Operational benefits and enlarged functionality of modern technology, but also the physical and the technological aging of conventional instrumentation and control (I&C) systems, are motivations for modernization of I&C systems in nuclear power plants (NPPs). In case of safety-relevant I&C systems, the licensing authorities require the demonstration of sufficient safety of the systems. In several countries ongoing research and development projects are directed to provide a scientific basis and engineering solutions for cost-effective assessment of software-based I&C systems important to safety in NPPs. International initiatives have been started to develop a harmonized safety assessment methodology. The Institute for Safety Technology (ISTec) has been engaged in national and international programs as well as in establishing a two-phase qualification approach, which comprises a generic plant-independent qualification of hardware and software components and a plant-specific system qualification phase. In both generic qualification and plant-specific system qualification, ISTec has been involved as an independent third-party assessor for the relevant state authority. This paper reports experiences from the ISTec involvement in the assessment projects, e.g., in the assessment of the I&C important to safety in the new German High-Flux Research Reactor (FRM-2) in Munich and in the assessments of I&C modernization in NPPs of foreign countries (Bohunice, Slovak Republic; Paks, Hungary; and others). Conclusions are drawn from the experiences with respect to the practicability of the two-phase qualification concept and from the major findings in the plant-specific IC assessments.
