Processing and modelling of non-stoichiometric zirconium carbide for advanced nuclear fuel applications

The properties of zirconium carbide are of interest for nuclear reactor core material applications, notably as the fission product barrier layer in TRi-structural ISOtropic (TRISO) coated fuel particles. It has been found to be mechanically superior to its more studied competing candidate SiC, capable of resisting higher temperature operational regimes for extended periods of time and having a higher neutron transparency. The situation regarding ZrC fission product retention capabilities has yet to be fully understood. Diffusion properties of fission products over the wide range of possible compositions (ZrC0.5-1.0) have not been comprehensively investigated and many gaps remain. Processing techniques for producing ZrC are generally time consuming and/or prone to O contamination. The reactive spark plasma sintering (RSPS) technique was applied to attempt rapid production of ZrC pellets with varied stoichiometric composition and low O contamination. It combines a reaction via the carbothermic reduction of ZrO2 by C and immediately followed by a high temperature and pressure sintering phase. The reaction phase was observed to be time and temperature-dependent and indifferent to applied pressure. With a reaction temperature of 2100 C it was possible to synthesise pellets within a 30 min treatment, a vast improvement over the typical carbothermic reduction time frames of typically more than 6 h. Oxygen contamination proved hard to completely eliminate in a single step process. Using density functional theory (DFT) both intrinsic and extrinsic defect structure formation energies and migration barriers were calculated. Vacancy-vacancy interactions were studied by modelling C vacancy pairs VC, it was confirmed VC will avoid coordinating on either side of a Zr atom. Carbon interstitial migration was shown to have a relatively low migration barrier provided the C was located as a trimer. Study of fission product atoms in ZrC revealed a preference for incorporating onto a vacant Zr lattice site and a strong affinity for clustering with VC. The high binding energies between Ag, Ba and Cs on a Zr lattice site with a neighbouring VC may in part explain why these are the fission products that are best retained by ZrC. The hop barrier for RuC to a neighbouring VC was found to be almost negligible suggesting a potentially fast VC mediated rapid diffusion path.Open Acces