Dynamic behavior of the Fast Reactor cores: the Symphony program

A fast reactor core is schematically constituted of Fuel Assemblies and Neutronic Shields, immersed in the primary coolant (sodium) which circulates inside the assemblies. Two main physical phenomena have a strong influence on the dynamic behavior of this system: the impacts between the beams and the interactions with the fluid. The impacts between the beams limit the relative displacements. The fluid leads to "inertial effects", with globally lower vibration frequencies, and "dissipative effects", with higher damping. Symphony is an important research program on the seismic behaviour of the fast reactor cores, developed from 1993 to 1998 at the CEA Saclay, with both experimental and theoretical parts. The experiments are at a representative scale, with Fuel Assemblies (or FA) and Neutronic Shields (or NS). Test are made "in air" (without fluid) and "in water", to study the influence of the fluid (the sodium). A numerical model has been built for the interpretation of the tests. The interpretation of the tests is made by using a simple and efficient numerical method, based on the Euler equations for the fluid and homogenization techniques, which yields low computational costs. Impacts between the beams are taken into account also. The gaps between the feet and the grid plate lead to high damping for the beams if the gaps are important. The fluid leads to a strong coupling between the FA and the NS in the whole core, and limits the relative displacement. (author)