Lixuan Lu

Analysis of on-line maintenance strategies for k-out-of-n standby safety systems

Abstract The objective of this paper is to compare the performance of three on-line test and maintenance strategies (corrective maintenance, preventive maintenance and predictive maintenance) for standby k -out-of- n safety systems. Each channel of the k -out-of- n system is modelled by an age-dependent unavailability model to reflect the effect of maintenance on the aging process. The system unavailability, the probability of spurious operation and the overall cost under the above maintenance strategies are analyzed and compared to obtain the optimal maintenance strategy. Sensitivity analyses are performed to reveal the effect of different model parameters on the system performance. A standby safety system in Canadian Deuterium–Uranium (CANDU) Nuclear Power Plants (NPPs), the Shutdown System Number One (SDS1), is used to illustrate the proposed analysis and the procedure. It is concluded that maintenance should neither be performed too frequently nor too rarely. When the system deteriorates very slowly, the corrective maintenance is more preferable than the preventive and predictive maintenance. When the failure rate of the system is high, the preventive maintenance results in the best system performance.

Joint Failure Importance for Noncoherent Fault Trees

There exist several risk importance measures in the literature to rank the relative importance among basic events within a fault tree. Most of the importance measures indicate how important a basic event is with respect to the top event of the fault tree. However, the mutual influence among the basic events should also be considered. This is particularly true in practice when making maintenance decisions with a limited resource. This paper investigates the Joint Failure Importance (JFI), which reflects the interaction among basic events, namely, the change in the Birnbaum Importance of one basic event when the probability of another basic event changes. Even though the JFI for coherent fault trees and its properties have been examined in the literature, the results cannot be easily extended to noncoherent fault trees. The current work has shown that, for both coherent, and noncoherent fault trees, the sign of the JFI can provide useful information. However, the properties of the JFI for noncoherent fault trees are more complex, and do not always share with those for coherent fault trees. The Shutdown System Number One (SDS1) in a Canadian Deuterium-Uranium (CANDU) Nuclear Power Plant (NPP) is utilized to illustrate the theoretical results developed in this paper.

Reliability modeling of networked control systems using dynamic flowgraph methodology

The recent trend in deploying communication networks in digital control systems to form Networked Control Systems (NCSs) brings the need for utilizing dynamic methods to assess the reliability of these systems. The methods should be able to capture the behaviour and interaction of the hardware, the software and the communication network in the NCSs. They should also be able to incorporate time dependency and multi-state behaviour. In this paper, it is demonstrated how the Dynamic Flowgraph Methodology (DFM) can be a promising method to fulfill these requirements. The behaviour and the effect of the communication network on NCS performance is emphasized. The information provided by analysis of the implemented model facilitates the improvement of the control system performance.

Configuration determination for k-out-of-n partially redundant systems

The k-out-of-n configuration is a widely adopted structure for partially redundant safety systems. This configuration ensures a high level of reliability and safety with limited financial and space resources. It also facilitates on-line Test and Maintenance (T&M) without having to shut the system down. One question a decision maker needs to answer when adopting k-out-of-n systems is: what is the best configuration for the application, i.e. how many channels in total need to be utilized and among these channels, how many channels need to function simultaneously in order for the system to function. There are various factors to consider in order to make this decision. This paper looks at this problem from a reliability engineers point of view. A quantitative analysis is performed for both unavailability and probability of spurious operation due to independent failure. In particular, the relative gain and/or loss of these quantities that occurs due to changing from one configuration to another are compared through rigorous mathematical analysis. The results provide important information that can be used when choosing system configurations to meet regulatory requirements and financial constraints. The two different configurations for shutdown systems in Nuclear Power Plants, the 2-out-of-3 system and the 2-out-of-4 system, are utilized as an example to illustrate the theoretical results.

Reliability evaluation of standby safety systems due to independent and common cause failures

Standby redundant systems are often adopted in critical applications such as the emergency shutdown systems (ESDS) in nuclear power plants (NPPs). One failure mode of the standby redundant systems is that they are not available when there is a demand. This is a serious safety issue. Another failure mode of the standby safety critical systems is that they function spuriously when there is actually no need. Once this occurs, the normal plant operation will be interrupted; certain equipment could be damaged; and restarting the plant could be very costly. The objective of this paper is to evaluate the unavailability and the probability of spurious operation of k-out-of-n systems when they are subjected to both independent and common cause failures (CCFs). A load-strength interference model is adopted for CCF analysis. A data mapping technique is utilized when there is no data available for a specific system. It is concluded quantitatively that the k-out-of-n system has a lower unavailability but a higher probability of spurious operation than the k-out-of-(n-1) system, under both independent failure and CCFs. This result complies with common sense and practical experience. The two different configurations adopted in different types of NPPs, the 2-out-of-3 system and the 2-out-of-4 system, are used to demonstrate the theoretical analyses that are developed in this paper. However, due to the lack of relevant data, the analysis of probability of spurious operation under CCFs are only explained in a qualitative manner

A comparison of Fault Trees and the Dynamic Flowgraph Methodology for the analysis of FPGA-based safety systems Part 1: Reactor trip logic loop reliability analysis

