GREENCDT Development of High Density/High Conductivity Fuels Concepts

Advanced nuclear fuels are seeking to improve upon the current UO₂ fuel material by providing increased thermal conductivity and uranium density. Increasing thermal conductivity in fuel material reduces maximum fuel temperatures, improving power-to-melt ratio and reducing fission gas release. Increasing uranium density improves fuel cycle length or allows for reduced enrichment. Several candidates exist, including uranium silicide, uranium nitride, uranium carbide and uranium boride compounds. However, a number of technical challenges around these materials exist, including knowledge on material properties as fabrication is scaled up, knowledge on fundamental properties at operating conditions, required fuel forms (monolithic, dispersion etc) and corrosion behaviour. This project will initially seek to identify suitable candidate fuel materials in an appropriate form (building on a current CDT project on clad-fuel interaction) before establishing an understanding of fuel behaviour under simulated operating conditions. Potential areas of exploration include;

• How variations in fabrication affect material properties and performance – many of the proposed materials are difficult to sinter conventionally, due to their high melting point, and affinity for forming oxide surface layers. The use of advanced manufacturing techniques (flash sintering, Spark Plasma Sintering etc) can produce high-density material, but may lead to inhomogenity if for example multiple pellets are fabricated simultaneously in an industrial setting. This is likely to lead to variations in material microstructure, thermal properties and impurity concentration across a batch and understanding these is crucial to establishing industrial-scale fabrication routes.
• Fundamental properties – several candidates are poorly understood, and establishing fundamental properties is necessary to understand if potential materials provide sufficient benefit to justify further investigation.
• Suitable fuel forms – Although current fuel is generally deployed as monolithic ceramic pellets contained in a metallic clad, a wide range of potential forms have been considered, which has a significant impact on other areas of interest. In consultation with Industrial Supervisors, potential forms of interest can be identified and their feasibility studied beyond the fundamental materials properties. For example the use of a UN dispersion fuel could lead to significant variations in as-fabricated properties than monolithic UN, particularly if advanced fabrication technology is also employed.