The impact of flow-accelerated corrosion on the crack developing in welded joints of NPP equipment and pipelines

Abstract

Thermal mechanical equipment or the pipelines of the second circuit of a VVER type NPP are mainly subject to two degradation mechanisms that reduce the strength of structural elements: flow-accelerated corrosion and fatigue damage. As a rule, the aforementioned degradation mechanisms bear no combined impact as they affect different zones of the object. However, welded joints are subjected to the impact of both mechanisms which results in initiation and development of cracks in welded joints and in their local thinning due to the low content of alloying elements or due to hydrodynamic features of the flows arising in back of taps, throttling devices, cone transitions and similar pre-connected sections. The goal of the study is to elucidate the effect of flow-accelerated corrosion on the development of fatigue cracks in welded joints. We used the formulas of fracture mechanics to describe the time-dependent undergrowth of cracks and empirical data to describe the dependence of flow-accelerated corrosion on the operation time and postulated residual defect structure. The proposed approach provided estimation the effect of flow-accelerated corrosion on the rate of crack developing in the equipment or pipeline. It is shown that flow-accelerated corrosion can both reduce and significantly increase the crack growth rate. The distribution of defects versus the operating time is obtained, taking into account the entire set of possible missed defects. The distribution obtained provides for more correct assessing of the reliability and safety of welded joints of the NPP equipment and pipelines.