Guidelines for defense-in-depth and diversity activities in digital instrumentation and control systems

Digital Instrumentation and Control (I and C) systems are becoming an ever-increasing part in I and C systems of nuclear power plants due to such features such as versatility, flexibility, and reduced sizes. The digital technology introduces a possibility that common-cause or common-mode failures (CCF or CMF) may cause redundant safety systems to fail in such a way that there is loss of safety function. A special form of CMF analysis called 'defense-in-depth and diversity' (D-in-D and D) analysis has been developed to identify possible common-mode failure vulnerabilities and to support a specific licensing action in digital systems. There are two main stages in D-in-D and D activities: both plan and analysis. How to plan and analyze D-in-D and D in digital I and C systems is important to minimize the possibility of CMFs and thus increase the plant reliability. This paper describes the guidelines for D-in-D and D activities in digital I and C systems