MATRICES FOR ACTINIDE WASTE IMMOBILIZATION IN THE CHEMICAL SYSTEM LN2O3–ZRO2–TIO2

Spent nuclear fuel (SNF) and high-level waste (HLW) from reprocessing of the SNF have high ecological threat. There are two broad solutions to the HLW problem. The first is to separate the long-lived nuclides of actinides and fission products from the waste, followed by neutron irradiation in order to transmute to stable or short-lived isotopes (partitioning and transmutation). The second approach is to separate actinides and fission products from HLW-waste streams and incorporate them into robust waste forms (matrices) for ultimate disposal in a deep underground repository (partitioning and conditioning). The main characteristics of the matrices are high loading of the waste, low leach rates of radionuclides in the presence of water and feasibility of industrial fabrication. Promising matrices for the isolation of the REE-actinide fraction are to be found in the system: Ln₂O₃–ZrO₂–TiO₂ (Ln – lanthanides). The properties of phases in this system are reviewed, including: isomorphism and compositions of the solid solutions, behavior under irradiation and stability in the presence of water, and the feasibility of synthesis by different methods. Important among these phases are the isometric pyrochlore-type structure with Ln₂(Ti,Zr)₂O₇ composition, monoclinic (Ln₂Ti₂O₇) and orthorhombic (Ln₄Ti₉O₂₄) titanates. These matrices accommodate from 45 to 65 wt.% of the waste and are appropriate for disposal in deep boreholes due to the high temperatures due to radioactive decay.