Kinetic and Thermodynamic Study of the Chemistry of Neoformed Phases during the Dissolution of Phosphate Based Ceramics
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0985-NN09-06
Kinetic and Thermodynamic Study of the Chemistry of Neoformed Phases during the Dissolution of Phosphate Based Ceramics Erwan du Fou de Kerdaniel1, Nicolas Clavier1, Nicolas Dacheux1, and Renaud Podor2 1 Institut de Physique Nucleaire d'Orsay, Orsay, 91406, France 2 Laboratoire de Chimie du Solide Mineral, Univ H. Poincare - Nancy I, BP 239, Vandoeuvre les Nancy, 54506, France
ABSTRACT Phosphate matrices are considered as potential candidates for the specific immobilization of tri- and tetravalent actinides. One of the main properties of interest of these matrices concerns their chemical durability. The chemical durability of the phosphate phases was studied with respect to the retention of actinides and developed using under- and over-saturation experiments. In this field, neoformed phases were precipitated. Lanthanides were used as surrogates for trivalent actinides while uranium and thorium were used for tetravalent actinides. These phases were extensively characterized through several analytical and spectroscopic techniques (SEM, EPMA, XRD, micro - Raman, TRLFS). Secondary phosphate phases such as Nd1-2xCaxThx-yUy(PO4,F) ½ H2O rhabdophane were identified during the dissolution of britholites. The precipitation of Nd1-2xCaxThxPO4 ½ H2O rhabdophane was obtained and it appeared that there was a segregation between neodymium, calcium and thorium after a few days to several months depending on the thorium weight loadings and the precipitation time, thorium precipitating as TPHPH and neodymium as NdPO4 ½ H2O. Quantification of the solubility constants showed very low values that means that these phases exhibit some beneficial properties for the retention of radionuclides. INTRODUCTION In the field of the specific immobilization of some radionuclides, several phosphate-based matrices (Th4(PO4)4P2O7 (β-TPD), β-TPD / monazite composites, monazite / brabantite solid solutions, and britholites) appeared to be promising materials for the immobilization of tri- and tetravalent actinides coming from the reprocessing of spent nuclear fuels and / or nuclear weapons [1-2]. Synthesis and sintering of monazite / brabantite solid solutions as well as britholites were performed using dry chemistry methods. One of the main properties required for such materials concerns their high chemical durability near and far from saturation equilibria. Two approaches combining kinetics (dynamic dissolution tests – multi parametric expression of the normalized dissolution rate (T, pH, medium)) and thermodynamics (identification of the neoformed phases as well as an assessment of the solubility constants) were developed. The retention of radionuclides was thus studied through the neoformed phases precipitating on the surface of the samples (under-saturation experiments). The evolution of the neoformed phases through oversaturation experiments was also examined.
EXPERIMENTAL SECTION The preparation of sintered compounds was developed using dry chemical routes. The reagents were ground mechanically then fired at high temperature (T
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