The Evaluation on Applying the Digital Safety System to Existing PWR Plants in Japan

To resolve the aging and obsolescence issues for instrumentation and control (I and C) systems at nuclear power plants, it is necessary to form a long-range maintenance and replacement plan for them systematically. To plan to replace the non-safety I and C system, the digital I and C system would be the most powerful choice from the view of maintainability and reliability because it has already been applied to our latest PWR plants. For the safety I and C system, even though digital safety system has already developed for the next PWR plants, it has never yet been applied to our plants. Therefore, how to apply digital safety systems to existing plants has been carefully studied. At first, the differences on safety functions, redundant architecture, and system interface between the next PWR plants and the existing plants have been studied. Then, the best replacement plan for safety I and C systems has been studied to obtain as much benefits as possible that are expected for the next plants, such as high reliability, availability, operability, maintainability and cost reduction. (authors)