Risk implications of digital reactor protection system operating experience

This paper summarizes an in-depth review of the US nuclear operating experience with the first generation of digital reactor protection systems. The accumulated operating experience from 1984 to 2006 on these first generation digital reactor protection system functions exceeds 1.27 million hours (145.5 yr). A review of failure event reports identified 141 specific events associated with these systems on seven US nuclear power plants. Twenty-six of these events involved some type of common cause failure mechanism (predominantly redundant sensors/channels being out of calibration), which temporarily rendered redundant portions of the overall trip function degraded. Most of these failures were found not to be unique to digital systems. Six of the common cause failure events were more severe and involved situations where incorrect addressable constant data sets were systematically loaded into all redundant computer channels due to personnel errors. One of these events involved a latent software design change error introduced during a software update, which would prevent proper operation, given an unlikely event involving failure of three out of four sensors of one type.Based upon this review of digital system operating experience, a series of risk assessment calculations were performed to evaluate the safety significance of the observed failure events. From the insights gained in this work, it is possible to develop a framework for establishing digital reactor protection system reliability requirements that can be related back to regulatory safety goal objectives and operating experience.     

On a Dynamic Display for Digital System Fault Detection

A new and simple method is presented for fault detection of logic circuits. This method describes hexadecimal dynamic mapping, an alternative "signature," of a digital system on the screen of an existing ordinary oscilloscope. This "signature" consists of a matrix of dots and ones, each dot and/or one representing the logic state of four channels, simultaneously. The number of channels may be increased to more than four at the expense of the simplicity of the hardware of the system that implements the system. The method, furthermore, lends itself to the generation of Karnaugh maps displayed on the screen of an oscilloscope for two or more variables. The true states of a Boolean function are displayed by ones while the false states by dots on this visual Karnaugh map. The generated map may be used in the minimization of combinatorial logic circuits, in the absence of other more advanced simulating techniques, or in the educational field for better comprehension of redundant circuits. The method presented herein, is based on the Lissajous patterns of digital signals, and it may also be used in matching digital signals and detecting phase difference and squareness of pulses, qualitatively.     

A multi-objective discrete reliability optimization problem for dissimilar-unit standby systems

A new methodology for the reliability optimization of a k dissimilar-unit nonrepairable cold-standby redundant system is introduced in this paper. Each unit is composed of a number of independent components with generalized Erlang distributions of lifetimes arranged in a series–parallel configuration. We also propose an approximate technique to extend the model to the general types of nonconstant hazard functions. To evaluate the system reliability, we apply the shortest path technique in stochastic networks. The purchase cost of each component is assumed to be an increasing function of its expected lifetime. There are multiple component choices with different distribution parameters available for replacement with each component of the system. The objective of the reliability optimization problem is to select the best components, from the set of available components, to be placed in the standby system to minimize the initial purchase cost of the system, maximize the system mean time to failure, minimize the system variance of time to failure, and also maximize the system reliability at the mission time. The goal attainment method is used to solve a discrete time approximation of the original problem.      

Optimal maintenance—replacement policy under imperfect maintenance

During a given planning period, in order to have the system working at or below a fixed failure rate, it may be necessary to replace the system to minimize the expected total cost. Preventive maintenance (PM) or replacement should be performed whenever the system reaches the maximum failure rate. This paper formulates a cost model and a branching algorithm to determine the number of PM interventions to be performed between each replacement and the number of such replacements during the planning period to minimize the expected total cost. Adjustment of the failure rate for the system degradation after each PM intervention and inflationary trends in cost factors are also incorporated. Numerical illustration and computational times have been presented.     

