Fault trees quantified

Despite its shortcomings, WASH-1400 provides at this time (1978) the most complete single picture of accident probabilities associated with nuclear reactors. The fault tree/event tree approach coupled with an adequate database is the best method available to quantify these probahililies. 

The accident sequence frequencies are quantified by linking the system fault tree models together as indicated by the event trees for the accident sequence and quantified with plant-specific data to estimate initiator frequencies and component/human failure rates. The SETS code solves the fault trees for their minimal cutsets the TEMAC code quantitatively evaluates ihe cm sols and provides best estimates of component/event probabilities and frequencies. 

The QRA was conducted by risk sts and design innel to determine the probability of explosive releases of the chemical. Fault tree analysis identified several combinations of equipment failures and operator errors that could cause the top event (reactor explosion), Failure data were obtained from plant ex ice and industry da%.ui,/uoes to quantify the fault trees to estimate the frequency of reactor explosions. The fault trees suggested several safety improv-... 

The use of event trees is sometimes limiting for liazard analysis because it may lack die capability of quantifying die potendal of die event occurring. Tlie analysis may also be incomplete if all inidal occurrences are not identified. Its use is beneficial in examining, rather dian evaluating, die possibilities and consequences of a failure. For this reason, a fault tree analysis (FTA) should supplement diis, to establish die probabilities of die event tree branches. Tliis topic was introduced in a subsection of Cliapter 16. 

In a more quantitative sense, cause-consequence analysis may be viewed as a blend of fault tree end event tree analysis (discussed in tlie two preceding cliapters) for evaluating potential accidents. A major strengtli of cause-consequence analysis is its use as a communication tool. For example, a cause-consequence diagram displays the interrelationships between tlie accident outcomes (consequences) and Uieir basic causes. The method can be used to quantify the expected frequency of occurrence of the consequences if the appropriate chita are available. 

Fault trees originated in the aerospace industry and have been used extensively by the nuclear power industry to qualify and quantify the hazards and risks associated with nuclear power plants. This approach is becoming more popular in the chemical process industries, mostly as a result of the successful experiences demonstrated by the nuclear industry. 

In this way, the fault tree can be quantified, which makes this technique very powerful for the reliability analysis of protection systems. The prerequisite is the availability of statistical reliability data of the different devices and instruments that is often difficult to obtain for multi-purpose plants, where devices can be exposed to very different conditions when changing from one process to another. Nevertheless, if the objective is to compare different designs, semi-quantitative data are sufficient. 

After the hazards have been identified, the second task is to determine the risks associated with them. Risk is defined as the likelihood of the hazardous event and the severity of the accident. Fault trees are often used to quantify the likelihood of hazardous events. The severity is usually defined as the degree, sometimes in terms of likelihood, of exposure to accidents. 

Once a fault tree has been developed, failure rate data for individual components in the system can be entered into the tree so that an estimate of the likelihood of the undesired event (the Top Event ) can be made. Frequently the quality of the failure rate data is poor nevertheless, through use of the Pareto Principle or 80/20 rule discussed above, a quantified analysis still provides useful insights because it identifies which items in the system contribute the most to system failure. Moreover, once the model has been developed, and preliminary estimates as to failure rates have... 

Development of a fault tree as shown in the previous sections can be very beneficial because the system logic shows just how events may occur. All the same, a fault tree becomes much more useful when quantified because management will be presented with a clear understanding as to which hazards contribute the most toward risk. This insight helps determine where investment resources are best spent in order to achieve the greatest reduction to that risk. 

The quantification of block diagrams follows the same general principles as used for quantifying fault trees and event trees (see Chapter 15). To calculate the probability of success for each path in... 

If a quantified method such as LOPA or Fault Tree Analysis is to be used then it will be carried out during Phase III in order to risk rank identified hazards. 

The primary events of the fault tree may be further decomposed. For example, the failure of the pump motor Ml might be caused by a failure of its stator or rotor windings, cables or such like. This would make sense if the motor itself were the object of the fault tree analysis. In practice the degree of decomposition (degree of detail) is determined by the boundaries (deUmitation) of the reliability data for describing component behaviour, which are needed for quantifying a fault tree. 

Table 9.19 Data for quantifying the fault trees of Figs. 9.28, 9.29, 9.30...

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