Determination of the region of vibroacoustic resonances of coolant and fuel assemblies in advanced enhanced-power reactors

Abstract

Computational methods and algorithms for determining the region of vibroacoustic resonances of fuel elements and fuel assemblies for existing and planned VVER, VVER-SKD have been developed. The results obtained are used to analyze the region of vibroacoustic resonances of the coolant and fuel assemblies in the new-generation reactors VVER-1200VVER- , -1700 In order to increase the fraction of electricity production by nuclear power plants in the next few decades, the plants will have to be operated in a partial power regime and in diverse transient regimes. Operation at constant (nominal) power will create additional low-cycle thermal loads on the equipment, which will increase the high-cycle loads which are due to more intense vibration of the equipment in nonstationary regimes. High-cycle dynamic loads can grow in some regimes with partial power [1,2]. Together with operating nuclear power plants in a partial power mode there arises the problem of increasing the service life from 30 to 50 and even 60 years. This requires upgrading of equipment and regimes at the design stage. Some of the main problems in the solution of this problem are determining and preventing conditions which result in resonance interaction of the acoustic oscillations of the coolant and vibrations of the equipment [1]. These problems arise in the development of new modifications of fuel elements and fuel assemblies. The experience gained in VVER operation shows that vibrations could be responsible for damage to fuel assemblies.The present article examines the problem of determining the conditions resulting in resonance interaction of acoustic vibrations of the coolant and equipment vibrations. The calculations permit determining the region of the resonance interaction of the natural frequency of the pressure oscillations of the coolant with the oscillations of the fuel elements, fuel assemblies, and shaft of the reactor. The methods and algorithms developed are used for quantitative evaluation of the natural frequency of the coolant pressure oscillations, figure of merit (Q) and transmission band of the existing and planned VVER and VVER-SKD.The dependence of the transmission band of the acoustic oscillations on the reactor regime is investigated to solve this problem. To this end, an acoustic model of the coolant in the core taking account of and neglecting the gas phase, formed during radiolysis, is used (Fig. 1) [3].Evaluations of Q and the transmission band of the acoustic oscillations in the coolant flows in a nuclear reactor have not been made before. The present article presents the results of calculations of Q and the transmission band for different VVER-1000 modifications.On the basis of the fact that the equations describing vibrational processes in hydraulic, acoustic, and electric systems are of the same form [4-7], a system of analogies holds (acoustic parameters of hydraulic elements -electric parameters): pressure drop ΔP, Pa -voltage, V; volume flow rate W v , m 3 /sec -current, A; acoustic compliance c, m 4 ·sec 2 /kg -cap...