Replacing the aging instrumentation and control system (I&C) in a nuclear power plant (NPP) is always a difficult task, even without the present energy-distribution disturbances in Europe and the more and more frequent plant-life-extension and power-up-rating projects. The NPP plant life-cycle tends to reach 50-60 years, while that of the I&C system is only 8-10 years. Not only the systems become obsolete: spare parts for the old system are practically nonexistent.

Putting into operation reliable, thoroughly tested, verified and validated, approved by the authorities new systems could significantly increase the outage time of the units. However, about 90% of the work can be done off-line, by means of full scope replica simulators, and in the same time the operators become trained to these new systems, too. Taking this approach the refurbishment does not increase the outage time needed for refuelling and repair, at least not significantly. The paper describes this methodology in detail.