The need for energy is growing in the world and the market of nuclear power is now once more expanding. Some issues of the current light-water reactors can be solved by the next generation of nuclear power, Generation IV, where sodium-cooled reactors are one of the candidates. Phénix was a French prototype sodium-cooled reactor, which is seen as a success. Although it did encounter an earlier unexperienced phenomenon, A.U.R.N., in which a negative reactivity transient followed by an oscillating behavior forced an automatic emergency shutdown of the reactor. This phenomenon lead to a lot of downtime of the reactor and is still unsolved. However, the most probable cause of the transients is radial movements of the core, referred to as core-flowering. This study has investigated the available documentation of the A.U.R.N. events. A simplified model of core-flowering was also created in order to simulate how radial expansion affects the reactivity of a sodium-cooled core. Serpent, which is a Monte-Carlo based simulation code, was chosen as calculation tool. Furthermore, a model of the Phénix core was successfully created and partly validated. The model of the core has a k_eff = 1.00298 and a neutron flux of (8.43+-0.02)!10^15 neutrons/cm^2 at normal state. The result obtained from the simulations shows that an expansion of the core radius decreases the reactivity. A linear approximation of the result gave the relation: change in k_eff/core extension = - 60 pcm/mm. This value corresponds remarkably well to the around - 60 pcm/mm that was obtained from the dedicated core-flowering experiments in Phénix made by the CEA. Core-flowering can recreate similar signals to those registered during the A.U.R.N. events, though the absence of trace of core movements in Phénix speaks against this. However, if core-flowering is the sought answer, it can be avoided by design. The equipment that registered the A.U.R.N. events have proved to be insensitive to noise. Though, the high amplitude of the transients and their rapidness have made some researcher believe that the events are a combination of interference in the equipment of Phénix and a mechanical phenomenon. Regardless, the origin of A.U.R.N. seems to be bound to some specific parameter of Phénix due to the fact that the transients only have occurred in this reactor. A safety analysis made by an expert committee, appointed by CEA, showed that the A.U.R.N. events are not a threat to the safety of Phénix. However, the origin of these negative transients has to be found before any construction of a commercial size sodium-cooled fast reactor can begin. Thus, further research is needed.

Résumé : Ce projet de maîtrise s’intéresse au comportement ultime d’une structure en acier, contrôlée sismiquement par des amortisseurs élastomères et des contreventements en chevron. Les séismes peuvent causer des dommages considérables quand les infrastructures et les bâtiments ne sont pas construits selon les normes et les techniques appropriées. Par conséquent, réduire l’impact des séismes revient particulièrement à construire des ouvrages sécuritaires en tenant compte bien entendu du paramètre économique. Ainsi Gauron, Girard, Paultre et Proulx ont étudié en 2009, un système de reprise de forces latérales, constitué uniquement de treventements en chevron montés en série avec des amortisseurs en caoutchouc naturel fibré ayant de nombreux avantages. Premièrement, le système reste élastique sous le séisme de design en réduisant les efforts sismiques linéaires par un facteur supérieur à R[indice inférieur d] = 3 par rapport à un cadre conventionnel. Deuxièmement, il est capable de contrôler les déplacements sous la limite du CNBC 2010 (Code National du Bâtiment du Canada 2010), et même de réduire ces derniers dans certains cas. Par conséquent, il permet de réduire les sections des poutres et des poteaux des cadres par rapport à une structure conventionnelle ainsi que les coûts de réparation après un séisme. Toutefois, le comportement à l’état limite ultime d’un tel système, ses limites et ses réserves de sécurité restaient à déterminer. Ainsi, l’objectif global de ce projet de recherche est de déterminer les différents mécanismes de ruine possibles de ce système, d’établir des limites et réserves de sécurité, et de préciser, après avoir formulé certaines recommandations, à quelles conditions il peut être utilisé dans le dimensionnement de nouvelles structures. Pour atteindre les objectifs fixés, deux essais quasi statiques ont été réalisés sur deux cadres en acier dimensionnés avec le système. Des essais dynamiques ont aussi été réalisés afin d’avoir les propriétés viscoélastiques des amortisseurs. Le premier essai a mis en évidence un mécanisme de ruine inattendu et prématuré qui a souligné un défaut majeur dans les connexions des diagonales avec l’amortisseur. Le second essai a révélé un des mécanismes de ruine envisagés initialement où le caoutchouc se déchire après l’initiation du flambement dans la diagonale comprimée. Les résultats expérimentaux ont montré que l’amortisseur constitue le maillon faible du système, et que des efforts parasites peuvent réduire significativement la capacité portante des structures dimensionnées avec un tel système. Dans les deux cas, les résultats ont montré que la méthode de dimensionnement du système tel qu’elle est définie actuellement mérite d’être améliorée. En ce sens, des recommandations relatives au dimensionnement des différents éléments des structures dimensionnées avec le système ont été élaborées, particulièrement en ce qui concerne le caoutchouc et les connexions. // Abstract : This thesis focuses on the ultimate behavior of steel structures, controlled seismically by elastomeric dampers and chevron bracings. Earthquakes can cause considerable damages when infrastructures and buildings are not built considering appropriate standards and technics. Therefore, mitigating the impact of earthquakes means essentially building safe structures by taking account of economic parameters too. Thus Gauron, Girard, Paultre and Proulx studied in 2009 a seismic force resisting system consisting only of chevron braces connected in series with fiber-reinforced natural rubber dampers that offers many benefits. First, the system remains elastic under the design earthquake by reducing linear seismic efforts by a factor of R[subscript d] = 3 compared to a conventional frame. Secondly, it allows to control the displacements under the limits of NBCC 2010 (National Building Code of Canada 2010), and even to reduce them in some cases. Therefore, it allows a reduction of sections of beams and columns of conventional frames and it prevents repairing costs of the structure after an earthquake. However, the ultimate limit state behavior of this system, its limitations and safety reserves have not been determined yet. Thus, the overall objective of this project is to determine the different possible failure mechanisms of the system, to set its limits and safety reserves, and to state after some recommendations, how it can be used in the design of new structures. To achieve these objectives, two quasi static tests were performed on two steel frames designed with the new system. Dynamic tests were also conducted to get the viscoelastic properties of the damping material. The first quasi static test revealed an unexpected and premature failure mechanism that pointed out a major flaw in the connections of the braces with the damper. The second test revealed one of the failure mechanisms originally expected where the rubber tears after buckling of the compression brace. The experimental results have shown that the damper is the weak element in the system, and that additional forces can significantly reduce the structural capacity of structures designed with the system. In both cases, the results have shown that the actual design method of the system should be improved. Thus, recommendations for the design of elements of structures designed with this system have been developed, particularly with regard to the rubber and brace connections.