Fault tree analysis impact event

Failure sequence modeling techniques such as fault tree analysis or event tree analysis are used to estimate tlie likelihood of incidents in facilities where historical data is unai ailable, or is inadequate to accurately estimate tlie likelihood of the liazardous incidents of concern. Otlier modeling tecluiiques may be required to consider tlie impact of external events (eartliquakes, floods, etc.), common cause failures, and human factors and hmnan reliability. 

There exist different methods like Fault Tree Analysis (FTA), Event Tree Analysis (ETA) and Monte Carlo Simulation (MCS) that can be applied and combined for the purpose of evaluating the frequency and probability of initiating events. However, the MCS can be handled much easier in order to accoimt for bormdary conditions like stochastic dependence, time dependence and physical impact (Hauschild Meyna 2007). The MCS has been apphed successfully for PSA in order to assess the safety of nuclear power plants (Devooght Smidts 1996, Woltereck2001) and especially for taking into account uncertain input data (BfS 2005). 

Representation Having defined what the operator should do (via task analysis) and what can go wrong, the next step is to represent this information in a form which allows the quantitative evaluation of the human-error impact on the system to take place. It is usual for the human error impact to be seen in the context of other potential contributions to system risk. Human errors and recoveries are usually embedded within logical frameworks such as fault tree analysis and event tree analysis. 

The estimated impact is then compared to hazard acceptance criteria to determine whether the consequences are tolerable without additional loss prevention and mitigation measures. If the identified consequences are not tolerable, the next step is to estimate the ffequency/probability of occurrence of the identified failure modes leading to loss of containment. For simple cases, frequency estimates are combined with consequences to yield a qualitative estimate of risk. For complex cases, fault tree analysis is used to estimate the frequency of the event leading to the hazard. These estimates are then combined with the consequences to yield a measure of risk. The calculated risk level is compared to a risk acceptance criterion to determine if mitigation is required for further risk reduction. 

Fault tree analysis (FTA) is a deductive method, which usually serves for quantification. Just like any method of systems analysis it requires in the first place a qualitative investigation of the system under analysis. After system failure or more generally the undesired or unwanted event (e.g. toxic release) has been defined, logic relationships with the so-called primary or basic events are identified and represented by a fault tree (vid. Fig. 9.8). The primary event may represent the failure of a technical component, an operator error or an impact from outside the plant like flooding or the spreading of a fire from neighbouring installations. 

This paper presents a new approach based on a combination of traditional predictive modelling and event/fault tree analysis techniques, which allows representing at the same time evolution of hazards and normal and abnormal (i.e. failures) performance of safety measures, e.g. variations of process parameters, analysis and inspections, through the food chain for a better estimation of the real impact of such deviations/failures on consumer health. 

Event Tree Analysis (ETA) uses the same logical and mathematical techniques as Fault Tree Analysis. However, whereas a fault tree analyzes how an undesirable top event may occur, an event tree considers the impact of the failure of a particular component or item in the system, and works out the effect such a failure will have on the overall system-risk or -reliability. Event trees are inductive fault trees are deductive. 

FMEA is simply an analysis tool that identifies all the ways a particular component can fail and what its effects would be at the subsystem level and ultimately on the system. FMEA is vastly different from fault tree analysis. Fault tree analysis is a top-down analysis of faults in a system. FMEA is a bottom-up analysis that identifies failures (not necessarily faults) in the system. The fault tree starts with the top-level or system-level concern (top event) and then works down to the events that lead to that top event. FMEA does exactly the opposite it starts with the components in the system and analyzes failures and how they impact the subsystan in which it is housed and what are the propagated effects across the syston. 

The use of fault-tree analysis, fw example, formalises parts of the safety argument A fault tree is a chain of assotions that proves that the top-level hazardous event cannot occur if particular combinations of underlying hazardous evoits are known not to be credible or possible. The production of a ftxmal proof is motivated by the same needs. However, in the case of formal proof, the complexities of the events concerned, and their impact in combination, can be subject to more thorough and rigorous analysis. 

The magnitude of risk from some event depends on the product of how often the analyst thinks an event will occur and how seriously the event impacts on the overall process. Therefore, it is. incumbent on the scientist to develop a quantitative sense of where the risks in an analysis exist, and how serious they are. The best systems analyst cannot perform this function only the person who the is most knowledgeable about the analytical procedure can function as the risk assessor. This person is normally the research chemist who developed the methodology and not the analyst who may run the procedure routinely. He or she is most familiar with the emerging methodology and has a basis (whether it be historical, intuitive or reasoned) to assign a factor of risk to the individual components of the analysis. Typical mechanisms for risk assessment studies include either the use of a "Fault Tree", which uses lists of major failures and associated minor failures which might cause them, or a "Failure Modes and Effects Analysis Model" (21) which uses lists of the ways a system can fail and the results of each failure. For this study, the "Failure Modes and Effects Analysis Model" was chosen. 

Are there documents that provide comprehensive analysis of all potential safety and health hazards of the worksite Are there documents that provide both the analysis of potential safety and health hazards for each new facility, equipment, material, or process and the means for eliminating or controlling snch hazards Does documentation exist outlining the step-by-step analysis of hazards in each part of each job, so that yon can clearly discern the evolution of decisions on safe work procedures If complicated processes exist, with a potential for catastrophic impact from an accident but low probability of such accident (as in nnclear power or chemical production), are there documents analyzing the potential hazards in each part of the process and the means to prevent or control them If there are processes with a potential for catastrophic impact from an accident but low probability of an accident, have analyses such as fault tree or what if been documented to ensure sufficient backup systems for worker protection in the event of multiple control failures ... 

Bow tie analysis is a tool that has become very popular in the last few years, especially because of the ease in which it can display cause-consequence of a particular hazardous condition. It is a qualitative tool that combines the fault tree to determine the causes and how the fault could occur, with the event tree, which documents the consequence of the hazardous condition. It became much better known in the mid-1990s when Royal Dutch/Shell used it to better understand the Piper Alpha disaster. The process industry uses it not only to assess the hazards and risks but also as a very effective communication tool to illustrate the cause-consequence-control and how it can impact a hazardous condition. In reality, it really isn t a new analytical tool, but rather, a very good visualization tool. 

ABSTRACT Technological advancements in area of sensor-based online maintenance systems have made the possibility of repairing some failed safety support systems of Nuclear Power Plants (NPP) such as electrical supply, I C systems, ventilation systems. However, the possibility of repair during accident situation is yet to be included into PSA level-1. Therefore, this paper presents a scheme of PSA level-1 by implementing an integrated method of Repairable Event Tree (RET) and Repairable Fault Tree (RET) analysis. The Core Damage Frequency (CDF) is calculated from consequence probabilities of the RET. An initiating event of Decay Heat Removal (DHR) systems of ASTRID reactor is analyzed. The proportionate CDFs estimated with repair and without repair have been compared and found that the recoveries can reduce CDF. In sum, this paper attempts to deal with the possibility of repair of some safety systems in PSA and its impacts on CDF of the NPP. 

External Events Analysis. This component of frequency analysis considers the impact of external events (sueh as earthquakes, tornadoes, floods, aircraft crashes, terrorism, and vandalism) as initiating events to undesirable event scenarios. Quantitative frequency information is then used in fault and event trees. 

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