Ceramics for the immobilization of plutonium and americium: Current progress of R&D of the VG Khlopin Radium Institute

Since 1990, the Laboratory of Applied Mineralogy and Radiogeochemistry of the V.G. Khlopin Radium Institute (KRI) has been developing several different types of crystalline host-phases acceptable for the economically feasible and environmentally safe immobilization of actinide wastes. We proposed that ceramics that are based on host phases similar to naturally occurring accessory minerals including zircon, (Zr,Hf...)SiO4; hafnon, (Hf,Zr,...)SiO4; baddeleyite (monoclinic zirconia), (Zr,Hf...)O-2; tazheranite (cubic zirconia), (Zr,Hf,Ca,Ti,...)O-2; garnet, (Ca,Fe,Gd,...)(3)(Al,Fe,Si,...)(5)O-12; perovskite, (Ca,Gd,...)(Al,Fe,Ti,...)O-3, and monazite, (La,Ce,...)PO4, are the most efficient materials for actinide immobilization in deep geological formations. Solid solution of Pu in zirconia, (Zr,Pu)O-2, could be used as a ceramic nuclear fuel that is competitive with mixed oxide fuel (MOX). To date, the following crystalline materials doped with Pu-239, Pu-238 and Am-243 have been successfully synthesized and studied at KRI: zircon; hafnon; cubic and tetragonal zirconia; monazite; aluminate garnet and perovskite. The maximum actinide loading was (in wt.% el.): Pu-239-37; Pu-238-10; Am-243-23. All Pu-Am-doped samples were made in air atmosphere under glove boxes conditions. Polycrystalline (ceramic) materials were made by sintering or melting of sol-gel, co-precipitated hydroxides, oxalates, and phosphates or ground oxide precursors; single crystals were grown by a flux method. It was demonstrated that all ceramic samples obtained are characterized by high chemical durability and typical normalized actinide losses in deionized water at 90degreesC do not exceed 10(-2)-10(-3) g/m(2) (without correction for ceramic porosity). However, investigation of long-term behavior of ceramic waste forms requires taking into account the results of accelerated radiation damage study and modeling of ceramic alteration by underground solutions. The principal features of Pu-Am-doped samples obtained so far at KRI and their synthesis conditions are discussed.