Potential Accident Scenarios Research Articles

Operational and safety analyses of a commercial PEMFC system

A commercial, transportable proton-exchange-membrane fuel-cell (PEMFC) system rated at 7 kW nominal power was the subject of an operational (reliability, availability, and maintainability) and process-safety analysis. The scope of the analysis included the system start-up to engine-run mode, the maintenance tasks, as well as the continuous power-production mode, carried out with a hazard and operability (HAZOP) study and a failure-scenarios' development. The failure and process-safety risks were categorized and the probabilities were assessed with a fault-tree analysis (FTA) using equipment-failure-rate data from the literature, if available. The results led to similar conclusions to those previously reported in similar studies, and the most important equipment/devices with respect to reliability and safety aspects were identified. While the most important uncertainty in the complete analysis comes from the generic equipment failure-rate data used, about three failures in the first year of operation are estimated together with an availability of about 98% (a point value figures, confidence interval analysis was also made). The process-safety analysis revealed four potential accident scenarios involving hydrogen releases, fires and explosions. Finally, detailed suggestions and recommendations for changes/improvements with respect to the reliability/availability, maintainability and safety aspects of the system analysed are presented.

Emergency planning and the Control of Major Accident Hazards (COMAH/Seveso II) Directive: An approach to determine the public safety zone for toxic cloud releases

The EU Control of Major Accidents Hazards Directive (Seveso II) requires an external emergency plan for each top tier site. This paper sets out a method to build the protection of public health into emergency planning for Seveso sites in the EU. The method involves the review of Seveso site details prescribed under the directive. The site safety report sets out the potential accident scenarios. The safety report's worst-case scenario, and chemical involved, is used as the basis for the external emergency plan. A decision was needed on the appropriate threshold value to use as the level of concern to protect public health. The definitions of the regulatory standards (air quality standards and occupational standards) in use were studied, how they are derived and for what purpose. The 10 min acute exposure guideline level (AEGL) for a chemical is recommended as the threshold value to inform decisions taken to protect public health from toxic cloud releases. The area delimited by AEGL 1 defines the population who may be concerned about being exposed. They need information based on comprehensive risk assessment. The area delimited by AEGL 2 defines the population for long-term surveillance when indicated and may include first responders. The area delimited by AEGL 3 defines the population who may present acutely to the medical services. It ensures that the emergency responders site themselves safely. A standard methodology facilitates discussions with plant operators and concerned public. Examples show how the methodology can be adapted to suit explosive risk and response to fire.

CFD‐based risk assessment for hydrogen applications

AbstractComputational fluid dynamics calculations for gas explosion safety have been widely used for doing risk assessments within the oil and gas industry for more than a decade. On the basis of predicted consequences of a range of potential accident scenarios a risk level is predicted. The development of applications using hydrogen as a clean energy carrier has accelerated in recent years, and hydrogen may be used widely in the future. Because of the very high reactivity of hydrogen, safe handling is critical. For most applications it is not realistic to perform an extensive risk assessment similar to what is done for large petrochemical installations. On the other hand, simplified methods, like venting guidelines, may have a questionable validity for hydrogen. The use of simple methods, if these actually are conservative, will in general predict too high consequences for the majority of scenarios, as these are not able to represent actual geometry and physics of the explosion.In this article a three‐step approach is proposed. The initial approach will be to carry out a “worst‐case” calculation evaluating the consequences if a full stoichiometric gas cloud is ignited. Mitigation measures can also be considered. As a second step, if potential consequences of the initial approach are not acceptable, the assumptions are refined and more calculations are performed to make the evaluations more realistic and reduce unnecessary conservatism of the chosen worst‐case scenarios. Typically a number of dispersion calculations will be performed to generate likely gas clouds, which are subsequently ignited. If estimated consequences are still not acceptable, a more comprehensive study, including ventilation, dispersion, and explosion, is performed to evaluate the probability for unacceptable events. © 2007 American Institute of Chemical Engineers Process Saf Prog 2007

