Passive System Reliability Assessment Research Articles

An active learning approach for reliability assessment of passive systems combining polynomial chaos expansion and adaptive sampling

Passive safety system is extensively employed in newly designed reactors to increase their inherent safety. However, the driving force is smaller, and the uncertainty of the thermal–hydraulic (T-H) process may potentially result in the inability of system to perform its intended function, which is often known as functional failure. Therefore, the passive systems reliability has received increasing attention. Unfortunately, the assessment of system failure probability utilizing Monte Carlo simulation (MCS) methods necessitates a significant quantity of repeated calculations employing the thermal–hydraulic code, which may be impractical in terms of computational cost. To enhance the assessment calculation efficiency, an adaptive surrogate model is proposed. The bootstrap method was employed to introduce an error estimate into the polynomial chaos expansion (PCE) model, which solves the obstacle of the PCE model in reliability evaluation applications due to the lack of error estimation. The method constructs an active learning approach through error estimation, strategically identifies and selects the best candidate samples, and iteratively updates the initial experimental design. The effectiveness of the method was verified through three benchmark cases, and it was employed for assessing the passive residual heat removal system (PRHRS) reliability within integral-type pressurized water reactor (IPWR200). The results demonstrated that the proposed method significantly diminishes the frequency of invocations to the T-H code and improves computational efficiency to a large extent. Furthermore, it can serve as a valuable guide for the design, operation, and reliability evaluation of the passive system.

Reliability Assessment of Passive Safety Systems for Nuclear Energy Applications: State-of-the-Art and Open Issues

Passive systems are fundamental for the safe development of Nuclear Power Plant (NPP) technology. The accurate assessment of their reliability is crucial for their use in the nuclear industry. In this paper, we present a review of the approaches and procedures for the reliability assessment of passive systems. We complete the work by discussing the pending open issues, in particular with respect to the need of novel sensitivity analysis methods, the role of empirical modelling and the integration of passive safety systems assessment in the (static/dynamic) Probabilistic Safety Assessment (PSA) framework.

Reliability assessment of passive systems using artificial neural network based response surface methodology

Many advanced Nuclear Reactor designs deploys passive systems for enhancing safety Passive systems rely on natural driving forces, such as natural circulation, gravity, internal stored energy etc., which do not require external power sources. The driving forces are weak, hence, the phenomenological failures becomes equally important as compared to mechanical failures for passive systems.Substantial efforts are underway towards improving reliability assessment methods for passive systems, however, consensus is not yet reached. Simplified and subjective assumptions made for generating the Response Surface may not necessarily be true for the complex systems.This paper describes ANN Based Response Surface (ANNBRS) methodology developed for reliability assessment of passive systems. The methodology utilizes Principle Component Analysis (PCA) to reduce dimensionality of system and optimizing efforts in designing training data for ANN model. The ANNBRS methodology is applied to Isolation Condenser System (ICS) to demonstrate improvements over the existing Response Surface based approaches.

Optimization of regression model using principal component regression method in passive system reliability assessment

The estimation of reliability of a passive system employed in innovative nuclear reactors has been a challenging assignment due to substantial involvement of natural convection and gravity in its function. This involvement warrants phenomenological failures to cast major influence on probabilistic safety assessment (PSA) of passive system. The phenomenological failures associated with passive systems cannot be evaluated directly by deterministic approach. Probabilistic method deploys thermal-hydraulic codes to evaluate availability of the system and generates simulated response surfaces comprising of large amount of data, which eventually consumes improbable computation time. Regression models can be used to generate large amount of data with limited number of thermal –hydraulic code runs. To make regression models more effective, it is necessary to evaluate effect of each input parameter that goes into them. Isolated condenser system (ICS) is one of the passive systems used in nuclear reactors. The performance of ICS is evaluated by measuring a parameter called integrated power ratio (IPR) with use of regression models. In present study, these regression models are optimized using principal component regression (PCR) method. PCR method eliminates multi-collinearity present within input parameters and thus retains only those parameters which significantly affect the output. Results obtained by PCR method are compared to the outcome of hypothesis testing. The exercise indicates that under given design conditions, at least one input parameter can be omitted from the regression model without any significant effect on IPR. This makes ICS more reliable due to reduction in its probability of failure owing to omission of a redundant input parameter.

A review: passive system reliability analysis – accomplishments and unresolved issues

Reliability assessment of passive safety systems is one of the important issues, since safety of advanced nuclear reactors rely on several passive features. In this context, a few methodologies such as Reliability Evaluation of Passive Safety System (REPAS), Reliability Methods for Passive Safety Functions (RMPS) and Analysis of Passive Systems ReliAbility (APSRA) have been developed in the past. These methodologies have been used to assess reliability of various passive safety systems. While these methodologies have certain features in common, but they differ in considering certain issues; for example, treatment of model uncertainties, deviation of geometric and process parameters from their nominal values, etc. This paper presents the state of the art on passive system reliability assessment methodologies, the accomplishments and remaining issues. In this review three critical issues pertaining to passive systems performance and reliability have been identified. The first issue is, applicability of best estimate codes and model uncertainty. The best estimate codes based phenomenological simulations of natural convection passive systems could have significant amount of uncertainties, these uncertainties must be incorporated in appropriate manner in the performance and reliability analysis of such systems. The second issue is the treatment of dynamic failure characteristics of components of passive systems. REPAS, RMPS and APSRA methodologies do not consider dynamic failures of components or process, which may have strong influence on the failure of passive systems. The influence of dynamic failure characteristics of components on system failure probability is presented with the help of a dynamic reliability methodology based on Monte Carlo simulation. The analysis of a benchmark problem of Hold-up tank shows the error in failure probability estimation by not considering the dynamism of components. It is thus suggested that dynamic reliability methodologies must be

