VGB PowerTech
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VGB PowerTech
Areva´s Monitoring Concept The Areva Integrated and Sustainable Concept of Fatigue Design, Monitoring and Re-assessment The paper was given at the “ANSYS Conference & 26th CADFEM Users’ Meeting 2008”, Darmstadt/Germany, October 22 to 24, 2008 Jürgen Rudolph and Steffen Bergholz Kurzfassung VGB PowerTech · Autorenexemplar · © 2008 Areva Fatigue Concept (AFC) – Ein geschlossenes Konzept zur Verfolgung und Optimierung der auf Ermüdung basierenden Schädigungseffekte von thermisch und mechanisch beanspruchten Kraftwerksbauteilen Die Vermeidung von Ermüdungsschäden an Komponenten der nuklearen Kraftwerkstechnik ist ein wichtiges Thema in Hinblick auf die Aufrechterhaltung der Anlagensicherheit sowie auf die Erhöhung der Verfügbarkeit. Die Ermüdungsüberwachung ist integrierter Bestandteil des Alterungs- und Lebensdauermanagements. Vor diesem Hintergrund und den sich ändernden Rahmenbedingungen, wie z.B. Neubau von KKW’s mit 60Jahren Laufzeit, der Laufzeitverlängerung bestehender Anlagen und der Berücksichtigung von medienbedingten Effekten, ist die Verbesserung der Nachweismethoden ein Hauptanliegen der F&EAktivitäten von Areva. Dabei wird das Hauptziel verfolgt, den aktuellen Erschöpfungsgrad der Kraftwerksbauteile realitätsnäher zu bestimmen. Der vorgeschlagene integrale und mehrstufige Ansatz trägt gleichzeitig zur Kostenoptimierung und Prüfzyklenminimierung bei. Die Ermüdungsüberwachung sollte bereits in der Inbetriebsetzungsphase einsetzen, da die hohen auftretenden Lasten bereits einen signifikanten Beitrag zum Erschöpfungsgrad leisten. Areva empfiehlt den Einsatz des Systems FAMOS (Fatigue Monitoring System) zur Bestimmung der realen transienten Lasten und Weiterverarbeitung im Sinne einer ersten Ermüdungsschnellbewertung. In einem mittelfristigen Zeitrahmen sollten die aktuellen Erschöpfungsgrade mittels einer detaillierten regelwerkskonformen Ermüdungsanalyse bestimmt werden. Eingangsdaten dafür sind die realen gemessenen Lasten, wobei die Spezifikation der Temperaturtransienten aus systemtechnischer Expertensicht erfolgt. Die Ermüdungsnachweise basieren auf FiniteElemente-Analysen unter Berücksichtigung der transienten Temperaturfeldbelastungen am Bauteil sowie der primären Belastungen. Potenzielle Problemzonen können auf diese Art und Weise zuverlässig identifiziert werden, was wiederum einen Beitrag zur Inspektionskostenreduktion leistet. Authors Dr.-Ing. Jürgen Rudolph Dipl.-Ing. Steffen Bergholz AREVA NP GmbH Erlangen/Germany. 58 Introduction F i g u r e 1 shows the typical arrangement of components within the primary circuit of a pressurised water reactor (PWR). The possible layout of measuring points for in-service fatigue monitoring is marked with full circles. Thus, the main role of the temperature measurement system can easily be identified. The principal modules of the Areva Fatigue Concept (AFC) are schematically shown and logically combined in F i g u r e 2 . It is obvious that for all plant life management activities, the quality of all fatigue analyses crucially depends on the determination of the real operational loads in the plant. However, the first analysis is carried out even before the initial start-up. In this design stage, the conservative approach may yield usage factors above 1.0 requiring more refined analyses during operation. The real transient loads are recorded and processed on site based on FAMOS (Fatigue Monitoring System). A simplified fatigue estimation method (one branch in Figure 2) is directly combined with the measuring system. This simplified fatigue estimation is carried out after every operational cycle. This procedure is highly automated and allows for an estimation of the recent usage factor. In this process, implausible measured data have to be eliminated. A further division into classes is carried out and usage factors are estimated by application of the rainflow algorithm. As temperatures are measured on the outer wall and fatigue relevant locations are usually located on the inner wall the inverse temperature field problem has to be solved appropriately as one modular task for the specification of realistic transients and subsequent thermomechanical analyses. Furthermore, temperatures are measured at thinwalled tubes within the piping system. So, temperature and stress profiles due to the measurements have to be transformed into profiles at the fatigue relevant locations (e.g. thick cross sections, nozzles, flanges and pipe bends). In the framework of the periodic safety inspection (PSI) a detailed fatigue check conforming to the code rules is carried out in order to determine the current state of the plant (see workflow in Figure 2). This fatigue check is based on monitored loads and updated specification of thermal transient loads and finite element (FE) analyses in connection with the local strain approach (strain life approach) to design against fatigue. There is still potential with respect to a less conservative specification of the thermal loads. Practicability still demands for the definition of some covering unit transients and the allocation of the real number of occurrences (pressure, temperature and mass flows). These unit transients