Revealing and Assessing the Deposit Amount on Heat-Exchanger Tubes of NPP Steam Generators According to the Operational Data of Routine Eddy Current Monitoring

One of the measures aimed at preventing unscheduled shutdowns of reactor units at nuclear power plants owing to inter-circuit leaks in steam generators consists in monitoring the formation of deposits on the surfaces of heat exchangers. The azimuthal and axial extension of the electrically conductive layer in the deposits, as well as its thickness, can be used to judge the intensity of corrosion. In addition, assessing the contamination of the heat exchanger tubes from the side of the second circuit is important to prevent the accumulation of deposits on the heat exchange surfaces and to plan measures for their removal in the course of chemical washing. Monitoring conductive deposits is based on the existence of a quantitative relationship between the energy parameters of a low-frequency signal of the axial eddy-current transducer on the absolute channel and the volume of the conductive layer in the deposit. The results of studies aimed at revealing and numerically assessing the thickness (volume) of deposits on the outer surface of heat-exchanger tubes in steam generators at nuclear power plants with VVER-440 and VVER-1000 reactor units are presented. For this purpose, operational data based on standard eddy-current monitoring have been used to assess the condition of the metal in heat-exchanger tubes obtained using the PIRATE software package. At the first stage, characteristic diagnostic parameters of eddy-current signal readouts from electrically conductive deposits on heat-exchanger tubes have been determined. Criteria were then formulated according to which readouts corresponding to the sites with deposits can be automatically detected. The next stage consisted in determining the parameters of the eddy-current signal readouts that make it possible to estimate the thickness of the deposits. An algorithm has been developed for constructing a calibration curve for assessing the volume and average thickness of electrically conductive deposits on the surface of heat-exchanger tubes.