Accident sequence

At this stage we examine the loss causation model, or accident sequence, and show the sequence of events that lead to an accident and subsequent loss. The sequence of events that lead to a near-miss incident are identical to those of an accident except no loss is experienced. 

Accidents are caused by a sequence of events, a combination of circumstances and activities similar to the snowball or domino effect. These events eventually culminate in a loss. The loss, caused by contact with a source of energy, may be an injury, damage, or business interruption. Due to some unexplained circumstance, sometimes called fortuity or luck, if the event does not end in loss, it is usually termed a near-miss incident. The factors leading up to an accident are there, but the event is interrupted, as there is no exchange of energy, or no contact with the energy, and therefore no injury, property damage, or loss is experienced. 

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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. 

Initiating Event Feed Shuts Off Reactor Dump Works Accident Sequence Number Frequency (events/yr) Consequence (impacts/event)... 

The models you use to portray failures that lead to accidents, and the models you use to propagate their effects, are attempts to approximate reality. Models of accident sequences (although mathematically rigorous) cannot be demonstrated to be exact because you can never precisely identify all of the factors that contribute to an accident of interest. Likewise, most consequence models are at best correlations derived from limited experimental evidence. Even if the models are validated through field experiments for some specific situations, you can never validate them for all possibilities, and the question of model appropriateness will always exist. 

Acceptable risk Accident (sequence) Acute hazard... 

A logic model that graphically portrays the range of outcomes from the combinations of events and circumstances in an accident sequence. For example, a flammable vapor release may result in a fire, an explosion, or in no consequence depending on meteorological conditions, the degree of confinement, the presence of ignition sources, etc. These trees are often shown with the probability of each outcome at each branch of the pathway... 

Inherently safer strategies can impact the accident process at any of the three stages. The most effective strategies will prevent initiation of the accident. Inherently safer design can also reduce the potential for propagating an accident, or provide an early termination of the accident sequence before there are major impacts on people, property, or the environment. 

A significant development of the study was the use of event trees to link the system fault trees to (lie accident initiators and the core damage states as described in Chapter 3. This was a response to the ditficulties encountered in performing the in-plant analysis by fault trees alone. Nathan Villalva and Winston Little proposed the application of decision trees, which was recognized by Saul Levine a.s providing the structure needed to link accident sequences to equipment failure. 

The Committee is unable to determine whether the absolute probabilities of accident sequences in WASH-1400 are high or low, but it is believed that the error bounds on those estimates are, in general, greatly understated. This is due in part to an inability to quantify common cause failures, and in part to some questionable methodological and statistical procedures. 

Provide a realistic evaluation of the potential consequences associated with hypoih.elic.i accident sequences, and... 

One of the products of a nuclear power plant PSA is a list of plant responses to initiating events (accident starters) and the sequences of events that could follow. By evaluating the significance of the identified risk contributors, it is possible to identify the high-risk accident. sequences and take actions to mitigate them. 

Although the consequences of the high-risk accident sequences may vary from one PSA to another, all PSAs attempt to evaluate realistically, the consequences of hypothetical accident sequences. Expending on the scope of the PSA, these evaluations may include an estimation of the number of latent cancers, the number of immediate fatalities, the probability of core damage, or a number of other consequence measures. 

When used to identify and evaluate significant risk contributors, as well as to assess the consequences of accident sequences, the PSA provides a comprehensive framework for making many types of decisions regarding reactor design, operation, and siting. These and other appi ications can be facilitated by the rational evaluation of the risks associated with a particular installation. 

The relationships between the importance measures is based on the assumption that the systems are not reconfigured in response to a component outage. If this is done, the basic definition of the importance measure is still valid but there is not such a simple relationship. Disregarding this complication, some interpretations of the importances may be made. The Bimbaum Importance is the risk that results when the i-th system has failed (i.e., it is the A, term in Equation 2.8-9). Inspection Importance and RRWI are the risk due to accident sequences containing the i-th system. Fussell- Vesely Importance is similar except it is divided by the risk so may be interpreted as the fraction of the total risk that is in the sequences contains the Q-th system. The Risk Achievement Worth Ratio (RAWR) is the ratio of the risk with system 1 failed to the total risk and is necessarily greater than one. The Risk Achievement Worth Increment (RAWI) is the incremental risk increase if system 1 fails and the Risk Reduction Worth Ratio (RRWR) is the fraction by which the risk is reduced if system 1 were infallible. 

The Boolean equation for the probability of a chemical process system failure is R = A (B-i-C (D-rE (B-l-F G+C). Using Table 2.1-1, factor (he equation into a sum of products to get the mincut representation with each of the products representing an accident sequence. 

Human errors may be dependent on the specific accident sequence displayed in the event tree, and, for that reason, may be included in the event tree. This requires the human-factors specialist to consider the context of the error in terms of stress, operator training in response to the accident, di.tgnosiic paiierns, environmental, and other performance-shaping factors. 

The event list is not necessarily hardware it may include anything germaine to the accident sequence such as human error. 

Function event trees are concerned with depicting functions that must happen to mitigate an initiating event. The headings of the function event tree are statements of safety functions that are required but that may fail in an accident sequence. 

If any of the responses are positive, the node should be included in the event tree. Each question must be examined in the context of each potential accident sequence because of effects on other systems,... 

Step I - Select the combinations of systems that enter the analysis. (This is equivalent to finding accident sequences in event tree analysis.)... 

Step 2 - Construct a global digraph model for each accident sequence. 

Step 3 - Partition digraph models into independent subdigraphs and find singleton and doubleton minimum cutsets of accident sequences. 

Merges system and functional equations into accident sequence cutsets ... 

ASEP Accident Sequence Evaluation Procedure NUREG/CR-4772. 1987... 

Since this text is not solely devoted to HRA, the above process is by necessity simplified. A more defined interaction is shown in Figure 4.5-2. Here HRA interacts with the development of the system models and feeds into not only the accident sequences but also the physical analysis of the inplant and explant accident progress,... 

Human error contributed to about 50% of the accident sequences m the RSS but none of the human error data came from the nuclear power industry. Furthermore, very high failure rates 0.5 to 0.1/action) were predicted but are not supported by the plant... 

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... 

The first step-in plant-system and accident-sequence analysis is the identification of earthquake-induced initiating events. This is done by reviewing the internal analysis initiating events to identify initiating events relevant to seismic risk. For example. Table 5,1 -5 shows the initiating events that were used in the Seismic Safety Margins Research Program for a PWR plant (Smith et al., 1981)... 

Assess the effects of these fire scenarios on accident sequences in event trees for fire-induced initiating events. 

Determining which accident sequences lead to which states requires a thorough knowledge of plant and process operations, and previous safety analyses of the plant such as, for nuclear plants, in Chapter 15 of their FSAR. These states do not form a continuum but cluster about specific situations, each with characteristic releases. The maximum number of damage states for a two-branch event trees is 2 where S is the number of systems along the top of the event tree. For example, if there are 10 systems there are 2 = 1,024 end-states. This is true for an "unpruned" event tree, but. in reality, simpler trees result from nodes being bypassed for physical reasons. An additional simplification results... 

An accident sequence source term requires calculating temperatures, pressures, and fluid flow rates in the reactor coolant system and the containment to determine the chemical environment to which fission products are exposed to determine the rates of fission product release and deposition and to assess the performance of the containment. All of these features are addressed in the... 

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