Among 4th Generation Nuclear Reactors, Lead-cooled Fast Reactors represent one of the most promising concepts. Lead stands above all metals because, ideally, it can solve issues associated with other types of coolant, including sodium (strong chemical reactivity) or lead-bismuth eutectic (activation under irradiation). Nevertheless, the bottleneck is lead aggressiveness towards structural steels, especially above 500-550 °C. Over this temperature range, which will be reached by in-core components, traditional materials cannot withstand erosion degradation and corrosion. To allow the development and construction of this new generation reactors it is necessary to protect structural steels from these phenomena. One possible solution is treating the material to obtain a surface layer able to survive operational conditions. A successful example is the aluminizing surface treatment, together with the controlled injection of oxygen into the coolant. This guarantees the formation of a protective alumina scale. However, the treatment has some drawbacks, such as fretting corrosion, low cycle fatigue and radiation-induced phase instability. A promising alternative consists in covering metals surface with ceramic coatings. Specific requirements must be achieved: high density and compactness, strong interfacial bonding, wear resistance and mechanical compatibility with steels. For this purpose, Pulsed Laser Deposition appear as the most suitable technique. In addition, it offers different others advantages: it can be carried out at room temperature and on every types of materials. An extensive characterization campaign has shown that PLD-grown Al2O3 is a perfect candidate for protecting steels in HLM environment. Previous works have also demonstrated strong adhesion, combined with a mix of metal- and ceramic-like properties. On the other hand, some of the main issues regarding the use of such coating concern the lack of self-healing properties, creep strength and resistance to thermal cycling, especially in the case of films grown on non-planar surfaces. Thus, further improvements are needed like the possible inclusion of a metallic bonding layer inside the system.
Tra i diversi prototipi presenti in Generation IV, il reattore veloce raffreddato a Piombo si pone come il più promettente. La scelta del Piombo come termovettore potrebbe risolvere tutti i problemi associati all’uso di altri metalli come l’instabilità chimica o l’attivazione sotto irraggiamento. Tuttavia, l’utilizzo del piombo ad oggi non è stato possibile per la sua aggressività nei confronti degli acciai strutturali dai 500 °C. Sopra questa temperatura, raggiunta all’interno del core, i materiali strutturali non sono in grado di resistere alla corrosione ed erosione. Per permettere l’impiego del piombo liquido è necessario proteggere le superfici con cui il piombo entra in contatto. Pertanto, uno dei possibili rimedi consiste nel trattare superficialmente i materiali interessati. In tal senso, il processo di alluminazione insieme al controllo dell’ossigeno nel termovettore porta alla formazione di un efficace strato di allumina. Allo stesso tempo questo tipo di film presenta alcuni svantaggi quali la scarsa resistenza al fretting e alla rottura per fatica, nonché la creazione di specie secondarie sotto irraggiamento. Un’interessante alternativa consiste nel ricoprire l’acciaio con rivestimenti ceramici. Per ottenere un rivestimento efficace, il film deve essere il più denso e compatto possibile, ben adeso, compatibile con il substrato e resistente ai fenomeni sopracitati. La deposizione laser-impulsato (PLD) oltre a produrre film di questo tipo presenta il vantaggio di poter essere condotta a basse temperature e su qualsiasi tipo di substrato. Diversi studi hanno dimostrato l’efficacia dell’allumina per PLD nella protezione degli acciai in contatto con piombo fluente. Il materiale possiede inoltre notevoli proprietà fisiche conferite da un comportamento intermedio ceramico/metallico. Rimangono comunque alcune perplessità in merito al suo utilizzo dovute principalmente alla mancanza di self-healing oltre che al comportamento sotto creep o ciclaggio termico, soprattutto nel caso di substrati non planari. Ulteriori studi sono richiesti per qualificare questo tipo di materiale, valutando ad esempio l’impiego di un bonding layer metallico.
Ceramic coated fuel claddings for lead-cooled fast reactors
VANAZZI, MATTEO
2014/2015
Abstract
Among 4th Generation Nuclear Reactors, Lead-cooled Fast Reactors represent one of the most promising concepts. Lead stands above all metals because, ideally, it can solve issues associated with other types of coolant, including sodium (strong chemical reactivity) or lead-bismuth eutectic (activation under irradiation). Nevertheless, the bottleneck is lead aggressiveness towards structural steels, especially above 500-550 °C. Over this temperature range, which will be reached by in-core components, traditional materials cannot withstand erosion degradation and corrosion. To allow the development and construction of this new generation reactors it is necessary to protect structural steels from these phenomena. One possible solution is treating the material to obtain a surface layer able to survive operational conditions. A successful example is the aluminizing surface treatment, together with the controlled injection of oxygen into the coolant. This guarantees the formation of a protective alumina scale. However, the treatment has some drawbacks, such as fretting corrosion, low cycle fatigue and radiation-induced phase instability. A promising alternative consists in covering metals surface with ceramic coatings. Specific requirements must be achieved: high density and compactness, strong interfacial bonding, wear resistance and mechanical compatibility with steels. For this purpose, Pulsed Laser Deposition appear as the most suitable technique. In addition, it offers different others advantages: it can be carried out at room temperature and on every types of materials. An extensive characterization campaign has shown that PLD-grown Al2O3 is a perfect candidate for protecting steels in HLM environment. Previous works have also demonstrated strong adhesion, combined with a mix of metal- and ceramic-like properties. On the other hand, some of the main issues regarding the use of such coating concern the lack of self-healing properties, creep strength and resistance to thermal cycling, especially in the case of films grown on non-planar surfaces. Thus, further improvements are needed like the possible inclusion of a metallic bonding layer inside the system.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/112082