Multifunctional aluminium oxide coatings for heavy liquid metals applications in lead-cooled fast reactors

Among all the innovative Generation IV concept reactors, Lead Fast Reactor (LFR) represents one of the most suitable and promising projects due to its intrinsic safety and the possibility to be employed in breeding, as well as burning configuration. Corrosion of structural steels appears to be the main bottleneck to the engineering feasibility of this nuclear systems, preventing their realization. The same problem concerns other applications involving the use of liquid metals, such as fusion reactors and accelerator driven systems. In order to solve the issue, from all the possible solutions, the PLD-grown alumina coating is one of the best choices. This nano-ceramic coating is characterized by chemical inertness at high temperatures, good mechanical compatibility and adhesion with steels and very high hardness and H/E ratio. Moreover, its good radiation resistance (due to the amorphous matrix and low neutron cross-section) makes alumina a perfect candidate. Most of the described properties of alumina are lost when the material starts to become crystalline, so it becomes essential to study how to stop or at least slow down the transformation process. Yttrium doping was chosen as a first step to understand the retardation effect of elements close to non-solubility in alumina. In this work, first of all, the thermal and radiation induced crystallization processes in undoped/doped alumina PLD-grown thin film, have been studied, by X-ray diffraction and TEM images analysis. TEM study allowed to find different models, to understand the grain growth process under irradiation in thin film. Then, by means of an accelerated deformation test, the adhesion and mechanical properties of alumina thin film have been studied in cylindrical configuration, to be able to imitate what would happen in a fuel rod. Nanoindentation and SEM analysis were used to understand deeper. the effects of Y in the PLD-grown alumina thin film. Beneficial effects of Yttrium have been demonstrated in several aspects cited above, only nanoindentation showed a little decrease in film hardness.

Tra tutti i nuovi prototipi di reattori nucleari, proposti dal Generation IV International Forum, quelli che utilizzano come mezzo termovettore il piombo fuso, risultano essere tra i più promettenti, per quanto riguarda l'efficienza e la sicurezza intrinseca. I metalli pesanti e quindi anche il piombo, allo stato liquido, producono effetti catastrofici a livello corrosivo. Per questo motivo tra tutte le possibili soluzioni, in questa tesi, si è discusso, l'utilizzo di film sottili di ossido d'alluminio, ottenuti per mezzo della pulsed laser deposition. Questo materiale presenta incredibili proprietà termomeccaniche e di resistenza alla radiazione legati alla sua microstruttura amorfa, ingegnerizzata per avere una dispersione omogenea di domini nanocristallini. Si è per cui studiato il processo di cristallizzazione indotto termicamente, per mezzo di analisi XRD e quello indotto da irraggiamento, per mezzo di analisi TEM, in film di allumina pura e di allumina dopata Yttria. La scelta di utilizzare Y come elemento dopante deriva dagli studi in letteratura, in cui è descritto essere in grado di ritardare il processo di cristallizzazione. Le proprietà meccaniche e morfologiche sono state studiate per mezzo di test di deformazione accelerata, nano-indentazione e microscopia elettronica.