are used as load data in the FE stress and strain analyses and evaluated in terms of the applied design code (ASME, KTA, RCC-M [1, 2 and 3]). The intermediate step of load determination by computational fluid dynamics (CFD) might be part of the workflow and will gain in importance in the future. The determination of realistic heat transfer coefficients as a function of time is an important task. The FE analyses always embrace 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. One code requirement is the additional check against progressive plastic deformation (ratcheting). All conceptual activities are supported by respective R&D activities as well as the development of specialised methods and tools (see lower workflow of Figure 2). Some of these activities will be exemplarily addressed. Figure 1. Primary circuit of a pressurised water reactor. VGB PowerTech 5/2009 Erection Areva´s Monitoring Concept Design data • component design • material data • design & operating loads Design analyses stress/fatigue & ratcheting Approval documents ------------------------Basis for monitoring and verification Licensed status Cycle counting methods Load data evaluation Ageing and life time management Operation Measured data FAMOS Transient catalogue (realistic transients) Simplified fatigue estimation • Plant optimization (operating modes, in-service inspection, maintenance) • Component availability Detailed code-based fatigue & ratcheting analyses Reliable verification documents • Early detection of failures Supporting functions: R&D Further development of codes Methods Tools Figure 2. Principal modules of the AFC. VGB PowerTech · Autorenexemplar · © 2008 First Design Analysis before Operation Before commissioning and operation of the plant a catalogue of presumed thermal transients is compiled. Consequently, these specified thermal loads have to be considered as design transients in contrast to the real transients based on temperature measurement during operation. In the past, the anticipated transients covered 40 years. Now, the period to be considered is even 60 years. The specification is done for level A and level B, e.g. normal and upset conditions as well as partially levels C and P (emergency and testing). The code based fatigue analysis considers level A and level B loads in the design phase. Design transients before operation are specified according to different plant models and experiences. As a general rule, the specified transients are conservative with respect to frequency of occurrence, temperature range, rate of temperature change and load type (thermal shock, thermal stratification). The usage factors calculated in the design fatigue analysis will considerably differ from the results of the more detailed consideration of the real operational (thermal) loads. As a consequence, usage factors around 1.0 are still tolerable in the design phase. They indicate the positions of further analysis which is another aspect of the advantageous application of the first design analysis before operation. This reliable identification of points of VGB PowerTech 5/2009 interest is valuable under different aspects. For example, these locations are selected for future instrumentation and non-destructive testing (NDT). Thus, the pre-operational analysis is a valuable aid for the specification of the testing concept. It allows for an estimation of the amount of testing required. Furthermore, it is still possible to exert influence on the design and construction of the components and the operation mode of the plant. The consequences of non-optimal operation modes can clearly be shown. Plant staff is sensitised from the very beginning to the influence of operation on the lifetime of components. It contributes to the development of a deepened consciousness of responsibility and safety. Thermal transients with low influence on the fatigue behaviour are identified as well. The design codes offer procedures for the exemption from fatigue analysis for non significant loads. The so-called six points check should be mentioned. This procedure is specified for primary circuit components in the ASME code [1], section III, division 1, paragraph NB3222.4 and nearly identically in the German KTA 3201.2 [2], paragraph 7.8.2. Note that all six points have to be simultaneously fulfilled. The points concerning pressure and temperature transients are of special interest in this context. Insignificant pressure ranges ∆pNS are calculated as a function of the design pressure pD, the admissible stress amplitude at the endurance limit Sa(N = 106) and the Sm value of the material at operational temperature SmT: 1 Sa(N = 106) ∆pNS < – · pD · ––––––––– 3 SmT (1) Insignificant temperature ranges ∆TNS are calculated as a function of the admissible stress amplitude at the endurance limit Sa (N = 106), Young’s modulus E and the linear thermal expansion coefficient α: Sa(N = 106) ∆T < ––––––––– 2·E·α (2) As far as non-primary circuit (class 2) components are concerned the German KTA rules give further exemption criteria in KTA 3211.2, paragraph 7.8.1.2.1. It should be mentioned that section B3000 (Design) of the recent issues of the French RCC-M code [3] does not contain a comparable procedure for a simplified fatigue