Appropriate radiation accident medical management: necessity of extensive preparatory planning

Despite the rareness of radiation accidents, their potential consequences can be very serious, and appropriate medical management requires sufficient preparatory planning. To identify necessary factors for sufficient preparatory planning, three different radiation accidents were analyzed, i.e. the accidents in Goiânia, Brazil, 1987; Lilo, Georgia, 1997; and Tokai-mura, Japan, 1999. These radiation accidents have been chosen specifically because they provide a wide spectrum of potential radiation accident scenarios. After a brief description of the accidents and the following medical management, the measures taken are analyzed in terms of diagnosing radiation-induced health damage, determining the cause, dealing with contamination/incorporation, pathophysiological and therapeutic principles, preparatory planning, national and international cooperation and training. Several important factors are identified that should be considered in preparatory planning, i.e. preventing delayed diagnosis and training of medical personnel. Due to limited national resources, an intensified international cooperation to manage medical radiation accidents is of great importance.

Comprehensive risk assessment for rail transportation of dangerous goods: a validated platform for decision support

Currently, the most advanced and well documented risk assessments for the transportation of dangerous goods by railway take into account: statistics-based loss of containment frequencies, specification of potential consequences for a given release situations using event tree methodology as an organisational tool and consequence calculation models to determine a risk figure known as CCDF (Complementary Cumulative Distribution Function). Such procedures for the risk assessment (including for example decision-making on preventive measures) may offer only a limited insight into the causes and sequences leading to an accident and do not allow for any kind of predictive analysis. The present work introduces an enhanced solution, and a related software platform, which attempts to integrate loss of containment causes and consequences with system's infrastructure and its environment. The solution features: the use of a detailed Master Logical Diagram, including fault/event tree analysis to determine a loss of containment frequency based on different initiating events, scenarios and specific basic data, the characterization of a resulting source term following a release situation, and the calculation of various potential impacts on the neighbouring site. Results are wrapped into a CCDF format for each selected traffic segment. The risk-related results are integrated on a software platform, structured as a decision support system using intelligent maps and a variety of GIS (Geographical Information System) data processing procedures. The introduction of the hot spot approach, allows us to focus on the most risk-relevant areas and to use information on various railway infrastructure elements (e.g. points, tunnels), are the basis of the new models employed. The software is applicable to any railway transportation system, comprising its technical infrastructure, rolling stock, human actions, regulation and management procedures. It provides the determination of the annual societal risk due to potential accident scenarios, while also revealing information on the potential causes of an accident taking into account spatial parameters. The approach and software have been validated by a case study done for a particular traffic segment of the Swiss Federal Railway company.

An Overview of Corrosion Issues for the Design and Operation of High-Temperature Lead- and Lead-Bismuth-Cooled Reactor Systems

The viability of advanced Pb- or Pb-Bi–cooled fast reactor systems will depend on the development of classes of materials that can operate over the temperature range 650–1200°C. We briefly review the current state of the technology concerning the interaction of Pb and Pb-Bi alloys with structural materials. We then identify the key challenges to successful use of materials in these systems and suggest a path forward to the development of new materials and operating methods to allow higher-temperature operation. Our focus is on the necessary trade-offs that must be considered and how these trade-offs influence R&D choices. Our analysis suggests that three classes of materials will be needed for successful deployment of a lead-alloy–cooled reactor system. A lower-temperature qualified material will be necessary for the pressure boundary. The structural and cladding materials will require 1000°C- and 1200°C-class materials. The 1000°C-class material will be exposed to the 1000°C coolant. The 1200°C-class material will be required for the cladding and structural materials in the core region. The higher-temperature material will be required to accommodate anticipated temperature transients from potential accident scenarios, such as a loss of flow.

