FLUID-STRUCTURE INTERACTION WITHIN A FUEL ASSEMBLY SUBJECTED TO SEISMIC EXCITATION

The interaction between fuel assemblies during a seismic or loss of coolant accident (LOCA) event is directly associated with safety and reliability issues for all types of nuclear reactors. This study concentrates on the modeling of a single fuel assembly represented by a cylinder subjected to external flow and an external forcing function at the base. The model investigates the response of the fuel assembly using continuum mechanics model. The general behavior of a cylinder supported at both ends and subjected to axial flow is summarized: The cylinder undergoes several flow-induced instabilities as the flow velocity increases. These instabilities start with a pitchfork bifurcation, resulting in the buckling of the cylinder. At higher flow velocities, period-doubling and torus instabilities are observed as well, eventually leading to chaotic oscillations of the cylinder It is shown that an increased confinement results in lower critical flow velocities for the first point of instability, resulting in buckling at lower flow velocities. The cylinder response to a base excitation is also considered and it is shown that the response amplitude can change depending on the frequency of base excitation.