Study of Actinides Incorporation in Thorium Phosphate-Diphosphate/Monazite Based Ceramics
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Study of Actinides Incorporation in Thorium Phosphate-Diphosphate/Monazite Based Ceramics Nicolas Clavier1, Nicolas Dacheux1, Renaud Podor2 and Philippe Le Coustumer3 1 Groupe de Radiochimie, Institut de Physique Nucléaire, Bât 100, Université Paris-Sud, 91406 Orsay, France 2 LCSM, Université H. Poincaré – Nancy I, BP 239, 54506 Vandœuvre-les-Nancy, France 3 CDGA, Université de Bordeaux I, BP 19, Avenue des facultés, 33405 Talence, France ABSTRACT Phosphate materials are usually considered as potential candidates to perform the immobilization of actinides coming from an advanced reprocessing of spent fuel in the field of an underground repository. Among them, Thorium Phosphate-Diphosphate (TPD) and monazites have been already extensively studied. The elaboration of TPD/monazite based materials was thus envisaged in order to immobilize simultaneously tri- and tetravalent actinides and a neutron absorber. Two chemical ways of synthesis were considered and the compounds were easily prepared in the powder and in the pellet form. A good chemical compatibility was found between TPD and monazite since the properties of both phosphates were kept in these composites. Moreover, the relative density of the pellets reached 90 – 95 % of the value calculated from XRD data. The normalized dissolution rates determined in acidic media did not exceed 5.10-4 g.m-2.day-1 which confirmed the very good durability of such materials during leaching tests. INTRODUCTION Thorium Phosphate-Diphosphate (Th4(PO4)4P2O7, TPD) [1] and monazites (LnPO4) [2,3] were already studied separately as potential matrices for the immobilization of actinides coming from an advanced reprocessing of spent fuel. Indeed, both structures allow the incorporation of large amounts of actinides in their structure since thorium can be replaced by smaller tetravalent cations in TPD (up to 47.6 wt. % for U(IV), 33.2 wt. % for Np(IV) and 26.1 wt. % for Pu(IV)), leading to the formation of Th4-xAnx(PO4)4P2O7 solid solutions [4,5], while monazites accept trivalent actinides (Am, Cm) to form Ln1-xAnxPO4 [6]. Moreover, both solids exhibit similar interesting properties such as an easy preparation in the powder and in the pellet form [7], a good resistance to radiation damages [8] and a strong chemical durability during leaching tests : the normalized dissolution rate at room temperature was found between (5.8 ± 0.3).10-6 g.m-2.day-1 (0.1 M HNO3) and (4.8 ± 0.3).10-8 g.m-2.day-1 (pH = 7) for TPD [9] and between 8.10-7 and 6.10-4 g.m-2.day-1 at 70°C for monazites for pH values ranging from 1.5 to 10 [10]. These values were systematically several orders of magnitude lower than that reported for materials such as borosilicate glasses [10]. Moreover, in the case of initial release of radionuclides from the material, the migration of the actinides to the biosphere will be delayed by the rapid precipitation of neoformed phosphate based phases in the back end of the initial dissolution. On this basis, TPD/Monazite based composites were prepared in order to immobilize simultan
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