Radiation-Induced Thermal Conductivity Degradation in SiC, ZrC and ZrN Ceramics Cladding Studied via Transient Grating Spectroscopy

Silicon carbide (SiC), zirconium carbide (ZrC), and zirconium nitride (ZrN) are attractive candidates for nuclear cladding materials due to their excellent thermal stability and corrosion resistance at high temperatures. SiC and ZrC can be used as a fission product barrier coating in TRISO fuel, while ZrN is being considered as a coating on Zirconium-based alloy for accident-tolerant fuels. However, the thermal conductivity of coatings can degrade under irradiation, which impacts fuel performance.

To assess this performance, SiC was irradiated with Si ions, and ZrC and ZrN with helium ions at 300°C and 700°C, over a dose range of 0.01 to 1 displacement per atom (dpa). Transient Grating Spectroscopy (TGS) was employed to measure thermal diffusivity and elastic modulus in the irradiated surface layers. The results showed significant reductions in thermal conductivity (>50%) at low doses, starting from 0.01 DPA for SiC, 0.5 DPA for ZrN, and 1 DPA for ZrC at 300°C. At 700°C, the conductivity degradation was much lower. To further understand the defects driving these changes, grazing incidence X-ray diffraction (GIXRD), Raman spectroscopy, and transmission electron microscopy (TEM) were used. The analysis showed that point defects were the main contributors to thermal conductivity degradation at 300°C, preceding the formation of extended defects. This research offers the first detailed insights into low-dose radiation effects on thermal conductivity, particularly for ZrC and ZrN, validating the use of ion irradiation and TGS as effective methods to predict the thermal behaviour of nuclear ceramics under irradiation.