Application of genetic algorithm for reliability allocation in nuclear power plants

Reliability allocation is an optimization process of minimizing the total plant costs subject to the overall plant safety goal constraints. Reliability allocation was applied to determine the reliability characteristics of reactor systems, subsystems, major components and plant procedures that are consistent with a set of top-level performance goals; the core melt frequency, acute fatalities and latent fatalities. Reliability allocation can be performed to improve the design, operation and safety of new and/or existing nuclear power plants. Reliability allocation is a kind of a difficult multi-objective optimization problem as well as a global optimization problem. The genetic algorithm, known as one of the most powerful tools for most optimization problems, is applied to the reliability allocation problem of a typical pressurized water reactor in this article. One of the main problems of reliability allocation is defining realistic objective functions. Hence, in order to optimize the reliability of the system, the cost for improving and/or degrading the reliability of the system should be included in the reliability allocation process. We used techniques derived from the value impact analysis to define the realistic objective function in this article.     

AHP‐IFM Target: An Innovative Method to Define Reliability Target in an Aerospace Prototype Based on Analytic Hierarchy Process

Reliability target definition is a crucial aspect of any reliability analysis. In literature, there are two types of analysis. The first one, called ‘bottom‐up’, goes back to the system's target using data of units through a fault tree analysis. Reliability data of components could be only partially available, particularly in the case of innovative systems. In the second type of analysis, called ‘top‐down’, starting from similar systems, the target of each unit is defined, by applying allocation techniques. Also, in this case, reliability data of similar systems might not be available, and the choice of the most appropriate technique could be tricky. The purpose of the present research is to combine the advantages of both usual approaches. The newly developed approach is based on the integrated factors method, whose values are adjusted trough a multicriteria method, the analytic hierarchy process, depending on the importance of each factor and each unit. The innovation of the proposed model consists in its dynamism, as most of the literature methods use constant weights for the factors involved in reliability allocation. No method takes into account the assignment of a different level of significance (weight) to different units of the system, simultaneously with the considered factors. The developed approach has been applied on an aerospace prototype system. The results show the goodness of the new method and its ability to overcome the problems noted in literature. Copyright © 2017 John Wiley & Sons, Ltd.     

空间站可靠性定额设计

象空间站这样的大型航天器的可靠性问题是十分重要而又十分复杂的，本文仅就其可靠性问题中的定额部分进行了研究，以俄罗斯相关资料上的可靠性指标资料，载人航天器故障统计资料作为原型资料，对未来某型空间站的总体可靠性以及其站上系统的可靠性进行了定额设计。     

Research on multiple-state industrial robot system with epistemic uncertainty reliability allocation method

Reliability allocation of industrial robot (IR) system is one of the important means to improve its whole life cycle, reduce maintenance cost, and characterize weak subsystems. The IR system is not only very complex but also has strong customization; meanwhile, its sample data are small, resulting in unclear degeneration and failure. Based on the above two epistemic uncertainties, a new methodology called multiple-state IR system reliability allocation method with epistemic uncertainty (MIRS-RAM-EU) is proposed. First, the Dempster-Shafer (D-S) evidence theory is used to quantify the epistemic uncertainty. Then, the Kolmogorov differential equations of MIR's subsystems are calculated. The reliability index of MIRS is allocated based on Birnbaum importance degree theory, and the reliability allocation coefficient of each IR subsystem is clearly expressed by this method. Finally, compared with traditional importance allocation method, the MIRS-RAM-EU is more efficient and accurate. This method is usefully directive for reliability evaluation of IR.     

