Probabilistic Safety Research Articles

Disturbances and safety analysis of linear adaptive cruise control for cut-in scenarios: A theoretical framework

Although adaptive cruise control (ACC) has been widely analyzed in the car-following process, the way it reacts to cut-in maneuvers has been largely ignored. The cut-in maneuvers may trigger multiple disturbances to traffic, resulting in traffic oscillations and corresponding rear-end crashes. Hence, this study provides a theoretical analysis framework to model the disturbance evolution for ACC under cut-in scenarios. Specifically, we first derive the general ACC dynamic evolution based on the widely adopted ACC (i.e., linear feedback control) by applying Caley-Hamilton theorem. Given that, two representative cut-in scenarios are designed to comprehensively understand the impact of cut-in vehicle behavior as well as control parameters on the safety and stability of the ACC system. Enable by the interpretability nature of ACC dynamic analytical solution, necessary and sufficient conditions for overshoot and potential safety risks are derived. The proposed framework is further applied to analyze the cut-in disturbance evolution of commercial ACC systems using field-test data and calibrated control parameters, by which the probabilistic safety and stability condition is provided. Through the above efforts, the framework is instrumental in robust ACC design under cut-in scenarios.

Learning Discrete-Time Uncertain Nonlinear Systems With Probabilistic Safety and Stability Constraints

This paper presents a discrete-time dynamical system model learning method from demonstration while providing probabilistic guarantees on the safety and stability of the learned model. The controlled dynamic model of a discrete-time system with a zero-mean Gaussian process noise is approximated using an Extreme Learning Machine (ELM) whose parameters are learned subject to chance constraints derived using a discrete-time control barrier function and discrete-time control Lyapunov function in the presence of the ELM reconstruction error. To estimate the ELM parameters a quadratically constrained quadratic program (QCQP) is developed subject to the constraints that are only required to be evaluated at sampled points. Simulations validate that the system model learned using the proposed method can reproduce the demonstrations inside a prescribed safe set while converging to the desired goal location starting from various different initial conditions inside the safe set. Furthermore, it is shown that the learned model can adapt to changes in goal location during reproductions without violating the stability and safety constraints.

A probabilistic risk-acceptance model for assessing blast and fragmentation safety hazards

There are many circumstances where decision-makers consider risks associated with explosions – from either natural or deliberate events – where the goal is clarity with respect to the actual safety and hazard risks posed to society and its people, systems and infrastructure. The paper describes how probabilistic safety and hazard modelling of blast and fragmentation can better inform a Quantitative Explosive Risk assessment (QERA). A QERA may be used to define an explosive safety distance based on the risk of explosive hazards being less than a societal acceptable risk. The concepts are illustrated with scenarios at a generic explosives ordnance (EO) site. In one scenario we demonstrate that current, deterministically based, regulations in Australia and internationally may be overly conservative. In other words, a deterministic based regulation may show that a building is located in an unsafe area, whereas a QERA can show, for the same building, that fatality risks are less than those deemed acceptable by society. Another example demonstrates the significant effect that uncertainty modelling, particularly that associated with post-detonation blast-loads, has on fatality risks.

Seismic fragility assessment of isolated structures by using stochastic response database

The seismic isolation system makes a structure isolated from ground motions to protect the structure from seismic events. Seismic isolation techniques have been implemented in full-scale buildings and bridges because of their simplicity, economic effectiveness, inherent stability and reliability. As for the responses of an isolated structure due to seismic events, it is well known that the most uncertain aspects are the seismic loading itself and structural properties. Due to the randomness of earthquakes and uncertainty of structures, seismic response distributions of an isolated structure are needed when evaluating the seismic fragility assessment (or probabilistic seismic safety assessment) of an isolated structure. Seismic response time histories are useful and often essential elements in its design or evaluation stage. Thus, a large number of non-linear dynamic analyses should be performed to evaluate the seismic performance of an isolated structure. However, it is a monumental task to gather the design or evaluation information of the isolated structure from too many seismic analyses, which is impractical. In this paper, a new methodology that can evaluate the seismic fragility assessment of an isolated structure is proposed by using stochastic response database, which is a device that can estimate the seismic response distributions of an isolated structure without any seismic response analyses. The seismic fragility assessment of the isolated nuclear power plant is performed using the proposed methodology. The proposed methodology is able to evaluate the seismic performance of isolated structures effectively and reduce the computational efforts tremendously.

