Event tree analysis construction

The analysis should use a logical approach which models how the event sequences progress from core damage to a radiological release. This is usually done by event tree analysis which models the accident sequence in a number of time frames and uses a set of nodal questions to model the sequence of events which occur. The construction of the event trees needs to be supported by thermal-hydraulic calculations and modelling of fission product release and transport inside the containment. 

Event tree construction and analysis (graphic and text) ... 

The frequencies of plant damage and public consequence are calculated using plant logic combined with component fragilities. Event and fault trees are constructed to identify tiic accident sequences and the damage that may result from an earthquake. In performing a plant system and accident-sequence analysis, the major differences between seismic and internal events analysis are given in Table 5.1-4... 

A dependency analysis determines which systems depend on other systems. This may require iterating the event tree construction to put a support system before the systems affected. 

A hazard tree is constructed by first identifying potential hazards. Starting with the hazard itself, it is possible to determine the conditions necessary for this hazard to exist. For these conditions to exist, a source that creates that condition must exist and so forth. Using this reasoning, a hierarchy of events can be drawn, which becomes the hazard tree. In a hazard analysis an attempt is made, starting at the lowest level in the tree, to see if it is possible to break the chain leading to the hazard by elimi-... 

The first step of the analysis is to identify the human actions and equipment failures that can lead to the failure of interest. An HRA event tree (Figure 5.14) is then constructed to depict the potential human errors (represented by capital English letters) and the potential equipment failures (represented by capital Greek letters). The series of events that will lead to the failure of interest is identified by an Fi at the end of the last branch of the event tree. All other outcomes are considered successes even though the propane release is not isolated in outcomes S2 and S3, so the process must be shut down. 

Figure 21.7.2 demonstrates die preliminary steps for a fault tree analysis (FTA) in addition, die TOP event, bounds, configurations, and unallowed events are specified, and die level of resolution is shown. Once all die limits have been detennined, die fault tree is constructed (Figure 21.7.3). Note tliat every branch of die fault tree ends in a basic fault or cause leading to die TOP event.

Unallowed events are those that are considered to be incredible or that, for some other reason, are not to be considered in the analysis. For example, wiring failures might be excluded from the analysis of an instrument system. Existing conditions are, for the purposes of the FTA, events or conditions considered certain to occur. The unallowed and existing conditions do not appear in the fault tree, but their effects must be considered in developing other fault events as the fault tree is constructed. 

Fault tree analysis is based on a graphical, logical description of the failure mechanisms of a system. Before construction of a fault tree can begin, a specific definition of the top event is required for example the release of propylene from a refrigeration system. A detailed understanding of the operation of the system, its component parts, and the role of operators and possible human errors is required. Refer to Guidelines for Hazard Evaluation (CCPS, 1992) and Guidelines for Chemical Process Quantitative Risk Assessment (CCPS, 2000). 

As a result of the AIC s efforts, we now have a process for investigating accidents in which we construct an event tree for each incident. The tree is quite similar to a fault tree from the quantitative risk analysis discipline, except that in the investigations we often sacrifice some structural rigor to get the most results in a reasonable time. Basically, the process uses a team to reconstruct the chronology of the incident and to construct the event tree. We try to include those who are most familiar with what actually happened, including the injured person(s) if any. We use the same basic method to investigate process failures, spills, injuries, or any other system failures. Emphasizing the system aspects of the failure removes much of... 

Inductive methods, such as check lists, Failure Mode and Effect Analysis (FMEA), event trees, decision tables, Analysis of Potential Problems (APP). These methods proceed from an initial cause of the deviation and construct a scenario ending with the final event. They are based on questions of the type What if ... 

HAZOP and What-If reviews are two of the most common petrochemical industry qualitative methods used to conduct process hazard analyses. Up to 80% of a company s process hazard analyses may consist of HAZOP and What-If reviews with the remainder 20% from Checklist, Fault Tree Analysis, Event Tree, Failure Mode and Effects Analysis, etc. An experienced review team can use the analysis to generate possible deviations from design, construction, modification, and operating intent that define potential consequences. These consequences can then be prevented or mitigated by the application of the appropriate safeguards. 