基于模糊故障树的军用气象物资包装可靠性分析

应用模糊故障树分析方法对军用气象物资包装可靠性进行了系统分析,简要介绍了模糊故障树分析方法的基本理论,利用专家判断和模糊集理论相结合的方法,评估了故障树底事件发生的模糊失效概率。并以"TFS-1通风干湿表包装"为例,建立了包装系统的故障树,采用下行法求解了引起顶事件发生的最小割集,定量分析计算,得出模糊失效率为0.0705,同时计算了各底事件的重要度。模糊故障树分析方法对于提高军用气象物资包装防护能力,确保物资装备质量,具有非常重要的意义。     

Reliability Allocation for Series-Parallel Systems subject to Potential Propagated Failures

To analyze the dependent failures in the early stage of system development, this paper considers the potential propagated failures in the reliability allocation process. Factors which can be used to not only measure the component importance but also to reflect the influence brought by propagated failures are proposed. Specifically, cooperative game theory is introduced to explore how the propagated failures affect the failure severity level. Failure rates are obtained by using the Alpha Factor Model with the consideration of dependence among components. Reliability improvement rate is also developed to proportionally assign the target improvement of system reliability to the corresponding components. Furthermore, reliability allocation frameworks for series, parallel and series-parallel systems are designed respectively to make the proposed model meet a wide range of applications. An illustrative example of a hydraulic cooling system is presented to show how the proposed approach is applied. The allocation results demonstrate that the proposed method can achieve a valid reliability improvement with the minimum error.     

An information fusion reliability allocation method considering limited sample size circumstances

As an essential part in product reliability design procedure, reliability allocation can connect the reliability index between the overall product and its multiple subsystems. To obtain a reasonable reliability allocation scheme, especially for limited sample size circumstances commonly encountered in aeronautic and astronautic fields, an information fusion allocation method is constructed in the current study. Grey relational analysis (GRA) is incorporated to analyze objective data, while analytic hierarchy process (AHP) is introduced to implement subjective data analysis. Then a final allocation scheme is derived by fusion of the binary information sources. A real application regarding an aeronautic product reliability allocation is given. The sensitivity of allocation result to limited life information circumstances and comparative analysis with reference methods are implemented to demonstrate the reasonability and effectiveness.     

产品设计过程中的可靠性增长研究

针对产品设计过程中的可靠性增长问题,提出把整个产品系统分为大样本子系统和小样本子系统,对于不同的子系统采用不同的可靠性增长方案和可靠性增长数学分析模型,充分挖掘所有的直接或问接数据信息,并利用到产品设计过程;建立产品的可靠性增长分析数据库系统,保存产品在设计过程中的数据,辅助同类产品设计．     

Reliability analysis against progressive failure of TLP tethers in extreme tension

The tension leg platform (TLP) is a moored floating structure whose buoyancy is more than its weight. The mooring system of TLP consists of number of tensioned tethers connected to the columns at the top and anchored to the seabed at the bottom. These tethers are vulnerable to failure due to extreme (maximum and minimum) tensions. The reliability study of these tethers is an important subject of study. The reliability may further deteriorate after the failure or removal of any of these tethers. In the present study, reliability analysis of an intact TLP and TLP with one tether missing has been carried out. A Von-Mises failure criterion has been adopted to define the failure of tether against maximum tension. The minimum tension failure occurs when the tethers slack due to loss of tension. The limit state functions for maximum and minimum tension cases have been derived. The computationally efficient algorithm First order reliability method (FORM) has been adopted for reliability calculations. Results are presented in terms of reliability indices and probabilities of failure for each sea state. The probabilities of failure so obtained for different sea states have been used for the assessment of annual and life time probability of failures. Reliability of the TLP with one tether missing has been compared with that of the corresponding intact TLP. Effect of wind has also been studied on the progressive failure of TLP tethers and the results have been compared with that of an intact TLP.     

Setting up reliability goals for systems

The quality of life of people of any country depends to a very large extent on the quality and reliability of products and services like transport, communications, power, broadcasting, telecasting, etc. Psychological and physical discomforts are felt by people only when things fail thereby reducing or removing the service. Everybody feels that ‘immortal’ services should be available, but services are created by man and are mortal like him. However, there is a need to have a reliability goal whereby one can guarantee the continuity of a service for a certain lengtht of time/distance with a known risk of failure. Reliability of services is dependent on the reliability of systems comprising them and the management. In this paper the author has tried to focus attention on the need to have reliability goals as distinct from availability goals. The paper gives the basis for setting up reliability goals for a system as well as its subsystems. The allocation of reliability goals for the subsystems is based on the operating experience with similar systems or on the basis of the knowledge of the system or both. Two typical cases of a nuclear power station and a particle accelerator are considered and reliability goals for both system and subsystems are arrived at. The author concludes by stating that reliability goals in respect of simple items produced in large quantities and used anywhere in the world are a necessity. This is so because cost of maintenance is bound to be large with such items.     