The use of Field Programmable Gate Arrays (FPGAs) in safety critical systems in nuclear power plants means that these systems must undergo a detailed reliability and safety analysis. Fault Tree Analysis (FTA) has seen extensive use in the nuclear power industry. However, FTA predates digital I&C systems, and only performs static analyses. Therefore, dynamic (time dependent) methodologies have been created to model and analyze digital I&C systems. One method is the Dynamic Flowgraph Methodology (DFM). DFM can model control loops and feedback, which are properties that FPGA-based systems include. This work presents a comparison of FTA and DFM analysis methods, for analyzing the reliability of a generic, one-parameter, one-channel FPGA-based reactor trip logic loop. The system was analyzed for two separate failure conditions, with the DFM and FTA results being compared. The DFM and FTA results were similar for simple systems using one time step, however the results were more different for multiple time steps and/or complex test systems. Issues with FTA were discovered pertaining to the oscillating clock states, leading to impossible MCS being returned by the FTA. Potential reasons for the different results returned by two methods are discussed, as is direction for future comparisons between these methods.

A Review of the Current State of FPGA Systems in Nuclear Instrumentation and Control

A Field Programmable Gate Array, or FPGA, is a form of integrated circuit that is programmed (configured) after it has been built. These devices have recently become a topic of interest for various applications in the nuclear field. Most of the recent work put into these FPGA systems is for the purpose of Instrumentation and Control (I&C) systems, but other applications include health physics, particle detectors, and pulse measurement systems. These new FPGA based systems are thought of as possible replacements for older, analog systems that are commonly used in Nuclear Power Plants (NPPs). Many of these systems are becoming obsolete, and it can be difficult to repair and maintain them. FPGAs possess certain advantages over traditional analog circuits, as well as microprocessors, for nuclear I&C applications. This paper provides an extensive literature survey on the current research into FPGA-based systems in NPP applications, such as shutdown systems, neutron monitors, and feedwater controls. Current plans and plans for future FPGA implementations are also discussed. Research from different countries in North America, Europe and Asia is discussed, covering a variety of NPP types (CANDU, Pressurized Water Reactors, Boiling Water Reactors, etc.). The main companies and organizations involved in the FPGA research and development are examined, and a direction for future research is presented.Copyright

An overview of multi-carrier modulation system for data transmission through smart grid

Smart Grid (SG) and Automated Metering Infrastructure (AMI) technologies are increasingly seen as a critical component to increase efficiency and reliability of the electric power delivery system, and to improve customer service. The promising smart grid system requires high speed data transfer from all the sensors on the system within a few power-cycles. The installed physical electrical network permits Power Line Communication (PLC) which has been an aspiration for the electronics industry especially for building automation. In this paper, different communication methods through AC power line carrier for more efficient energy control are discussed. Among these the Spread-spectrum technique is recognized as an efficient tool against a variety of interferers and hostile channel properties, such as time-variant attenuation and selective fading. An interactive spread spectrum technique for data transmission through smart grids is also presented.

Design and reliability prediction of a distributed landing gear control system

Purpose – Proper function of landing gear plays a crucial role in the safe operation of an airplane. Traditional landing gear control system utilizes centralized control technology. The relatively heavy wire harness and low reliability accompanied with this technology make it logical to transfer from traditional control to real‐time distributed control. This paper aims to look into a new landing gear control system based on time‐triggered architecture (TTA).Design/methodology/approach – In this paper, a new landing gear control system based on TTA is proposed. The reliability of the proposed system is investigated using a combination of Markov analysis and MIL‐HDBK‐217 methods.Findings – The results show that by integration of TTP/C and TTP/A technologies, the advantages of both are achieved. A very high level of reliability is obtained. This increases the confidence when adopting distributed landing gear control technology.Originality/value – The paper presents a new landing gear control system based on ...

Reliability and Safety Assessment of a Conceptual Thermochemical Plant for Nuclear-Based Hydrogen Generation

Hydrogen is a clean fuel that can help to reduce greenhouse gas emissions, as its oxidation does not emit carbon dioxide (a primary greenhouse gas). Generation of hydrogen has attracted much recent worldwide attention. A promising method to generate hydrogen is to use heat from nuclear power plants. The advantages of using nuclear heat are capabilities of large-scale generation of hydrogen and zero greenhouse gas emissions. Nuclear energy is expected to have an important role for hydrogen generation in the future. In this paper, reliability and probabilistic safety assessments of a conceptual nuclear-hydrogen plant will be analyzed. There are two main methods to generate hydrogen from nuclear energy. They include: 1) thermochemical processes and 2) electrochemical processes. The conceptual plant of this paper is based on a Cu-Cl thermocycle developed by Atomic Energy of Canada Limited (AECL) and the Argonne National Laboratory (ANL). Using a flowsheet of the hydrogen plant created by an Aspen Plus simulation by ANL, four fault-trees are constructed for potential risk scenarios. Based on the results from the fault tree analyses (FTA), the risk levels of the hydrogen generation plant under different accident scenarios can be calculated. Based on the results, potential problems encountered in Cu-Cl cycle are identified and possible solutions will be recommended for future improvements.Copyright