Ion Irradiation Studies for the Development of Novel Nuclear Fuels

Following the Fukushima Daiichi accident, important efforts were made to improve the materials used in nuclear reactors, from the fuel/clad combination to structural materials, in order to provide greater tolerance towards accidents. This study discusses the fundamental stability of two types of ceramic materials after irradiation: uranium oxide surrogates with 10 vol. % high thermal conductivity additives (aluminum oxide and silicon carbide) and nitride ceramics (silicon nitride and zirconium nitride). Ion irradiations were performed using 20 MeV Ni⁶⁺ ions for the fuel surrogate samples and 15 MeV Ni⁵⁺ ions for the nitrides ceramics. Midrange (around 3 um) doses varied from 1 to 50 dpa and temperatures from 300 to 700 °C. Thermal conductivity was measured using laser flash analysis and thermoreflectance. Volumetric lattice swelling of the ceramics was determined by grazing incidence X-ray diffraction. Defect production and evolution were tracked using Transmission Electron Microscopy, while nanoindentation was performed to quantify the ceramics’ mechanical properties evolution.

The thermal conductivity of the fuel surrogates showed significant enhancement following sintering as measured by laser flash while novel measurement techniques such as transient grating spectroscopy presented some limitations. Differences in sintering behavior compared to traditional UO₂ were observed and chemical compatibility issues occurred as low as 1150 °C for SiC. Both ceramics presented a decrease of their mechanical properties above 15 dpa. Microcracks due to anisotropic swelling were not observed, but parallel clustered dislocation lines were found adjacent to the second phase grains, suggesting high stresses in the matrix.

Irradiation studies in nitride ceramics helped fill the current gap present in the literature. Coherent behavior with past work was observed on thermal conductivity evolution as well as mechanical properties and defect formation up to 15 dpa and 500 °C. Failure of the sintering aid in Si₃N₄ was observed above these conditions. Different behavior was observed in both nitrides at 50 dpa and 700 °C, where lattice swelling increased pas potential saturation values. Unreported cavity formation was witnessed, in both materials, under all irradiation conditions, with stable number density and slight size increase above 15 dpa and their origin remains to be determined.