check. Hence, it depends on the code to be applied if this kind of fatigue analysis is admissible for nuclear power plant components. Areva’s Advanced Fatigue Monitoring System (FAMOS) The acquisition of realistic operational data in the power plant is one essential pillar of the AFC. Its role as the basis for a fast fatigue estimation and detailed data processing (e.g. for realistic load determination) is pointed up in Figure 2. Areva disposes of a fatigue monitor- 59 Areva´s Monitoring Concept Thermocouples Mounting strap Cover shell T1 Hot Cold T7 T2 T3 T4 y [mm] T5 T6 T [K] Figure 3. FAMOS measurement section. VGB PowerTech · Autorenexemplar · © 2008 ing system called FAMOS since the early 1980ies [4]. At that time, German licensing authorities demanded for the realisation of a comprehensive programme of measurement in the nuclear power plant of Grafenrheinfeld/ Germany in order to get detailed information on the real component loadings during plant operation. It was shown that the real operating conditions partially differed significantly from the design data. It should be pointed out that all measured data fell into admissible limits. At this occasion, the advantages of monitoring the real operating conditions and using the acquired data as an input for fatigue analyses based on realistic loading data became obvious. This gave a strong development impetus towards a sophisticated fatigue monitoring system. As a consequence, the German nuclear power plants as well as many plants in other countries were equipped with FAMOS (more than 20 nuclear power plants since 1988; 20 to 50 measurement sections and up to 150 thermocouples per plant). The objectives of FAMOS [4] can be summarised as follows: –– determining the fatigue status of the most highly stressed components, –– identifying operating modes which are unfavourable to fatigue, –– establishing a basis for fatigue analysis based on realistic operating loads and –– using the results for lifetime management and lifetime extension. In fact, the practical application resulted in various improvements of operating procedures, transient catalogues and fatigue results. FAMOS proved to be a reliable basis for the integrity concept, inspection plans and lifetime extension considerations. In the following the technical bases of FAMOS are shortly described. F i g u r e 3 shows the application of thermocouples at the outer surface of a piping section. The standard configuration consists of a mounting strap with the thermocouples and a second clamp- 60 ing strap used to press the thermocouples to the surface of the pipe. In 1996, a new measurement section which significantly improved the thermal sensitivity was successfully introduced. The FAMOS special instrumentation features consist of seven temperature transducers around one half of the pipe circumference in order to detect thermal stratification. This arrangement of thermocouples resolves the course of temperature as a function of time and the vertical coordinate y. In case of anticipated plug flow the application of only two thermocouples (T1 and T7 according to Figure 3) is sufficient. The practical installation of FAMOS in a nuclear power plant starts with the generation of a so-called FAMOS manual. This process contains a weak point analysis identifying components in the primary circuit and its safety systems as well as the secondary circuit that are expected to be particularly prone to fatigue damage. All design documents and operating experience are considered. A measuring point plan is elaborated and all activities are co-ordinated with the plant operator and if required with independent experts. As an example, points of interest within the primary circuit of a nuclear power plant are marked with full circles in Figure 1. In this case the following components are fatigue monitoring locations: the surgeline, the pressuriser spray lines, the nozzle of the residual heat removal system at the cold loop, the volume control system, the nozzle at the primary loop, the recuperative heat exchanger and the feedwater nozzle at the steam generator. The measured temperatures are to be transferred to the fatigue most severe locations. TInside, 2 TOutside, 2 TMedium, 2 TMedium, 1 TInside, 1 ṁ TOutside, 1 FAMOS TMedium Figure 4. Determination of realistic inner wall transients. Design & construction Operation modes Thermodynamic loading data Stress & fatiqueanalysis Lifetimemonitoring Figure 5. Interaction between operating mode, loading data and stress & fatigue analysis. Nowadays, this is done by FE analyses based on the measured data as input parameters. In the future, this task should be fulfilled automatically considering the capacitive corrections between thin and thick cross sections as it is shown F i g u r e 4 . This requires the simultaneous solution of the inverse temperature field problem and the automatic correction of the measuring tolerance deviation. Thus, the required transient temperatures will be directly available at the fatigue critical locations. Of course, the transient temperatures Tinsid e,2 in Figure 4 can be used as input data for a simplified as well as a detailed code based fatigue analysis. However, at least the detailed code based fatigue analysis