Finite Element Analysis of the Residual Stresses Arising During the Fabrication of TRISO Coated Nuclear Fuel

TRistructural ISOtropic coated particle fuels (TRISO CPF) have been developed as a possible fuel solution for high temperature nuclear reactors, which offer the possibility of nuclear cogeneration-powered industrial facilities and hydrogen production.<br/>One of the main characteristics of TRISO fuel is the presence of many layers of very different materials, such as uranium dioxide in the kernel, multiple types of pyrolytic carbons and silicon carbide. Each of the layers has its purpose and the multiple layer approach is adopted to minimise the risk for fission product containment failure, but the materials in TRISO can be very heterogenous when their thermomechanical properties are concerned. This is most relevant during the fabrication process, as the chemical vapour deposited (CVD) layers require very high and variable temperatures for their formation. The difference in the materials expansion coefficients and elastic moduli generates residual thermal stresses in the particle during the cool down phase to room temperature, which precedes their use in a reactor.<br/>An “ABAQUS” finite element model was built to reproduce the fabrication process of TRISO particles adopted in various HTGR programs and mock-up particles for testing. Understanding the initial stress and bonding state of the layers is crucial for predicting performance and failure in the reactor. The simulations revealed the presence of important stress distributions in the particles after fabrication, with high values of tensile hoop stresses in the inner fuel kernel (up to 250 MPa) and even higher compressive hoop stresses (up to 600 MPa) in the silicon carbide layer. The effect of thermal creep in uranium dioxide and silicon carbide during fabrication of the particle and the fuel elements has also been investigated, and it showed an overall increase in the stress values in each layer of the particle. This is most evident if annealing processes at 1950°C for up to 18 hours are applied to the final product. The reason for this is related to the creep-driven dimensional change occurring at high temperatures and stresses, which eventually leads to greater cooldown strains when the temperature returns to low values. Due to the assumption of perfectly bonded layers used in this work, the results do not exclude the possibility for the layers to debond (delamination) before the particles are used in a reactor. However, the behaviour of inter-layer interfaces is the subject of current research at the Department of Materials of Imperial College London, and this work represents a preliminary step in the development of an improved Peridynamics model for TRISO.