Diversity is the general approach used for decreasing common cause failure (CCF) risks of instrumentation and control (I&C) systems. Differences in equipment, development and verification technologies, implemented functions, etc. can mitigate the potential for common faults. Diversity and defense-in-depth (D3) is the required attribute of Nuclear Power Plant (NPP) I&C systems important for safety. One of the key theoretical and practical problems is diversity estimation and optimization of used version redundancy capacity. Existing NUREG guidelines published in reports CR-7007 and CR-6303 present the technical basis for establishing acceptable mitigating strategies that resolve D3 assessment findings. These approaches work in the terms of diversity attributes and associated criteria aimed at the potential for CCF vulnerabilities and make possible to choose I&C system architecture based on combinations of diversity criteria. But they do not provide measures (diversity indexes or metrics) to calculate reliability of such system in the context of CCF. In the report some other techniques for diversity assessment are analyzed, as well as advantages and disadvantages of these techniques in comprising with NUREG-based method are described. Possibilities of their joint applications and tool support are considered. The combined use of reliability models, diversity metrics and reliability indexes of system components allows enhancement of estimation sensitivity, making sufficiency criteria for diversity and redundancy more concrete and choice of technical solutions more informed and confident at the early stages of NPP I&C system design. The problems for future research are stated as creation of Safety-Case-oriented techniques and tools for multi-version NPP I&C systems assessment.

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