Mechanical characterisation of heat resistant power plant materials by creep and fatigue testing in controlled gas atmospheres

The continuing increase of steam parameters of fossil fuelled high efficiency power plants and new combustion concepts for the capture and storage of carbon dioxide lead to harsher service conditions for the components and structural materials of such facilities. The present work introduces a test concept that allows testing of candidate materials under simultaneous mechanical and corrosive loading. The material's reaction can be directly investigated under simulated temperature, load and corrosion conditions of modern installations. First results obtained for different heat resistant steels suggest a strong influence of the environmental medium on the fatigue and creep behaviour. Such findings complement the data that is available from the classical qualification process of the materials and may support the material selection for new power plant installations.     

Meeting the challenges related to material issues in chemical industries

Reliable performance and profitability are two important requirements for any chemical industry. In order to achieve high level of reliability and excellent performance, several issues related to design, materials selection, fabrication, quality assurance, transport, storage, inputs from condition monitoring, failure analysis etc. have to be adequately addressed and implemented. Technology related to nondestructive testing and monitoring of the plant is also essential for precise identification of defect sites and to take appropriate remedial decision regarding repair, replacement or modification of process conditions. The interdisciplinary holistic approach enhances the life of critical engineering components in chemical plants. Further, understanding the failure modes of the components through the analysis of failed components throws light on the choice of appropriate preventive measures to be taken well in advance, to have a control over the overall health of the plant. The failure analysis also leads to better design modification and condition monitoring methodologies, for the next generation components and plants. At the Indira Gandhi Centre for Atomic Research (IGCAR), Kalpakkam, a unique combination of the expertise in design, materials selection, fabrication, NDT development, condition monitoring, life prediction and failure analysis exists to obtain desired results for achieving high levels of reliability and performance assessment of critical engineering components in chemical industries. Case studies related to design, materials selection and fabrication aspects of critical components in nuclear fuel reprocessing plants, NDT development and condition monitoring of various components of nuclear power plants, and important failure investigations on critical engineering components in chemical and allied industries are discussed in this paper. Future directions are identified and planned approaches are briefly described     

Examination of issues related to the development and implementation of real-time operational safety monitoring tools in the nuclear power industry

In recent years, risk and reliability techniques have been increasingly used to optimize deterministic requirements and to improve the operational safety of nuclear power stations. This paper discusses the historical development and current status of implementation of real-time operational safety monitoring tools in the nuclear power industry worldwide. A safety monitor is defined as a PC-based risk management tool, based on a plant specific PSA, which can be used to manage plant safety during the day-to-day operation of a nuclear power plant by planning maintenance activities and providing advisory information to plant operational staff in order to avoid high risk plant configurations. As this technique has only been applied in a few plants worldwide, the technology is still evolving and there are several technical and implementation-related issues which still need to be resolved. This paper attempts to summarize all such issues and describe how they have been addressed in several different applications of this technology around the world.     

High Temperature Corrosion on high temperature materials – Metallographic,scanning electron microscopic and microanalytical tests

High temperature corrosion – oxidative damage of metallic materials from hot gas and other fluid and solid components – shortens the life of gas turbines. By comparing the results of laboratory tests under constant conditions with the results of the operation of gas turbines the facts of failure should be clarified. Metallographie examinations, but especially SEM, Microprobe and AES help to clear the facts of failures in the microscopic zones. These tests will help when selecting materials and their protective coatings with the purpose of higher rentability of the plant.     

Modellbasierte Optimierung und experimentelle Charakterisierung der Thermo‐ und Kryo‐Mechanik werkstoffhybrider Bauteile

Model based optimisation and experimental characterisation of thermo and cryo‐mechanics of hybrid material components The design of hybrid material components with thermo‐mechanical requirements is supported by optimisation techniques. From a set of examples some general conclusions will be derived. Severe requirements come from cryogenic environment not only because of individual material behaviour but also due to possible mismatches between different materials to be combined. This also puts specific requirements for test set ups for determination of related material and component properties.     

Requirements for operational fatigue strength of high quality Cast components

For the fatigue design of high quality cast components the information about service loading, geometry, material behaviour depending on alloy, casting technology, heat treatment and surface state must be available. The influence of these parameters on operational strength is demonstrated on the examples of selected ductile cast iron materials, aluminium alloys and nickel base alloys as well as cast steel. The proof-out of the components must be supported by appropriate quality requirements in design, provision of material and manufacture. However, for safty components, their functionability and fatigue life must be guaranteed by service strain measurements and fatigue tests under variable amplitudes, too.     

