Operator action event tree

Operator action event trees are treelike diagrams that represent the sequence of various decisions and actions that the operating team is expected to perform when confronted with a particular process event. Any omissions of such... 

Four of the scenarios assodated with irradiated foel/taiget storage in the K-, L-, and P-Reactor Disassembly Basms 0Disassembly Basin draindown. Disassembly Basin overfill, and inadvertent criticality) were combined in a probabilistic manner in the analysis discussed m the PHA (Ref 8-17) Since each of these events contains numerous end-states (depending upon automatic and/or operator actions, fiiult-tree probabilities, and the pt cs of each event), a Monte-Carlo type method (Ref. 8-17) was used vdiich integrates the fiidt-tree end states of all four events (and thdr respective sub-events) into one consequence versus fiequency curve. It is... 

In any given situation, there may be different levels of dependence between an operator s performance on one task and on another because of the characteristics of the tasks theraseb e.s. or because of the manner in which the operator was cued to perform the tasks. Dependence levels between the performances of two (or more) operators also may differ. The analyses should account for dependency in human-error probabilities. In addition, each sequence may have a set of human recovery actions that if successfully performed will terminate or reduce the consequences of the sequence. This information, coupled with a knowledge of the system success criteria leads to the development of human success and failure probabilities which are input to the quantification of the fault iices or event trees. With this last step, the HRA is integrated into the PSA, and Pl. ise 4 is complete. 

Techniques for quantifying Techniques used were basically from NUREG/CR-1278 or PRA Procedun s Guide, Chapter 4. (l)THERP (1980 version NUREG/CR-1278 with appropriate factors for a tree was used in procedure defined events. (2) A time dependent modeling approach was used to quantify operator actions during a sequence. THERPmethixlr. were ii ,cd as described in WASH- Id ... 

Hall, R. E. et al., Post-Event Human Decision Errors Operator Action Tree/Time Reliability Correlation, BNL, November 1982. 

The committee s analysis was conducted on several levels. First, members investigated the causal factors for each of the seven events listed in Tables 2-1 and 2-2. They then developed a notional causal tree for each of the two events in Table 2-2 that were analyzed in depth. For illustrative purposes, a causal tree developed by the committee for the December 3-5,2000, incident at JACADS appears at Appendix F. The tree is a standard tool in reliability analysis and is particularly useful in human reliability analysis where operator actions contribute either positively or negatively to an incident. Lastly, the committee provides a series of general and specific observations about the events. 

Each of the initiating events fi>r the Disassembly Basin draindown event leads to between 10 and 25 fiult tree id states, dq>ending on automatic and operator actions fiiat are credited, and errors or random Mures that occur. Further complicating tiie analyses is that a number of different damage states may occur. For these reasons, a staristical approach to the event has been used to quantify the consequ ces and the fiequendes. (Ref 8-36,8-17). 

Operator action (when assumed in the foult tree branch) is assumed to begin 60 minutes afler the event takes place Q>lus a 60 minute standard deviation to cover a 2 hour emergency re nse action time). 

As the name suggests Fig. 11/3.7.2-1 is like an event tree diagram, which shows various decisions and actions concerned people are expected to do. This operator action tree is mainly used for post accident cases to pinpoint an operator s failure. This is very much used in accident analysis in air crashes. The level of breakdown of the main task is crucial. Too many breakdowns may pose a problem in handling. From visual inspection it is possible to identify the crucial operation that can initiate the event. The figure will give a clearer idea. 

An event tree is a graphical representation of event sequences with probabilities, following an initiating event. An event tree provides the systematic model of the time sequence of event propagation through a series of barriers, normal plant actions, and/ or operator intervention and incidental consequences. Now, short discussions have been put forward to see how an event tree is constructed with the help of a number of steps there are basically seven steps Involved, and these are defined in Table V/2.2-1. 

Each event tree describes the plant response to the most representative (not necessarily most limiting) event in a category. Limiting cases may be additionally studied in sensitivity and uncertainty analyses. In defining the plant response, credit is taken for all classes of safety systems as long as they are realistically expected to respond to the event. Moreover, credit is taken for proceduralised operator actions that are expected to be performed. 

Operator actions and containment systems that address, prevent, or mitigate the severe accident phenomena are considered on the containment event tree. The operator actions and systems that are explicitly modelled on the containment event tree are ... 

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