Reliability Of Passive Systems 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.

Rigorous Reliability Testing of an Encapsulated Thermal Pyrolytic Graphite (TPG) Heat Spreader for Passive Thermal Management Applications

Continued microelectronic miniaturization has resulted in higher power density, increasing the need for high performance thermal management. One possible solution for board-level thermal management is to fully encapsulate a high thermal conductivity material, such as Thermal Pyrolytic Graphite (TPG; ~1700 W/mK) in a metal (such as 6061 aluminum), producing an efficient passive heat spreader for thermal dissipation. The metal shell provides sufficient mechanical strength at a small detriment to the overall thermal conductivity. Reliability and consistent performance over time are critical for thermal management systems, especially in Defense and Aerospace applications. MIL-STD-883H provides test parameters for reliability involving exposure to mechanical shock (Method 2002.5), mechanical vibration (Method 2005.2), and thermal shock (Method 1011.9), with traditional evaluation criteria involving microelectronic device performance and/or a visual examination. For this work, the reliability of aluminum encapsulated TPG test coupons was evaluated with more demanding criteria: a visual examination, X-ray inspection, acoustic microscopy, and thermal conductivity performance testing, both before and after the previously discussed MIL-STD-883H exposures. The encapsulated coupons showed excellent robustness; the thermal conductivity properties remained constant with an average value around 965 W/mK, and the non-destructive imaging revealed excellent bond quality for the metal encapsulation, with no evidence of delamination. This more stringent evaluation criteria after MIL-STD-883H testing provides a better way to evaluate the reliability of passive thermal management systems. The study results also demonstrate the metal encapsulated TPG heat spreader as a superior thermal management solution with high performance and high reliability for demanding applications.

Application of REPAS to analyze the sump clogging issue following a LOCA and its impact on the reliability of the ECCS long-term core cooling function

Sump clogging has been identified as a relevant issue after an accident occurred in the Barsebäck-2 nuclear power plant in Sweden (1992). Following a steam line Loss of Coolant Accident (LOCA) due to the inadvertent opening of a safety relief valve, the jet stripped some insulation material from nearby pipes. The insulation debris were transported to the inlet of the strainers for the drywell spray system, thus clogging the intake. The accident was not serious but showed that the Emergency Core Cooling System (ECCS) could have failed. As a consequence, several actions have been undertaken by international organizations, regulatory bodies and nuclear power plant owners to characterize this issue, and propose solutions and improvements. In the present paper, the Reliability Evaluation of Passive Safety Systems (REPAS) methodology is applied to analyze the sump clogging issue and its effect on the long-term core cooling function. Originally developed to evaluate the reliability of passive systems, REPAS is here applied for the first time to an active system. The application is performed using the TRAC/RELAP Advanced Computational Engine (TRACE) code, developed by the USNRC, to simulate a generic three-loop PWR, with an active decay heat removal system.

METHOD OF QUALIFICATION OF PASSIVE SAFETY SYSTEMS OF MODULAR NUCLEAR REACTORS WITH CIRCULATION CIRCUIT FLOWS

Low-power modular reactors are a promising direction for increasing the safety of nuclear power, because accident management in modular reactors is carried out only by passive safety systems (without electric pumps). Critical for the safety of modular reactors are accidents with a violation of the tightness of the natural circulation circuits of passive safety systems. The main limitations of using traditional accident modeling approaches with deterministic codes to qualify the reliability and operability of passive safety systems of modular reactors are related to the possibility of negative effects of "code differences" and "code user differences", as well as the unfoundedness of code verification/validation results. An original qualification method has been developed to ensure the safety conditions of the passive safety systems of the Westinghouse low-power modular reactor (SMR) in the event of accidents with a violation of the tightness of the natural circulation circuits. The assumptions adopted in the developed method ensure the conservatism of qualification results. Based on the preliminary calculation qualification of the natural circulation circuits of the SMR passive safety systems, it was established that for the relative sizes of the leaks, greater than 5% of the pipeline cross-section, a violation of safety conditions and drainage of the active zone may occur less than 24 hours after the start of the accident. It is necessary to modernize the SMR with regard to systems for diagnosing leaks in natural circulation circuits of passive safety systems and isolating damaged sections of circuits.