Evaluation of the dependencies related to passive system failure

A major issue to be addressed in safety and risk studies related to advanced reactors is the reliability of the implemented passive safety features. The passive safety system operation is a quite complex process. This complexity gives rise to unpredictable failure patterns. While there are a number of well-established failure analysis (physics-of-failure) models for individual components, these models do not hold good for complex systems as their failure behaviours may be totally different. Failure analysis of individual components does consider the environmental interactions but is unable to capture the system interaction effects on failure behaviour. These models are based on the assumption of independent failure mechanisms. Dependency relationships and interactions of components in a complex system might give rise to some new types of failures that are not considered during the individual failure analysis of that component. The approach to the passive system reliability assessment based on independent modes of failure begins by identifying critical parameters, as input to basic events, corresponding to the failure modes, arranged in a series system configuration. Within this methodology, the selected system critical parameters are properly modelled through the construction of probability functions. The application of the methodology to a realistic thermal–hydraulic passive system design is illustrated. The analysis reveals that the critical parameters are not suitable to be chosen independently of each other, mainly because of the expected synergism between the different phenomena under investigation, with the potential to jeopardize the system performance. This conclusion allows the implementation of the proposed methodology, by properly capturing the interaction between various failure modes.

Reliability Prediction of Passive Systems Based on Bivariate Probability Distributions

The focus of the present study is passive system reliability assessment, with reference to the thermal-hydraulic passive systems (i.e., relying on natural circulation). An approach based on system-relevant performance parameters is introduced to provide system-significant availability and reliability figures, within a reliability physics framework.The method exploits the fact that for thermal-hydraulic passive systems to perform as expected to accomplish the required mission, parameters must lie between certain limits according to defined safety criteria. Some relevant physical parameters are worth considering as significant indicators of thermal-hydraulic passive system performance, for instance coolant flow or exchanged thermal power. Within this methodology, the selected representative parameters defining the system performance are properly modeled through the construction of joint probability functions in order to assess the correspondent functional reliability. The application of the methodology to a realistic passive system design is illustrated.The results are shown to point out the relevance of the passive system functional reliability aspects with respect to the classical mechanical component malfunctions, serving as a foundation for continuous improvement of the passive system reliability assessment process.This paper aims to remedy some of the limitations following on from applying the functional reliability approach to the passive system reliability problem, as highlighted in an earlier paper [Nuclear Technology, Vol. 144, p. 145 (Nov. 2003)]. This concerns essentially the assumption of independence between the marginal distributions to construct the joint probability distributions to evaluate system reliability.

Failure Mode and Effect Analysis Application for the Safety and Reliability Analysis of a Thermal-Hydraulic Passive System

The inclusion of passive safety-related systems within the advanced reactor design claims high system availability and reliability. A detailed system and safety analysis applying the failure mode and effect analysis (FMEA) approach is required as a primary step for the development of a methodology aimed at the reliability assessment of passive systems. The present study concerns thermal-hydraulic passive systems that are designed for decay heat removal and rely on natural circulation that foresee a heat exchanger immersed in a cooling pool. The main purposes of the work are to identify important accident initiators, find out the possible consequences to the plant deriving from component malfunctions, individuate possible causes, identify mitigating features and systems, and classify accident initiators in initiating events of accident sequences. A qualitative overview of accident sequences could be derived from the FMEA tables looking at consequences’ description and preventive and corrective actions. Failure probabilistic evaluations are included as well to point out the probabilities and frequencies to have the plant in fault and/or unavailability conditions during passive system operation, therefore ensuring a complete set of initiating events of reactor accident sequences. Finally, important feedback to the design activities will derive from the FMEA study performed for safety assessment purposes. An important lesson elicited from the analysis is that measures against common-cause failures can reduce significantly the probability of failure of the system.

The analytic hierarchy process as a systematic approach to the identification of important parameters for the reliability assessment of passive systems

Passive systems play a crucial role in the development of future solutions for nuclear plant technology. A fundamental issue still to be resolved is the quantification of the reliability of such systems. In this paper, we firstly illustrate a systematic methodology to guide the definition of the failure criteria of a passive system and the evaluation of its probability of occurrence, through the identification of the relevant system parameters and the propagation of their associated uncertainties. Within this methodology, we propose the use of the analytic hierarchy process as a structured and reproducible tool for the decomposition of the problem and the identification of the dominant system parameters. An example of its application to a real passive system is illustrated in details.

Passive System Reliability Analysis: A Study on the Isolation Condenser

This paper deals with the reliability assessment of passive systems that have been developed in recent years by suppliers, industries, utilities, and research organizations, aimed at plant safety improvement and substantial simplification in its implementation. The present study concerns the passive decay heat removal systems that use, for the most part, a condenser immersed in a cooling pool. The focus of the paper is a reliability study of the isolation condenser system foreseen for advanced boiling water reactors (BWRs) for the removal of the excess sensible and core decay heat from the BWR by natural circulation. Furthermore, an approach aimed at the thermal-hydraulic performance assessment (i.e., the natural circulation failure evaluation) from the probability point of view is given. The study is not plant-specific-related but pertains to the conceptual design of the foregoing system.