Radiation tolerant nanoceramic coatings for lead fast reactor nuclear fuel cladding

Over the past decade, several advanced reactor concepts have been explored as the potential eet of next-generation nuclear systems -else known as Generation IV (GIV). Among these systems, lead fast reactors (LFRs) are particularly interesting owing to inherent safety features and to possible con gurations as nuclear waste incinerators or adiabatic reactors that burn self-generated waste. The major bottleneck for the development of LFRs regards the quest for suitable structural materials. Some of the most prominent issues in this regard are common to nuclear energy systems in general, and are typically found in high-level radiation damage GIV concepts. Anyhow, the most important problem is speci cally related to the use of lead as a coolant- namely, corrosion degradation of steels. Thus, the very feasibility of LFRs ultimately depends on the development of high-performance materials that are capable of withstanding high-levels of radiation damage in an extremely corrosive environment. While self-passivation through oxygen injection and control is an e ective corrosion protection method up to 500 C, other strategies are required for protecting components in service at higher temperatures -namely, fuel cladding. Most of these options are incompatible with or require the injection of considerable amounts of oxygen into the coolant. On the contrary, some types of oxide coatings are both independent from and compatible with oxygen injection, which might be necessary anyhow for protecting components which operate below 500 C. In this case, the coatings would allow decoupling the problem of corrosion protection at low and high temperatures. Anyhow, coatings have always been regarded as a taboo in the nuclear eld, mainly due to the lack of self-healing properties, among other intrinsic drawbacks. The lack of self-healing properties is a daunting problem. On one hand, fuel cladding is exposed to an extremely harsh environment, in which the combination of high temperatures with an intense neutron radiation eld ultimately results in ever growing stress and strain. On the other hand, the structural integrity of the coatingsubstrate system as a whole must be guaranteed at all times, meaning that a coating must be able to withstand high-level radiation damage, and to accommodate the stresses and strain imposed by fuel cladding. In this framework, this Ph.D. Thesis deals with the design, the processing, the characterization and the testing under LFR-relevant conditions of a corrosion resistant and radiation tolerant nanoceramic coating as an enabling technology. In order to meet requirements, a custom process -namely room temperature PLD- is proposed according to a bottom-up approach. The main advantage of PLD is that it allows tailoring the structural features and the mechanical properties of the coatings through nano-scale engineering. The process is rst optimized for coating plates of the steels of interest, and is then upgraded to coat tubes up to 10 cm long, proving that an industrial scale-up is possible in principle with proper engineering. Recent studies have highlighted the outstanding radiation tolerance and mechanical properties iii Contents iv of nanomaterials. Following these results, the coating -namely, a bi-phase Al2O3 nanocompositeis designed to evolve towards a nanoceramic structure during irradiation in-service. This structure is meant to overcome the major intrinsic problems associated with the use of Al2O3 as the coating material, such as void swelling and low fracture toughness, among others. The characterization of the coating in the as-deposited state is performed by a wide variety of techniques, including SEM, TEM, XRD, nanoscratch, nanoimpact, microindentation, microscratch. Overall, the coating attains an unusual ensemble of metal-like mechanical properties which can be explained in terms of structural features. Detailed analyses by TEM reveal that the nanostructure of the coating consists of a homogeneous dispersion of randomly-oriented -Al2O3 nanodomains in an amorphous Al2O3 matrix. Importantly, this nanostructure is shown to be stable upon annealing up to 800 C. The remaining analyses show that the fracture strength and the adhesive strength are remarkable, and that the latter is likely higher than the former. The e ectiveness of the coating is evaluated under LFR-relevant conditions with speci c concern for corrosion and high-level radiation damage. As a general statement, PLD-grown Al2O3 performs e ectively as a corrosion barrier under the investigated conditions -namely, short tests at 550 C and 600 C in stagnant lead. The evolution of structural features and mechanical properties following heavy ion irradiation up to 20, 40, 150 and 450 dpa is analyzed by SEM, TEM, XRD, nanoindenation, nanoimpact, microindentation and microscratch. The analyses reveal that the changes in the mechanical properties of the coating are due to athermal radiation-induced crystallization, which drives the evolution of the initially bi-phase nanocomposite coating towards a radiation tolerant nanoceramic structure. This statement is supported by the total absence of void swelling, which in turn is observed in irradiated sapphire. A model of evolution is proposed to correlate structural features and mechanical properties with the prevailing deformation modes at di erent radiation damage levels. Importantly, the exposure to end-of-life levels of radiation damage does not compromise the structural integrity of the coating-substrate system. In conclusion, PLD-grown Al2O3 is a suitable and promising coating material for LFR nuclear fuel cladding.