The reliability of nuclear power plants (NPPs) has an influence on power generation safety and stability. The reliability of NPP equipment and pipelines (E&P), and the frequency of in-service inspections are directly linked with damage mechanisms and their development rates. Flow accelerated corrosion (FAC) is one of significant factors causing damages to E&P because these components experience the influence of high pressure, temperature, and high flow velocity of the inner medium. The majority of feed and steam path components made of pearlitic steels are prone to this kind of wear. The tube elements used in the coils of high pressure heaters (HPH) operating in the secondary coolant circuit of nuclear power plants equipped with a VVER-1000 reactor plant were taken as the subject of the study. The time dependences of changes in the wall thickness in HPH tube elements are studied proceeding from an analysis of statistical data of in-service nondestructive tests. A method for determining the initial state of the E&P metal wall thickness before the commencement of operation is proposed. The article presents a procedure for predicting the distribution of examined objects' wall thicknesses at different times of operation with determining the occurrence probability of damages caused by flow accelerated corrosion to calculate the time of safe operation until reaching a critical state. A function that determines the boundary of permissible values of the HPH wall thickness distributions is obtained, and it is shown that the intervals of in-service inspections can be increased from 6 years (the actual frequency of inspections) to 9 years, and the next in-service inspection is recommended to be carried out after 7.5 years of operation. A method for determining the existence of FAC-induced local thinning in the examined object has been developed. The developed approaches and obtained study results can be adapted for any pipelines prone to wall thinning to determine the frequency of in-service inspections (including an express analysis based on the results of a single nondestructive in-service test), the safe operation time, and quantitative assessment of the critical value reaching probability.
