Immobilization of separated actinides in apatites

Natural apatites provides a means of assessing the long-term performance of actinides host phases in high-level nuclear waste forms. Based on geological observations, the idea for using a nuclear waste disposal medium based on a synthetic analogue of natural mineral has been evaluated. A preliminary study on natural britholites coming from Algerian Hoggar has been carried out. It has shown that these minerals are able to incorporate large quantities of U, Th, Pu and fission rare earths in the crystal lattice. Based on these geological observations and on the well-known physico-chemistry of apatites, we have identified the structural formula for a potential conditioning matrix: Ca9Nd(SiO4)(PO4)5F2 in which actinides wastes (trivalent or tetravalent) will be introduced in partial substitution of neodymium. We have then developed an elaboration process based on the synthesis of britholic ceramics. The total substitution of SiO4 by PO4 in the apatic lattice appears to be possible. The X-ray diffraction confirms that a solid solution exists between the two limit compounds: Ca10-xLnx(SiO4)x(PO4)6-xF2 with x between 0 and 6. This solid solution follows the Vegard's law. Chemical immobilization occurs through process of atomic scale fixation of the radionuclide as this latter participate to the mineral stoichiometry. This participation modifies the cell parameters which increase with the simultaneous introduction of lanthanide and silicate ions