Steffen Bergholz

AREVA Fatigue Concept - A Three Stage Approach to the Fatigue Assessment of Power Plant Components

Within the continuously accompanying licensing process for NPPs until the end of their operational lifetime, the ageing and lifetime management plays a key role. Here, one of the main tasks is to assure structural integrity of the systems and components. With the help of the AREVA Fatigue Concept (AFC), a powerful method is available. The AFC provides different code-conforming fatigue analyses (e.g. according to the wide spread ASME code [1]) based on realistic loads. In light of the tightening fatigue codes and standards, the urge is clearly present that, in order to still be able to comply with these new boundaries, margins which are still embedded within most of the fatigue analyses in use, have to be reduced. Moreover, thermal conditions and chemical composition of the fluid inside the piping system influences the allowable fatigue levels, which have come under extensive review due to the consideration of environmentally assisted fatigue (EAF) as proposed in the report [2]. Therefore, for highly loaded components, some new and improved stress and fatigue evaluation methods, not overly conservative, are needed to meet the increasingly stringent allowable fatigue levels. In this context, the fatigue monitoring system FAMOS, central module of AFC, is able to monitor and record the real local operating loads. The different modules of the AFC are schematically represented in Figure 1.

Fatigue Monitoring Approaches for Power Plants

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.In terms of the nuclear industry, the ageing management of power plant components is nowadays a main issue for all actors: states, regulatory agencies, operators, designers or suppliants. As regards fatigue assessment of nuclear components stringent safety standards imply the consideration of new parameters in the framework of the fatigue analysis process:• new design fatigue curves, consideration of environmental fatigue (EAF) parameters and• stratification effects.In this general context AREVA developed the integral approach AREVA Fatigue Calculation (AFC) with new tools and methods in order to live up to operators’ expectations: Simplified Fatigue Estimation (SFE), Fast Fatigue Evaluation (FFE) and Detailed Fatigue Check (DFC). Based on real measured thermal loads and superposed mechanical loads the Fast Fatigue Evaluation (FFE) process allows a highly automated and reliable data processing to evaluate cumulative usage factors of mechanical components. Calculation and management of results are performed within the fatigue assessment software FAMOSi (FAatigue MOnitoring System integrated), thus impact of operating cycles on components in terms of stress and fatigue usage can be taken into account in order to plan optimized decisions relating to the plant operation or maintenance activities.This paper mainly describes the fatigue and structural health methodologies developed within the AREVA Fatigue Concept (AFC).Copyright

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

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.Copyright

Automatic Fatigue Monitoring Based on Real Loads - Calculation Example of a Flange

The ageing management of power plants is nowadays a main issue for all nuclear industry actors: states, regulatory agencies, operators, designers or suppliers. Consequently, lots of operators have to deal with demanding security requirements to ensure the operation of power plants. Regarding with fatigue assessment of nuclear components, stringent safety standards are synonymous of new parameters to take into account in the fatigue analysis process, for instance: new design of fatigue curves, consideration of environmental parameters or stratification effects. In this context AREVA developed within the integral approach AREVA Fatigue Concept (AFC) new tools and methods to live up to operators’ expectations. Based on measured thermal loads, the Fast Fatigue Evaluation (FFE) process allows for highly-automated and reliable data processing to evaluate time-dependant cumulative usage factors of mechanical components. Calculation and management of results are performed with the software FAMOSi, thus impact of operating cycles on components in terms of stress but also with regard of fatigue can be taken into account to plan an optimized decision related to the plant operation or maintenance activities.The FFE process was exemplary applied in 2012 in the EnBW Power Plant of Neckarwestheim (GKN II) to perform an informative fatigue diagnose of a spray line flange for different operating cycles. This paper describes the calculation methodology but also some relevant results to point out the benefits of this method to the ageing management of mechanical parts.Copyright

Fatigue Monitoring System and Post-Processing of Temperature Measurements: Surge Line Under Stratification Loading