炭素电极损伤的敲击响应粗糙集分析检测方法

利用小波分解和粗糙集研究敲击法炭素电极检测技术中的敲击后响应信号,得到了炭素电极损伤的敲击响应粗糙集分析检测方法。文章利用db9小波对敲击后响应信号进行8层分解,对得到的9个小波通道利用粗糙集删除各冗余属性通道,根据删除后知识系统的描述精度确定相应属性的重要度。研究结果表明其中D5、D6、D7、D8四个通道为鉴别正品与次品的有效通道。在研究中提出了二元属性重要度分析的方法,以将重要度为零的可简约属性成对删除,提高属性简约效率,找到知识表达系统的核。并通过机理分析证明,正品与次品敲击信号差异主要来自对产品特征频率基频和低次谐波的共振,从理论上解释了碳素电极敲击法的检测原理。     

A continuous‐time Markov chain model for redundancy allocation problem: An economic analysis

Reliability of an engineering system can be improved by investing on redundant (spare) parts. However, the cost‐efficiency of such an investment is a significant concern that needs to be taken into consideration in practice. To do so, a continuous‐time Markov chain (CTMC) model is presented in this paper to analyze the system's reliability by allocating redundant components. The developed model can also capture the system's repair and failure conditions by defining appropriate states in CTMC. Subsequently, the net present value (NPV) approach is utilized for a variety of scenarios to investigate the effectiveness of investment on spare parts using the break‐even point (BEP) analysis. Afterwards, a comprehensive analysis is carried out to examine the impact of input parameters including interest rate, initial cost of investment, and periodic profit on the decision making process to find the optimal number of spare parts.     

Energy and Timing Measurement with Time-Based Detector Readout for PET Applications: Principle and Validation with Discrete Circuit Components

A new signal processing method for PET application has been developed, with discrete circuit components to measure energy and timing of a gamma interaction based solely on digital timing processing without using an amplitude-to-digital convertor (ADC) or a constant fraction discriminator (CFD). A single channel discrete component time-based readout (TBR) circuit was implemented in a PC board. Initial circuit functionality and performance evaluations have been conducted. Accuracy and linearity of signal amplitude measurement were excellent, as measured with test pulses. The measured timing accuracy from test pulses reached to less than 300 ps, a value limited mainly by the timing jitter of the prototype electronics circuit. Both suitable energy and coincidence timing resolutions (~18% and ~1.0 ns) have been achieved with 3 × 3 × 20 mm(3) LYSO scintillator and photomultiplier tube-based detectors. With its relatively simple circuit and low cost, TBR is expected to be a suitable front-end signal readout electronics for compact PET or other radiation detectors requiring the reading of a large number of detector channels and demanding high performance for energy and timing measurement.     

CDF front end electronics: The rabbit system

A new crate-based front end system has been built featuring low cost, compact packaging, fast readout, command capability, 16 bit digitization, and a high degree of redundancy. The crate can contain a variety of instrumentation modules and is designed to be placed near the detector. Remote, special purpose processors direct the data readout. Channel-by-channel pedestal subtraction and threshold comparison in the crate allow the skipping of empty channels. The system is suitable for the readout of a very large number of channels.     

False targets vs. redundancy in homogeneous parallel systems

System defense against natural threats and disasters that have a stochastic nature includes providing redundancy and protecting system elements. The defense against strategic intentional attacks can also include deploying false targets aimed at misleading the attacker. Distribution of the available resources among different defensive means is an important problem that arises in organizing the defense of complex civil infrastructures, industrial systems or military objects. The article considers defense resource allocation in a system exposed to external intentional attack. The expected damage caused by the attack is evaluated as system unsupplied demand. The defender distributes its limited resource between deploying redundant genuine elements and false elements, both of which are targets of attack. The attacker attacks a subset of the elements and distributes its limited resource evenly among the attacked elements. Two cases are considered: in the first one the number of attacked elements and the vulnerability of each genuine element are fixed and the defense resource distribution is determined as a solution of an optimization problem; in the second one the number of attacked elements is the attacker's free choice variable and the element's vulnerability depends on a contest determined by the defender's and attacker's resources allocated to each element. The defender's optimal resource distribution strategy is determined as a solution of a two-period minmax game. It is shown that the optimal number of genuine elements decreases monotonically with the growth of the element cost and vulnerability, whereas the optimal number of false elements demonstrates non-monotonic behavior. The contest intensity is an important factor influencing the optimal defense resource distribution. It cannot be ignored when the defense strategy is determined, and it thus also impacts the attack strategy.     

