The analysis of processes of loading formation towards fuel rods in the coolant flow of the fuel assemblies within the Water-Water Power Reactor
Students Name: Kuzmich Mariia Oleksandrivna
Qualification Level: magister
Speciality: Nuclear Рower Еngineering
Institute: Institute of Power Engineering and Control Systems
Mode of Study: full
Academic Year: 2020-2021 н.р.
Language of Defence: англійська
Abstract: Kuzmich M.O. (performer), Semerak M.M. (supervisor). The analysis of processes of loading formation towards fuel rods in the coolant flow of the fuel assemblies within the Water-Water Power Reactor. Master’s thesis. – Lviv Polytechnic National University, Lviv, 2020. Extended abstract. The problem of ensuring the operational reliability component of technical systems has been and remains one of the keys in modern engineering technologies. As for nuclear energy, it is no exception. Thus, the issue of ensuring a high level of operational reliability and maintaining the required resource of the active zone of nuclear reactor, as well as critical for the safety of nuclear power units fuel rods in the structure of the fuel assembly is a priority. Vibration processes in nuclear reactors significantly determine the reliability of equipment and safety of operation of the unit as a whole. Thus, increased vibrations have repeatedly been the cause of failures of fuel assemblies by the mechanisms of vibration wear of elements and components of fuel assemblies, which led to premature unloading of fuel and significant economic losses [1]. In this regard, there is a very important task to ensure vibration strength and vibration resistance of fuel assemblies. The dynamic influence of the coolant flow on the structural elements of reactor systems initiates and maintains their mechanical oscillations (vibrations). Vibrations in the longitudinal turbulent flow of the coolant lead to damage and the formation of defects in the shells of fuel rods and the release of radioactive fission products in the circulation circuit of the coolant [2, 3]. Identifying the mechanisms of vibration of fuel assemblies is the most important stage of creating methods of calculation and experimental analysis of vibration processes and developing scientifically sound recommendations for constructive solutions aimed at increasing the resistance of fuel assemblies as a whole and their individual elements to dynamic coolant effects. Analysis of hydrodynamically disturbed vibrations is impossible without describing the processes of flow interaction with streamlined surfaces of a mechanical oscillatory system [4]. The most complete information about the vibrational behavior of fuel assemblies can be obtained based on a combination of theoretical research and experimental methods of obtaining individual parameters of the studied processes. The master’s qualification work is devoted to the study of the processes of formation of hydrodynamic loads and the vibrations initiated by them in the longitudinal flow of the coolant on the fuel rod bundles in the structure of WWER fuel assemblies. Study object of research – is a bundle of fuel rods in the structure of fuel assemblies of WWER type reactors in terms of excitation and maintenance of vibrations in the longitudinal turbulent flow of the coolant. Scope of research – dependence of vibration parameters initiated by hydrodynamic loads on the characteristics of turbulent coolant flow. Goal of research – determining the influence of the flow structure on the vibration intensity of the fuel rod beam; description and study of the regularities of the process of formation of hydrodynamic loads on the bundles of fuel rods of WWER fuel assemblies. Keywords: vibration of fuel element beams, the fuel assemblies WWER, pressure fluctuation , random hydrodynamic loads. In the first part of the work the main consequences of the influence of vibrations on the reliability of NPP equipment are considered, the review of theoretical and experimental researches of hydrodynamically excited vibrations of fuel rods is carried out. Based on this, the maximum deflection of the fuel rod in the fuel assembly beam is estimated. The second part considers the hydrodynamic structure of the coolant, defines the characteristics of turbulent flows, the peculiarities of their formation and interaction with the structural elements of a nuclear reactor. The third part describes the methods of experimental studies of vibrational processes of fuel rods under the action of turbulent water flow using full-scale models. Methods of research and processing of experimental data are presented. The fourth part, based on experimental data available on the vibration characteristics of fuel rods and pressure pulsations, reflects the role of turbulent flow hydrodynamics at the inlet and in the region of the beamforming hydrodynamic loads, and vibrations, respectively. The influence of input devices, such as shut-off and control valves, throttle washers, ADP on the structure of turbulent flow was analyzed. It is established that the intensity of vibrations is determined not only by the flow velocity in the beam but also significantly depends on the pulsation characteristics of the flow. Also in this section, the ability of random hydrodynamic loads to excite and maintain beam vibrations was analyzed, the intensities of vibrations of fuel rods that occur in the turbulent flow of the coolant under their impact are considered. Based on the processing of experimental data, data were obtained to estimate the vibrational displacements of the fuel rod beam at the initial hydrodynamic section depending on the intensity of pressure pulsations at the inlet to the beam in front of the lower support lattice. The obtained results show that the flow rate of the coolant does not unambiguously determine the intensity of vibrations of the fuel rods. The pulsation characteristics of the flow are important. To reduce vibrations, it is necessary to form a flow structure with low levels of pressure pulsations at the entrance to the fuel rods. References: 1. Fuel failure in water reactors: causes and mitigation: Proceedings of a Technical Meeting. (2003). Bratislava. 2. Review of fuel failures in water cooled reactors (2010). IAEA Nuclear Energy Series. Vienna. No. NF-T-2.1. 3. Structural behaviour of fuel assemblies for water cooled reactors: proceedings of a Technical Meeting. (2005) Cadarache. 4. Frolov, K.V., Makhutov, N.A., Kaplunov, S.M. (2002). Dynamics of designs of hydroelastic systems. Moscow: Nauka.