Extending the operating life of nuclear power plants implies to monitor the civil engineering structures ageing. Structural monitoring of the prestressed concrete containment is one of the main components of the EDF's strategy to justify that safety requirements will be reached until the end of operating life. Containment structures are instrumented to check their behaviour during construction (especially tendon tensioning), during periodical pressure tests and for prestressing losses monitoring. Nonetheless current instrumentation was designed for 30-40 years of operation. Operating life extension up to 60-80 years requires then developments for an appropriate and sustainable structural monitoring system for reliable long term surveillance.This paper will present our approach that leads to the definition of an Optimum Surveillance System (OSS). This concept defines the required sensors and measurements that are necessary to ensure efficient surveillance of the pressure concrete containment with an operating life of 60 years.This Optimum Surveillance System includes on one hand the sensors which deliver global displacement and which can be replaced (such as pendulum) and, on other hand, the sensors dedicated to local phenomena, initially embedded in concrete (such as strain meters) and thus inaccessible. An equivalent system is then required to maintain the OSS in case of failure of an embedded sensor. For that purpose EDF has developed new devices.This paper will also deepen the specific work made about how to acquire and validate measurements of deformation in the concrete. This work is structured around two principal themes:1.Reinstatement of vibrating wire strain meters owing to innovative electrical excitation approach combined with appropriate signal processing techniques.2.Development and installation of an accurate surface extensometer when embedded strain meters were out of order.A two-step approach was adopted for the validation of the surface extensometer's behaviour for long term monitoring deformation:1.Step 1: laboratory experiments.2.Step 2: installation on prestressed concrete containment and comparison with embedded strain meters.The positive results led to the successful validation of surface extensometer for both decennial pressure testing and long term monitoring. The uncertainty associated to surface extensometer is coherent with creep rate measured on containment building and is about a few microstrains per year.The two proposed developments ensure capturing all the necessary information for suitable monitoring of the long term behaviour of prestressed concrete containment.
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