Availability Model of Critical NPP I&C Systems Considering Software Reliability Indices

Providing the high availability level for the Instrumentation and Control (I&C) Systems in Nuclear Power Plants (NPP) is highly important. The availability of the critical NPP I&C systems depends on the hardware and software reliability behavior. The high availability of the I&C systems is ensured by the following measures: structural redundancy with choice of the I&C system configurations (two comparable sub-systems in the I&C system, majority voting "2oo3", "2oo4", etc.); maintenance of the I&C system, which implies the repair (changing) of no operational modules; using the N-version programming; software updates; automatic software restart after temporary interrupts caused by the hardware fault. This paper proposes solution of the following case: the configuration of the fault-tolerant I&C system with known reliability indexes of hardware (failure rate and temporary failure rate) is chosen, the maintenance strategy of hardware (mean time to repair, numbers of repair), methods to forecast the number of software failures and the failure rate is specified. To solve this issue, the availability model of the fault-tolerant I&C system was developed in the discrete-continuous stochastic system form. We have estimated the influence of the I&C system on the operational software parameters. Two configurations of I&C systems are presented in this paper: two comparable subsystems in I&C system, and I&C system with majority voting "2oo3".