Scientific paradigms of structural safety of aged plants—Lessons learned from Russian activities

Abstract

The study of the effects behind the degradation of components and materials is becoming increasingly important for the safe operation of aged plants especially when it comes to life extension. Since the Russian nuclear community began to examine life extension issues nearly 15 years ago, there is much to learn from these pioneering studies. At the Ninth International Conference entitled ‘Material Issues in Design, Manufacturing and Operation of Nuclear Power Plants Equipment’ held in St. Petersburg, 2006, recent data were introduced regarding the ageing effects of mechanical properties of various kinds of steel and welding joints of Russian NPP components. The meeting was organized by the Central Research Institute of Structural Materials (CRISM) “Prometey” in cooperation with the IAEA and JRC-EU. In reviewing the recent data presented at the Ninth Conference, the authors believe that the paradigms of structural integrity issues in aged plants are now reasonably well established in (1) fracture mechanics and irradiation hardening of reactor vessels and core internals and (2) thermal ageing and annealing effects. However, the first author, G. Saji, believes that the current approach of low-cycle fatigue is still unable to prevent and predict environmentally assisted cracks such as demonstrated in the IGSCC issues in the down-comer pipes of RBMK plants and various steam generator corrosion issues. This fundamental flaw stems from design codes, which do not incorporate the basic knowledge of electrochemical corrosion mechanisms as represented by the corrosion current.