Design of a New Reactor to Work at Low Volume Liquid/Surface Solid Ratio and High Pressure and Temperature: Dissolution
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Design of a New Reactor to Work at Low Volume Liquid/Surface Solid Ratio and High Pressure and Temperature: Dissolution Rate Studies of UO2 Under Both Anoxic and Reducing Conditions. A. Martínez-Torrents1,2, J.Giménez2, I.Casas2, J. de Pablo1,2. 1
CTM Centre Tecnològic, Plaça de la ciència 2, 08243 Manresa, Spain Departament of Chemical Engineering, Universitat Politècnica de Catalunya, Diagonal 647 H-4, 08028 Barcelona, Spain. 2
ABSTRACT A flow-through experimental reactor has been designed in order to perform studies at both high pressure and high temperature conditions. A chromatographic pump is used to impulse the leachant throughout the reactor in order to work at very low flows but high pressures. Therefore, high surface solid to volume leachant ratios, similar to the ones predicted in the final repository, can be obtained. The reactor allows working at different atmospheres at pressures up to 50 bars. The temperature inside the reactor can be set using a jacket. Using this new reactor the evolution of uranium concentrations released from an UO2 sample was studied at different conditions. The results show that at hydrogen pressures between 5 and 7 bars, hydrogen peroxide does not seem to significantly oxidize the uranium (IV) oxide. Uranium concentrations in those experiments remain between 10-8 mol·l-1 and 10-9 mol·l-1. INTRODUCTION The study of the behavior of the Spent Nuclear Fuel Matrix is critical for the deep geological repository safety assessment. As an approach to the chemical behavior of the Spent Nuclear Fuel Matrix, it is possible to use unirradiated UO2 as an analogue. Casas et al. [1] studied the dissolution kinetics of UO2 under oxidizing conditions proposing a first mechanism of oxidation-dissolution of UO2. Later the same group studied the role of pe, pH and carbonate on the solubility of UO2 at reducing conditions [2]. The oxidative dissolution mechanism was improved by De Pablo et al. [3,4,5] adding the effect of Temperature, pH, carbonate concentration and oxygen partial pressure and a mechanism of the dissolution of UO2 due to the uranium-carbonate complexation was proposed[5]. Due to the radiolysis of water, species like H2, O2 and H2O2 are formed. Clarens et al. [6] added the concentration of hydrogen peroxide as a new parameter to the oxidative dissolution mechanism studies and studied the effect of the pH in the dissolution on UO2 in H2O2 solutions. Precipitation of the uranium peroxide Studtite was observed at different concentrations of hydrogen peroxide adding complexity to the dissolution mechanism. Casas et al. [7] added a new parameter to the dissolution experiments: pressure. They designed a reactor in order to perform experiments up to 100 bars and temperatures up to 100 oC. The reactor was continuously stirred and they determined UO2(s) dissolution rates in a hydrogen peroxide and carbonate media as a function of pressure and temperature. Lately, Casas et al. [8] improved the knowledge in the effect of carbonate and hydrogen peroxide concentration in UO2 dissolution observi
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