New Synthesis Route and Characterization of Siderite (FeCO3) and Coprecipitation of 99Tc
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0985-NN12-08
New Synthesis Route and Characterization of Siderite (FeCO3) and Coprecipitation of 99Tc Isabelle Llorens1, Massoud Fattahi1, and Bernd Grambow2 1 Laboratoire Subatech, Ecole des Mines de Nantes, 4 rue Alfred Kastler, Site de la Chantrerie, Nantes, 44307, France, Metropolitan 2 laboratoire Subatech, Ecole des Mines de Nantes, 4 rue Alfred Kastler, Site de la Chantrerie, Nantes, 44307, France, Metropolitan ABSTRACT Technetium-99 is a long-lived product of nuclear fission. In the ground-waters of European disposal sites Tc is expected to be in the tetravalent state since the geochemical conditions are reducing due to the presence of redox couples such as Fe(II)/Fe(III), and the presence of reducing minerals such as pyrite and siderite (FeCO3). Siderite is expected to be formed as well as container corrosion product. Experiments of coprecipitation of Fe2+ and Tc(IV) were conducted in the laboratory under an inert atmosphere at room temperature. High resolution transmission electron microscopy shows that the coprecipitate is around 3 nm sized particles. Analytical electron microscopy indicates that the precipitate contains 30 atom % of Tc and 70 atom % Fe. The Tc was homogeneously distributed. Selected Area Electron Diffraction doesn’t reveal the formation of siderite, which suggests that the presence of technetium in high concentration inhibit the formation of this phase. INTRODUCTION Geological disposal of fuel wastes from nuclear reactor is currently studied in many countries. Deep underground disposal would provide a long-term barrier between the emplaced wastes and the biosphere. Retardation of radionuclide transport through this geological barrier involve various natural processes including ion exchange, coprecipitation, and sorption. To access the effectiveness of this retardation, information relative to the interaction between geological material and radionuclides, and to the chemical behavior of radionuclides in aqueous solutions are needed. Technetium-99 is a fission product with a thermal neutron fission yield of 6.13% and has a half life of 2.13 x 105 y [1]. Some Tc99 has been released into the environment as the result of nuclear weapon testing, and Tc99 is an important component of high level wastes. Since no isotope of Tc is stable, knowledge of its geochemistry is limited to observation at the site of an ancient natural reactor at Oklo as well as to experimental investigations. Geochemically, Tc exists in two stable oxidation states depending on the redox conditions. In aerated water, Tc exists as soluble and mobile TcO4- whereas the tetravalent form will dominate in groundwater with low redox-potentials. The environmental redox chemistry of Tc is thus very important in governing its mobility. Several studies have been carried out on interactions between technetium and iron-containing minerals [2-8], essentially magnetite. The results are contradictory. If Haines et al. [9] evidenced the reduction of Tc(VII) to Tc(IV) by Fe(II) at the magnetite surface befor the precipitation of technetium
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