On systems availability

Reliability has always been one of the most important criteria for designing any type of system. Unfortunately, increase in reliability may produce a design which, once broken clown, will take a long time to repair, thus reducing the total time for which the system is available for use. The relatively new concept of availability accounts for this trade-off. This paper investigates the problem of optimal availability allocation in a multi-component system. Two methods are considered, namely dynamic programming and the discrete maximum principle. Numerical examples are presented to illustrate these concepts.     

Reliability and redundancy apportionment using crisp and fuzzy multiobjective optimization approaches

The reliability of a multistage system with several components in each stage can be improved either by using more reliable components, or by adding redundant components in parallel in any stage. In many practical situations where reliability enhancement is involved, the decision making is complicated because of the presence of several mutually conflicting goals. For example, in the reliability based design of a system, the designer may be required to maximize the reliability and minimize the cost, weight or volume. This work considers the problem of reliability allocation for multistage systems with components having time-dependent reliability. Two multiobjective optimization techniques are presented, coupled with heuristic procedures, to solve the mixed integer nonlinear programming problems. A generalization of the problem in the presence of vague information results in an ill-structured reliability apportionment problem. The solution of such multiobjective problems is also presented in the present work using the techniques of fuzzy optimization.     

Reliability Analysis of a Composite Wind Turbine Blade Section Using the Model Correction Factor Method: Numerical Study and Validation

Reliability analysis of fiber-reinforced composite structures is a relatively unexplored field, and it is therefore expected that engineers and researchers trying to apply such an approach will meet certain challenges until more knowledge is accumulated. While doing the analyses included in the present paper, the authors have experienced some of the possible pitfalls on the way to complete a precise and robust reliability analysis for layered composites. Results showed that in order to obtain accurate reliability estimates it is necessary to account for the various failure modes described by the composite failure criteria. Each failure mode has been considered in a separate component reliability analysis, followed by a system analysis which gives the total probability of failure of the structure. The Model Correction Factor method used in connection with FORM (First-Order Reliability Method) proved to be a fast and efficient way to calculate the reliability index of a complex composite structure.     

Reliability Optimization by Considering Time‐Dependent Reliability for Components

Reliability optimization is an important and challenging topic both in engineering and industrial situations as its objective is to design a highly reliable system that operates more safely and efficiently under constraints. Redundancy allocation problem (RAP), as one of the most well‐known problems in reliability optimization, has been the subject of many studies over the past few decades. RAP aims to find the best structure and the optimal redundancy level for each subsystem. The main goal in RAP is to maximize the overall system reliability considering some constraints. In all the previous RAP studies, the reliability of the components is considered constant during the system's mission time. However, reliability is time‐dependent and needs to be considered and monitored during the system's lifetime. In this paper, the reliability of components is considered as a function of time, and the RAP is reformulated by introducing a new criterion called ‘mission design life’ defined as the integration of the system reliability function during the mission time. We propose an efficient algorithm for this problem and demonstrate its performance using two examples. Furthermore, we demonstrate the importance of the new approach using a benchmark problem in RAP. Copyright © 2017 John Wiley & Sons, Ltd.     

Reliability aspects of man-machine interface

Issues related to the reliability of man-machine interfaces (MMIs) are discussed from a multi-faceted point of view. Reliability concerning the human operator, the user of MMIs, is discussed using the stimulator data obtained under abnormal plant conditions. Based on the analysis, requirements for MMI design are derived. Cognitive task analysis is also performed to derive design requirements related to human problem solving.Reliability of software, one of the major components of the MMI, is then discussed from the viewpoint of software diversity. Other issues are also discussed to reduce design and implementation errors in the software development process.An experience in carrying out the evaluation of MMIs, one of the critical issues in developing dependable MMIs, is presented to clarify the methodological issues to be solved in the future.The discussions included in this paper show the necessity for a multi-faceted approach to the reliability of MMI both as an integrated object and as a component in a complex human-machine system.     