CONSTRUCTING THE FAULT TREE. Fault tree construction begins at the top event and proceeds, level by level, until all fault events have been traced to their basic contributing events or basic events. The analysis starts with a review of system requirements, function, design, environment, and other factors to determine the conditions, events, and failures that could contribute to an occurrence of the undesired top event. The top event is then defined in terms of sub-top events, i.e., events that describe the specific "whens and wheres" of the hazard in the top event. Next, the analysts examine the sub-top events and determine the immediate, necessary, and sufficient causes that result in each of these events. Normally, these are not basic causes, but are intermediate faults that require further development. For each intermediate fault, the causes are determined and shown on the fault tree with the appropriate logic gate. The analysts follow this process until all intermediate faults have... 

EVALUATING THE FAULT TREE. After a fault tree is constructed, it can be input to a fault tree analysis computer program, such as FTAP, IRRAS, or WAM. The output from the computer program is a list of MCSs which cause the top event to occur. For each of the MCSs, the analysts describe the consequences associated with that cut set. Table 4.25 shows a typical worksheet used to document the consequences associated with MCSs. 

Fault Tree. When direct data allowing to calculate the probability of a failure mode are not available or this failure form is complex, it is proposed the elaboration of a fault tree. It is a method of multidisciplinar analysis that begins with the selection of a failure mode or event that is tried to avoid. The event is developed into its immediate causes, and the sequence of events continues until basic causes are identified. The fault tree is constructed showing the logical event relationships that are necessary to result in the top event. The fault tree reaches terminal events whose probability must be calculated or estimated. These events can be basic events, which do not require to be explained by means of other previous events, or events which are not developed because it is not considered necessary or for lack of information. 

The above layout would be cumbersome to construct for each fault tree generated, so many fault tree analysis programs allow CCF groups to be created. Basic events are added to the group and a Beta factor between 0 and 1 specified. This is normally represented visually on the fault tree by displaying a p symbol next to the included events, as shown in Fig. 4A-4A 4 below. 

The probabilistic analysis of a plant is usually performed by the construction of event trees, for any single group of similar initiating events, and of fault trees, for any single system or component whose fault probability is important for the study of the various accident sequences. 

We start our analysis with hazard event identification and construction of the fault tree for the hazard. For the system presented in the Section 3.1, we can describe hazard as operational system failure . This negative event can occur only if the time resource, which is defined for restoring of the failed tram, is over crossing. This event may occur only in one of two possible situations ... 

As stated earlier, two types of fault trees will be constructed to demonstrate the use of this system safety analytical technique. The first, which will be referred to as a positive fault tree analysis, will identify the events necessary to achieve a top desired event of no accidents. The second, or negative fault tree, will be constructed to show those events or conditions that will lead to a top undesired event of a fire in a manufacturing facility. 

Fault tree analysis as applied to system safety relies on preliminary hazard analyses (PHA) or other analysis techniques to identify major undesirable events. After constructing a tree, a system safety team applies qualitative or quantitative analyses to elements. To perform quantitative analysis on a tree, team members must apply a probability to each event cause. Today, computer systems make the... 

Fault Tree Analysis - working backwards from a failure or potential failure, this logical procedure identifies all the possible causes, and hence, the origins of that failure. The fault tree consists of branches connected by AND and OR nodes - all the branches below an AND node need to coincide for the event above the node to occur, but only one of the branches below an OR node is required to condition the same. In these terms, a cause-effect map of failure is constructed. The advantage of such analysis is that it codifies a common understanding of the intrinsic logic of failure possibility. 

WJe now have a process for investigating accidents in which we construct an Event Tree for each incident. The Tree is quite similar to a Fault Tree from the Quantitative Risk Analysis discipline, except that in the investigations we often sacrifice some structural rigor to get the most results in a reasonable time. 

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