Effect of Gadolinium Doping on the Air Oxidation of Uranium Dioxide

AbstractResearchers at the Pacific Northwest National Laboratory (PNNL) investigated the effects of gadolinium oxide concentration on the air oxidization of gadolinium oxide-doped uranium dioxide using thermogravimetry and differential scanning calorimetry to determine if such doping could improve uraniumdioxide's stability as a nuclear fuel during potential accident scenarios in a nuclear reactor or during long-term disposal. We undertook this study to determine whether the resistance of the uranium dioxide to oxidation to the orthorhombic U3O8 with its attendant crystal expansion could be prevented by addition of gadolinium oxide. Our studies found that gadolinium has little effect on the thermal initiation of the first step of the reported two-step air oxidation of UO2; however, increasing gadolinium oxide content does stabilize the initial tetragonal or cubic product allowing significant oxidation before the second expansive step to U3O8 begins.

Modeling of ice formation and condensation on a cryogenic surface

Current designs for next-generation fusion devices feature superconducting magnets for confining and managing the plasma. Often the design also features water as the primary circuit coolant. Thus, a potential accident scenario is steam injection from the primary circuit into the magnets’ cryogenic chamber (cryostat). An important question to be answered for the above scenario is whether the pressure suppression created by the cryogenic surfaces is sufficient to circumvent cryostat damage. Providing an answer to this question is the objective of the Experimental Vacuum Ingress Test Apparatus (EVITA). This paper presents pre-experiment calculations on the ice layer growth and condensation formation within EVITA. The analyses focused on two scenarios: the power of the injected vapor ( Ps) equals the removal power of the cryogenic system ( P ln) and P v exceeds P ln. The first scenario produced a pressure increase of 30 mbar for P v= P ln=10 kW while the second scenario produced pressurization to the 2 bar EVITA limit when P v=10 and P ln=5 kW. The primary conclusion is that the saturation temperature at 3 mbar and the heat transfer coefficient of the film condensate are important variables since they greatly influence the pressurization.

Probabilistic analysis of the inadvertent reentry of the Cassini spacecraft's radioisotope thermoelectric generators.

As part of the launch approval process, the Interagency Nuclear Safety Review Panel provides an independent safety assessment of space missions--such as the Cassini mission--that carry a significant amount of nuclear materials. This survey article describes potential accident scenarios that might lead to release of fuel from an accidental reentry during an Earth swingby maneuver, the probabilities of such scenarios, and their consequences. To illustrate the nature of calculations used in this area, examples are presented of probabilistic models to obtain both the probability of scenario events and the resultant source terms of such scenarios. Because of large extrapolations from the current knowledge base, the analysis emphasizes treatment of uncertainties.

Commentary: Determining If and When Impacts of Potential Accidents Need to be Evaluated in a NEPA Analysis

Determining if and when a potential accident scenario is subject to requirements of the National Environmental Policy Act (NEPA) of 1969 has long been an open question. This issue is routinely addressed by some agencies, yet is virtually ignored by others. Certainly, not all federal proposals merit a detailed analysis of accident scenarios. Yet, how does one determine under which circumstances an analysis of accident scenarios should be subject to a NEPA analysis versus those that are not? The implications of this issue are particularly important with respect to low-probability high-consequence events. Beyond the obvious risk of increased exposure to litigation, this question is of interest for other reasons, not the least of which being that such an analysis can substantially increase schedules and cost. This paper examines this question and provides a tool consisting of seven tests for assisting practitioners in reaching a determination.

Siting criteria based on the prevention of deterministic effects from plutonium inhalation exposures.

