Identification of Emergency Mode of Deteriorated Heat Transfer in Channels of Advanced Light Water Reactors with Supercritical Thermodynamic Parameters
Authors:   Institute for Safety Problems of Nuclear Power Plants, NAS of Ukraine, Sharaievskyi І. G. 
Published: December 2024

From the standpoint of physical analysis of the micro-kinetics of the near-wall layer of the supercritical coolant, as well as the emergency dynamics of the heat transfer process from the surface of the fuel rods under these conditions, the features of the emergency mode development of deteriorated heat transfer in the channels of advanced light water reactors with supercritical thermodynamic parameters are analyzed. The defining physical differences in the onset of the deteriorated heat transfer mode in the reactor core with supercritical thermodynamic parameters are specified. The fundamental differences between the procedures for operational diagnostics of heat exchange crises on the surface of fuel rods, which may arise in water-cooled reactors under subcritical parameters, and the development of the emergency mode of
deteriorated heat transfer in the core under supercritical parameters are physically substantiated. The fundamental impossibility of applying deterministic approaches to the implementation of existing monitoring and diagnostics systems for the current operational state of subcritical reactors of the VVER type to solve the problem of early detection of the initial phases of the deteriorated heat transfer mode is justified. Taking into account the defining features of the molecular kinetics of the onset and development of the deteriorated heat transfer mode on the surface of fuel rods in the reactor core with supercritical thermodynamic parameters, the main diagnostic principles for the automatic identification of this emergency thermal-hydraulic mode, which must be timely detected in future reactors with supercritical coolant parameters, are specified. In this context, the fundamental principles of building advanced in-core monitoring systems for light water reactors with supercritical parameters are formulated. It is justified that the onset of the emergency mode of deteriorated heat transfer on the surface of fuel rods in a supercritical light water reactor must be detected at the initial phase of this emergency mode. It is shown that the core of the mathematical software for advanced in-core monitoring systems for reactors with supercritical thermodynamic parameters of the light water coolant should consist of intelligent procedures for automatic recognition of the initial phases of the pseudoboiling mode, which precedes the pseudo-film boiling mode and the destruction of the fuel rod cladding in the deteriorated heat transfer mode. A mathematical model
of preventive diagnostics of the emergency mode of deteriorated heat transfer is proposed, based on procedures for constructing a separating hyperplane in a multidimensional feature space. The adequacy of the developed model for automatic recognition of the pseudo-boiling mode in an experimental cylindrical channel under the conditions of a thermal-hydraulic stand with supercritical thermodynamic parameters was verified based on the spectral parameters of the acoustic emission process during the pseudo-phase transition process. A 100% reliability of correct identification of the pseudo-boiling mode was achieved.