Synthesis, Characterization, Sintering and Leaching of Thorium - Uranium (IV) Phosphate Diphosphate Solid Solutions from
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Synthesis, Characterization, Sintering and Leaching of Thorium – Uranium (IV) Phosphate Diphosphate Solid Solutions from Low-Temperature Precursors Nicolas Dacheux1, Nicolas Clavier1, Renaud Podor2 and Philippe Le Coustumer3 1 Radiochemistry Group, IPNO, Bât. 100, University of Paris-Sud-XI, 91406 Orsay, France 2 LCSM, University of H. Poincaré – Nancy I, BP 239, 54506 Vandœuvre les Nancy, France 3 CDGA, University of Bordeaux I, BP 19, Avenue des facultés, 33405 Talence, France
ABSTRACT In the field of the immobilization of high level activity and long life radwaste for a deep underground repository, several phosphate matrices were already proposed to delay the release of actinides to the biosphere. Among them, Thorium Phosphate-Diphosphate (TPD) and associated solid solutions were considered. In order to improve the homogeneity of the samples prepared, a new way of preparation based on the precipitation in closed containers, at 150°C, of the Thorium-Phosphate HydrogenPhosphate hydrate (TPHP) or associated Thorium – Uranium (IV) Phosphate HydrogenPhosphate hydrate (TUPHP) was developed. Well crystallized and single phase TUPD solid solutions of general formula Th4-xUxP6O23 were prepared after heating above 950°C. Dense TUPD pellets were also obtained through a two-steps procedure involving a uniaxial pressing of the powder at room temperature then a heating treatment at 1250°C for 10 hours. Several properties of the pellets such as their chemical durability during leaching tests were considered carefully. All the normalized dissolution rates of powdered and sintered TUPD solid solutions remained low by comparison to that reported for several other ceramics studied in the same objective.
INTRODUCTION On the basis of several interesting properties, phosphate matrices like britholites Ca9Nd(PO4)5(SiO4)F2 [1], monazites MIIIPO4 [2] and associated brabantites MIIMIV(PO4)2 [3], sodiumzirconium phosphate NaZr2(PO4)3 (NZP) [4], or thorium phosphates like Th4(PO4)4P2O7 (TPD) [5] could be potential candidates for the immobilization of actinides coming from an advanced reprocessing of spent fuel or for the final disposal of non recyclable plutonium. Indeed, phosphate materials and minerals are usually considered to be very low-soluble [6,7] (we can report, for example, the good retention of thorium and uranium in phosphate ores containing monazites, brabantites or apatites) and appear generally resistant to radiation damages [1,2]. In this aim, Thorium Phosphate Diphosphate (Th4(PO4)4P2O7, TPD) was already prepared through several ways of synthesis [8]. Several reasons led us to propose this material for the immobilization of tetravalent actinides (U, Np, Pu). As the largest tetravalent cation of the periodic table, Th4+ (VIIIrcat.= 1.05 Å) could be easily replaced by one of the other smaller tetravalent actinides (VIIIrcat. equal to 1.00 Å, 0.98 Å and 0.96 Å for U, Np and Pu, respectively) [9]. Moreover, this material is very easy to prepare in the powder and in the pellet form [10] and is highly resistant to aque
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