Event tree analysis quantification

A fault tree may either stand alone or be coupled to an event tree to quantif" bability. The top event in either case is the abjective of performing the analysis. If tht is the reliability of a system under specific conditions - then that is the top event. If it is to qua iify a node of an event tree the top event title is that of that particular node subject to the condi ons imposed by the preceding modes. 

The relevant data for the quantification of the event tree analysis are the conditional probabilities for the branch points. There is considerable uncertainty in the phenomena that would occur and consequently the probabilities used are often based on expert judgement. 

Fault Tree Analysis FTA and Event Tree Analysis (ETA) (Figure 2) are alternative methods of depiction of causes of top events and lends themselves better at both expansion and quantification. 

The development of the HRA event tree is one of the most critical parts of the quantification of human error probabilities. If the task analysis lists the possible human error events in the order of ihcir potential occurrence, the transfer of this information to the HRA event tree is fadlitutcd. Each potential eiTor and success is represented as a binary branch on the HRA event tiec. with subsequent errors and successes following directly from the immediately preceding ones. Cure should be taken not to omit the errors that are not included in the task analysis table but might affect the probabilities listed in the table. For example, administrative control errors that affect a task being performed may not appear in the task analysis table but must be included in the HRA event tree. 

If the results of the qualitative analysis are to be used as a starting-point for quantification, they need to be represented in an appropriate form. The form of representation can be a fault tree, as shown in Figure 5.2, or an event tree (see Bellamy et al., 1986). The event tree has traditionally been used to model simple tasks at the level of individual task steps, for example in the THERP (Technique for Human Error Rate Prediction) method for human reliability... 

INTEGRATION WITH HARDWARE ANALYSIS. The error probabilities obtained from the quantification procedure are incorporated in the overall system fault trees and event trees. 

Fault tree analysis (FTA) is a deductive method, which usually serves for quantification. Just like any method of systems analysis it requires in the first place a qualitative investigation of the system under analysis. After system failure or more generally the undesired or unwanted event (e.g. toxic release) has been defined, logic relationships with the so-called primary or basic events are identified and represented by a fault tree (vid. Fig. 9.8). The primary event may represent the failure of a technical component, an operator error or an impact from outside the plant like flooding or the spreading of a fire from neighbouring installations. 

The experience is that for I C systems approval the probabilistic goals are set and needs to be fulfilled. Reasonable consideration of software reliabihty is desired. This could lead to sometimes senseless way of involvement of software faults into Fault Trees and their quantification. The sensitivity analysis of system tolerance to software faults and their common cause aspects is much more meaningful and could reveal the weak points of the I C design. Even if this analysis is mostly quahtative unless we have applicable methodology to estimate particular basic events prob-abftistic parameters, the Fault Tree Analysis Method represents a good base to demonstrate a sound fault tolerant design. 

The assessment should confirm that the framework for making these expert judgements is sound and the basis for the judgement is stated and shown to be valid as far as possible. This should take account of the thermal-hydraulic analysis that has been carried out, analyses for other similar plants and applicable research data. The quantification of the containment event trees should take account of the interdependences between the various phenomena that are being modelled. 

Human error quantification is one of the most bitterly disputed areas of risk analysis. Risk analysts starting from the reasonably successful experience of quantifying hardware reliability try to treat human reliability in the same way, so that it can be integrated into their fault tree and event tree analyses. Psychologists doubt the possibility of doing this because ... 

Upon identifying the list of potential hazards and its contributing factors, which could be achieved by several methods including HAZard and OPerability studies (HAZOP) (Villemeur (1992)), Failure Mode and Effects Analysis (FMEA) (Military Standard (1980)), Fault Tree Analysis (FTA) (Henley and Kumamoto (1992)), etc, the next step is to quantify these events for the risk estimation phase. Quantification of risk considers two parameters, namely,... 

After the serious hazards have been identified with a HAZOP study or some other type of qualitative approach, a quantitative examination should be performed. Hazard quantification or hazard analysis (HAZAN) involves the estimation of the expected frequencies or probabilities of events with adverse or potentially adverse consequences. It logically ties together historical occurrences, experience, and imagination. To analyze the sequence of events that lead to an accident or failure, event and fault trees are used to represent the possible failure sequences. 

The investigation has shown how the safety of the system can be improved. At the same time its availability is increased, although this was not the express objective of the analysis. Some of the results were already obtained in the qualitative part of the analysis. The quantification of the fault trees brought further insights and enabled one to identify areas of unbalanced safety measures. The latter are characterized by largely differing contributions of an individual initiating event to the expected frequency of an explosion (vid. Table 9.48). The proposals for... 

When presenting an overall risk picture (step 4) the analyst s should present a nuanced and balanced risk picture to the decision-makers based on the cause analysis and consequence analysis. The analysis should establish a risk picture covering ah the dimensions (A, C, C, U, P, S, K). The probabhity quantification of for example a top event in a fault tree is presented along with uncertainty assessments, and a sensitiv-... 

Over the next several years, the PRA was detailed to the point it included detailed fault trees of the mechanical, electrical, and instrumentation and control systems and the scope was expanded to include shutdown, fire, flood events, and large release frequency and off-site dose quantifications. Core damage frequency PRA was supported by extensive plant thermal-hydraulic analysis to justify success criteria. Extensive testing and thermal-hydrauUc analysis, to support containment integrity during core melt sequences, underpirmed the large release PRA. 

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