Scaling, Passive Systems, and the AP-1000

The development of the AP-1000 design and of its precursor the AP-600 started in the aftermath of the Chernobyl event (1986) when the need came from the scientific and technological community for a resilient system against deliberate threats by humans. The “passive system” design concept became relevant. The first AP-1000 entered into operation around 3 decades after that event. This paper discusses the issue of how much the progress in nuclear science and technology since the end of the 1980s has affected the AP-1000 design. Five interconnected areas are identified: (1) reliability of passive systems, (2) scaling and uncertainty, (3) coupling between three-dimensional neutron physics and thermal hydraulics, (4) consideration of large-break loss-of-coolant accidents, and (5) simulation of instrumentation and control systems. All these areas are relevant for the AP-1000 and standard pressurized water reactors; however, the areas (1) and (2) have specific applicability for the AP-1000 and constitute the main concerns of this paper. The conclusion from qualitative investigation is that the safety demonstration of the AP-1000 did not take full benefit from progress in these areas, namely, inadequacies characterize the scaling database and the processes for determining the reliability of thermal-hydraulic passive systems did not receive proper attention.

Risk-informed design optimization method and application in a lead-based research reactor

Risk-informed approach has been widely applied in the safety design, regulation, and operation of nuclear reactors. It has been commonly accepted that risk-informed design optimization should be used in the innovative reactor designs to make nuclear system highly safe and reliable. In spite of the risk-informed approach has been used in some advanced nuclear reactors designs, such as Westinghouse IRIS, Gen-IV sodium fast reactors and lead-based fast reactors, the process of risk-informed design of nuclear reactors is hardly to carry out when passive system reliability should be integrated in the framework. A practical method for new passive safety reactors based on probabilistic safety assessment (PSA) and passive system reliability analyze linking is proposed in this paper. New three-dimension frequency-consequence curve based on risk concept with three variables is used in this method. The proposed method has been applied to the determination optimization of design options selection in a 10 MWth lead-based research reactor(LR) to obtain one optimized system design in conceptual design stage, using the integrated reliability and probabilistic safety assessment program RiskA, and the computation resources and time consumption in this process was demonstrated reasonable and acceptable.

A Predictive Dynamic Approach to Evaluating the Reliability of Passive Systems

A Predictive Dynamic Approach to Evaluating the Reliability of Passive Systems

Application of Chernoff bound to passive system reliability evaluation for probabilistic safety assessment of nuclear power plants

There is an increasing interest in passive safety systems to minimize the need for operator intervention or external power sources in nuclear power plants. Because a passive system has a weak driving force, there is greater uncertainty in the performance compared with an active system. In previous studies, several methods have been suggested to evaluate passive system reliability, and many of them estimated the failure probability using thermal–hydraulic analyses and the Monte Carlo method. However, if the functional failure of a passive system is rare, it is difficult to estimate the failure probability using conventional methods owing to their high computational time. In this paper, a procedure for the application of the Chernoff bound to the evaluation of passive system reliability is proposed. A feasibility study of the procedure was conducted on a passive decay heat removal system of a micro modular reactor in its conceptual design phase, and it was demonstrated that the passive system reliability can be evaluated without performing a large number of thermal–hydraulic analyses or Monte Carlo simulations when the system has a small failure probability. Accordingly, the advantages and constraints of applying the Chernoff bound for passive system reliability evaluation are discussed in this paper.

The combination method of functional failure and device fault for passive safety system in nuclear power plant

Functional failure is one of the important causes for passive system failure, and some methods are developed for functional failure evaluation based on the thermal–hydraulic (T-H) model. However, the system equipment configuration is the prerequisite of the T-H behavior analysis and setting in the model, and the system configuration and the functional failure have one-to-one logical relationship which should be considered in the passive system reliability evaluation. In this paper, a logical combination method is put forward based on Fault Tree (FT) tool, and the simplification way is also proposed to improve the calculation efficiency. The Passive Containment Cooling System (PCCS) in AP1000 is analyzed as an example, the result shows that the functional failure under the configuration of one or more of the three parallel lines in valve gallery are available is the main contributor to the system failure, for the plant site in a subtropical area.

