Xps and Sem Studies on the Corrosion of UO 2 Cointaining Plutonium in Demineralized and Carbonated Water.
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XPS AND SEM STUDIES ON THE CORROSION OF UO2 COINTAINING PLUTONIUM IN DEMINERALIZED AND CARBONATED WATER. J. Cobos1, T. Wiss, T. Gouder, V.V. Rondinella. European Commission, Joint Research Centre, Institute for Transuranium Elements, Postfach 2340, 76125 Karlsruhe, Germany. 1 CIEMAT, Avda Complutense 22, E-28040 Madrid, Spain. [email protected] ABSTRACT An oxidation and dissolution study has been performed on UO2 pellets containing ~10 and ~0.1 wt. % 238Pu, ~10 wt. % 239Pu and on undoped UO2 to investigate the effects of radiolysis and composition on the corrosion behavior of spent fuel. The so-called alphadoped UO2 is used to simulate the alpha-radiation field of different types of commercial LWR spent fuel after different storage times. Leaching experiments in demineralized and carbonated water at room temperature under oxidizing conditions showed that relatively high amounts of 238Pu were released. The leached surfaces were examined with X-ray Photoemission Spectroscopy (XPS), and the progressive surface oxidation was monitored. The oxidation of the U(IV) during the leaching experiments, in the materials doped with 238 Pu resulted in precipitation of U(VI) phases: enhanced formation of studtite for the strongest radiation field and shoepite at low radiation field was observed on the surface of the pellet . Essentially no precipitation of Pu-rich phases was directly observed. Leaching in carbonated water and characterization of UO2 containing 239Pu under the same experimental conditions were performed and the results compared to those for alpha-doped UO2. The chemistry effects due to the presence of Pu in addition to alpha-radiolysis were investigated. INTRODUCTION Reducing conditions are expected in the European geologic repositories for nuclear high level waste disposal. Under such conditions UO2 is stable. However, due to radiolysis, local-oxidising conditions cannot be excluded. The radiolysis of water produces both molecular and radical products [1,2]. The production of the oxidant reactants by radiolysis is accompanied by the production of an equal number of reducing species. Molecular hydrogen is formed as a primary reducing product in the radiolysis of water and being rather inactive at room temperature and at low concentrations it may be responsible for the excess of the oxidising species [3]. After a few hundred years of storage, alpha emissions will dominate the radiation field in and around the spent nuclear fuel [4,5]. Since the penetration rate of the alpha particles emitted in the spent nuclear fuel is short (~40µm), radiolysis leading to oxidative dissolution would occur very near the interface spent fuel – water [6]. The rate of groundwater flow in contact with the waste is expected to be sufficiently slow to permit saturation of water with radionuclides. The interaction of oxidising species from the radiolysis of groundwater with the fuel surface may cause a migration front of radionuclides. Redox-sensitive nuclides like uranium, the main constituent of the fuel matrix, become more solubl
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