Neutron noise observations in German KWU built PWRs and analyses with the reactor dynamics code DYN3D

Abstract

Low-frequency neutronic noise with magnitudes of up to ten percent of the reactor power have been observed in Konvoi-type PWR in Germany in the last years. Several attempts were made to identify the reasons for increased magnitudes of the neutronic fluctuations in comparison with pre-Konvoi reactors, and various hypotheses have been created to explain this effect.In this paper, results of noise simulations performed with use of the reactor dynamics code DYN3D are presented. Fluctuations of the coolant inlet temperature and mass flow rate were considered. Besides of un-correlated fluctuations, correlated temperature fluctuations were simulated. In a preliminary analysis with harmonic temperature oscillations, the importance of existing correlations between temperature fluctuations in the individual fuel assemblies was pointed out. In our analyses, the correlations between fluctuations in the individual fuel assemblies were obtained based on an experimentally validated coolant mixing model. However, the features of the neutronic noise found in the simulations do not correspond to the measurements.Obviously, more complex mechanisms than only temperature and/or mass flow fluctuations have to be considered. Simulated generic fluctuations of the local moderator density, that were introduced independently from thermal hydraulics, indicate that potentially deformations or vibrations of the fuel rod lattice or turbulent fluctuations of the coolant temperature and density inside the fuel rod bundles leading to variations of the local moderator content might be responsible for the observed neutronic noise. Therefore, advanced models coupling neutronics, thermal hydraulics, turbulence and mechanical modelling have to be developed.