Anwendung dilatometrischer Messungen bei der Entwicklung von Glasloten und reaktiven Metallloten zum Fügen von Hochtemperaturbrennstoffzellen

The principle of operation of solid oxide fuel cells (SOFCs) is very simple. However, the fact that very different materials are used for the individual components requires advanced thermal joining techniques to join them in a functional manner. Two very distinct designs have established themselves for the two different intended applications: decentralised power generation (stationary SOFCs) on the one hand, and power converters for vehicles (mobile SOFCs) on the other hand. As a consequence, alternative techniques for joining the individual components are also required. The principal joining process for the stationary SOFC design consists of joining individual steel plates with a glass sealant in an electrically insulating way so that they form an SOFC stack. For the mobile fuel cell design, the SOFC stack consists of individual thin steel cassettes. The window frame of the cassettes, which is made of ferritic chromium steel, is brazed to the ceramic layer of the zirconium oxide solid electrolyte using a filler metal. The material used is a silver‐based brazing filler metal which contains only small amounts of copper oxide (CuO) and titanium hydride (TiH₂) as wetting agents. Both joining processes must be applicable in normal atmospheric air, i. e. under oxidative conditions. R&D activities continue for improving the efficiency and long‐term operational stability of the technology to such an extent that SOFCs will become ready for the energy sector market. The two joining techniques described cannot yet be considered standard processes. They, too, will require continuous improvement with respect to reproducibility, endurance and strength of the joints. The Special Joining Techniques working group at Forschungszentrum Jülich uses specially modified dilatometric techniques as suitable quick replacement methods for studying and measuring the joining characteristics of the materials without having to manufacture complex and expensive SOFC stacks. The shrinkage processes in the glass sealant joints are simulated and measured in the μm range using a special dilatometer. In this way, the amount of glass sealant – which is decisive for tightness and bonding – and the process parameters can be determined in advance. With a vertical dilatometer, the melting behaviour of the reactive silver filler metals is examined with respect to melting point shift, viscosity and void ratio, and as a function of the metal additives (Al) and the process atmosphere.     

Historical perspective on failure rates for US commercial reactor components

To support the development of probabilistic risk assessments of US commercial nuclear power plants, significant effort has been expended to develop generic failure rates for components. Generic failure rates indicate industry-average performance of components, rather than component performance at a specific plant. Most publicly available, generic failure rate databases are typically based on data collected in the 1970s and 1980s for US nuclear power plants. Recent data analysis programs sponsored by the US Nuclear Regulatory Commission and data collection programs sponsored by the Institute of Nuclear Power Operations provide an opportunity to compare more recent failure rate estimates with those obtained in the 1970s and 1980s. These recent results indicate that many component generic failure rates are now lower than observed in the 1970s and 1980s. Suggestions for up-to-date failure rates are presented. Also, failure to run rates for standby components are presented for both short- and longer-term run times.     

Assessment of fire vulnerability for nuclear power plant safety systems by combining fault-and success-oriented logic with physical models

The time behaviour of potential accident sequences may carry important information regarding nuclear power plant (NPP) safety operation and shutdown. In the case of external and environmental events, the ability of NPP components to operate correctly can be changed dramatically in a short time. In contrast to the failures caused by internal events, these two groups of undesirable events may lead to dynamic dependent failures among components of one or several systems. Such kinds of failure should be taken into account in the models of NPP behaviour. To evaluate how successfully the tasks of the safety systems will be carded out, logical models such as fault trees are usually used. The fault trees are not efficient at describing the short-term changes of the failure probabilities for system components. A method that has some advantages over the pure fault tree logic is proposed. The main features of the method are demonstrated by using examples.     

Prediction of deformation and failure of modified 9Cr–1Mo steel under creep-fatigue interaction

Abstract

Plant components often undergo loadings which have aspects of both fatigue and creep by experiencing repeated start-up/shut-downs between steady-state operation at high temperature. In such a case, structural materials show inelastic deformation different from that observed under pure fatigue or pure creep conditions. This finding prompted the development of “unified” constitutive models in which cyclic and creep deformations are treated in a unified way. The author’s group has been developing such a unified model for modified 9Cr–1Mo steel which is widely used in ultrasupercritical fossil power plants. Although the latest model has been demonstrated to be capable of describing deformation behaviour under various loading conditions, including cyclic and steady-state loadings, the model still has room for improvement, such as the need to represent accelerated deformation in the tertiary creep regime. In this study, an attempt is made to improve the capability of the model from various perspectives. A failure prediction model which can deal with various failure modes is also formulated. The deformation and failure lives predicted using these models show reasonable agreement with the results of various load sequence tests obtained on modified 9Cr–1Mo steel.     