Seismic Response Analysis of Nuclear Power Plant Structures and Equipment due to the Pohang Earthquake

The probabilistic seismic safety assessment is one of the methodology to evaluate the seismic safety of the nuclear power plants. The site characteristics of the nuclear power plant should be reflected when evaluating the seismic safety of the nuclear power plant. The Korea seismic characteristics are strong in high frequency region and may be different from NRC Regulatory Guide 1.60, which is the design spectrum of nuclear power plants. In this study, seismic response of a nuclear power plant structure by Pohang earthquake (2017.11.15. (KST)) is investigated. The Pohang earthquake measured at the Cheongsong seismic observation station (CHS) is scaled to the peak ground acceleration (PGA) of 0.2 g and the seismic acceleration time history curve corresponding to the design spectrum is created. A nuclear power plant of the containment building and the auxiliary buildings are modeled using OPENSEES to analyze the seismic response of the Pohang earthquake. The seismic behavior of the nuclear power plant due to the Pohang earthquake is investigated. And the seismic performances of the equipment of a nuclear power plant are evaluated by the HCLPF. As a result, the seismic safety evaluation of nuclear power plants should be evaluated based on site-specific characteristics of nuclear power plants.

Fragility Analysis of A Scaled Model of Reinforced Concrete Column in Accordance with Similitude Law

Many studies are conducted in several fields for fragility analysis of structures or elements which is a probabilistic seismic safety analysis in consideration with uncertainty of seismic loading. It is hard to directly conduct fragility analysis for an infrastructure with social importance due to its size. Therefore, a fragility analysis for an infrastructure mainly conducted in element level or conducted with scaled model built in accordance with similarity law. In this article, fragility analysis for prototype and scaled model of reinforced concrete column was conducted with numerical models which had been updated by the results of shaking table test and pseudo dynamic test. As a result, response stress from the numerical analysis result of prototype model was higher than that from scaled model due to different stiffness ratios between steel and concrete. However, the probability of failure for scaled model was higher than that for prototype model because failure criteria for scaled model was down due to similarity law. Also it was evaluated that probability of failure by using log normal standard deviation of response stresses by spectrum matched accelerograms was more reliable than probability of failure by using existing coefficient of variation normally used.

A probabilistic bridge safety evaluation against floods.

To further capture the influences of uncertain factors on river bridge safety evaluation, a probabilistic approach is adopted. Because this is a systematic and nonlinear problem, MPP-based reliability analyses are not suitable. A sampling approach such as a Monte Carlo simulation (MCS) or importance sampling is often adopted. To enhance the efficiency of the sampling approach, this study utilizes Bayesian least squares support vector machines to construct a response surface followed by an MCS, providing a more precise safety index. Although there are several factors impacting the flood-resistant reliability of a bridge, previous experiences and studies show that the reliability of the bridge itself plays a key role. Thus, the goal of this study is to analyze the system reliability of a selected bridge that includes five limit states. The random variables considered here include the water surface elevation, water velocity, local scour depth, soil property and wind load. Because the first three variables are deeply affected by river hydraulics, a probabilistic HEC-RAS-based simulation is performed to capture the uncertainties in those random variables. The accuracy and variation of our solutions are confirmed by a direct MCS to ensure the applicability of the proposed approach. The results of a numerical example indicate that the proposed approach can efficiently provide an accurate bridge safety evaluation and maintain satisfactory variation.