Reliability assessment of locking systems

The aim of this work is to predict the failure probability of a locking system. This failure probability is assessed using complementary methods: the First-Order Reliability Method (FORM) and Second-Order Reliability Method (SORM) as approximated methods, and Monte Carlo simulations as the reference method. Both types are implemented in a specific software [Phimeca software. Software for reliability analysis developed by Phimeca Engineering S.A.] used in this study. For the Monte Carlo simulations, a response surface, based on experimental design and finite element calculations [Abaqus/Standard User’s Manuel vol. I.], is elaborated so that the relation between the random input variables and structural responses could be established. Investigations of previous reliable methods on two configurations of the locking system show the large sturdiness of the first one and enable design improvements for the second one.     

Bayesian decision analysis and reliability certification

Reliability certification is set as a problem of Bayesian Decision Analysis. Uncertainties about the system reliability are quantified by assuming the parameters of the models describing the stochastic behavior of components as random variables. A utility function quantifies the relative value of each possible level of system reliability after having been accepted or the opportunity loss of the same level if the system has been rejected. A decision about accepting or rejecting the system can be made either on the basis of the existing prior assessment of uncertainties or after obtaining further information through testing of the components or the system at a cost. The concepts of value of perfect information, expected value of sample information and the expected net gain of sampling are specialized to the reliability of a multicomponent system to determine the optimum component testing scheme prior to deciding on the system's certification. A component importance ranking is proposed on the basis of the expected value of perfect information about the reliability of each component. The proposed approach is demonstrated on a single component system failing according to a Poisson random process and with natural conjugate prior probability density functions (pdf) for the failure rate and for a multicomponent system under general assumptions.     

A comprehensive reliability allocation method for design of CNC lathes

In the design and development of computerized numerical control lathes, an effective reliability allocation method is needed to allocate system level reliability requirements into subsystem and component levels. During the allocation process, many factors have to be considered. Some of these factors can be measured quantitatively while others have to be assessed qualitatively. In this paper, we consider seven criteria for conducting reliability allocation. A comprehensive failure rate allocation method is proposed for conducting the task of reliability allocation. Example data from field studies are used to illustrate the proposed method.     

A new efficient algorithm for computing the imprecise reliability of monotone systems

Reliability analysis of complex systems by partial information about reliability of components and by different conditions of independence of components may be carried out by means of the imprecise probability theory which provides a unified framework (natural extension, lower and upper previsions) for computing the system reliability. However, the application of imprecise probabilities to reliability analysis meets with a complexity of optimization problems which have to be solved for obtaining the system reliability measures. Therefore, an efficient simplified algorithm to solve and decompose the optimization problems is proposed in the paper. This algorithm allows us to practically implement reliability analysis of monotone systems under partial and heterogeneous information about reliability of components and under conditions of the component independence or the lack of information about independence. A numerical example illustrates the algorithm.     

Estimation Techniques for Common Cause Failure Data With Different System Sizes

Modeling of system lifetimes becomes more complicated when external events can cause the simultaneous failure of two or more system components. Models that ignore these common cause failures lead to methods of analysis that overestimate system reliability. Typical data consist of observed frequencies in which i out of m (identical) components in a system failed simultaneously, i = 1,…, m. Because this attribute data is inherently dependent on the number of components in the system, procedures for interpretation of data from different groups with more or fewer components than the system under study are not straightforward. This is a recurrent problem in reliability applications in which component configurations change from one system to the next. For instance, in the analysis of a large power-supply system that has three stand-by diesel generators in case of power loss, statistical tests and estimates of system reliability cannot be derived easily from data pertaining to different plants for which only one or two diesel generators were used to reinforce the main power source. This article presents, discusses, and analyzes methods to use generic attribute reliability data efficiently for systems of varying size.