Siting criteria are established by regulatory authorities to evaluate potential accident scenarios associated with proposed nuclear facilities. The 0.25 Sv (25 rem) siting criteria adopted in the United States has been historically based on the prevention of deterministic effects from acute, whole-body exposures. The Department of Energy has extended the applicability of this criterion to radionuclides that deliver chronic, organ-specific irradiation through the specification of a 0.25 Sv (25 rem) committed effective dose equivalent siting criterion. A methodology is developed to determine siting criteria based on the prevention of deterministic effects from inhalation intakes of radionuclides which deliver chronic, organ-specific irradiation. Revised siting criteria, expressed in terms of committed effective dose equivalent, are proposed for nuclear facilities that handle primarily plutonium compounds. The analysis determined that a siting criterion of 1.2 Sv (120 rem) committed effective dose equivalent for inhalation exposures to weapons-grade plutonium meets the historical goal of preventing deterministic effects during a facility accident scenario. The criterion also meets the Nuclear Regulatory Commission and Department of Energy Nuclear Safety Goals provided that the frequency of the accident is sufficiently low.

THE USE OF EVENT TREES IN OIL SPILL PREVENTION APPLICATIONS1

ABSTRACT This paper investigates the applicability of event tree analysis, a method for identifying and analyzing potential accident scenarios, to the problem of oil spill prevention. It is shown that when a generic set of safety functions (identified from an analysis of 10 oil spill case studies) is used, event trees do provide an accurate and useful representation of the sequences of events leading up to and following an oil spill. Possible insights resulting from an event tree analysis and particular insights from the case studies are discussed.

The recent fragmentations of LEO upper stages

Between March 1991, and March 1993, three USSR/CIS upper stages from two different launch vehicle families broke up at moderate LEO altitudes, creating more than 300 long-lived debris. All three vehicles had successfully completed their satellite delivery missions before the breakups occurred 1–21 days after launch. Two of the vehicles, a Kosmos second stage and a Zenit second stage, apparently experienced multiple fragmentations. Joint Russian-American accident investigations have characterized the fragmentation events, uncovered potential accident scenarios, and recommended corrective measures for future missions. As seen repeatedly in recent years, residual propellants continue to be an apparent source for launch vehicle breakups. International cooperation can not only assist in better defining the near-Earth space environment, but also can improve space operations and mitigate the unnecessary growth of the Earth's satellite population.

Abnormal EEG coherence at rest and during photic stimulation in drug-naive schizophrenic patients

Selecting representative accident scenarios (RAS) is one of the most discussed and important aspects of the HAZOP process, which is the main part of risk analysis. During that process, several uncertainties can occur, which may lead to critical oversights with further consequences for life and property. These mainly concern the semi-quantitative process of risk ranking, especially the evaluation of the frequency and severity of the categories of potential accident scenarios. According to our experience, other sources of uncertainty, which are hardly taken into account at this stage of the analysis, are connected with the effects of the type and performance of the safety barriers and protection measures as well as the impact of the quality of HAZOP analysis on the risk ranking process. The latter aspect depends on many continuously changing factors that are generally related to the safety culture that exists in a specific organization. These aspects are always the focal area for discussion by analysts, and these were hardly taken into account in our previous research.In this study, both aspects, the effects of the protection layers and the quality of hazard identification analysis on the selection of RASs, are considered. The major idea is connected with the extension of the classical HAZOP study by the application of a modified risk ranking method to identify potential accident scenarios. For that process, we propose applying appropriate correction indexes concerning both aspects. The impacts of the safety layers were assessed by the efficacy index (EI), which evaluates the effectiveness of the safety barriers, and the effects of the quality of HAZOP analysis were assessed by the quality index (QI). Both indexes were used to develop the risk correction index (RCI) that was used to modify the final risk, which was the basis for selecting the RASs. The analysis was performed in two ways: 1. traditional and 2. with fuzzy logic support. Both approaches were applied to the case study concerning the storage of liquefied natural gas (LNG). The results illustrated the sensitivity of the risk ranking matrix to the risk correction index and proved the advantages of the fuzzy risk ranking methodology in relation to the traditional approach. The proposed RAS selection process supports the credibility of the risk analysis by taking into account its further application in the subsequent stages of risk analysis and then in emergency planning.

