Event sequence diagram

RISKMAN embodies PLG s scenario-based, engineering-approach displays and calculates event sequence diagrams, fault trees, and event trees. It allows for system-specific data updates and accoums for dependencies between systems. 

Event Sequence Diagrams (ESDs) and Event Trees (ETs) were used to logically di plant re.spon.se to various initiator.s in a two step process ... 

Use of structured techniques, such as event trees or event sequence diagrams Identification of significant occurrences on a time-scale, e.g. flux monitor trip and start of insertion of control rods ... 

Event Trees and Event Sequence Diagrams are structures that depict the current state of the analyst s knowledge about function and system dependencies. Event Sequence Analysis is used to represent all possible success paths for initiating events. This can be done on much more detailed level than function event trees. In particular, different possibilities of achieving the same safety function (e.g. by... 

For severe accidents, reported parameters are similar to those for LWRs, as described in Ref. [13]. The inventory of the moderator and the shield tank water as a function of time should be included, as these sources of water can delay the progression of the accident significantly. An event sequence diagram, with timing, is particularly helpful. 

The OWR protective systems were modeled with event tree diagrams for the time sequence following an initiating event to fuel damage or safe shutdown. Fault trees were used to find the probability of failure of each protective system in a particular event tree. 

A widely used method to study event sequences that can lead to incidents involves logic diagrams, an example of which is shown in Figure 4.1. Through the method illustrated in the figure, it is determined that a runaway reaction will happen only if both the cooling system fails and the reactor contents... 

Figure 23-1 shows the hazards identification and risk assessment procedure. The procedure begins with a complete description of the process. This includes detailed PFD and P I diagrams, complete specifications on all equipment, maintenance records, operating procedures, and so forth. A hazard identification procedure is then selected (see Haz-ard Analysis subsection) to identify the hazards and their nature. This is followed by identification of all potential event sequences and potential incidents (scenarios) that can result in loss of control of energy or material. Next is an evaluation of both the consequences and the probability. The consequences are estimated by using source models (to describe the... 

The implementation of calendar A keeps a separate unordered vector of events for each instmctor in a hashtable, keyed by the instmctor. The calendar s internal interactions are described in the sequence diagram in Figure 3.7, with each arrow indicating an operation request. Upon receiving an addEvent request, the calendar first creates a new event object. It then looks up the event vector for the current instmctor in its hashtable, creating a new vector if none exists. The new event is added to this vector, and the hashtable is updated. The Java code for this design is shown on the next two pages. 

A sequence diagram is an alternative presentation of the same information as shown in a collaboration diagram and is better at showing the sequence of events but the collaboration diagram also shows the links and objects. As an example, Figure 6.36 shows the sequence of the propagation of changes. 

Sequence diagrams are a more elaborate graphical depiction of a timeline, and allow the investigator to present related events and conditions in parallel branches. These sequence diagrams are also known as causal factor charts. 

On the basis that a picture is worth a thousand words, a sequence diagram can be used as an effective tool for commimication about the incident sequence, the initiating events, and their causal factors. Sequence... 

Next the team develops a chronology of events based on the available known times and sequences. This document is usually referred to as a timeline. (See the detailed description of timeline development in Section 9.3.) Unconfirmed assumptions regarding chronology should be clearly identified as unconfirmed, and action should be initiated to verify assumptions. Many investigators use relatively simple timelines (instead of sequence diagrams) with the logic tree methods because the logic tree itself shows the interactions of events and conditions. 

The next phase of investigation involves developing a preliminary chronological description of the sequence of events that led to the failure. Timelines can be developed in various formats and levels of detail, from simple lists of events to complex sequence diagrams or causal factor charts, usually dependent upon the particular circumstances of the investigation being conducted. 

Quality assurance tests should be applied to enhance the process. A completeness test can help identify gaps or omissions in the chronology of events, while a sufficiency test can check the logic in a sequence diagram by comparing two adjoining facts to verify if one fact always leads to the second fact. 

Once the evidence has heen collected, a timeline or sequence diagram developed, and the actual scenario confirmed, the investigation can proceed to the next stage, the identification of causal factors. These causal factors are the negative events and actions that made a major contrihution to the incident. 

There are a number of quality assurance checks that should be considered before identifying the final list of causal factors. It is important to test for sufficiency of the information when compiling a sequence diagram. This test for sufficiency may be performed by asking one or more of the following questions, when comparing two adjoining facts in the sequence of events ... 

These sequences are in general represented by event trees, also called accident sequence diagrams. Several examples of event trees are shown in Figs. 10.2, 10.3, 10.4 and 10.5. Releases of materials under pressure can additionally be accompanied by missile flight of fragments of the pressure boundary (e.g. vessels or pipework). If a gas is not flammable but toxic atmospheric dispersion follows its release. This is also true in cases of delayed ignition. 

Sequence Diagram Guard (trans.), event synchronisation. 

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