Biased Monte Carlo unavailability analysis for systems with time-dependent failure rates

In this paper we consider systems made of components with time-dependent failure rates. A proper analysis of the time-dependent failure behaviour is very important for considerations of life-extension of safety critical systems such as nuclear power plants. This problem is tackled by Monte Carlo simulation which does not suffer from the additional complexity introduced by the model parameters' time inhomogeneity.The high reliability of the systems typically encountered in practice entails resorting to biasing techniques for favouring the events of interest. In this work, we investigate the possibility of biasing the system failures to be distributed in time according to exponential laws. The drawbacks encountered in such procedure have driven us towards the adoption of biasing schemes relying on uniform distributions which distribute failures over the system life more evenly.     

Bruchmechanische Überlegungen über die Auswirkung von Heißrissen

Fracture Mechanical Considerations on the Effect of Hot Cracks. There exists a lot of literature (1, 2) concerning the mechanisms of the origin of hot cracks. Something essential about the effects of hot cracks on the mechanical behaviour of components hardly can be found. These hot cracks can be valuated only by a quantification of the cracks and by an estimation of their behaviour under operational loadings. It is shown how by means of fracture mechanic material laws and calculation methods the growth of hot cracks under operational conditions can be evaluated. By a simple example of a fatigued vessel the systematic procedure is demonstrated and the results of calculation are explained. This procedure can be applied on the most technical problems as the operational stresses generally are below the yield point. For the region of the yield point and above it neither experimental nor theoretical results are available at the moment. The possibility of simplified calculations of burst stress of vessels is mentioned, which is derived from experiments. Experimental data (11) confirm the chosen hypothesis in so far as the burst stress can be realistically or rather conservatively evaluated by this method. It is concluded: With regard to the fact of unavoidable hot cracks for some certain austenitic steels the authors are convinced that basing on the examples and using the well known data of an austenitic steel as well as modified fracture mechanical rules the up to now empirically assumed stability of hot cracks is practically existent against intolerable crack growth. The given examples make it quite significant that the tolerable defect can be rather big without any critical consequences. If the operational conditions are at or above the level of the tolerable stresses, then safety precautions exist in so far as by means of non destructive – for instance during inservice inspections – volumetric examinations can be carried out. This will enable the detection of a critical defect or of one which becomes critical. Whilst the classic hot crack ranges in a micro dimension and therefore remains undetectable for the non destructive testing methods available at present the increased defect is detectable. Furtheron it should be pointed out that this detectable defect will surely be below the critical size and therefore will not be dangerous.     

The MDTA-based method for assessing diagnosis failures and their risk impacts in nuclear power plants

In the emergency situations of nuclear power plants (NPPs), a diagnosis of the occurring events along an accident progression or as initiating events is crucial for managing or controlling a plant to a safe and stable condition. If the operators fail to diagnose the occurring event(s), their responses to a given event can eventually become inappropriate or inadequate. This paper presents an analytical method for assessing the potential for a diagnosis failure (or misdiagnosis) and its consequences for human behaviour and plant safety. The method largely comprises of three steps as follows: (1) Analysis of the potential for a diagnosis failure, (2) Identification of the human failure events (HFEs) that might be induced due to a diagnosis failure, and (3) Quantification of the HFEs and their modeling into a PSA model. The paper also presents a pilot application of the proposed method to the small loss of coolant accident of a Korean NPP.     

Hydro turbine failure mechanisms: An overview

Turbine failure not only increases the plant down-time and brings about revenue losses but also poses a serious threat to the life of the operational and maintenance personnel especially when the power house is underground. There are basically four main failure modes identified in open literature. The current article presents an overview of these four hydro turbine failure modes based on an extensive literature review and also field observation from the Tala Hydropower Plant, Bhutan. Moreover, the report provides information about different turbine parts that are more likely to be liable to damage under these failure modes. It also attempts to elucidate some recommended methods to prevent/mitigate against these failure modes.     

