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Availability Model of Critical NPP I&C Systems with K-phase Erlang Distribution of Software Update

This paper is the continuation of the research devoted to enhancing the adequacy of reliability model of Nuclear Power Plant (NPP) Instrumentation and Control (I&C) Systems considering software reliability. The reliability model of NPP I&C systems is a basement from which the availability, safety, risk, and other important characteristics of the system could be assessed. The availability function of a critical NPP I&C system depends on the hardware and software reliability and maintenance. The high availability value of the critical I&C systems could be ensured by following: structural redundancy; maintenance of the system; using the N-version programming; software updates. Thus the NPP I&C system reliability model to ensure its high level of adequacy and applicability has to take into account both software and hardware failures, as well as realistic distribution of software fault correction and K-phase Erlang Distribution of software updates are specified.