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AbstractAbstract
[en] In Sodium-cooled Fast Reactors (SFR), hypothetical failure of the core cooling system or the plant protection system may lead to a severe accident scenario. In such a scenario, core materials (fuel and cladding) melt down generating a hot molten mixture called corium. This corium may interact with the coolant (liquid sodium) leading to Fuel Coolant Interaction (FCI) which can generate energetic events and hence jeopardize the reactor structures. The yield of these energetic events strongly depends on the state of the corium-sodium mixture prior to the energetic event. Therefore, the knowledge of the features of the mixture composed of three-phases (i.e., corium, liquid sodium, and sodium vapor) is crucial. The lack of knowledge on the phenomenology of the interaction emphasizes the need to study it with the help of experiments. PLINIUS-2, the future large-mass experimental platform of CEA Cadarache, will be dedicated to experiments aiming at understanding the interaction phenomenology of prototypic corium with coolant (sodium and water). The present research aims to develop a high-energy X-Ray imaging system for this facility, to visualize and better understand the corium-sodium interaction. An image-processing algorithm to analyze the three-phase repartition is also developed to contribute to the improvement of numerical modeling. This Ph.D. research has been executed in three steps. In the first step, a bibliographic study of the past experiments was carried out to better understand the physics of the interaction and the mechanism of fragmentation during corium-sodium interaction. This bibliographic study, along with a statistical analysis of the particle size distribution data of various experiments conducted in the past, revealed that the particles formed in these tests are extremely fine fragments with characteristic diameters smaller than 1 mm. Due to the small particle size and the detection limitations of corium fragments in sodium with our X-Ray system, clouds of particles were detected instead of individual particles. In the second phase, the simulation of clouds of corium particles followed by the designing of phantoms (3D mock-ups) representing the 3-phase medium was carried out. Simulations of clouds of corium fragments in liquid sodium and vapor were performed using the CEA Cadarache in-house tool MODHERATO. Based on the results obtained from the simulations, certain phantoms were designed to conduct some physical experiments. These phantoms representative of the FCI interaction zone were manufactured to experimentally evaluate the performance of the radioscopy system and to facilitate the development and calibration of the image processing software. The third step of this work was dedicated to performing experiments with the phantoms and analyzing the radiographic images by developing an image processing algorithm. Experiments were carried out with phantoms in several configurations with the X-Ray radiography system at the CEA Cadarache KROTOS facility. The radioscopic images obtained were treated by developing a new comprehensive image processing and analysis code called PICSEL to identify the three phases composing the medium. Further verification and validation of the PICSEL software were carried out on a test conducted between corium and water at the KROTOS facility under the Euro-Chinese project 'ALISA'. Thus, in this Ph.D. research, an X-Ray imaging system was qualified to visualize the corium-sodium interaction in the future PLINIUS-2-FR facility. A qualitative analysis of the images produced by this system was also performed using the PICSEL software to better characterize the evolution of the three-phase mixture and understand the FCI phenomenon, knowledge of which is deemed essential to improve the safety and designs of future sodium-cooled fast reactors. (author)
[fr]
Dans les reacteurs a neutrons rapides refroidis au sodium (RNR-Na), une defaillance dans le systeme de refroidissement du coeur ou de protection du reacteur peut conduire a un accident grave. Dans un tel scenario, les materiaux du coeur (combustible et acier) fondent pour former un melange appele corium susceptible d'interagir avec le refrigerant (sodium). Une telle interaction (Fuel Coolant Interaction, FCI) peut generer des evenements energetiques aptes a mettre en danger les structures du reacteur. La violence de l'explosion depend essentiellement de l'etat du melange corium-sodium. Connaitre les caracteristiques des trois phases en presence dans la zone d'interaction (corium, sodium liquide, vapeur de sodium) est donc crucial. Le manque de connaissances de la phenomenologie de l'interaction conduit a l'etudier au moyen de dispositifs experimentaux. PLINIUS-2, la future plateforme experimentale du CEA Cadarache consacree aux grandes masses, sera dediee a l'etude de la phenomenologie de l'interaction corium-refrigerant (eau et sodium). L'objectif du present travail est de developper, pour cette installation, un systeme d'imagerie basee sur des radiographies X afin de visualiser et de mieux comprendre l'interaction corium-sodium. Un algorithme de post-traitement de ces images a aussi ete ecrit afin de les analyser, d'en deduire la repartition des trois phases dans le systeme et de contribuer ainsi a l'amelioration de la modelisation numerique. Ce travail de these a ete conduit en trois etapes. La premiere etait consacree a une etude bibliographique des experiences passees d'interaction corium-sodium et des mecanismes de fragmentation mis en jeu. Cette etude, completee d'une analyse statistique des distributions de particules obtenue alors, a montre que les particules generees lors de ces experiences sont extremement fines avec des diametres caracteristiques inferieurs au mm. Ces tres petites tailles de particules combinees aux limites de detection de notre systeme d'imagerie nous ont amenes a travailler plutot sur la detection de nuages de particules que de particules individuelles. Dans une deuxieme etape, la simulation du nuage de particules de corium suivie de la conception de fantomes (maquettes 3D) representant les trois phases du milieu ont ete realisees. Les simulations de nuages de fragments dans le sodium liquide et la vapeur ont ete realisees avec le logiciel CEA MODHERATO. Sur la base des resultats de ces calculs, certains fantomes ont ete fabriques afin de generer des images 'reelles' a l'aide d'une chaine d'imagerie existant au CEA. Ces fantomes representatifs de la zone d'interaction corium-sodium ont ete fabriques pour faciliter le developpement et la calibration du logiciel de traitement d'images. La troisieme etape de cette these etait dediee a la conduite des experiences avec les fantomes 3D et au developpement du logiciel de traitement des images ainsi obtenues. Les radiographies X des fantomes ont ete generees a l'aide du systeme d'imagerie de l'installation KROTOS du CEA Cadarache. Les images ont ensuite ete traitees avec le logiciel appele PICSEL que nous avons developpe afin de localiser les trois phases en presence dans la zone d'interaction. PICSEL a ensuite ete applique a une experience d'interaction corium-eau realisee dans l'installation KROTOS dans le cadre du projet ALISA de collaboration Europe-Chine; ceci a permis une premiere validation du logiciel. Ainsi, lors de cette these, un systeme de radioscopie X a ete defini pour visualiser l'interaction corium-sodium dans les experiences de la future installation PLINIUS-2 du CEA Cadarache. Une analyse qualitative des images produites par ce systeme (experiences menees avec des fantomes) est aussi realisee avec le logiciel PICSEL developpe lors de cette these afin de mieux caracteriser le melange, de mieux comprendre les mecanismes de l'interaction et de contribuer ainsi a l'amelioration de la surete des reacteurs a neutrons rapides. (auteur)Original Title
Radioscopie X pour les interactions corium-sodium lors d'un scenario d'accident grave
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10 May 2019; 237 p; 119 refs.; Available from the INIS Liaison Officer for France, see the INIS website for current contact and E-mail addresses; Energie Nucleaire
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