Handling of design and licensing changes in the Russian Federation

Design changes in NPP units that aim at eliminating or minimizing the effects of ageing mechanisms are usually the result of complex analyses and of R&D conclusions. Design changes can also be the result of feedback, both positive and negative, from OE in similar units. Design changes are implemented taking into account the recommendations from international organizations (including the IAEA), and lessons learned from events and accidents around the world. These usually require preventive and compensatory measures in order to remove weaknesses and avoid analogous situations in the future. For example, as a result of experience accumulated from the operating steam generators PGV-1000 and PGV-1000M in WWER-1000 units, some abnormal hydrodynamic regimes deviating from the design were observed in the heat exchange section, and increased damage to the steam generator pipe nozzles was observed. High stresses (including residual stresses) in the piping joints were noted. As a result, the steam generator design was altered, and these changes either reduced or completely eradicated the degradation phenomena, thereby increasing steam generator reliability and serviceability