Bifurcations in Mathematical Model of Nonlinear Vibration of the Nuclear Fuel Rod

The paper deals with nonlinear phenomena that occurs during vibration of nuclear fuel rod (FR). The FR is considered as a system consisting of two impact-interacting subsystems FR cladding (zircalloy tube) and fuel pellets stack placed inside FR cladding. Between both subsystems, there is a small radial clearance. The FR is bottom-end-fixed, and at eight equidistant levels, the FR cladding is supported by spacer grids (SG). Both subsystems are modelled by means of finite element method for one-dimensional Euler-Bernoulli continua. During fuel assembly (FA) motion caused by pressure pulsations of the coolant, the FR vibrates and impacts can possibly occur between FR cladding and fuel pellets stack. The paper focuses on qualitative change of vibration with change of bifurcation parameters clearance between FR cladding and fuel pellets stack and stiffness of spacer grids cells. The change of vibration quality is shown by extremes of relative radial displacements of both continua in discretization nodes and by phase trajectories. Dependence of impact motion on modal properties of both subsystems is shown.