Highly Dense Graphite Matrix: A New Material for the Conditioning of Radioactive Wastes

In continuation of 20 yr of experience with high-temperature gas-cooled reactor graphite fuel element fabrication (molded fuel spheres and monolithic block fuel elements), NUKEM introduced a new graphite-based material with an inorganic binder produced by molding. A mixture of nickel and sulfur is used as the inorganic binder for natural graphite powder. The fabrication is facilitated by the low temperature of 500°C in generating a chemically and thermally stable nickel sulfide. The newly developed material is suitable as a matrix for the fixation of high-active waste and as a corrosion-resistant layer of the steel supporting tube for final disposal of spent fuel elements. The material of natural graphite/nickel sulfide is distinguished by high density, high corrosion resistance, a low leaching rate, good thermal conductivity, an appropriate coefficient of thermal expansion, and high thermal stability due to the high melting point of nickel sulfide ( 790°C). Currently, a pilot plant for fabrication of containers for spent fuel elements is being erected. Additionally, development work is in progress to obtain basic design data for a high-level waste fixation facility.