Development of safety assessment methodology on fuel element failure propagation in SFR and its application to Monju

Probabilistic and deterministic safety assessments and experimental studies on local fault (LF) propagation in sodium-cooled fast reactors (SFRs) have been performed in many countries because LFs have been historically considered as one of the possible causes of severe accidents. Adventitious fuel pin failures have been considered to be the most dominant initiators of LFs in these probabilistic assessments because of its high frequency of occurrence during reactor operation and possibility of subsequent pin-to-pin failure propagation. Four possible mechanisms of fuel element failure propagation from adventitious fuel pin failure (FEFPA) were identified from a state-of-the-art review of open papers. All the mechanisms for FEFPA analysis including thermal, mechanical and chemical propagation are modeled into a safety assessment code which is applicable to arbitrary SFRs by developing some needed but missing methods. Furthermore, an assessment on FEFPA of Japanese prototype fast breeder reactor (Monju) was performed using this methodology. It was clarified that FEFPA was highly unlikely and limited at most within one subassembly in Monju owing to its redundant and diverse detection and shutdown systems for FEFPA even assuming the propagation. These results also suggested future possibility of run-beyond-cladding-breach operation which would enhance the economic efficiency in Monju.

Possible Probabilistic Safety and Longevity Evaluations of the Metal in the Studs of the No. 2 Generating Set at the Sayano-Shushenskaya HPP

Use of the probabilistic approach to evaluation of the safety and longevity of the metal in the studs of the No. 2 generating set at the Sayano-Shushenskaya HPP is examined.

Options for the Treatment of Uncertainty in Seismic Safety Assessment of Nuclear Power Plants

This paper proposes options for improving seismic safety analysis for nuclear power plants. Cumulative absolute velocity (CAV) has been considered and analyzed for describing different load-cycle-dependent phenomena, e.g., fatigue failure and soil liquefaction. Fatigue damage estimation is based on load-cycle counting, which can be performed by counting zero or level crossings if power spectral density (PSD) of load time history is known. Correlation between CAV and moments of PSD of load time history is derived in this paper. A fatigue failure criterion is developed in terms of CAV. P-box theory is proposed for improving uncertainty modeling of probabilistic seismic safety assessment. P-boxes are represent upper and lower bounds of the distribution function specified by left side and right side distribution functions. The utilization of the p-box method has an advantage if the conditional probability of a particular failure mode of a component is known approximately only, and the sample size of damage histories is small or inconsistent or modeling of component failure is uncertain.

Analysis of the operator action and the single failure criteria in a SGTR sequence using best estimate assumptions with TRACE 5.0

Steam Generator Tube Rupture (SGTR) sequences in Pressurized Water Reactors are known to be one of the most demanding transients for the operating crew. SGTR are a special kind of transient as they could lead to radiological releases without core damage or containment failure, as they can constitute a direct path from the reactor coolant system to the environment.The first methodology used to perform the Deterministic Safety Analysis (DSA) of a SGTR did not credit the operator action for the first 30min of the transient, assuming that the operating crew was able to stop the primary to secondary leakage within that period of time. However, the different real SGTR accident cases happened in the USA and over the world demonstrated that the operators usually take more than 30min to stop the leakage in actual sequences. Some methodologies were raised to overcome that fact, considering operator actions from the beginning of the transient, as it is done in Probabilistic Safety Analysis.This paper presents the results of comparing different assumptions regarding the single failure criteria and the operator action taken from the most common methodologies included in the different Deterministic Safety Analysis. One single failure criteria that has not been analysed previously in the literature is proposed and analysed in this paper too. The comparison is done with a PWR Westinghouse three loop model in TRACE code (Almaraz NPP) with best estimate assumptions but including deterministic hypothesis such as single failure criteria or loss of offsite power. The behaviour of the reactor is quite diverse depending on the different assumptions made regarding the operator actions. On the other hand, although there are high conservatisms included in the hypothesis, as the single failure criteria, all the results are quite far from the regulatory limits. In addition, some improvements to the Emergency Operating Procedures to minimize the offsite release from the damaged SG in case of a SGTR are outlined taking into account the offsite dose sensitivity results.

