Core mechanical dynamics experiment in the Phenix reactor

International audienceASTRID is a project for an industrial prototype of a 600 MWesodium cooled Fast Reactor, led by CEA. A consequent program is inprogress for the development and the validation of numerical toolsfor the simulation of the dynamic mechanical behavior of the FastReactor cores, with both experimental and numerical parts. The coresare constituted of Fuel Assemblies (or FA) and Neutronic Shields (orNS) immersed in the primary coolant (sodium), which circulatesinside the Fluid Assemblies. The FA and the NS are slenderstructures, which may be considered as beams, from a mechanicalpoint of view.The dynamic behavior of this system has to be understood, for designand safety studies. Two main movements have to be consideredglobal horizontal movements under the effect of a seismic excitation,and a radial opening of the core. The fluid presence leads to complexinteractions between the structures at a distance. The dynamicbehavior of the core is strongly influenced by contacts between thebeams and by the interactions with the sodium, which both limit theirrelative displacements. Numerical methods and models are built todescribe and simulate this dynamic behavior. The validation of thenumerical tools is based on the results of different experimentalprograms, already performed or in progress.The paper presents the interpretation of tests performed in 2013 in thePhenix reactor. The French Phenix reactor was definitively shutdownin 2009 and is currently at an early stage of the decommissioningprocess. Before unloading the core, it has been decided to performone last experimental campaign aimed at testing the mechanicaldynamic behavior of the core. The interpretation of the tests highlycontributes to the validation of the simulation methods. Relativelygood comparisons have been obtained between the theoretical andexperimental results, for the static excitation (stiffness of the bundle)and for the dynamic response (characteristic times). The tests confirmthat the fluid leads to a significant decrease of the frequencies.Uncertainties remain on the significant damping which seems to bepresent, and may be due to the fluid or to the structures