Three-Dimensional Model of Vibration and Impacting of Instrument Tubes in a Boiling Water Reactor and Transfer from Mechanical to Neutron Noise

The vibration and impacting of an instrument tube against the surrounding fuel boxes in a boiling water reactor (BWR) was studied by means of a three-dimensional, one-modal model. The transfer of the tube displacements to neutron fluctuations of the fission chamber-instrument tube assemblies was calculated by employing the adjoint function technique and utilizing a two-dimensional, two-group diffusion theory. The results were used to interpret the particular measured noise spectra of an instrument tube in the 1300-MW Gundremmingen C BWR. The root-mean-square of the displacements was estimated from the experimental neutron data.