ROLE OF PASSIVE SAFETY SYSTEMS IN PREVENTION AND MITIGATION OF SEVERE ACCIDENTS AT NUCLEAR POWER PLANTS

Fukushima Daiichi accidents led to the worldwide update of the NPP safety requirements. Particular attention was paid to severe accidents both during the design of new NPP units and during the safety reassessment of existing ones. Performed safety reassessment of operating units was focused on development and implementation of technical means and strategies directed to prevention and/or mitigation of severe accidents. Technical solutions that are used in modern designs of reactor facilities involve several options for providing of heat removal from the core and containment. Such options include heat removal systems from the reactor core and from the containment of reactor facility. Most of such systems are based on passive principals of operation and do not require or require a minimum action of operating personnel. At the same time, reactor facilities that are operated in Ukraine are based on old VVER designs, which do not include passive safety systems for long-term heat removal (especially for the case of total station blackout, which was in the focus during safety reassessments, so-called “stress tests”). Thus, the necessity of such systems at Ukrainian NPPs with VVER-1000 and VVER-440 was discussed together with analysis of existing types of passive systems and their prevalence. Based on the results of performed analysis the feasibility of implementation at Ukrainian NPPs of a passive heat removal system from the core for VVER-1000 and from the confinement for the VVER-440 was identified. The possibility of implementation of such systems at Ukrainian NPPs requires a separate analysis. In addition, a general information on current state of passive systems reliability assessment was described. It should be noted that despite the reliability of passive safety systems is one of the important issues it is still poorly studied and require additional analyses.

Passive Ultra High Frequency RFID sensor with reference tag for crack detection of aluminum alloy structure

It can be ensured safe and reliable operation of a metallic structure by monitoring the positions which are easy to crack. Because the channel changes with the environment, a tag sensor in the ultra high frequency (UHF) is vulnerable to interference. This paper aims to investigate the reliability of passive RFID sensor systems with a reference tag for crack detection of aluminum alloy structures when the condition of reading distance and surrounding environment changes. The impedance and gain of tags are simulated, which provides theoretical basis for experimental analysis. With an increase of crack depth, the impedance of the sensor tag shifts to the low frequency direction, and the reference tag changes slightly. When dual tags are placed orthogonally, the gain of the sensor tag changes greatly, which means that it is more sensitive to crack depth. The backscatter power of the sensor tag shows a decline trend, and the power of the reference tag remains unchanged. The sensor features a linear response with the variation of crack depth. The sensitivity of crack characterization is better when dual tags are placed orthogonally.

A Bayesian framework of inverse uncertainty quantification with principal component analysis and Kriging for the reliability analysis of passive safety systems

In this work, we propose an Inverse Uncertainty Quantification (IUQ) approach to assigning Probability Density Functions (PDFs) to uncertain input parameters of Thermal-Hydraulic (T-H) models used to assess the reliability of passive safety systems. The approach uses experimental data within a Bayesian framework. The application to a RELAP5-3D model of the PERSEO (In-Pool Energy Removal System for Emergency Operation) facility located at SIET laboratory (Piacenza, Italy) is demonstrated. Principal Component Analysis (PCA) is applied for output dimensionality reduction and Kriging meta-modeling is used to emulate the reduced set of RELAP5-3D code outputs. This is done to decrease the computational cost of the Markov Chain Monte Carlo (MCMC) posterior sampling of the uncertain input parameters, which requires a large number of model simulations.

Multivariate polynomial fit: Decay heat removal system and pectin degrading Fe3 O4 -SiO2 nanobiocatalyst activity.