基于霍尔效应的印刷机能耗检测方法研究

目的采用霍尔元件法搭建印刷机群能耗数字化检测平台。方法通过建立印刷机能耗指标模型框架体系,形成以总耗电量指标、均值类指标、耗电比例类指标等为主的印刷机耗电评价标准,完善能耗评价准则。采用灵敏度高、稳定性好的霍尔元件作为能耗检测元件,并分析霍尔效应、磁平衡式电流和电压霍尔传感器的检测电路。采用伏安法功率测量方式分别检测各负载电路的电流与电压,通过A/D转换模块实现检测数据数字化,并通过硬件控制核心对采集的数字信号进行运算、输出和显示,采用CAN总线实现硬件控制系统与上位机之间数字信息交互。结果搭建了印刷机群能耗数字化检测系统平台,实现了外围设备与上位机间的数据交互,实现了印刷机群以数据为核心,通过能耗检测、能效分析、能源成本控制以及能源指标优化,提高了能源利用效率。结论能耗检测的实现既能减少冗余电量损耗,又有利于监测电路稳定性,利用霍尔元件法实现印刷机群能耗检测的方法是可行的。     

考虑设备劣化及返工的质量批量模型研究

为了得到最大质量批量，研究了返工成本边际效应下的期望利润值计算方法，将生产过程分为两个阶段：无需返工和需要返工的连续加工。给出了质量控制策略下批量计算公式以及考虑边际成本下的最大利润所对应的质量批量。采用蒙特卡洛仿真和Design Expert 8.0.6软件拟合，通过数字实验，对比了返工成本与未考虑返工成本情况下的期望利润值。研究显示，考虑返工成本边际效应下的质量批量大于一般的经济批量，所得到期望利润也最大。     

一种新的分布式无线传感器网络定位方法

针对无线传感器网络节点能源受限的特征,以系统最小硬件开销为设计原则,提出了一种适用于基于测距的分布式定位方法(3/2-NANDB),该方法可在不增加单个独立节点硬件开销的情况下,利用附加的外部控制系统发射一个旋转定向波束充分挖掘节点间的冗余信息,有效排除节点位置的模糊性,从而可完全确定只有两个邻居节点的节点位置和部分只有一个邻居节点的节点位置,达到减少GPS携带节点数量、最大化网络内部可定位节点数目、扩大网络观察范围和延长无线传感器网络存活时间等目的.而利用该方法的节点二义性排除算法,还可以辅助其他现有的基于三邻居(3-NA)的定位算法提高整体定位性能.     

A method for evaluating fault coverage using simulated fault injection for digitalized systems in nuclear power plants

The fault coverage for digital system in nuclear power plants is evaluated using a simulated fault injection method. Digital systems have numerous advantages, such as hardware elements share and hardware replication of the needed number of independent channels. However, the application of digital systems to safety-critical systems in nuclear power plants has been limited due to reliability concerns. In the reliability issues, fault coverage is one of the most important factors. In this study, we propose an evaluation method of the fault coverage for safety-critical digital systems in nuclear power plants. The system under assessment is a local coincidence logic processor for a digital plant protection system at Ulchin nuclear power plant units 5 and 6. The assessed system is simplified and then a simulated fault injection method is applied to evaluate the fault coverage of two fault detection mechanisms. From the simulated fault injection experiment, the fault detection coverage of the watchdog timer is 44.2% and that of the read only memory (ROM) checksum is 50.5%. Our experiments show that the fault coverage of a safety-critical digital system is effectively quantified using the simulated fault injection method.     

