Predicting Vibroacoustic Resonances in the Cores of NPPs Equipped with LWR-Type Power-Generating Reactors

It has been shown that measures aimed at reducing the fretting wear of fuel assemblies and fuel rods, i.e., measures reducing the impact of cyclic loads and vibration on these components, should be taken to increase the fuel campaign and improve the fuel cycle. It has been revealed that these loads and vibration are excited mainly as a consequence of unsteady hydrodynamic processes in the coolant and imbalanced operation of the circulation pumps. It is shown that for substantiating the possibility of increasing the fuel campaign, it is necessary to identify the conditions under which the vibration level exceeds the permissible values, to identify the factors causing this exceeding, and to prevent or limit their influence. To solve these problems, the data from technical diagnostics systems consisting of miscellaneous measurement channels are used. The importance of developing CFD codes for studying the reactor’s thermal-hydraulic model and, in particular, for constructing the temperature distribution fields over the cross section at the core outlet is pointed out. It has been stated, as a recognized conclusion, that it is impossible to study the vibration characteristics of fuel assemblies (FAs) in the operating reactor, and that attempts to model a number of most important processes that determine the totality of hydrodynamic loads at experimental facilities are not met with success. It is pointed out that there are a number of insufficiently studied phenomena and processes in the NPP equipment components that affect the vibration state of fuel assemblies and fuel rods, but in the existing system codes they are either considered in a very simplified manner or are not taken into account at all. It is shown that the developed methods and algorithms for quantitatively estimating the natural frequencies, q-factor and acoustic vibration bandwidths are suitable for VVER-type reactors. The conclusion stating that to prevent amplification of the fuel rod and fuel assembly vibrations under the effect of acoustic waves, it is necessary and sufficient to take measures for shifting their frequencies to beyond the bandwidth has been substantiated.