The AREVA Integrated and Sustainable Concept of Fatigue Design, Monitoring and Re-Assessment

Abstract

The prevention of fatigue damages in components is a major responsibility during the entire operation of every nuclear power plant. Hence, fatigue is a central concern of AREVA’s R&D activities in the view of changing boundary conditions: modification of the code based approaches, life-time extension, new plants with scheduled operating periods of 60 years (e.g. EPR, BWR1000) and improvement of disposability. Simultaneously, an integrated approach to the fatigue issue is the way to an optimization of costs and plant operation as well as a minimization of non-destructive testing requirements. The AREVA fatigue concept provides for a multiple step process against fatigue before and during the entire operation of nuclear power plants. Indeed, fatigue analyses are undertaken at the design stage and for Plant LIfe Management & Plant License EXtension (PLIM-PLEX) activities. The quality of all fatigue analyses crucially depends on the determination of the real operational loads including the high loads of the initial start-up in the commissioning phase. It has to be pointed out that mainly thermal transient loading is fatigue relevant for nuclear power plant components. AREVA utilizes a measuring system called FAMOS (Fatigue Monitoring System) recording the real transient loading continuously on site. The direct processing of the measured temperatures is used for a first fast fatigue estimation after every operational cycle. This procedure is highly automated and allows for a rough estimation of the recent partial usage factor as well as the qualitative comparability of the data (loads, fatigue damage increment). In the framework of the decennial Periodic Safety Inspection (PSI) a detailed fatigue check conforming to the code rules (e.g. [1, 2, 3]) is carried out in order to determine the current state of the plant. This fatigue check is based on the real loads (specification of thermal transient loads based on measurements) and finite element analyses in connection with the local strain approach to design against fatigue. The finite element analyses always include transient thermal determination of the temperature field and subsequent determination of (local) stresses and strains. The latter analyses might be simplified elastic plastic or fully elastic plastic. Another Code requirement is the additional check against progressive plastic deformation (ratcheting) which is demanded by the design code (e.g. [1, 2, 3]). In the case of the elastic plastic approach much care has to be taken with respect to the application of an appropriate material law. Advanced nonlinear kinematic material laws are favored at AREVA at the present time in order to carry out realistic ratcheting simulations. One alternative to this approach is the application of the so called direct method based on the shake down theorems [25]. As a conclusion, one essential benefit of the integrated AREVA fatigue concept can easily be identified: Locations of potential fatigue failure are reliably identified and all efforts can be concentrated on these fatigue critical components. Thus, expensive costs for inspection can be essentially reduced. Of course, one requirement is the application of a temperature measurement system in the power plant. The concept itself is supported and its further development is ensured by numerous R&D activities, derived methods and tools as well as the further development of design codes. For example, it is planned to integrate direct measurements of fatigue damage, more sophisticated analysis concepts for fatigue damage (application of short crack fracture mechanics to fatigue crack growth), to combine fatigue damage monitoring and models for 3D crack growth simulation and to develop an alternative approach of high cycle fatigue initiation based on damage models in the integrated AREVA concept.

Thermal–mechanical stress analysis of pressurized water reactor pressure vessel with/without a preexisting crack under grid load following conditions

“…Most previous research on NPP component safety assessment, including our own [11][12], is based on a stress analysis of components using simplified design transients. For accurate structural integrity assessment of NPP components, it is necessary to perform structural fatigue evaluation under more realistic loads [13][14][15]. In this regard, thermal-mechanical stress analysis of NPP components under the grid load-following mode might be necessary for accurate fatigue evaluation.…”