Herein, multivariate Lagrange's interpolation polynomial (MLIP) and multivariate least square (MLS) methods are used to derive linear and higher‐order polynomials for two varied applications. (1) For an effective fabrication of Pectin degrading Fe3O4‐SiO2 Nanobiocatalyst activity (IU/mg). Here, the three parameters namely: pH value, pectinase loading and temperature as independent variables are optimized for the maximal of anobiocatalyst activity as a dependent variable. (2) For a passive system reliability estimation of decay heat removal (DHR) of a nuclear power plant. The success criteria of the system depend on three types temperature that do not exceed their respective design safety limits and are considered as dependent variables and 14 significant parameters were used as independent variables. Statistically, the validation of these multivariate polynomials are done by testing of hypothesis. Comparative study of the proposed approach gives significance results in the first application have the optimum conditions for maximum activity using linear MLIP method is: 58.64 with pH = 4, pL = 250 and Temp = 4°C. The maximum activity using second order MLIP method is 59.825 and method of MLS is 59.8249 with the optimized values of an independent variables pH = 4, pL = 300 and Temp = 8°C depicted in Table 1. In DHR system, the significance results are obtained and depicted in Table 2.

Comparison of classical and reliable controller performances for seismic response mitigation

Natural hazards like earthquakes, high winds, and tsunami are a threat all the time for multi-story structures. The environmental forces cannot be clogged but the structures can be prevented from these natural hazards by using protective systems. The structural control can be achieved by using protective systems like the passive, active, semi-active, and hybrid protective systems; but the semi-active protective system has gained importance because of its adaptability to the active systems and reliability of the passive systems. Therefore, a semi-active protective system for the earthquake forces has been adopted in this work. Magneto-Rheological (MR) damper is used in the structure as a semi-active protective system; which is connected to the current driver and proposed controller. The Proportional Integral Derivative (PID) controller and reliable PID controller are two proposed controllers, which will actuate the MR damper and the desired force is generated to mitigate the vibration of the structural response subjected to the earthquake. PID controller and reliable PID controller are designed and tuned using Ziegler-Nichols tuning technique along with the MR damper simulated in Simulink toolbox and MATLAB to obtain the reduced vibration in a three-story benchmark structure. The earthquake is considered to be uncertain; where the proposed control algorithm works well during the presence of earthquake; this paper considers robustness to provide satisfactory resilience against this uncertainty. In this work, two different earthquakes are considered like El-Centro and Northridge earthquakes for simulation with different controllers. In this paper performances of the structure with and without two controllers are compared and results are discussed.

Time-invariant reliability assessment for a passive heat removal system based on FORM methodology with a new approach for finding the design point

Passive systems are widely becoming used in the nuclear industry to improve the safety level of nuclear power plants. However, the functioning parameters of the passive systems are extremely tied to uncertainty, and the reliability assessment of these systems is one of the important open issues in the passive safety system analysis area. So far, there is no common method to estimate the reliability of passive safety systems. The FORM method with the HL-RF algorithm is a popular iterative method to gain reliability index. However, when the nonlinearity of a system increases, the method may cause numerical instability. In this study, the reliability methodology for passive safety systems (RMPS) is applied to evaluate the reliability of the passive heat removal system (PHRS) of the VVER-1000 reactor. A simple and efficient reliability algorithm is proposed to overcome the numerical instabilities of the HL-RF algorithm of FORM based on the effective search direction vectors with an appropriate step size to find the design point. The accuracy of the proposed algorithm is shown through the stochastic response surface method (SRSM). Results show that the proposed algorithm is simple, reliable, fast, and can find the reliability index with high efficiency and accuracy.

An efficient method for passive safety systems reliability assessment

Safety by passive systems is a key design feature for new generation Nuclear Power Plants (NPPs). The Passive Containment Cooling System (PCCS) of the AP1000 NPP is a typical passive safety system, by which the heat produced in the containment is transferred to the environment through natural circulation and atmosphere is used as ultimate heat sink. Then, the climate conditions at the plant location influence the system performance. Monte Carlo (MC) simulation of random scenarios of embedding Thermal-Hydraulic (T-H) response of the passive system is commonly used for passive safety systems reliability assessment. However, T-H codes are usually computationally burdensome, in this paper, an effective way to overcome this issue and estimate passive safety system reliability is proposed and applied to the PCCS of the AP1000 NPP, showing that the T-H model runs can be reduced for efficient reliability assessment.

