Oxidation and Irradiation Damage of Carbide Materials for Accident Tolerant Nuclear Fuel Cladding

The inherent weakness of zirconium (Zr) based nuclear fuel cladding has been shown when performing outside of standard operating conditions such as a loss of coolant accident (LOCA). These vulnerabilities were showcased during the Fukushima Daiichi incident in Japan 2011. The development of accident tolerant fuel (ATF) cladding has been proposed as a method of mitigating the risks of such accidents. ATF cladding concepts include the complete redesign of fuel cladding, or, the application of surface coatings which act as a barrier to oxidation during accident scenarios. In this thesis several novel carbide materials have been investigated to determine their suitability as ATF cladding materials. Experiments investigating the oxidation resistance of Ti2AlC, (Nb,Ti)2AlC and SiC under environments imitating a LOCA is presented in Chapter 3. The oxidation testing was conducted using steam following a temperature regime beginning at low temperatures and increasing to 1100˚C. All three samples demonstrated superior oxidation resistance in comparison to Zircaloy-4 which is currently used in light water reactors (LWR). The oxidation resistance of Ti2AlC was found to be poor in comparison to experiments conducted in literature at both higher temperatures and oxidation times. It is believed that the increased oxide thickness results from the formation of less protective α-TiO2 layers during the initial low temperature oxidation rather than a protective α-Al2O3. (Nb,Ti)2AlC was found to have a lower weight gain in comparison to Ti2AlC due to the formation of a protective NbAlO4 oxide layer. SiC was found to have the lowest weight increase following this oxidation regime due to the production of an amorphous SiOx layer. Chapters 4 and 5 investigate the effect of irradiation induced damage on nanoscale Al-NbC coating layers deposited on Si substrates. Ion beam irradiations were conducted at room temperature and 450˚C. Following room temperature irradiations, crystalline Al and NbC were detected using grazing incidence X-ray diffraction (GIXRD), with amorphous regions containing a mixed AlNb structure found using transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) in conjunction with energy dispersive X-ray (EDX) chemical analysis. Following high dose irradiation, interfaces became less distinct, and the intermixing between coating layers was found using EDX line scans. It is suggested that the irradiation caused the removal of carbon (C) atoms from the NbC layer, which were subsequently implanted into the NbC-Si interface. High temperature irradiation also caused the breakdown of coating interfaces, with the addition of a phase transformation creating Al3Nb. Chapter 6 investigated the swift heavy ion irradiation of 6H-SiC using 93.6MeV lead (Pb) ions. Following irradiations, crystallinity was conserved, however, Raman spectroscopy detected Si-Si, disordered Si-C and C-C bonding which was not detected in pristine 6H-SiC. TEM imaging found the existence of stacking faults in the material. However, no evidence of single ion tracks was found