Inert matrices for the transmutation of actinides: fabrication, thermal properties and radiation stability of ceramic materials

In this study, ceramic binary or ternary compounds were tested for their suitability to serve as inert matrices for transmutation of the minor actinides Np and Am, i.e. to achieve a reduction in their quantities by irradiation in nuclear reactors without formation of new actinides from the matrix. Five materials (MgAl2O4, ZrSiO4, CeO2, SiC and Si3N4) were selected and produced without and with Am-241 using the well known soaking method. Special emphasis was put on the characterization of the materials and the measurement of thermal properties like heat capacity and thermal diffusivity. Irradiation damage was investigated by ion implantation experiments. X-ray diffraction measurements of MgAl2O4, CeO2 and ZrSiO4 were performed on the as-produced specimens to provide the basis for evaluating expected lattice parameter changes during self-irradiation by a-decay in future experiments. In addition, ceramic samples containing 20 wt % UO2 were produced to demonstrate that matrix materials can accept large amounts of an oxide similar to americium oxide. An assessment of thermodynamic data for the systems SiC + UO2 and Si3N4 + UO2 was used to calculate pseudobinary phase diagrams. Structural, thermal and thermodynamic aspects of all selected matrices, and the reasons for their selection are discussed. (C) 1998 Elsevier Science S.A.