Characterization of High-Temperature Ceramics for Nuclear Fuels Applications

Ultra-high-temperature ceramic compounds (UHTCs) are a broad class of ceramics that include transition and actinide metal carbides, nitrides, and borides. The physical properties of these materials, such as their elevated melting temperature and enhanced thermal conductivity, enable remarkable resilience against modification in extreme environments. As such, these materials are being implemented in advanced aerospace and energy technologies where they will be exposed to these extremes during operating conditions. Specifically, within the nuclear energy sector, UHTCs such as uranium carbides and nitrides will be used in environments consisting of extreme mixed radiation fields (e.g., fission fragments & neutrons), elevated temperatures (T > 2000K), corrosive conditions, and evolving chemical composition. This work describes the synthesis of both transition (e.g., ZrC) and actinide metal carbides/nitrides (e.g., UC) and the accumulation of structural and chemical modifications under far-from-equilibrium conditions.

Proposed within this research project is the use of advanced experimental characterization approaches including high-resolution synchrotron X-ray diffraction, spallation neutron total scattering, and Raman spectroscopy. Utilizing leading edge Sol-Gel synthesis approaches enabled detailed assessment of structural and chemical characteristics with low impurity contamination, enabling thorough investigations into ion beam induced sample modifications. Despite their structural similarities, the performance of all UHTCs studied under energetic heavy ion irradiation differs greatly. For instance, the unit cell expansion of ZrC follows an unexpected, two-stage profile up to the maximum fluence studied 5×10¹³ cm^-2. In contrast, UC and UN behave in a different manner studied under identical irradiation conditions. UC follows a standard one-stage swelling trend with clear saturation dA/A_0,sat = 15.2 ± 0.1 %, whereas UN does not demonstrate any indication of swelling saturation to the maximum fluence studied 8×10¹³ cm^-2. This result carries significant impact in nuclear fuel-forms research and demonstrates the effect of the inherently complex nature of transition metal and actinide carbides under extreme conditions. Detailed structural studies of the UC and UN samples under both ambient and far-from equilibrium conditions elucidate how the SHI induced defects may lead to defect accumulation/clustering, phase segregation, and chemical modifications (e.g., oxidation).