Accident scenario

The subsequent step is to identify the various scenarios which could cause loss of control of the hazard and result in an accident. This is perhaps the most difficult step in the procedure. Many accidents have been the result of improper characterization of the accident scenarios. For a reasonably complex chemical process, there might exist dozens, or even hundreds, of scenarios for each hazard. The essential part of the analysis is to select the scenarios which are deemed credible and worst case. 

Hazard analysis does have limitations. First, there can never be a guarantee that the method has identified all of the hazards, accident scenarios, and consequences. Second, the method is very sensitive to the assumptions made by the analysts prior to beginning the procedure. A different set of analysts might well lead to a different result. Third, the procedure is sensitive to the experience of the participants. Finally, the results are sometimes difficult to interpret and manage. 

QRA results can guide decision makers in their quest for continuous improvement in risk reduction, but zero risk is an unattainable goal. Any activity involves some risk. Even if it were hypothetically possible to eliminate the risk of every accident scenario in a QRA, some risk would still remain because no QRA examines every possible accident scenario. At best a QRA identifies the dominant contributors to risk from the system as it existed at the time of the analysis. Once those are eliminated, other minor risk contributors (including many that were left out of the original QRA because they were negligible contributors, as well as new risks introduced by changes to eliminate the original risks) remain as the new dominant risk contributors. 

In any case, like frequency analysis, examining the uncertainties and sensitivities of the results to changes in boundary conditions and assumptions provides greater perspective. The level of effort required for a consequence analysis will be a function of the number of different accident scenarios being analyzed the number of effects the accident sequence produces and the detail with which the release, dispersion, and effects on the targets of interest is estimated. The cost of the consequence analysis can typically be 25% to 50% of the total cost of a large QRA. 

Consequence Phase 3 Develop Detailed Quantitative Estimate of the impacts of the Accident Scenarios. Sometimes an accident scenario is not understood enough to make risk-based decisions without having a more quantitative estimation of the effects. Quantitative consequence analysis will vary according to the hazards of interest (e.g., toxic, flammable, or reactive materials), specific accident scenarios (e.g., releases, runaway reactions, fires, or explosions), and consequence type of interest (e.g., onsite impacts, offsite impacts, environmental releases). The general technique is to model release rates/quantities, dispersion of released materials, fires, and explosions, and then estimate the effects of these events on employees, the public, the facility, neighboring facilities, and the environment. 

Layer of protection analysis (LOPA) is a simplified form of event tree analysis. Instead of analyzing all accident scenarios, LOPA selects a few specific scenarios as representative, or boundary, cases. LOPA uses order-of-magnitLide estimates, rather than specific data, for the frequency of initiating events and for the probability the various layers of protection will fail on demand. In many cases, the simplified results of a LOPA provide sufficient input for deciding whether additional protection is necessary to reduce the likelihood of a given accident type. LOPAs typically require only a small fraction of the effort required for detailed event tree or fault tree analysis. 

Frequency Phase 1 Perform Qualitative Study, Typically Using HAZOP, FMEA, or What-if Analysis. To perform a qualitative study you should first (1) define the consequences of interest, (2) identify the initiating events and accident scenarios that could lead to the consequences of interest, and (3) identify the equipment failure modes and human errors that could contribute to the accident... 

Frequency Phase 3 Use Branch Point Estimates to Develop a Ere-quency Estimate for the Accident Scenarios. The analysis team may choose to assign frequency values for initiating events and probability values for the branch points of the event trees without drawing fault tree models. These estimates are based on discussions with operating personnel, review of industrial equipment failure databases, and review of human reliability studies. This allows the team to provide initial estimates of scenario frequency and avoids the effort of the detailed analysis (Frequency Phase 4). In many cases, characterizing a few dominant accident scenarios in a layer of protection analysis will provide adequate frequency information. 

A valuable QRA result is the importance of various components, human errors, and accident scenarios contributing to the total risk. The risk importance values highlight the major sources of risk and give the decision maker a clear target(s) for redesign or other loss prevention efforts. For example, two accident scenarios may contribute 90% of the total risk once you realize that, it is obvious that you should first focus... 

A simplified form of event tree analysis using selected accident scenarios and order-of-magnitude estimates to determine whether additional protection is needed... 

This section describes how both hypothetical and real accidents are analyzed. These methods varying greatly in complexity and resource requirements, and multiple methods may be used in an analysis. A simple method is used for screening and prioritization followed by a more complex method for significant accident scenarios. Some methods give qualitative results more complex methods give quantitative results in the form of estimated frequencies of accident scenarios. The process systems in Figures 3.3.1-1 and 3.3.1-2 are used in the examples. 

The hazards associated with the process may noi, be in generic information, Checklists idcnlify iiazards but not the accident scenarios that lead to the hazards. 