一种基于并行结构的宽带实时数字下变频技术

提出一种并行处理结构,通过多个数字下变频通道并行处理,能有效降低采样率和数据处理速率,解决超宽带数字中频处理的难题。分析了并行数字下变频技术的原理推导,分析了多路NCO输出相位差及影响信噪比的误差因素,为具体工程设计提供指标分配依据。最后通过实验平台验证方法的正确性。     

Selection of an optimum readout frequency in digital measurements

Summary As a result of the above research we have proved the existence of an optimum readout frequency for digital measurements and determined the optimum operating conditions for a given speed of a digital converter, number of measuring channels, and spectral width of the measured processes.     

A robust cell formation approach for varying product demands

In cellular manufacturing environments, manufacturing cells are generally formed based on deterministic product demands. In this paper, we consider a system configuration problem with product demands expressed in a number of probabilistic scenarios. An optimization model integrating cell formation and part allocation is developed to generate a robust system configuration to minimize machine cost and expected inter-cell material handling cost. A two-stage Tabu search based heuristic algorithm is developed to find the optimal or near optimal solutions to the NP-hard problem. Numerical examples show that this model leads to an appropriate compromise between system configuration costs and expected material handling costs to meet the varying product demands. These example problems also show that the proposed algorithm is effective and computationally efficient for small or medium size problems.     

Distribution system reliability enhancement using optimal capacitor placement

Failure statistics of most utilities indicate that distribution systems make the greatest individual contribution to the unavailability of supply to customers. Optimal capacitor placement in distribution systems has a number of advantages such as reducing losses, improving voltage profile, improving power factor and so on. The conventional objective function of the optimal capacitor placement consists of the total cost of losses and investments. Since capacitors supply reactive loads locally, they improve the load-carrying capability of the lines and therefore play the same role as redundant lines. Thus, optimal capacitor placement can also improve the reliability indices of a distribution system. Therefore two new objective functions are defined here. The first one is defined as the sum of reliability cost and investment cost. The second is defined by adding the reliability cost, cost of losses and investment cost. The latter is a comprehensive objective function which is the resultant of the conventional and reliability-based objective functions. This problem is solved using a particle swarm optimisation-based algorithm. The effectiveness and applicability of the proposed approach are examined using a distribution network.     

Economic design of a circular consecutive--out-of-:F system with -step Markov dependence

A circular consecutive-k-out-of-n:F system consists of n components arranged along a circular path. The system fails if and only if at least k consecutive components in the system fail. The system reliability, the expected system life, and the expected number of failures are obtained under the assumption that the failure rate of a component depends on the number of consecutive failed components that follow it. A procedure to find the optimal k and a simulation procedure to search the near-optimal k are proposed with illustrative numerical examples. An expected cost per unit time is considered as the objective function to be minimized.     

Optimal testing policy for a computer system with intermittent faults

A computer system with intermittent faults fails with probability p when it is used in hidden faults. Periodic tests are scheduled at times kT(k = 1, 2,…) to detect a hidden fault. The mean time, the expected number of tests and the expected cost until detection of a fault or system failure are derived, using the theory of Markov renewal processes. An optimal testing time T^* to minimize the expected cost is discussed. A finite T^* is given by a unique solution of an equation.     

Optimal maintenance models with minimal repair, periodic overhaul and complete renewal

This paper investigates the maintenance decision situation in which three actions, minimal repair, periodic overhaul and complete renewal, may be applied to the system under consideration. A new mathematical model is proposed to describe system improvement due to the maintenance action of an overhaul that differs from the virtual age approach by considering a direct reduction on the system's failure rate. Based on this improvement model, two cost models for determining the optimal overhaul interval and the number of overhauls in a renewal cycle, that minimize the expected unit-time cost or the total discounted cost, are established. Existence conditions of optimal solutions are obtained and special cases of the two cost models are discussed.