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AbstractAbstract
[en] In the framework on 4. generation reactors, the CEA is developing new grades of austenitic steels that will be usable, for example, for the cladding of fuels for sodium-cooling fast neutron reactors (RNR-Na). Thanks to their excellent mechanical properties and good corrosion resistance, they can be used up to 100 dpa, although their service life may be limited by the phenomenon of swelling under irradiation. Swelling is due to the formation of cavities in the material following irradiation and can cause geometric deformations and weaken the fuel claddings. The reference alloy, developed thanks to previous R and D on French RNRs, is an austenitic 15Cr/15Ni titanium stabilized steel called AIM1. This work focuses on studying and understanding the mechanisms leading to the formation of cavities under irradiation to contribute to the development of a more swell-resistant AIM2 grade. Different chemical and microstructural optimizations were investigated using an analytical approach. Three model alloys were used to study the double stabilization of titanium and niobium and several model microstructures were defined to highlight the role of microstructural parameters influencing swelling (dislocations, solutes, nanoprecipitates). Characterizations by SEM, DRX and DNPA have allowed a better understanding of the microstructural evolutions of the three grades, model microstructures and also to study their ability to form a fine network of nanoprecipitates. Very high-dose irradiations with Fe3+ ions (2 MeV and 10 MeV) to induce the formation of cavities have highlighted the major role of microstructure on swelling resistance. A new methodology for the study of swelling induced by ion irradiation has been proposed. It allows a statistical study of cavity formation and is based on the use of scanning microscopy. Indeed, the new detectors can acquire high definition images that can contain several thousand cavities on the same micrograph. These images are then analyzed using a supervised learning artificial intelligence algorithm to automatically recognize the cavities but as well as different objects present in the microstructure (precipitates, grain joints, etc.). An example of a study of the effect on the swelling of the irradiation damage gradient, characteristic of heavy ion irradiation, is presented as an illustration of this methodology called MEBIA. Cluster dynamic calculations simulated the impact of nanoprecipitates and the initial density of dislocations on swelling. These results inspired the creation of new microstructures that were irradiated and began to be characterized. This work will have to be continued to validate the relevance of optimized microstructures. Results presented in this manuscript illustrate the difficulties encountered in studying the microstructures of austenitic steels irradiated at very high doses, but it shows that new approaches can also be put in place to facilitate this work. (author)
[fr]
Dans le cadre des recherches sur les reacteurs de 4eme generation, le CEA developpe de nouvelles nuances d'aciers austenitiques qui seront utilisables, par exemple, pour le gainage de combustibles de reacteurs a neutrons rapides a caloporteur sodium (RNR-Na). Grace a d'excellentes proprietes mecaniques et une bonne resistance a la corrosion, ils sont utilisables jusqu'a une centaine de dpa, meme si leur duree de vie peut etre limitee par le phenomene de gonflement sous irradiation. Le gonflement est du a la formation de cavites dans le materiau suite a l'irradiation et peut provoquer des deformations geometriques et fragiliser les gaines de combustible. L'alliage de reference, developpe grace aux precedents travaux sur les RNR francais, est un acier austenitique 15Cr/15Ni stabilise au titane appele AIM1. Ce travail porte sur l'etude et la comprehension des mecanismes menant a la formation des cavites sous irradiation pour contribuer au developpement d'une nuance AIM2 plus resistante au gonflement. Differentes optimisations chimiques et microstructurales ont ete etudiees en adoptant une demarche analytique. Trois alliages modeles ont permis d'etudier la double stabilisation titane et niobium et des microstructures modeles ont ete definies pour mettre en evidence le role de parametres microstructuraux influencant le gonflement (dislocations, solutes, nano-precipites). Des caracterisations par MEB, DRX et DNPA ont permis une meilleure comprehension des evolutions microstructurales des trois nuances et des microstructures modeles et aussi d'etudier leur capacite a former un fin reseau de nano-precipites. Des irradiations a tres hautes doses aux ions Fe3+ (2 MeV et 10 MeV) pour provoquer la formation de cavites ont permis de mettre en avant le role preponderant de la microstructure sur la resistance au gonflement. Une nouvelle methodologie de l'etude du gonflement induit par l'irradiations aux ions a ete proposee. Elle permet une etude statistique de la formation des cavites et repose sur l'utilisation de la microscopie a balayage. Les nouveaux detecteurs permettent en effet l'acquisition de cliches haute definition pouvant contenir sur une meme micrographie plusieurs milliers de cavites. Ces cliches sont ensuite analyses a l'aide d'un algorithme d'intelligence artificielle a apprentissage supervise pour reconnaitre automatiquement les cavites mais egalement differents objets presents dans la microstructure (precipites, joints de grain, etc). Un exemple d'etude de l'effet sur le gonflement du gradient de dommage d'irradiation, caracteristique des irradiations aux ions lourds, est presente comme illustration de cette methodologie appelee MEBIA. Des calculs par dynamique d'amas ont permis de simuler l'impact des nano-precipites et la densite initiale de dislocations sur le gonflement. Ces resultats ont inspire la creation de nouvelles microstructures qui ont ete irradiees et ont commencees a etre caracterisees. Ce travail devra etre poursuivi pour valider la pertinence des microstructures optimisees. L'ensemble des resultats presentes dans ce manuscrit illustre les difficultes rencontrees pour etudier les microstructures des aciers austenitiques irradies a tres forte dose mais il montre que des approches nouvelles peuvent aussi etre mises en place pour faciliter ce travail. (auteur)Original Title
Contribution au developpement d'aciers austenitiques avances resistants au gonflement sous irradiation
Primary Subject
Source
18 Nov 2019; 188 p; 143 refs.; Available from the INIS Liaison Officer for France, see the INIS website for current contact and E-mail addresses; Sciences des Materiaux
Record Type
Report
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Thesis/Dissertation
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Country of publication
ALLOYS, CARBON ADDITIONS, CRYSTAL DEFECTS, CRYSTAL STRUCTURE, DEFORMATION, EPITHERMAL REACTORS, IRON ALLOYS, IRON BASE ALLOYS, LIFETIME, LINE DEFECTS, LIQUID METAL COOLED REACTORS, RADIATION EFFECTS, REACTORS, REFRACTORY METAL COMPOUNDS, STEELS, TRANSITION ELEMENT ALLOYS, TRANSITION ELEMENT COMPOUNDS
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