Study of WWER Reactors Neutronic Noise Spectral Images in Irregular Thermоhydraulic Regimes of Core Zones

І. G. Sharaevsky, Т. S. Vlasenko, L. B. Zimin,
А. V. Nоsоvskyi, N. М. Fіаlkо, G. І. Sharaevsky

Institute for Safety Problems of Nuclear Power Plants,
NAS of Ukraine, 12, Lysogirska st., Kyiv, 03028, Ukraine



The information on the peculiarities of the manifestations of potentially dangerous thermohydraulic anomalies, that occur unpredictably during the operation of the WWER reactor core, is generalized. Considerable attention is paid to the threat of uncontrolled occurrence of latent pre-emergency and emergency operating states in fuel assemblies during the generation of the vapor phase on the surface of fuel elements, including the heat transfer
crisis with subsequent boiling of the coolant. It is noted that under these conditions, heat transfer, which should occur exclusively in the convective mode, is also characterized by the uncontrolled occurrence of the boiling process, from the activation of the first centers of vaporization to the formation of a developed bubble structure of the coolant. The limited possibilities of modern technical means of monitoring the technical condition of WWER, which do not provide detection of these anomalies and preemergency heat transfer regimes, are considered. Given the data on neutron flux fluctuations, it is shown that the noise components at the output of standard electron emission neutron flux meters carry important diagnostic information on thermohydraulic processes in the volume of the core, which is currently lost. With this in mind, it is concluded that it is necessary to immediately develop a methodology for the operational identification of random spectral realizations of neutron noise, which is registered at the outputs of neutron detectors of existing in-reactor control systems. The nature of the necessary reliable data on the spectral structure of the main types of diagnostic images (signatures or patterns) of neutron noise, which is registered at the output of standard neutron flux detectors in accordance with the main types of thermohydraulic anomalies, is determined. The analysis of known data and performed with the participation of authors of reactor experiments on the basis of which it is proved that the spectral structure of auto- and coherent signal density at the output of standard neutron flux detectors contains comprehensive diagnostic information on the type and location of vapor-liquid structures of WWER reactor core. With this in mind, the information characteristics of individual spectral ranges of neutron noise are analyzed and conclusions are made about their direct correspondence to the characteristic thermohydraulic processes as objects of diagnostics.

Keywords: WWER, core, neutron noise, spectral characteristics of signals, thermohydraulic anomalies, diagnostics.


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