The ageing management of power plants is nowadays a main issue for all nuclear industry actors: states, regulatory agencies, operators, designers or suppliers. Consequently, lots of operators have to deal with demanding safety requirements to ensure the operation of power plants particularly in the context of lifetime extension. With regard of the fatigue assessment of nuclear components, stringent safety standards are synonymous of new parameters to take into account in the fatigue analysis process such as for instance: new design of fatigue curves particularly for austenitic stainless steels, the consideration of environmentally assisted fatigue (EAF) and stratification effects. In this context AREVA developed within the integral approach AREVA Fatigue Concept (AFC) new tools and methods to live up to operators expectations. The last mentioned stratification issue will be focused on in the framework of this dedicated paper. Based on measured thermal loads, the Fast Fatigue Evaluation (FFE) process allows for highly-automated and reliable data processing to evaluate time-dependent cumulative usage factors of mechanical components. This method has recently been extended to the consideration of stratification loading with surge line application. The paper presents the latest AREVA research and development activities on the FFE method applied to a surge line under stratification thermal loading. An additional CFD analysis was performed in order to calculate realistic thermal loadings during start-up conditions of nuclear power plant conditions. The FFE methodology was used to calculate thermal stress at all relevant locations. This approach opens the possibility of a realistic CUF calculation. The methodology, the principle results and benefits are presented in the paper.Copyright

Automatic Fatigue Monitoring Based on Real Loads and Consideration of EAF: Live Demonstration

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.Copyright

Detailed Fatigue Checks Based on a Local Monitoring Concept

The Detailed Fatigue Calculation (DFC) of nuclear power plant components is part of the three staged approach to lifetime assessment and lifetime management of the AREVA Fatigue Concept (AFC). It is applied for fatigue relevant components showing high usage factors by application of the Simplified Fatigue Evaluation (SFE) and/or the Fast Fatigue Evaluation (FFE), e.g. the preceding stages of the AFC. The quality of the fatigue lifetime assessment essentially depends on one hand on the fatigue model assumptions and on the other hand on the load data as the basic input. In the case of nuclear power plant components thermal transient loading is most fatigue relevant.The issue of qualified determination of the real operating loads is crucial and closely connected to the fatigue monitoring strategy to be applied in the plant. As regards fatigue monitoring, two possible basic approaches are practiced: global and local concepts.The first mentioned relies on signals from the standard plant instrumentation in connection with transfer functions whereas the second one requires additional measurement sections at fatigue relevant locations. As a compensation to the additional instrumentation effort, the application of a local fatigue monitoring strategy paves the way of delivering continuously (at a frequency of 1 Hz) realistic load data. Disposal of these data constitutes the first step within the flowchart of fatigue assessment. The according methods of qualified processing of these data are discussed in detail. The processing of arbitrary operational load sequences and the derivation of representative model transients are essential steps. Appropriate cycle counting approaches and the consideration of Environmentally Assisted Fatigue (EAF) by way of Fen-factors are addressed in this context.Within the fatigue evaluation model the appropriate consideration of cyclic plasticity effects and the identification of fatigue damaging events are central modules. Plasticity correction is equally proposed in a staged approach by the application of established and more advanced Ke-factors within the simplified elasto-plastic analysis, the application of direct methods (such as the simplified theory of yield zones) and the general elasto-plastic analysis based on appropriate material models.These three stages are characterized by increasing calculation effort and (usually) decreasing degree of conservatism. Their application is case dependent. Additionally, the general elasto-plastic analysis entails a cycle-by-cycle ratcheting check based on an appropriate material model as part of the detailed fatigue check (not elaborated in detail in this paper).The application of the integrated AFC approach is explained by way of a representative example.Copyright

Zur Methodik der Ermüdungsbewertung von Komponenten der nuklearen Kraftwerkstechnik

Kurzfassung Der Nachweis der Betriebsfestigkeit ist wesentlicher Baustein des Sicherheitskonzeptes sowie des Alterungs- und Langzeitbetriebsmanagements kerntechnischer Anlagen. Hoher Aufwand zur Lastidentifikation (Fatigue Monitoring) und starke Regelwerksbindung sind kennzeichnend. Besonderheiten gegenüber anderen Technikbereichen stellen die Dominanz thermozyklischer betrieblicher Belastungen sowie relevante Beanspruchungsamplituden im niederzyklischen Bereich (LCF) dar. Das verfolgte Nachweiskonzept wird in seiner regelwerksseitigen Einbettung im ersten Teil des Beitrages erläutert. Darüber hinausgehend werden Möglichkeiten aufgezeigt, bestehende Margen zu quantifizieren. Hierzu wird ein neuer Ansatz der mechanismenorientierten Simulation des thermozyklischen Ermüdungsvorgangs auf Basis der Kurzrissbruchmechanik im zweiten Teil beschrieben.