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Model checking has been successfully used for detailed formal verification of instrumentation and control (I&C) systems, as long as the focus has been on the application logic alone. In safety-critical applications, fault tolerance is also an important aspect, but introducing I&C hardware failure modes to the formal models comes at a significant computational cost. Previous attempts have led to state space explosion and prohibitively long processing times. In this paper, we present an approach to model and formally verify protection functions allocated to one or several I&C systems, accounting for hardware component failures and delays in communication within and between the systems. Formal verification is done with model checking, whose feasibility on such complex systems is achieved by utilizing the symmetry of I&C systems: the components of the overall model that do not influence the checked requirements are eliminated, and the failing components are fixed. Generation of such abstracted models, as well as subsequent verification of their requirements and symmetry with the NuSMV symbolic model checker, is handled by a software tool. In addition, we explore how to specify formal requirements for systems of the considered class. Based on a case study built around a semi-fictitious nuclear power plant protection system that achieves reliability by means of redundancy, we demonstrate how failure tolerance of even detailed system designs can be formally verified. INDEX TERMS Formal verification, model checking, nuclear I&C systems, fault tolerance.

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Formal methods -such as model checking -have definite advantages over more commonplace verification techniques. By providing proof of the analyzed systems' correctness, they are especially useful in domains that are under regulatory supervision, like the nuclear industry. The foremost challenge for wider adoption of model checking is the effort and the expertise required for formalizing functional requirements into verifiable properties. A particular challenge in verifying the application software of industrial process control systems is taking into account the different sequencing and timing issues that arise from, e.g., the dynamic behavior of the plant processes being controlled. In this paper, we review specification languages that are aimed at making formal methods more accessible. We have collected 1079 sample formal properties from practical model checking projects in the nuclear industry, and identified repeatedly occurring property types. We present our findings, and based on the sample data, evaluate the applicability of different approaches on user-friendly property specification.