FBCRI Based Real-time Path Planning for Unmanned Aerial Vehicles in Unknown Environments with Uncertainty

This paper presents an FBCRI(feedback based compositional rule of inference) based novel path planning method to satisfy the requirements of real-time navigation, smoothness optimization and probabilistic obstacle avoidance. With local path-searching behaviors in regional ranges and global goal-seeking behaviors in holistic ranges, the method infers behavior weights using fuzzy reasoning embedded with feedback, and then coordinates the behaviors to generate new reference waypoints. In view of the deterministic decisions and the uncertain states of a UAV(unmanned air vehicle), chance constraints are adopted to probabilistically guarantee the UAV's safety at a required level. Simulation results in representative scenes prove that the method is able to rapidly generate convergent paths in obstacle-rich environments, as well as highly improve the path quality with respect to smoothness and probabilistic safety.

Adaptive and Sequential Gridding Procedures for the Abstraction and Verification of Stochastic Processes

This work is concerned with the generation of finite abstractions of general state-space processes to be employed in the formal verification of probabilistic properties by means of automatic techniques such as probabilistic model checkers. The work employs an abstraction procedure based on the partitioning of the state-space, which generates a Markov chain as an approximation of the original process. A novel adaptive and sequential gridding algorithm is presented and is expected to conform to the underlying dynamics of the model and thus to mitigate the curse of dimensionality unavoidably related to the partitioning procedure. The results are also extended to the general modeling framework known as stochastic hybrid systems. While the technique is applicable to a wide arena of probabilistic properties, with focus on the study of a particular specification (probabilistic safety, or invariance, over a finite horizon), the proposed adaptive algorithm is first benchmarked against a uniform gridding approach taken from the literature and finally tested on an applicative case study in Biology.

Novel probabilistic dynamic safety metric formulations for flight path determination

This article details a novel method for the determination of safe flight paths dynamically following an in-flight distress event. The method is based on probabilistic safety metrics which also include the touchdown and evacuation/rescue phases after landing. Two case studies simulating in-flight distress events, one from the west and the other from the east coast are presented using these formulations for a quantitative analysis. It is found that the nearest landing sites are not always the safest ones showing the benefits of the newly developed safety metrics. Finally, the path safety levels are plotted as a function of mission safety probability values using innovative polar plots that provide useful information to pilots.

A simulation-based approach to the approximation of stochastic hybrid systems

We study the problem of approximating a stochastic, possibly hybrid, system by means of some abstracted model to the purpose of simplifying the analysis of properties such as probabilistic safety and reachability. We suppose that the property to be analyzed depends on the behavior of some output signal of the system and that the model is designed in order to reproduce that signal as close as possible, for the different possible realizations of the stochastic input affecting the system. The idea developed in this paper is to assess the quality of a model as an approximation of a stochastic system by testing how close are their output signals over a finite number of input realizations. Under suitable assumptions, we show that, with high confidence, the quality assessed on a few input realizations is guaranteed to hold also for all the unseen ones except for a set of pre-defined probability e. The proposed approach can be applied to an arbitrary system, the only requirement being to be able to run multiple simulations of its behavior for different input realizations.

Design of Probabilistic Pilot Model with Fuzzy Controller for Microburst Escape

Pilot should control the aircraft manually when encountering low altitude wind shear during takeoff and landing. For wind shear escape and flight safety research, an effective human pilot model together with wind shear and flight dynamics model should be built with high fidelity. A skill-based human pilot model was built which can describe pilots’ characteristics such as experiences, skills, emotions, reaction abilities, etc. A fuzzy controller was designed for lateral and longitudinal escape control in pilot model. Since single pilot could not represent a group of pilots’ control behavior, some of the model parameters were set to be stochastic, then the Monte Carlo method was adopted to obtain a numerical approximation of safety analysis results. With the probabilistic pilot model, escape strategies and safety analysis can be studied by simulation with high fidelity.