still requires the specification of thermal transients as an intermediate step. Detailed Data Processing and Specification of Thermal Transients An important connecting link between the recording of measurements and the stress and fatigue analysis is the specification of thermal loads. For this task it is essential to use the specific knowledge of system and process engineers to identify the relevant load mechanism. Consequently, it is possible to develop smarter operational modes or more favourable hardware designs to achieve lower fatigue values which are necessary for an extended lifetime ( F i g u r e 5 ). Additionally, this procedure is a plausibility check of the available data. As a result, implausible data are detected and adjusted. The staff is sensitised to the consequences of certain operational actions, preferably beginning with the commissioning and initial start-up phase. Note that the transients can be significantly different in one power plant compared to another one due to the numerous influences. In spite of the strict regulations of the operational manuals the operator still takes an active influence on the operation of the power plant. If there is no existing compilation of the load cases which must be considered, e.g. from the permission of the plant, such a compilation must be created. This can be done specific for the system or for the whole pressure retaining boundary. In some cases such an overall load VGB PowerTech 5/2009 Areva´s Monitoring Concept case compilation was carried out for German power plants for the pressure retaining boundary together with the secondary side systems and the most important neighbouring systems such as the chemical and volume control system (CVCS) and the residual heat removal system (RHRS). All the information is used for the specification of the plant specific thermodynamic loads in official reports and transient handbooks. For thermal loads three types of specification are state of the art: –– Design specification for new plants or backfitting projects. This type of specification is based on theoretical analysis as well as measurements in comparable plants. It is necessary to reflect all possible load cases in the lifetime of the plant. –– Determination of the actual fatigue value (note that it is only possible to examine an upper limit of this number). Here all the events in the present lifetime must be considered which are relevant for loading. Normally this exercise will be repeated every ten years. –– Overall specification for all possible and hypothetical load cases which must be considered for the lifetime. The first step to specify thermal loads is the identification of the operational processes lead- ing to relevant transients of temperature and/or pressure. For these events an appropriate number of model transients are selected. Usually, it is necessary to split up these model transients into subclasses which are different, e.g. in the temperature difference of the transient. In a next step all the (relevant) events in the considered lifetime period are allocated to the specified model transients (or the corresponding sub-classes) and all the factors defining the model transient or the corresponding subclasses must be specified conservatively. All allocated events must be covered. Occasionally, an iterative process is required in order to achieve suitable model transients with an acceptable measure of conservatism. As the location of the existing measurements is not identical to the locations of maximum stresses it is necessary to translate the information of the outside transients to the inner wall and the medium including the appropriate heat transfer coefficients (see Figure 4). Special tools are used for the determination of these parameters. The thermodynamic loads are harmonised with the system specification, the transient handbook, the instructions of the operating manual, isometric diagrams, data of the FAMOS system and other inputs such as test instructions, logic diagrams and other work reports. If hypothetical load cases must be considered as well, the experience of normal events must be transferred to these theoretical boundary conditions (e.g. thermal stratification can be expected between pressuriser temperature and main coolant temperature). Knowledge of the processes and the operation of the systems is essential. As an example, a spray event under emergency conditions can be derived from a spray event of the start-up of the plant. Normally, only part of the required information is available. Obviously, the exchange of experiences and information between similar plants is synergetic and is a valuable aid in the permanent improvement of the entire process of data evaluation (Figure 2). This holds especially true for the German KONVOI plants which have a very similar basic design. If the conditions are not identical it is still possible to acquire additional information by transfer and re-scaling to hypothetical boundary conditions. Again, missing information has to be complemented based on an excellent knowledge of systems and processes of the plant. Finally, the specified thermodynamic loading data are the basis for the code based fatigue and ratcheting analyses. It should be pointed out that