Power Uprate Effect on Flow Accelerated Corrosion of Pipelines and Equipment of the Condensate-feed Path in VVER-1000 Nuclear Power Units

Abstract

In recent years, domestically constructed NPP units equipped with VVER-1000 reactors have undergone a power uprate, following the global trend in nuclear power generation. This has become possible due to changes in the working medium operating parameters that affect flow-accelerated corrosion (FAC) processes in the secondary circuit condensate-feedwater and wet-steam paths. The aim of the study was to evaluate how the power uprate of VVER-1000 nuclear power units affects the metal thinning rate caused by flow-accelerated corrosion and what are the zones of condensate-feedwater path pipeline and equipment components that are most susceptible to this thinning. Preliminarily, taking the Balakovo NPP Unit 1 as an example, its secondary circuit’s thermal cycle process loop was modeled to determine both the absolute values and relative changes of the working medium parameters that influence the FAC rate in various pipeline and equipment components. As a result, the values of temperature, flowrate, pH, and oxygen concentration were obtained for different sections of the condensate-feedwater path components during power unit operation at nominal and uprated power levels. The calculation results have shown that, when the VVER-1000 nuclear power unit operates at 112% of nominal power, the working medium temperature and flowrate in all sections of the condensate-feedwater path main pipelines were increased, while the pH_t value became slightly lower. It has also been found that the oxygen concentration changed very insignificantly, so that it can have almost no effect on FAC processes. Using the calculated values of the above-mentioned parameters and applying the RAMEK-1 software (certified at the Federal Environmental, Industrial and Nuclear Supervision Service of Russia), the flow-accelerated corrosion rates of the condensate-feedwater path pipelines have been determined for the Balakovo NPP VVER-1000 Unit 1 operating at 100%, 104%, and (as a prospect) 112% power levels. The degree to which each of working medium parameters affects the change in the total FAC rate for different condensate-feedwater path pipeline sections has been determined. The obtained FAC rate calculation results can be used to provide a computational and experimental basis for optimizing in-service monitoring programs for the condensate-feedwater path pipelines of VVER-1000 nuclear power units operating under power uprate conditions.