Monitoring in order to extend operation life of nuclear power plants
The worldwide average age of nuclear power plants (NPP) is 29 years. Most of NPP were built with a 40 year operating lifetime hypothesis. Therefore plant life extension has become a major issue for the operators.
The main ageing effects of nuclear reactor components are well-known degradations of physical properties : fatigue, stress corrosion, fretting, fouling, thermal and irradiation embrittlement. Due to long term ageing processes, concrete structures are also concerned by life extension issues.A majority of replaceable components
Effective methods of surveillance of the material sensitivity should be implemented during operation to evaluate the ageing process of the equipments. Some items are critical for safety, others are part of a critical system and candidate for run-to-failure or production loss.
Continuous surveillance is the basis for timing and effective maintenance, repair and replacement, within a reliable and cost-effective condition monitoring based maintenance strategy.
High added-value technologies such as optical fiber can optimize the service life maintenance : one application is the thermal mapping of steam generator so as to locate and measure internal deposits on the internal surface (Cementys ThermoLux® SG).Reactor vessels and confinement buildings are difficult to replace
The fast neutrons exposure surveillance of the reactor vessel surface (ferritic steel) helps to optimize the loading process of fuel assemblies in order to control the irradiation embrittlment and extand the remaining lifespan. Testing reactor vessel encapsulated specimens added to various non-destructives evaluations of components are part of a large periodic in-service inspection program.
Precast confinement buidling is subject to concrete creep and hoop tendon relaxation causing prestress losses and concrete micro-cracks.A systematic approach to monitor and analyze these phenomena is the only suited method to cope with the ageing process of the confinement building and concrete structures.
Our expertise in concrete investigations identifies the failure modes and potential irreversible damages over time. Our measurement solutions confirm a « normal » behaviour of a nuclear infrastructure.
This « monitoring know-how » is well-known by French Nuclear Operator EDF, dixit Alexis COURTOIS (Nuclear Concrete Structure Expert) :
« EDF starts monitoring its energy prodution infrastructures during construction : hydroelectric dams, confinement buildings and cooling tower structures. The investigation data collection and analysis provides structural health diagnosis and prognosis. These informations enable EDF to be compliant with its safety regulatory requirements and to ensure a long-tem and optimal asset management. The EDF engineering branch has developed a recognized know-how from the design of a monitoring system to data processing, characterizing the structural health integrity and anticipating the possible maintenance activities ».Not all the sensing technologies are adapted for long-term surveillance
Vibrating wire sensors have a strong track record for confinement building instrumentation : MicroVib® RR (Cementys product : strain extensometer resistant to radiation) can insure long-term measurement (back ground of more than 50 years in France with such vibrating wire sensors).
Optical fiber sensors are also good candidates for long term monitoring strategies. With a track record of 20 years, it represents a good solution to provide long-term measurement in harsh and low-radiation environments. With this technology, it is now possible to measure, with no restrictions, all along of an optical fiber : vibration, strain and temperature : real-time shape of prestressed hoop tendons (Cementys SensoLux® TM) is an example, or vibrating noise mapping of the confinement building or components (Cementys SensoLux® DAS).
Last generations of NPP (APR1400, VVER1200, AP1000, EPR) already benefit from these technologies so as to increase again their safety level.
Making durable the existing nuclear infrastructure to ensure the installed power capacity and increasing the sustainability of next built reactors are the main goals of monitoring programs and lifetime management.
Nuclear & Military Business Development Manager