the quality and accuracy of the thermomechanical analyses highly depends on the plausibility of the specified transient loads. Together with the information about e.g. fluid dynamical or seismic loads the following ACHEMA 2009 Halle 10.1, Stand B22-C23 VGB PowerTech · Autorenexemplar · © 2008 Probenahme- und Analysensysteme • Thiedig-Probenahmeeinrichtungen für alle Messaufgaben • Engineering kompletter Systemlösungen • Hohe Funktionalität und Lebensdauer • 70 Jahre Know-how und Innovation Digox 601 silica • Medientransport erfolgt ohne Pumpen • Geringer Chemikalienverbrauch • Automatische Zweipunktkalibrierung • Individuell wählbare Messhäufigkeit für jeden Kanal Dr. Thiedig – Engineering Solutions [email protected] • www.thiedig.com Areva´s Monitoring Concept tion and the detection of fatigue critical locations. Furthermore, the investigation of the results allows for the detection of anomalies. Begin D ≥ 1.0 Component geometry and (thermal) loading Load-time-functions (transients) D < 1.0 End Fatigue check Usage factor yes Simplified check no Sn,m<3Sm (steel) Sn,m<4Sm (cast iron) yes Elastic fatigue analysis Counting method according to ASMEcode + linear damage accumulation rule no Sn,m<3Sm (steel) Sn,m<4Sm (cast iron) no yes Simplified elasticplastic fatigue analysis Ke-approach Fatigue curves (strain lite curves plotted as pseudostress amplitudes) Elastic-plastic fatigue analysis non-linear FE yes (optional) Ratcheting check It is important to distinguish this cycle based method for simplified stress based fatigue (SSBF) from the frequently applied event based fatigue method for the approximate assessment of fatigue damage increments based on the recording and evaluation of so-called counting lists of load cases. In the design phase the calculated fatigue effects are related to certain presumed events. Such an event, to which a defined set of fatigue load cycles is assigned, may be for instance the unit startup, the unit shutdown, power increase or decrease, exchange of water, emergency shutdown etc. The fatigue calculation assigns a fraction of fatigue damage to each of these events. The fatigue fraction is then multiplied by the presumed number of occurrences of the event during the component’s planned service life. The cumulated fatigue is obtained by linear summation of all fatigue fractions of all the events considered (Miner’s rule). Event-based fatigue methods compare the numbers of occurrences of operational events with the presumed events from the design phase. The evaluation does not consider occurred loads. Thus, it is not investigated whether the course of events during operation complies with those from the design phase. As an example, different start-up events may cause different fatigue usage contributions due to different numbers of hot standby hours and the derived different thermal loads. Figure 6. Code-based fatigue analysis. steps in the chain of certification can be done. VGB PowerTech · Autorenexemplar · © 2008 It is pointed out that the FAMOS system should be installed in the power plant from the very beginning of operation, i.e. in the commissioning and initial start-up phase. This phase is often characterised by the highest loads of the entire lifetime of the power plant. If the loads of this phase are unknown the initial pre-damage state cannot be specified. The specification of the initial starting-up phase has to be done in a very conservative way in that case. Simplified Stress-based Fatigue Estimation The results of the temperature measurement are to be processed quickly in order to get a first estimation of the fatigue state. One important task before the simplified and automated evaluation is the verification of the acquired data. Detection and adjustment of implausible data are parts of this process. These plausibility and quality checks of the measured data have to be done by experienced spe- 62 cialists. The result is a pre-processed database for (automated) data evaluation and fatigue assessment. Detailed Code-based Fatigue and Ratcheting Analysis C o d e - b a s e d Fa t i g u e A n a ly s i s In the very first step of the simplified fatigue assessment procedure the changes of temperatures are subject to a rain flow cycle counting algorithm [5]. In this process the temperature ranges at the locations of measurement are identified, counted and classified. These thermal load cycles are input data for a stress and fatigue assessment of the monitored components based on conservative analytical computation formulae. This rough real time fatigue estimation is done after every operational cycle and allows for a direct comparison of thermal loads and fatigue damage increments as well as an evaluation of the actual fatigue usage. The detailed code-based fatigue analysis is usually carried out after a certain time period of plant operation (e.g. ten years) in the framework of the periodic safety inspection. Loading data of the operational period as well as anticipated loads of future operation are used as essential input parameters. Hence, usage factors are calculated for the current state of the plant and until the end of life (e.g. 40 or 60 years). The fatigue analysis for primary circuit components is based on design codes such as the