Laboratory experiments with impacting fuel rods

Abstract

Vibration surveillance and diagnostics of fuel rods and fuel assemblies are important tasks in NPPs. They are so much the more difficult since the commonly used accelerometers do not operate properly under the extreme conditions of the core of reactor. Therefore vibration (or impact) of fuel rods can only be detected indirectly from neutron noise or pressure fluctuation signals. Thus accurate knowledge of vibration phenomena and measurability is very important. Experimental results on models without limiter give good coincidence with theoretical calculations. Spectra measured on impacting rod become smoother with increasing impacting level. Spectra of fuel rods have a wider range in impacting rate and higher level of smoothing than spectra of model rod have. The impacting rate strongly depends on mechanical properties of the rod. By the experiments, one can state that as for Fourier spectra the only thing caused by the impacts is the smoothening. This, however, has the very remarkable consequence, that disappearance of certain peaks does not always reflect a decrease in the vibration amplitude, but can also mean the opposite case — a vibration increased to such an extent that the rod is impacting on its surroundings. Thus there is a higher chance to give faulty diagnosis by Fourier spectra only. Consequently investigation of fuel rod vibration requires increased caution.