Synthesis and Characterization of Cs-Bearing Apatites
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INTRODUCTION The French Act, dated 30 December 1991, related to "research concerning the elimination of radioactive wastes", defines the research programs including the study of new conditioning processes and long-term surface storage of these wastes. The research of new conditioning processes mainly concerns separated long-lived radionuclides. Among radioactive elements in high-level radioactive wastes, cesium is one of the radioelements which need to be confined in specific matrix. This element has two isotopes of concern, 135Cs (2.3 millions years of period) and ' 37Cs (30 years of period with a very high thermal power (0.07W/g)). This implies that the matrix for cesium immobilization must have a high leaching resistance and have to be thermally stable. Its vaporization during matrix elaboration is a serious problem. This work concerns cesium immobilization into an apatitic structure. Apatitic structure minerals form a large family of compounds, natural or synthetic isomorphs, that generally crystallize in the hexagonal system (spatial symmetry group P63/m). They have the general chemical formula Me10(XO4 )6Y2 where Me generally represents a bivalent cation, (XO4 ) a trivalent anion and Y a monovalent anion. A well known representative term is the fluorapatite Ca 10(PO4 )6F2. Numerous elements can be substituted in the apatitic lattice and in this structure, cesium is substituted in the calcium site. Based on geological observations, the idea for using a nuclear waste disposal medium
based on synthetic analogue of natural apatite is being evaluated at the CEA (Commisariat d l'Energie Atomique) (1-6). One of the most convincing demonstrations of the stability of the apatite lattice is provided by apatites that have incorporated significant amounts of actinides and fissiogenic products several millions years ago (7,8). In Oklo reaction zones, cesium has been confined into apatites : in fact, its radioactive descendant, "'Ba can be found in theses structures (9). This work concerns the incorporation of cesium into britholites by synthesis at high temperatures below the volatilization temperature of cesium (10). Densification of this material allows on the one hand cesium waste's volume reduction and on the other hand chemical inertia.
EXPERIMENTAL We studied the solid solution CaLnYCS,(PO4)t(SiO4)1.X with Ln--trivalent cation, X=2F', 02., FO 1, 2. Ratios between coefficients are following : 2
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