ASME code [1], the German KTA rules [2] or the French RCC-M code [3]. The code based procedure [1, 2] is exemplified in Figure 6. The result of this simplified fatigue assessment is usually conservative (idealised thermal shock assumption, thermal stratification with sharp separation). Although the correlation of the real temperature ranges is fairly simple it is suitable for a comparison of different real sequences of loads and allows for a qualitative evaluation of the mode of opera- The simplified elastic-plastic fatigue analysis (not to be confused with the simplified stressbased fatigue estimation explained above) based on elastic FE analysis and plasticity correction (fatigue penalty or strain concentration factors Ke) e.g. according to paragraph 7.8.4 of [2] or equally NB 3228.5 of [1] is known to yield conservative results [6,7]. In VGB PowerTech 5/2009 Areva´s Monitoring Concept space. Classical kinematic material models (Prager, Besseling etc.) often do not describe this translation [11] in a sufficiently accurate way. As a consequence, they fail in the simulation of experimentally proved load paths. Both considerable over- and underestimations are possible. Allowable limit 1.0 gin Mar Fatigue usage U Commissioning and initial start-up Consideration of more realistic operational data Implementation of improvements Operating without fatigue monitoring 0 Start of fatigue monitoring 40 years 60 years Figure 7. Development of fatigue usage factor considering local monitoring and improvements. VGB PowerTech · Autorenexemplar · © 2008 the practical application this will often yield high calculated usage factors. As a consequence, the branch of elastic-plastic fatigue analysis methodology based on non-linear FE analysis of Figure 6 will often be used for fatigue design. This is associated with an increased calculation effort. Computing times for complex 3D geometries and numerous transients (coupling of transient thermal and mechanical analyses) are significant. Under these circumstances the specified transients have to be rearranged in a small set (often 7 to 10) of covering transients for calculation purposes. There is still potential for improvement at the interface between “transient catalogue” and “detailed code based fatigue & ratcheting analyses” in Figure 2. The possible modification of design codes with respect to more severe fatigue curves [8] and the consideration of environmental, surface, load sequence and other effects [9] will significantly influence the code based fatigue design. Of course, these developments are attentively followed and actively accompanied (see “supporting functions” in Figure 2). As an example, surface and load sequence effects can be described on a mechanistic basis by application of short crack fracture mechanics. For the practical fatigue calculation process, conservative assumptions will have to be scrutinised in order to compensate the more severe fatigue assessment approach. Fi n i t e E l e m e n t M o d e l l i n g Requirements FE analyses as one essential pillar of the codebased fatigue approach have to meet strict quality requirements. Results obtained by different commercial FE codes and by different engineers must be comparable. The appropriate idealisation and discretisation strategy has to be formulated in terms of mandatory rules. Where applicable such rules VGB PowerTech 5/2009 should be verified by checking calculations and convergence tests. The elaboration and formulation of modelling guidelines is part of the quality management process. Note that special rules apply for thermal transient analyses in terms of discretisation requirements and time step sizes. The compliance with such rules ensures an accurate determination of notch stresses, thermal stresses and stress gradients. Ratcheting: Incremental E l a s t i c - p l a s t i c A n a ly s i s Ratcheting as a possible damage mechanism is a design code relevant issue. The ratcheting phenomenon is defined in paragraph NB3213.33 of the ASME code [1] as follows: “Ratcheting is a progressive incremental inelastic deformation or strain which can occur in a component that is subjected to variations of mechanical stress, thermal stress, or both”. In contrast, the term Shakedown is defined in paragraph NB-3213.34 of [1] as follows: “Shakedown of a structure occurs if, after a few cycles of load application, ratcheting ceases. The subsequent structural response is elastic, or elastic-plastic, and progressive incremental inelastic deformation is absent. Elastic shakedown is the case in which the subsequent response is elastic”. In fact, ratcheting is a complex phenomenon of cyclic plasticity. The realistic simulation of the process of strain accumulation requires the application of the incremental theory of plasticity. In the case of the elastic-plastic approach, care has to be taken with respect to the application of an appropriate material law. The development of appropriate material laws for ratcheting simulation is the subject of numerous international research activities [10]. The key for any successful ratcheting simulation is the description of the kinematic hardening, i.e. the translation of the yield surface in stress It is important to point out that available