One-dimensional bimodal model of vibration and impacting of instrument tubes in a boiling water reactor

The vibration and impacting of an instrument tube in a boiling water reactor (BWR) have been studied using a one-dimensional bimodal model. Four modal nonlinear boundary conditions have been applied, and a set of coupled nonlinear equations describing the temporal evolution of two continuous modal amplitudes have been obtained. These equations have been numerically solved by means of a generalized Runge-Kutta algorithm for stochastic equations. The theoretical results have been compared with experimental in-core neutron noise measurements performed in a 1300-MW BWR, Gundremmingen C, and have been used to interpret the particular vibration behavior of one instrument tube.