The MORT Event Tree

In general, the remainder of this chapter focuses on the explanation of the various MORT event tree symbols and their use and meaning. Since the tree is an analytical model, the information presented in the previous chapter (on fault tree analysis) will be helpful and should be reviewed. 

The primary symbols used for most analytical trees have been used in the MORT event tree as well. These include the rectangle (primary or top event, and secondary, contributory or main events), the diamond (undeveloped event), the circle (basic event), the and gate, the or gate, the oval (conditional or constraint symbol), and the triangle (transfer gate or symbol). In addition to these symbols, the MORT 

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The primary symbols used for most analytical trees have been used in the MORT event tree as well. These include... 

Black—event or factor not applicable. Depending on the nature of the top event, there may be some areas of the comprehensive MORT event tree that simply do not apply to the particular investigation. In this case, the event and/or an entire event block is colored black or simply crossed out. Although it may be obvious in many instances when a set of events is not applicable, the... 

Figure 36-8 shows the general features of a MORT event tree. This example gives a flavor for MORT diagrams... 

The project evaluation tree is an analytical tree to be used primarily as a graphic check in basically the same manner as the management oversight and risk tree (see Chapter 18). The PET chart, however, contains fewer than 200 event symbols and no transfers the MORT chart contains approximately 1,500 events symbols, multiple transfers, transfers within transfers, and drafting breaks. The PET chart is divided into three branches procedures, personnel, and plant and hardware. 

The symbols used on the MORT chart are basically those used for other analytical trees (Chapter 10) and fault tree analysis (Chapter 15). They include the rectangle as the general event symbol, the circle as the base event symbol, the diamond as an undeveloped terminal event, the and gate, the or gate, and the ellipse as a constraint symbol (Rgs. 18-1 and 18-2). 

Figure 18-1 Event Symbols. The General Event symbol, Basic Event symbol, and Undeveloped Terminal event symbol are the same as commonly used for other fault trees. The use of the stretched circle for a Satisfactory Event and the scroll for a Normally Expected Event are unique to the MORT chart.

MORT investigation utilizes a color-coding system, as follows, to help identify those areas on the event tree where additional investigation or analysis is warranted ... 

In addition to these, the MORT chart also uses a rounded rectangle, or elongated circle, to represent a satisfactory event (an event that may have contributed to an accident or incident but whose existence is essential for normal system operation). Also, instead of the house symbol common to fault tree analysis to represent those events that are considered normal and expected in a typical system, MORT uses a scroll symbol. Figure 13.2 shows the MORT symbols as described here. 

Figure 13.3 The top branches of the MORT analytical event tree (Source DOE. ...

Cut-Set As pertains to fault tree analysis (FTA) and/or the Management Oversight and Risk Tree (MORT), a defined set of events, under the top event, that can be isolated from the remainder of the fault tree and examined as contributory to the occurrence of the top or primary event. 

The Regulatory Body also has certified investigators, who use the Management Oversight and Risk Tree (MORT) to review some of the root cause analysis performed by the utilities and, in specific cases, carry out fully independent root cause analyses of the same events. 

Many methods of reducing human error are also based on the determination of whether human error actually occurred. Therefore, among these methods are included fault tree analysis and MORT event analysis. 

Eigure 5.11 shows the main elements of MORT (Johnson, 1980 Knox and Eicher, 1992). They are displayed as events in a logical tree of, in total, about fifteen hundred basic events. Each event corresponds to a question to be addressed in accident investigations and SHE audits. The MORT diagram is not based on systematic research, but is rather a codified collection of the practical experience and judgement existing at that time. 

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