What-ll/Checklist combines the creative, brainstorming features of a What-lf with the systematic features of a Checklist. The What-lf analysis considers accidents beyond the checklist the check list lends a systematic structure to the What-lf analysis. A What-lf7Checklist examines the potential consequences of accident scenarios at a more general level than some of the more detailed I l liA methods. It can be used for any type of process at any life cycle stage. 

The results of a What-If/Checklist analysis are documented like the results of a What-lf analysis as a table of accident scenarios, consequences, safety levels, and action items. The results may also include a completed checklist or a narrative. The PrHA team may also document the completion of the checklist to illustrate its completeness. The PSM rule requires detailed... 

The process begins with initial system and accident definition for which accidem the probabilities and consequences must be determined to give the risk (Figure 6,3-1). Item 1, (he event tree is central to PSA because it diagrams the accident scenarios to connect accident imtiaiors to consequences. 

Potential accident scenarios and flood locations were identified from plant drawings and tlic RHR system fault tree that identifies the equipment and support needed for RHR system operation. The equipment location was correlated with flood areas with consideration for plant features which may impede or divert the flow. The flood scenarios identify the effect on systems required to prevent core damage. Quantification accounts for the rate of rise of the flood relative to the critical level in each specific plant area. The time available for any recovery action is calculated from tiic volume and the flow rate. 

The dominant internal event accident scenarios were used in conjunction with conservative source terms (100% of the core melts, releasing 100% of the noble gases, 100% nf iodine and cesium, and 1% solids), and )ecific meteorological data to estimate e consequences to be well below the lOCFRlOO siting guidelines. 

Its unique design suggests several accident scenarios that could not occur at other reactors. For example, failure to supply ECC to 1/16 of the core due to the failure of an ECC inlet valve. On the other hand, some phenomena of concern to other types of reactors seem impossible (e.g., core-concrete interactions). The list of phenomena for consideration came from previous studies, comments of an external review group and from literature review. From this, came the issues selected for the accident progression event tree (APET) according to uncertainty and point estimates. 

This volume consists of two parts Chapters 1-6 and Chapters 7-9. Chapters 1 through 6 offer detailed background information. They describe pertinent phenomena, give an overview of past experimental and theoretical research, and provide methods for estimating consequences. Chapter 2 describes the phenomena covered, identifies various accident scenarios leading to each of the events, and describes actual accidents. In Chapter 3, principles such as dispersion, deflagration, detonation, blast, and radiation are explained. 

This volume does not address subjects such as toxic effects, explosions in buildings and vessels, runaway reactions, condensed-phase explosions, pool fires, jet flames, or structural responses of buildings. Furthermore, no attempt is made to cover the frequency or likelihood that a related accident scenario will occur. References to other works are provided for readers interested in these phenomena. 

Turbulence in a vapor cloud explosion accident scenario may arise in any of three ways ... 

Accident scenarios leading to vapor cloud explosions, flash fires, and BLEVEs were described in the previous chapter. Blast effects are a characteristic feature of both vapor cloud explosions and BLEVEs. Fireballs and flash fires cause damage primarily from heat effects caused by thermal radiation. This chapter describes the basic concepts underlying these phenomena. 

Chapter 2 discussed the possible influence of atmospheric dispersion on vapor cloud explosion or flash fire effects. Factors such as flammable cloud size, homogeneity, and location are largely determined by the manner of flammable material released and turbulent dispersion into the atmosphere following release. Several models for calculating release and dispersion effects have been developed. Hanna and Drivas (1987) provide clear guidance on model selection for various accident scenarios. 

You can quickly identify these plant sections by reviewing process flow diagrams and valving arrangements. Isolation points are defined by control valves or powered block valves that can be remotely activated. Process hazard analysis techniques help you identify the maximum credible accident scenarios. (Note that manual valves should not be considered reliable isolation points unless they are located to be accessible following a major accident. However, remotely-activated valves can only be considered reliable isolation points if there are adequate reliability engineering and maintenance programs in place.)... 

Given the limited resources that a plant can provide for training, a critical question arises concerning emphasis which must be given to individual or team training. Many accident scenarios involve well-trained personnel who failed to work collectively under the particular conditions of the task. We need, therefore, some guidelines to judge the relevant importance of individual versus team performance for different types of tasks. 

The first step in a process plant building risk assessment is to identify specific accident scenarios that endanger building occupants. As discussed in Chapter 2 and illustrated in Table 2.1, accident scenarios are sequences of events that lead to an outcome of concern. The specific outcomes of concern are those involving explosions or fires that could impact buildings in process plants. 

The risk assessment process begins by identifying specific accident scenarios that apply to the facility under review. Steps include ... 

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