The Cassini mission risk assessment framework and application techniques

The Cassini space mission is intended to investigate the planet Saturn and its ring system. Due to the low intensity of solar radiation, solar batteries are not adequate for this mission and radioisotope thermoelectric generators (RTGs) are utilized to supply the electric power to the spacecraft systems. The presence of a radioactive material on board and the consequences of a potential accident during the launch and near-earth phases of the mission require a scrupulous process of risk assessment to be executed before launch approval is granted at the highest level. For this purpose, the probabilistic risk assessment (PRA) approach has been chosen and applied throughout the analysis process. This paper describes the Cassini PRA methodology and presents the steps which have been accordingly executed to develop potential launch accident scenarios and their quantification.

Safety analysis of manual tasks in automatic production systems— Implications for design

A safety procedure for application in the design of automatic production systems is outlined. It involves the specification on safety requirements for a particular system on the basis of general safety requirements and of the results of a safety analysis. During the safety analysis, work tasks inside the danger zone of the system and potential accident scenarios related to these tasks are identified, and safety measures are evaluated. Various checklists have been developed to support each step in the analysis. The method for safety analysis has been tested in field studies of three automatic production systems in operation: two mechanical systems and one process plant. The aim has been to evaluate the method and to collect operational experience concerning accident risks and preventive measures to be considered in design. The main results of the studies are presented, and considerations for the use of the method during the design phases are discussed. It is concluded that the approach that has been selected satisfies the needs of a safety procedure for use in design.

Human error models as predictors of accident scenarios for designers in road transport systems

Models of human error in technological systems have so far been used mainly retrospectively to explain accidents which have happened. Unless they can be used by designers of transport systems as prospective predictors of potential accident scenarios, their value as tools for accident prevention is limited. This paper reviews the theoretical and practical difficulties involved in closing the gap between human error models and the needs of transport system designers. The main conclusions are that cognitive psychologists should: (1) produce a pool of production rules which give an adequate description of driver behaviour in practice, plus a confusability index for those rules; and (2) document cases of erosion of rules by road users and develop a diagnostic tool for ‘susceptible’ rules.

Use of safety analysis in automatic production systems

Rundmo, T., Sten, T. and Kjellén, U., 1990. Use of safety analysis in automatic production systems. Journal of Occupational Accidents, 12: 139–147. The majority of occupational accidents in automatic production systems happens during maintenance, testing or during handling of irregularities in the production process where operators and maintenance personnel are required to intervene. Methods for safety analysis in such systems have to appraise these various operational conditions. Norsk Hydro has contracted SINTEF to develop a method for safety analysis in automatic production systems, and to test and evaluate the method in three case studies. The analysis identifies work tasks inside the danger zone of a production system and potential accident scenarios connected to these tasks. Various checklists have been developed to support each step in the examination. The method has been tested in case studies at two automatic mechanical production systems and one chemical process plant at Norsk Hydro. The article presents the method for safety analysis and results from the case studies.

Acute health effects from accidental releases of high toxic hazard chemicals

Abstract This paper describes a comprehensive methodology for predicting impacts on surrounding populations due to accidental spills of toxic chemicals. The work was motivated by the need for a reasonably accurate and quick method for analysing potential accident scenarios as part of our emergency planning work for industrial clients in the USA. An integrated assessment methodology, including vaporization release rate estimation, transient atmospheric dispersion, and resultant receptor concentration—time history is presented. Dose contours, which show exposure in terms of concentration in time for downwind affected areas are also generated, and health impacts are quantified using probit equations. Results are compared for releases of acrolein, ammonia, chlorine, and sulphur dioxide. Implications and criteria for plant emergency response actions are presented.

A survey of recent driver steering behavior models suited to accident studies

A literature survey for the years 1975–1980 has been carried out in the area of driver models describing steering behavior. The purpose of this search was to locate models which could be applied to potential roadway accident scenarios. A range of candidate driver models was found and their main features briefly outlined. Where possible, model parameters and sample response time histories have been included. It was found that no well-developed and validated model for the detailed study of accidents yet exists. The paper concludes with a brief assessment of the merits of the various models presented.