze the interaction between UO2 surfaces and H2O2 solutions of different concentrations as a function of time. SFM allows for topographic changes to be observed at the solid-liquid interface, which is of interest in processes such as corrosion, passivation, and mineralization (20-24).

Experimental Section We used two disks of unirradiated UO2(s) that were cut from a pellet supplied by ENUSA (Empresa Nacional del Uranio S.A., Spain). Each disk weighed 0.92 g and was 10 mm in diameter and 1.1 mm thick. The disks were mechanically polished to 1 µm roughness prior to the leaching experiments. The specific surface area of each disk was calculated to be 7.1 × 10-4 m2 g-1, based on the surface area of a UO2 pellet (25) and taking into account the ratio between surface area and mass of the two geometries. Each UO2 disk was put into a methacrilate cylindrical shaped batch reactor, 6 cm i.d. and 8 cm high. The leaching solutions used consisted of 200 cm3 of 5 × 10-4 and 5 × 10-6 mol dm-3 of hydrogen peroxide, respectively. In both experiments, deaerated 0.1 mol dm-3 NaClO4 was used as ionic medium. The initial pH was 5.8 ( 0.1, and experiments were carried out at room temperature. The S/V ratio of the experiment was 3.3 m-1. Light incidence on H2O2 solutions was prevented. During the experiments, nitrogen gas was continuously bubbled through the solutions to prevent oxygen intrusion in order to study only the effect of hydrogen 10.1021/es0492891 CCC: $27.50

 2004 American Chemical Society Published on Web 11/12/2004

FIGURE 1. Uranium concentration as a function of time during the UO2 surface cleanup. peroxide, which is the main molecular oxidant expected in the repository. Both uranium and hydrogen peroxide concentrations in solution were analyzed as a function of time. Uranium concentration was determined by ICP-MS (Elan 6000, Perkin-Elmer). Hydrogen peroxide concentration was measured using a chemiluminiscence method with luminol and cobalt as reagents (26) and a Camspec CL-1 chemiluminiscence detector (detection limit: 10-7 mol dm-3). Before putting the disks into the vessels, the UO2 surface was cleaned by using a 2 × 10-3 mol dm-3 HCO3- solution in a flow-through reactor coupled to the SFM head at a flow rate of 0.1 mL min-1. This step allowed for the removal of any surface layer oxidized beyond UO2, due to the formation

highly stable carbonate-U(VI) aqueous complexes (27). Surface cleanup was monitored by comparing the topographic profiles during the data acquisition. Ex-situ SFM was used at different times throughout the experiment to observe the evolution of the UO2 surface in contact with H2O2. Prior to each observation, the UO2 disk was removed from the batch reactor and rinsed with ethanol and MilliQ water; this procedure was carried out in a glovebox in order to minimize oxygen intrusion. Ex-situ measurements were conducted at room temperature using an Extended Multimode SFM head with a Nanoscope IIIa electronic controller (Digital Instruments Veeco Metrology Group) (28). Imaging was carried out in tapping mode, an intermittent-contact technique used to reduce lateral and frictional forces. Although tapping mode yields slightly lower resolution results than contact mode, it minimizes artifacts that may result from the continuous scanning of the sample surface by the tip. NCH Nanosensor Pointprobes, manufactured with monocrystalline silicon, were used for SFM analysis. The probe tips are pyramidal in shape, with a nominal tip radius of 10 nm, a spring constant of approximately 35 N/m, and a resonant frequency of about 300 kHz. In the literature, dissolution experiments coupled to SFM observations have been focused on materials with nearly atomically flat surfaces that present excellent resolution (29). Fluid cell SFM is usually performed on materials that exhibit fast dissolution rates, in the range of 10-10-10-6 mol m-2 s-1 (30), thus allowing the process to be studied in situ. In

FIGURE 2. SFM images of the UO2 surface: (a) before cleaning the surface; (b) after cleaning the surface with HCO3- solution for 180 min.

