No abstract

The fatigue assessment of power plant components based on local fatigue monitoring approaches is an essential part of the integrity concept and modern lifetime management. An integral approach like the AREVA Fatigue Concept (AFC) basically consists of two essential modules: realistic determination of occurring operational thermal loads by means of a high end fatigue monitoring system and related highly qualified fatigue assessment methods and tools. The fatigue monitoring system delivers continuously realistic load data at the fatigue relevant locations. Consequently, realistic operational load sequences are available as input data for all ensuing fatigue analyses. This way, realistic load data are available and qualified fatigue usage factors can be determined. The mode of operation of the fatigue monitoring system will be explained in the framework of a live demonstration by means of the FAMOSi (i = integrated) demonstration wall. The workflow starts with the continuous online measurement of outer wall temperatures transients on a pipe. Visualization is implemented within the FAMOSi viewer software. In a second step, inner wall temperatures are directly calculated. In a third step, the resulting linearly elastic stress history will be calculated as the basis for subsequent code conforming fatigue assessment. Subsequently, the related advanced fatigue assessment methods of the three staged AFC-approach are addressed.

The fatigue check of components has to be considered as an essential module of the safety concept as well as the ageing and lifetime management of nuclear power plants. It is based on special safety requirements in the design phase as well as in the framework of the ageing and lifetime management. Considerable efforts of load identification (fatigue monitoring) as well as the strict code conformity are characteristic features. The predominance of thermal cyclic loadings as well as relevant stress/strain amplitudes in the low cycle fatigue regime (LCF) are peculiarities compared to other technical domains. The code based fatigue concept is explained in the first part of the contribution. Furthermore, potentials for the quantification of existing margins are shown. On this, a new approach of mechanistic simulation of the thermal cyclic fatigue process based on short crack fracture mechanics is described in detail in the second part.

Modern state-of-the-art fatigue monitoring approaches gain in importance not only as part of the ageing management of nuclear power plant components but also in the context of conventional power plants and renewables such as wind power plants. Consequently, lots of operators have to deal with demanding security requirements to ensure the safe operation of power plants and to cope with plant lifetime extension (PLEX) related issues.AREVA disposes of a long tradition in the development of fatigue and structural health monitoring solutions. Nuclear and conventional power plant applications require the qualified assessment of measured thermo-mechanical loads. The methodology is transferable to mechanical loading conditions such as those of wind energy plants. The core challenge is the identification and qualified processing of realistic load-time histories. The related methodological requirements will be explained in detail. This paper mainly describes the fatigue and structural health methodologies developed within the AREVA Fatigue Concept (AFC).

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.