experimental data and research results mostly refer to mechanical loading. By contrast, components of nuclear power plants are mainly subject to thermal transient loading conditions. This requires special emphasis on thermal ratcheting issues. It does not only demand for the determination of the material parameters as a function of temperature but for the additional implementation of a differential temperature term. An expanded version of the Ohno & Wang material law is actually being implemented as a user routine within the commercial FE code ANSYS® [12]. Consideration of Fatigue Damage beyond Recent Code Requirements D e s c r i p t i o n o f Fa t i g u e D a m a g e B a s e d o n S h o r t C r a c k Fr a c t u r e Mechanics Instead of using the abstract notion of a usage factor, fatigue damage can be interpreted as the initiation and propagation of microscopically, physically and mechanically short cracks to an engineering crack size of about 0.5 to 3.0 mm. Fatigue curves of un-notched (polished) standard specimens resulting from strain controlled tests usually describe this limiting damage criterion. This is one characteristic feature of the local strain approach. The design fatigue curves of the applicable design codes [1 to 3] are based on these types of tests. The phase of technical crack initiation to an engineering crack size as part of the real fatigue damage process can alternatively be described by means of short crack fracture mechanics [13,14]. The main advantages of this approach are listed below: –– realistic description of the fatigue damage process, –– determination of more accurate usage factors, –– additional consideration of the influence of surface roughness by a fracture mechanical interpretation as an initial damage (crack), –– consideration of size and gradient effects, –– implicit consideration of mean stress effects by simulation of the crack closure behaviour, –– realistic interpretation of load sequence effects by consideration of the transient crack closure behaviour under variable amplitude loading and –– evaluation of NDT findings on the basis of 63 Areva´s Monitoring Concept experimentally approved crack propagation laws. The concept has already been extended to multiaxial loading conditions [15] and is successfully applied for the simulation of fatigue damage of engineering components with emphasis on mechanical loadings [16]. The strengthening of the approach for thermomechanical loading conditions is part of current research activities. The calculated local elastic-plastic stresses and strains and the derived fracture mechanical parameters serve as input data for the fracture mechanics based laws of crack initiation and crack propagation. The short crack model should be combined with replica measurement of the crack growth. Hence, measured and modelled crack lengths can be compared in a very early stage of fatigue damage. As a result, NDT findings of short fatigue cracks can be directly allocated to the real usage of the material. The short crack approach is based on parameters of elastic-plastic fracture mechanics. Hence, it is appropriate for the simulation of low cycle fatigue conditions typical for nuclear power plant components. D i r e c t S c a n n i n g o f Fa t i g u e D a m a g e VGB PowerTech · Autorenexemplar · © 2008 A long-term R&D project has been launched with the aim of being able to detect fatigue damage directly on the component based on NDT methods. Basic research results of various German institutes give evidence that the different stages of fatigue damage in power plant steels can be successfully related to NDT data [17]. A new practical approach to the evaluation of fatigue damage based on the fractal dimension of deformation structures is promising [18] in this context. It was found that the fractal dimension of the deformation structure derived from the surface topography increases during fatigue load. Simultaneously, the fractal dimension of the changing magnetic domain structure shows the same behaviour. Thus, magnetic noise measurements collect information on small sample regions. The scaling parameter “fractal dimension” is derived and correlated to fatigue life [18]. Areva aims at the application and verification of this method. Conclusions The Areva integrated and sustainable concept of fatigue design, monitoring and re-assessment is an expression of the significance of design against fatigue of nuclear power plant components. New plants with scheduled operating periods of 60 years, lifetime extension, modification of the code-based approaches and the improvement of operational availability are driving forces in this process. This is an expression of the sense of responsibility as well as an economic requirement. 