ASTRID is a project for an industrial prototype of a 600 MWe sodium cooled Fast Reactor, led by CEA. A consequent program is in progress for the development and the validation of numerical tools for the simulation of the dynamic mechanical behavior of the Fast Reactor cores, with both experimental and numerical parts. The cores are constituted of Fuel Assemblies (or FA) and Neutronic Shields (or NS) immersed in the primary coolant (sodium), which circulates inside the Fluid Assemblies. The FA and the NS are slender structures, which may be considered as beams, from a mechanical point of view.

The dynamic behavior of this system has to be understood, for design and safety studies. Two main movements have to be considered: global horizontal movements under the effect of a seismic excitation, and a radial opening of the core. The fluid presence leads to complex interactions between the structures at a distance. The dynamic behavior of the core is strongly influenced by contacts between the beams and by the interactions with the sodium, which both limit their relative displacements. Numerical methods and models are built to describe and simulate this dynamic behavior. The validation of the numerical tools is based on the results of different experimental programs, already performed or in progress.

The paper presents the interpretation of tests performed in 2013 in the Phénix reactor. The French Phénix reactor was definitively shutdown in 2009 and is currently at an early stage of the decommissioning process. Before unloading the core, it has been decided to perform one last experimental campaign aimed at testing the mechanical dynamic behavior of the core. The interpretation of the tests highly contributes to the validation of the simulation methods. Relatively good comparisons have been obtained between the theoretical and experimental results, for the static excitation (stiffness of the bundle) and for the dynamic response (characteristic times). The tests confirm that the fluid leads to a significant decrease of the frequencies. Uncertainties remain on the significant damping which seems to be present, and may be due to the fluid or to the structures.

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