Preliminary study of the effects of ageing on the long-term performance of NPP pipe

Most of today's operating nuclear plants are facing long-term operation (LTO) issues caused by the time degradation and/or deterioration suffered by the system, structure, and components (SSCs). These phenomena are known as ageing and are responsible for the change of material properties and, in turn may affect the structural integrity of plant SSCs. The paper deals with the analysis of the performance of a primary pipe of a typical PWR subjected to ageing mechanisms. To the aim an inverse space marching method is applied. From reconstructed temperature it is possible to determine e.g. temperature values at surfaces that are difficult to reach and inspect. Accordingly, based on the thermal gradient across the pipe wall, the residual thickness of the pipe may be determined and used for structural capacity verification. Analytical and numerical (thermo-mechanical) analyses are performed considering several thinning rates. The effects of both homogeneous and heterogeneous thinning are also investigated. The results suggest that an excessive (general or local) thinning may affect the strength capacity of pipe. The performance of the pipeline confirms the possibility of the life extension if the thinning rate is kept below 0.5 mm/year, even when the plant operating conditions are outside the prescribed operating limits.