FIGURE 3. Evolution of uranium (b) and hydrogen peroxide (O) concentrations in two different solutions, as a function of time: (a) [H2O2]0 ) 5 × 10-6 mol dm-3; (b) [H2O2]0 ) 5 × 10-4 mol dm-3. VOL. 38, NO. 24, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 4. (15 × 15) µm2 SFM images of UO2 surface: (a) initial surface; (b) after 69.6 h; (c) after 141.6 h; (d) after 314.4 h of contact with H2O2 5 × 10-4 mol dm-3. contrast, UO2 is a sintered material with a 5-10 µm grain size, exhibits poor cleavage, and has a very slow dissolution rate of 10-10-10-12 mol m-2 s-1 (19, 27, 31). For this reason, the surface needs to be polished to make it as flat as possible. To ensure that SFM measurements were recorded at the same point on the surface, marks were made with a diamond tip. Relocating this area on the surface was made possible using an optical microscope coupled to the SFM apparatus. At the end of the experiments, the UO2 surfaces were analyzed using an environmental scanning electron microscope (ESEM; Electroscan 2020), which allows microphotographs to be obtained at pressures around 50 Torr. An X-ray diffraction (XRD, Bruker D5005) machine with a Cu LR radiation source was used to identify the secondary phases that formed on the UO2 surface.

Results and Discussion During the initial observation period, in-situ SFM images of the solid UO2 surface in contact with the HCO3- solution were continuously recorded at a rate of one image per minute. After 30 min, the topographic profiles remained unchanged, indicating that the oxidized layer had likely been removed due to the complexing effect of the carbonate (27). The uranium concentration profiles obtained with ICP-MS showed a rapid initial increase with time, reaching a maximum value 6658

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at 45 min (2 × 10-7 mol dm-3). After 180 min, the uranium concentration reached a constant value (7 × 10-8 mol dm-3), as seen in Figure 1. In-situ SFM images of initial and cleaned UO2 surfaces are compared in Figure 2, where it can be seen that the surface of the grain is better defined due to the dissolution of the oxidized layer. After the cleaning step, the disks were put into the batch reactors. The evolution of both uranium and hydrogen peroxide concentrations, in solution, as a function of time is shown in Figure 3a,b. In the experiment at 5 × 10-6 mol dm-3 H2O2 concentration (Figure 3a), the uranium concentration in solution increases for the duration of the experiment; in contrast, a steady-state value is observed in the experiment at 5 × 10-4 mol dm-3 H2O2 concentration (Figure 3b). UO2 dissolution rates were calculated from the variation in uranium concentration during the first 5 days. The values obtained were 1.1(( 0.5) × 10-9 mol m-2 s-1 for the high H2O2 concentration and 8(( 0.5) × 10-11 mol m-2 s-1 for the low H2O2 concentration. These dissolution rates are slightly higher than those obtained using powdered UO2 (10-50 µm) in ref 19: 4 × 10-10 mol m-2 s-1 for 5 × 10-4 mol dm-3 of H2O2 and 4 × 10-11 for 10-5 mol dm-3 of H2O2. Ex-situ SFM images were recorded at different elapsed times throughout the experiments. In Figure 4, the initial UO2 surface is compared to the surface after 69.6, 141.6, and

FIGURE 5. Topographic profiles illustrating the height variation between the initial UO2 surface (a) and the surface after 69.6 h (b) in the experiment with 5 × 10-4 mol dm-3 H2O2. The point denoted as R is an unchanged reference point. 314.4 h in the experiment carried out at 5 × 10-4 mol dm-3 H2O2 concentration. As it can be seen, during the first 3 days no precipitation was observed. Dissolution rate can be calculated from analysis of the topographic profiles. Figure 5 illustrates the procedure: height difference between initial and 69.6 h profiles are quantified with reference to an unchanged point (this point is indicated with R in Figure 5) (32). This procedure represents a semiquantitative analysis of the dissolution progress because absolute surface changes can only be obtained in the presence of unalterable reference points (24). Measured heights (h) varied between 4.5 and 8.6 nm day-1. Dissolution rates (in mol m-2 s-1) were calculated to be (2.4-4.6) × 10-9 mol m-2 s-1, by using the following equation:

TABLE 1. Results of the Roughness Analysis of the Solid Surface [H2O2] ) 5 × 10-4 M

