Events accident-initiated event

An occurrence involving equipment performance or human action, or an occurrence external to tlie system tliat causes system upset. In this document an event is associated with an accident either as the cause or a contributing cause of the accident or as a response to the accident-initiating event. 

Accident-Initiated Event An event (or the first event in an event seqnence) that is caused by a movement-related transportation accident (e.g., a train derailment or a barge gronnding). 

Non-Accident-Initiated Event An event due to causes rmassociated with the movement-related aspects of transportation (e.g., improperly closed valves or pressure build-up due to contamination). 

In a safety risk analysis, initiating transportation events may include both accidents and non-accidents. Depending on the mode of transport. Table 3.2 details some of the types of accident-initiated events that should be considered in the risk analysis process. 

Accident-initiated events can generally be identified based on historical data for the industry or current operations/activities. Information from both actual... 

The following are examples of potential causes, or contributors to, accident-initiated events, and may be considered across various modes ... 

Non-accident-initiated events differ in that the event is not associated with an external impact during transport. Typical non-accident-initiated events that may be reviewed as part of a transportation risk analysis may not be mode-specific, and could include ... 

Step 3 Selection of Consequence Measures Transportation risks arise from the release of hazardous materials as a result of accident- and/or non-accident-initiated events. The types of impacts that may be evaluated in a QRA include impacts to people, property, the environment, disraption of supply chain activities, and other financial concerns. 

The chance of an incident is generally a function of the distance traveled. Thus, the frequency of an accident is often expressed as an accident rate per mile. Contributions from non-accident-initiated events are typically expressed on a frequency-per-hour or per-year basis. Thus, the duration of the hazardous materials movement is a key parameter. Figure 5.3 illustrates the basic calculation sequence for one trip or movement. If multiple trips are made, the total risk is equal to the number of trips times the risk per trip. The basic calculation sequence will have minor variations for each mode of transport and can be broken down into greater detail as needed. Increased detail might include different accident rates and lengths for each segment of a route or might explicitly address the accident rates and release probabilities for different accident causes. Inputs to the analysis that may be altered or may influence the calculation include ... 

On the other hand the method is not well suited to discover hazards which result from the simultaneous failure of several components. Hence, it is often used to prepare the application of other methods of safety analysis, for example event tree and fault tree analyses (cf. Sects. 9.1.2 and 9.1.2.7). It then serves primarily to identify accident initiating events, i.e. such events which would lead to an accident, should the safety equipment of a system fail. 

A technical system normally has a number of standby components (components which become active only after demand). Components of the monitoring and safety systems belong to this category. These systems are devised to cope with accident initiating events. They form the barriers between the initiating and the undesired event. The latter only occurs if all barriers fad. The situation is shown schematically in Fig. 9.10. Obviously in the same system initiating events can occur for whose control various barriers may become effective, and initiating events which directly lead to the undesired event, for example the spontaneous failure of a chlorine pipe. 

Detailed What-ir Hazop FMECA Identify hazards Identify accident-initiating events Estimate worst-case consequences ... 

It should determine the accident initiating events. Was it a failure Did the accident occnr as part of normal operations What kinds of factors led to the initiating events Were people involved What was the state of the equipment involved in the accident ... 

The safety systems are designed so that the plant can survive a set of potentially hazardous events called initiating events. Five initiating events are defined loss of coolant accident (LOGA), loss of feed water due to e.g. main feed water pump failures (LOFW), loss of online power (LOOP), disturbances in normal plant operation without the loss of primary coolant (Transient), and the loss of DC power. Depending on the initiating event there are different success criteria... 

Accident initiation events considered in design-basis evaluation in the safety analysis report SG leaks Fuel failure propagation Rupture of primary piping Pimg) failme or reactivity transients... 

The overpressure in the containment can arise after a loss-of-coolant accident initiating event or a progression of an accident to severe accident conditions (after primary depressurization). In such cases, the primary steam is released in the hermetic zone and expands from the reactor hall to the bubble condenser. 

Table 2.1-2 is a list of potential accident-initiating events (initiators), which applicants are specifically requested to address in SAR Chapter 15. Regulatory Guide 1.70 asks that the potential causes of each of these initiators be identified, and the estimated frequency of occurrence of each plausible initiator be assigned to one of the following categories ... 

Define the type of accident initiating events to be considered (e.g. component failures, human error)... 

Type of accident initiating events that need to be considered Determined by hazard study 2 method... 

Despite these severe difficulties, probability trees lend themselves conveniently to significant insights into accident-initiating event sequences, even if basic event data are not available. Qualitative analyses of probability trees can proceed one step further with quantitative rankings of the relative importance of accident sequences. The relative assessment and selection of given mitigating measures is also possible. 

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. 

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

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 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. 

After having studied the peer comments about some important classes of initiating events, we remain unconvinced of the WASH-1400 conclusion that they contribute negligibly to the overall risk. Examples include fires, earthquakes, and human accident initiation. 

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. 

An event tree is a model of the process response to an accident initiator. The initiators are... 

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. 

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