Specific aspects for Cernavoda - Unit 1 NPP life assurance

Full text: The main scope of a Plant Life Management Program is to operate the NPP in a safe manner and at a competitive cost during the reactor life. To achieve this goal, it is important to continuously evaluate the degradation of the main structures and components of the NPP. Background -Cernavoda NPP design life is 30 years. Compared with this target, the operation history is not long (Unit 1 is in commercial operation since 1997). It is still important to begin a plant life management program early to identify the critical components and structures, to establish the data needed for their monitoring and to find methods to mitigate their degradation. A specific aspect for Cernavoda NPP - Unit 1 is the long delay between the fabrication of the main components and the start-up. Most components were procured 10-15 years before start-up. First criticality was achieved in 1996, but the containment perimeter wall sliding was complete in 1983, the Calandria vessel was installed in 1985, the Steam Generators were in position in 1987, the fuel channels were installed in 1989. In evaluating the history of these components, the preservation period must be observed. For Unit 2, which will be in service around 2005, the delay will be longer. For this reason, CNCAN (the Romanian Regulatory Authority) imposed, as a condition to resume the work, to evaluate the ageing of the existing components and structures in order to establish their acceptability for use in the plant. The results of this evaluation can be used as references for subsequent evaluations. Plant Life Assurance Programme - The first step of a PLIM programme is to identify the components and structures that are important for the plant life management. Critical components and structures selection is done using the following criteria: safety criteria - components and structures whose failure can cause a release of radioactivity or which have to mitigate the release of radioactivity in case of a failure of other systems/components; availability criteria - components and structures whose failure can cause a shut down or a reduction of power of the plant. The critical components and structures are of two categories: long-life passive components - Calandria vessel, Calandria internals, fuel channels, steam generators, reactor cooling pipes, pressurizer, containment building, concrete and steel structures, pressure vessels, heat exchangers, cables; short life active components - reactivity mechanisms, turbine, generator, diesel generator, pumps, valves, motors, air compressors, fans, instrumentation and control systems, electrical systems. For these components and structures, the potential degradation mechanisms must be identified and a safety margin defined. The next step is to estimate the actual ageing of these structures and components. This can be done by reviewing the design and the history of fabrication, installation, operation and maintenance. Good records and a database of operating conditions are essential. Periodic inspections (such as in-service, containment components, etc) results give also important information for evaluating the ageing degree. Main Activities ageing studies for Cernavoda NPP - Unit 1 began in 1994. They referred to the improvement of knowledge about ageing mechanisms, the selection of critical components, data collection and data processing. Also a special programme for Environmental Qualification up-dating has been developed by the owner of the plant. The installation of a multi-channel Data Acquisition System at Cernavoda NPP - Unit 1 is the scope of an ongoing IAEA Programme (ROM 04/027). The first steps of a Plant Life Management Programme started this year. The main proposed goals of the PLIM programme are: to review the existing records regarding the history of the critical components and structures (design, fabrication, installation, preservation, operation conditions, maintenance) in order to establish a reference database; to estimate the actual degradation of the plant using the results of the previous activities on ageing and to compare with the safety margins; to find the methods to mitigate the degradation of the long-life passive components; to establish a Condition Oriented Maintenance Programme (instead of the Time Oriented Maintenance, which is now used) for the short life active components. (author)