Formation of Studtite during the Oxidative Dissolution of UO 2 by Hydrogen Peroxide: A SFM Study F. CLARENS, † J. DE PABLO, †,‡ I. Dı ´EZ-PE ´REZ, § I. CASAS, † J. GIME ´NEZ,* ,† AND M. ROVIRA ‡ 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 UO 2 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 UO 2 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 (UO 4 ‚ 4H 2 O). 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. 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 4 He 2+ radiation (13), during the corrosion of UO2 pellets containing plutonium (14), and on the Chernobyl ‘lavas’ (15). These phases have been also identified in unirradiated UO2 leaching experiments at different hydrogen peroxide con- centrations (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 m 2 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 cm 3 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 * 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: francisco.javier.gimenez@upc.es. † ETSEIB, Universitat Polite `cnica Catalunya. ‡ Centre Tecnolo ` gic de Manresa, Universitat Polite `cnica Catalunya. § Universitat de Barcelona. Environ. Sci. Technol. 2004, 38, 6656-6661 6656 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 38, NO. 24, 2004 10.1021/es0492891 CCC: $27.50  2004 American Chemical Society Published on Web 11/12/2004