A non linear model for the PWR fuel assembly seismic analysis

In seismic PWR core analysis, the non linear behavior of fuel assemblies has to be studied taking .into account variations of stiffness and damping due to the slippage of rods in spacer grids. Based on the linear CEAʹs assembly model, principally composed of two beams representing sets of rods and thimbles, a linear model is described. In the second part of this paper the slippage and loss of contact of rods on a grid relic is analyzed. Based on the Coulomb friction model and plasticity analogy, a grid model constituted of elastoplastic hinge and non linear springs is proposed to simulate the progressive slippage of rods inside the cells. In the third part, a non linear assembly model is designed and validated with experimental data. This model represented with success the increase of damping and decay of frequency when the magnitude of the dynamic loading increases.