64 It is pointed out that the fatigue concept is widely supported by measured data. Although the FAMOS system does not interfere the operation of the plant the results of the fatigue monitoring can be the basis for decisions of optimised operating modes and thus influence the fatigue usage U. This is shown schematically in F i g u r e 7 . The application of fatigue monitoring in an early phase of plant operation permits an early implementation of improvements and consideration of more realistic operational data in the detailed fatigue analysis. As a rule of thumb the actual operating conditions result in lower fatigue usage than those applied in the design calculations. Of course, additional loadings may occur as a result of the mode of operation or faulting conditions within specific systems. The main modules are the first design analysis before operation, the advanced load monitoring system, the simplified fatigue assessment, the detailed data processing combined with the specification of occurring thermodynamic loads and the code based fatigue and ratcheting analysis. The fatigue monitoring process should be considered from the very beginning (initial start-up) until the end of life (e.g. 60 years). As all modules are closely connected, it is reasonable to apply the approach as a whole with an additional cost reduction effect compared to separate solutions. Thus, the integrated fatigue approach makes a significant contribution to the operational availability and the protection of investment. [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] Acknowledgements [14] Of course, the implementation of the Areva integrated fatigue concept depends on the coordination of numerous specialists of various disciplines in a cooperative effort. This paper gives a descriptive overview of the ongoing process. The authors wish to express special thanks to the following colleagues for their valuable contributions, helpful discussions and amendments to the manuscript: K. Degmayr, G. Desfontaines, M. Franz, F. Guena, P. Gilles, N. Gillet, R. Gurdal, S. Krüger, W. Kleinöder, H. Lang, C. Pöckl, P. Quinot, G. Richter, G. Schön, K. Schramm, P. Ulmer, A. Willuweit and K. Wirtz. [15] [16] [17] References [1] [2] 2007 ASME Boiler & Pressure Vessel Code, Section III, Division 1 – Subsection NB: Class 1 Components, Rules for Construction of Nuclear Power Plant Components. KTA 3201.2 (06/96): Components of the Reactor Coolant Pressure Boundary of Light Water Reactors. Part 2: Design and Analysis. [18] RCC-M Edition 2000, Section I, Subsection B: Class 1 Components. Design and Construction Rules for Mechanical Components of PWR Nuclear Islands. Kleinöder, W., and Pöckl, C.: Developing and implementation of a fatigue monitoring system for the new European pressurized water reactor EPR. Proceedings of the International Conference on Nuclear Energy for New Europe 2007, Portoroz, September, 10 to 13, 2007. Matsuishi, M., and Endo, T.: Fatigue of metals subjected to varying stresses. Proceedings of the Kyushu branch of the Japanese Society of Mechanical Engineers, March 1968. Adams, S.A.: An Alternate Simplified ElasticPlastic Analysis Method. ASME Code Correspondence. Technical Support Document, January 19, 2005. Hoffmann, M., Seeger, T., and Führing, H.: Proposal for an improvement of the Ke-procedure of the ASME-Code. Technical University of Darmstadt, FF-2/1985, 1985. O’Donnell, B.: Proposed Section III Fatigue Design Curves for Air Environments.ASME Code Correspondence, November 7, 2007. Effect of LWR Coolant Environments on the Fatigue Life of Reactor Materials. Final Report. NUREG/CR-6909 ANL-06/08. Argonne National Laboratory, February 2007. Bari, S., and Hassan, T.: Anatomy of coupled constitutive models for ratcheting simulation. International Journal of Plasticity 16 (2000), pp. 381/409. Rahman, S.M. and Hassan, T.: Advanced Cyclic Plasticity Models in Simulating Ratcheting Responses of Straight and Elbow Piping Components, and Notched Plates. Proceedings of PVP-2005, Denver, Colorado, USA. ANSYS Rev. 11 - Multi Purpose Finite Elemente Code. Vormwald, M.: The Consequences of Short Crack Closure on Fatigue Crack Growth Under Variable Amplitude Loading. Fatigue Fract. Engng. Mater. Struct. 14 (1991), No. 2/3, pp. 205/225. Vormwald, M., Heuler, P., and Krae, C.: Spectrum Fatigue Life Assessment of Notched Specimens Using a Fracture Mechanics Based Approach. ASTM STP 1231, 1994, pp. 221/240. Savaidis, G., and Seeger, T.: Consideration of Multiaxiality in Fatigue Life Prediction Using the Closure Concept. Fatigue Fract. Engng. Mater. Struct. 20 (1997), No. 7, pp. 985/1004. Savaidis, G., Savaidis, A., and Seeger, T.: Engineering Components under Multiaxial Loading. Fatigue Analysis and Lifetime Evaluation. MP Materialprüfung 43 (2001), No. 3, pp. 76/86. Palm, S., Gertkemper, H., Knoch, P., and Maile, K.: A study of the early stages of fatigue damage in power plant steels. Proceedings of the 31st MPA-Seminar in conjunction with the Symposium on Materials & Components Behaviour in Energy & Plant Technology, Stuttgart, Germany, October 13-14, 2005. Schreiber, J., and Kröning, M.: Evaluation of fatigue damage of steel for nuclear power stations by the fractal features of deformation structures and their noise behaviour (in the German language). Proceedings of the 31st MPA-Seminar in conjunction with the Symposium on Materials & Components. h VGB PowerTech 5/2009 DVD-VGB_2007-e 04.07.2008 15:15 Uhr Seite 1 VGB PowerTech-DVD More than 10,000 digitalised pages with data and expertise (incl. search function for all documents) Please fill in and return by mail or fax I would like to order the VGB PowerTech-DVD 1990 to 2007 (single user license). 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