TALL-3D open and blind benchmark on natural circulation instability

Design and safety analysis of the currently developed pool type liquid metal cooled fast nuclear reactors is currently impaired by limited operational experience for such systems and insufficient confidence in the predictive capabilities of the applied modelling. Understanding of pool-reactor thermal-hydraulics is crucial for assessment of reactor performance and passive safety systems reliability. Credibility of the analysis tools can be established in the process of code validation, which includes open and blind benchmarks against integral experiments.TALL-3D is a lead-bismuth eutectic (LBE) loop built to provide experimental data for validation of standalone and coupled system thermal-hydraulics (STH) and computational fluid dynamics (CFD) codes. This paper summarizes the results of the open and blind benchmark exercise, performed using experimental data on natural circulation instability in liquid metal flows from the TALL-3D facility.An approach for selection of experimental data for benchmark and tests for model input calibration is presented. A list of parameters, initial and boundary conditions are defined based on modelling limitations and sources of experimental uncertainty. A set of requirements and assessment criteria for the blind calculations were specified. Results of simulations are compared to experimental data. Implications of the benchmark test results for the codes validity and lessons learned are reported.

Reliability Level Evaluation of Passive Fire Protection System in Hospital Buildings (Case Study: Grandmed Lubuk Pakam Hospital)

This research focuses only on passive fire protection systems in-hospital buildings because the Hospital is public buildings that many people use as facilities for health needs. Hospital residents are not only healthy people, but most sick people are not able to save themselves because of that the building safety system must be reliable especially fire protection systems. This study uses descriptive analysis method by checking directly in the field against the variables that have determined and then analyzed, based on the weight of the AHP method after that each variable and sub-variable will be assessed with a Likert scale according to its suitability with regulations and standards, then weights will be multiplied by the value of conformity to get the reliability value. The results are the level of reliability of the building fire passive protection system 2,250 it includes which in the category enough. This is to increase the value of the level of reliability, it must fulfill the design guidelines in the form of standards.

Dependency consideration of passive system reliability by coupled stress-strength interference/functional relations of parameters approach

Thermal-hydraulic passive systems are currently being deployed in evolutionary nuclear reactors and often incorporated in the design of innovative ones. Passive safety systems are known to be safe, simple and economical but are characterized with uncertainties. To ensure reliable operations of these safety based front-line systems, it is essential to evaluate their reliability with dependable methods. One of the main principles in which the reliability of the systems is based is functional failure analysis (FFA) which is related to their performance analysis. FFA has its physics concept developed on the stress-strength interference (SSI) model of fracture mechanics. In this work, the SSI approach (an independency method) was coupled with the functional relations of critical parameters (CPs) approach (a dependency method) with mass flow rate required for cooling mission as the performance factor. The coupled approach is an improvement on the SSI principle as dependency is introduced into it. The approach is essential as dependency consideration of influencing factors of reliability is of critical concern. To demonstrate the approach, natural circulation system of the System-integrated Modular Advanced ReacTor (SMART) was adopted. The presented approach gave realistic results and in agreement with existing dependency-based reliability methods.

Reliability assessment of passive residual heat removal system of IPWR using Kriging regression model

Passive safety systems have been incorporated into new design nuclear power plants to enhance the power plants’ inherent safety. However, due to the lack of the experimental data and operational experience, evaluating the reliability of passive systems is a challenging task. Many uncertainties are involved in the thermal hydraulic process during startup and operation. This may make the system unable to accomplish its expected function even though the hardware is available, known as functional failure. The evaluation of system functional failure probabilities based on the direct Monte Carlo method may be computationally impractical. In order to reduce the computational load, the Kriging regression model was constructed to avoid a large number of thermal hydraulics simulations. Compared with other metamodels, the Kriging regression model obtained better accuracy. The proposed method was applied to evaluate the reliability of a passive residual heat removal system of IPWR200 during a station blackout accident. The Kriging regression model not only ensures the accuracy of the reliability assessment, but also greatly reduces the runs of T-H codes. Furthermore, global sensitivity analysis aimed at determining the contributions of input parameters was carried out. Results indicate that the probability of passive residual heat removal system failing functionally is estimated to be 1.94e-4 and that insufficient decay heat removal is the most likely failure mode. Sensitivity analysis results identified five key uncertain parameters, which is significant for system performance. The sensitivity information can provide guidance for enhancing passive safety systems design and operation, which is of great importance to the safety and reliability of passive safety systems.