Plant and safety system model

The design and development of a digital computer-based safety system for a nuclear power plant is a complex process. The process of design and product development must result in a final product free of critical errors; operational safety of nuclear power plants must not be compromised. This paper focuses on the development of a safety system model to assist designers, developers, and regulators in establishing and evaluating requirements for a digital computer-based safety system. The model addresses hardware, software, and human elements for use in the requirements definition process. The purpose of the safety system model is to assist and serve as a guide to humans in the cognitive reasoning process of establishing requirements. The goals in the use of the model are to: (1) enhance the completeness of the requirements and (2) reduce the number of errors associated with the requirements definition phase of a project.     

Herstellung und Umformverhalten austenitischer Polymerkernsandwichverbunde

Production and forming behaviour of austenitic steel sandwich composites with a polymer core layer Three‐layered symmetrical sandwich combines offer a great lightweight construction potential due to their structural construction and her good recession behavior at simultaneously high strength. By the combinability of the qualities of the combined partners sandwich combines as good solution offer their services if it is about high strength at good resistance in the lightweight construction. Depending on a functional request all sorts of materials, metals, polymer and others, are used as deck and nuclear material. This one is a central problem for the construction, the processing and later use of sandwich systems in comparison with the monolithic materials, terrible forming behaviour of the combined partners with strongly different Youngth Modulus as well as the question of the adhesion of the different material layers. Nowadays the industrial use of such sandwich materials is still limited. In the context of the project “Untersuchungen zur Kompatibilisierungsmethode ”Umformen“ für unterschiedliche Metall ‐ Nichtmetall – Systeme” promoted by the DFG three layered Sandwich materials were manufactured and analyzed for their characteristics concerning the adhesive strength of the sandwich partners and the forming behaviour. The task existed in the development, the production and investigation of new sandwich materials, which can be adapted to special requirements regarding their functional characteristics. Only the combined view of the material and constructional components leads to system‐oriented materials, construction units, concepts and structures. On the basis of an industrially manufacturing process an adapted press‐joining process for sandwich materials was developed. At the beginning of the project polymer foils were used as core materials, then in th developed pres‐joining process different polymers granules were used as core material. Thereby fiber‐reinforced PA and PP granules were used. As skin layers high‐grade steel sheets of the quality 1,4404 and aluminum plates AlMg3 were used. Depending upon manufacturing processes for the polymer foil an epoxy resin adhesive was used and for the polymer granules three different adhesion agents were used. A reproducible manufacturing could be proven by the angle peeling test. The stretching and deep‐drawing characteristics of the sandwich materials as a function of the core layers could be pointed out in the Erichsen‐Test and in first deep‐drawing investigations. So the Youngth Modulus in the comparison to the polymer foil could be increased by fiber‐reinforced polymer granules. It was possible to change the deep‐drawing behaviour clearly.     

An approach to integrating surveillance and maintenance tasks to prevent the dominant failure causes of critical components

Surveillance requirements and maintenance activities in a nuclear power plant aim to preserve components' inherent reliability. Up to now, predictive and preventive maintenance mainly concerned plant staff, but the US Nuclear Regulatory Commission Maintenance Rule released in July 1991 will have significant impact on how nuclear power plants perform and document this maintenance. Reliability Centered Maintenance (RCM) is a systematic methodology to establish maintenance tasks for critical components in plant with a high degree of compliance with the goals of the Rule. RCM pursues the identification of applicable and efficient tasks to prevent these components from developing their dominant failure causes, and, in turn, towards achieving proper levels of components availability with low cost. In this paper, we present an approach for identifying the most suitable set of tasks to achieve this goal, which involves the integration of maintenance activities and surveillance requirements for each critical component based on the unavailability and cost associated with each individual task which is performed on it.     

珊溪水电厂巡检数据库系统的设计与实现

水电厂设备巡检数据库系统已在多个水电站应用多年,然而由于各水电厂特点不同、管理模式各异,对巡检参数的要求也不同,每一次应用都需重新完善和优化。通过对温州珊溪水电厂的软件设计与优化,该系统的应用更加方便、可靠,很好地满足了电厂的运行要求,实现了电厂减人增效的目的。     

The effect of the threshold stress on the determination of the Weibull parameters in probabilistic failure analysis

In the analysis of brittle materials and components the probability of failure is commonly modelled using a two-parameter Weibull distribution. Occasionally, a three-parameter model is used when the material shows significant threshold behaviour. In this paper two methods for determining the three-parameter constants are discussed. Two theoretical two- and three-parameter distributions are then analysed to examine the number of samples needed to determine the parameters accurately. The two-parameter models are the best fits of the three-parameter models and their failure distributions are very similar to the three-parameter distributions. It is concluded that far more specimens need to be tested than is usually the case to be confident that the correct distribution has been found.