Assessment of the Statistical Distributions of Structural Demand Under Earthquake Loading

Many existing probabilistic seismic safety assessment studies assume that structural demand conditional to a seismic intensity follows a lognormal distribution. Based on the analysis of five reinforced concrete structures subjected to real earthquake records scaled to several intensities, an in-depth analysis of this assumption is performed using adequate statistical methods. The considered demand parameters are the chord rotation, curvature, shear force, and inter-story drift. The results found indicate that the lognormal distribution is suitable for the probabilistic modeling of the considered demand parameters. In addition, the normal distribution was also found suitable for the shear force demand.

Probabilistic Critical Safety Wheel Wear Sizes of Chinese Railway Freight Cars

Critical safety wheel wear size is investigated through considering the fatigue safety effect on RD2 type axle of China railway freight cars. The wheel wear size increase results in a rise of the wheel-rail contact forces, which increase the fatigue stress history of the axle in service. Therefore, the size must be controlled to maintain the axle safety in service before the next overhaul inspection. A field investigation was firstly made on the wear size change regularity. Then, the size related wheel-rail impact forces are solved by a non-linear Hertzian contact theory with a vehicle multi-body dynamic model. In addition, a statistical method is further developed for incorporating the impact forces into the wheelset service load history. Finally, a wear size related critical state equation is established for ensuring the axle safety in service. A stationary solution is statistically obtained for the critical wear sizes with respect to the axle circumferential crack and semi-elliptical crack. It is concluded that the critical safety wear size should be controlled in the range of 100 mm at the statistical level of survival probability 0.99 and confidence 95%.

Probabilistic Critical Fatigue Safety State of the RD2 Type Axle of China Railway Freight Car

Critical fatigue safety state is investigated on RD2 type axle of China railway freight cars. Attention is paid on the grooves near axle boxes, where more fatigue cracks were early appeared even result in at least six derailed accidents. Load history was obtained by vehicle dynamics simulation combining with on-line inspection. Braking loads and effects of maintenance and off-round of wheels are also incorporated. Semi-elliptical and external circumferential cracks are employed for crack growth simulation. Crack shape change and shear stressing of the semi-elliptical crack are taken into account. New crack growth rate modeling in which covers from cracking threshold to toughness related fracture state is applied for residual life evaluation. Critical fatigue safety state is defined as the crack size from which the axle can be safely operated to next overhaul inspection. Critical crack size is estimated step-by-step with the crack shape change. Results show that the external circumferential crack is more dangerous than the semi-elliptical crack. Application more than two years in production verifies that the present assessment is available and reliable.

Fire tests to validate fire simulation codes: numerical results

Abstract Fire simulations have gained more and more significance, particularly in the context of deterministic as well as probabilistic fire safety analyses for operating nuclear power plants (NPP). An “International Collaborative Project to Evaluate Fire Models for Nuclear Power Plant Applications (ICFMP)” has been started in 1999, to make use of the experience and knowledge of a variety of expert institutions in this field worldwide to assess and improve, if necessary, the state-of-the-art with respect to modelling fires for application to nuclear installations/plants. In the frame of ICFMP two fuel fire experiments have been selected for the Benchmark Exercise No. 4 and four cable tray fire tests for the Benchmark Exercise No. 5, all of these performed in the fire compartment OSKAR of the iBMB (Institut für Baustoffe, Massivbau und Brandschutz) in Braunschweig. In this paper an overview of the calculated results will be given. The simulations of the two large fuel pool fire experiments still indicate difficulties to simulate under-ventilated conditions. The results provide a first indication on the uncertainties of such simulations, although it has to be considered, that the fire load was quite large compared to the compartment volume. The ignition of cables trays as well as the flame propagation on these trays could still not be simulated in an adequate way by the fire codes applied. Different boundary conditions such as pre-heating or the use of more complex cable insulation materials make the situation even more complex.