[H2O2] ) 5 × 10-6 M

time (h)

roughness (nm)

time (h)

roughness (nm)

initial 69.6 141.6 314.4

6.7 7.2 22.9 32.4

initial 52.8 144 576 840

11.9 10.6 11.6 10.5 8.7

rdiss ) h (m)‚F (kg m-3)/PA of U (kg mol-1)‚t (s) (1) where F is the UO2 density (10 960 kg m-3), PA is the atomic weight of uranium, and t is the elapsed time. Interestingly enough, this dissolution rate is on the same order of magnitude as the one determined by measuring the evolution of uranium concentration in solution. After 141.6 h, precipitation of a yellow secondary phase onto the UO2 surface is observed. This precipitation is much more evident in the SFM image taken at 314.4 h as can be seen in Figure 4. In the experiment carried out at an H2O2 concentration of 5 × 10-6 mol dm-3, no precipitation was observed even after 840 h. In this experiment, by using the same procedure explained above, the measured height varied between 0.64 and 0.98 nm day-1, which corresponds to a dissolution rate of (3.4-5.2) × 10-10 mol m-2 s-1. This value is higher than the one determined from uranium release by ICP-MS, 8(( 5) × 10-11 mol m-2 s-1, which is not surprising because the small variation in height was determined in zones of the UO2 surface where dissolution was more evident. Surface roughness analysis was performed on a 5 µm2 area, using the Nanoscope software. Surface roughness values (Ra) are given in nanometers and are calculated by an algorithm related to the heights measured on the surface. Roughness results are presented in Table 1, and a significant increase of Ra is obtained only when the secondary phase precipitates. At the end of the experiment (314.4 h), the UO2 disk on which the precipitate formed was analyzed using ESEM. A microphotograph of the secondary phase is shown in Figure 6. The particle size of the precipitate was determined to be ∼200 nm in length. The X-ray diffractogram obtained from XRD analysis is shown in Figure 7, where peaks corresponding to two different solid phases can be seen; one corresponds to UO2, and the second one corresponds to the uranium peroxide, namely, studtite (UO4‚4H2O) (33). The precipitation

FIGURE 6. Microphotograph of the UO2 disk obtained by ESEM after 314.4 h of contact with 5 × 10-4 mol dm-3 H2O2. of this phase is assumed to be responsible for the steadystate uranium concentration observed in Figure 3b. Once the secondary phase was identified as studtite, its precipitation rate was estimated from SFM images by using the following equation:

rppt ) h (m)‚F (kg m-3)/PA of U (kg mol-1)‚t (s) (2) where h is the average height of the phase formed (we estimated this value to be 35 and 50 nm in the SFM images taken at 141.6 and 314.4 h, respectively); F is the density of studtite, equal to 3460 kg m-3; PA is the atomic weight of uranium; and t is the elapsed time. Respective time values of 72 and 244.8 h were used, which represent the difference between SFM images taken at 69.6 h (no precipitation) and the SFM images taken at 141.6 and 314.4 h. The calculated precipitation rate ranged between (8 and 32) × 10-10 mol m-2 s-1. No available data regarding studtite precipitation rates have been found in the literature to compare with the result obtained in this work. VOL. 38, NO. 24, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 7. X-ray diffractogram of the UO2 disk showing the characteristic peaks of uraninite (UO2) and studtite (UO4‚4H2O). To our knowledge, this study is the first one that clearly demonstrates the formation of studtite at a very low S/V ratio (3.3 m-1). Previous studies by Dı´az-Arocas et al. used ratios of ratios of 100, 500, and 1000 m-1 (17), and Macnamara et al. used 1 g of spent fuel in 8 mL of water (12). Studtite precipitation is very dependent on hydrogen peroxide concentration at the S/V ratio used in this work. Finally, when studtite began to precipitate, the uranium concentration measured in solution was 1.8 × 10-6 mol dm-3. At this value schoepite is still undersaturated (34, 35), indicating that studtite must have a lower solubility constant. This result is in agreement with the calorimetric data reported by Kubatko et al. (10), who determined that studtite is thermodynamically the dominant phase in the presence of peroxide. From these results, the mobility of uranium can be modeled in environments where studtite has been identified. Furthermore, insight into identifying systems where the formation of this secondary phase is likely to take place is gained.

Acknowledgments Thanks are due to Aurora Martı´nez-Esparza for her interest in our work. We thank the five anonymous reviewers for their comments and suggestions. We also thank Josep Elvira and Jose´ Ma. Manero for the X-ray diffraction and ESEM analysis, respectively. This work was financially supported by ENRESA (Empresa Nacional de Residuos, Spain), the European Union (SFS and NFPRO projects), and the Spanish “Ministerio de Ciencia y Tecnologı´a (MCyT)” by means of the ‘Ramo´n y Cajal” Programme.

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Received for review May 12, 2004. Revised manuscript received September 13, 2004. Accepted September 22, 2004. ES0492891

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