Event tree analysis evaluation

Event Tree An event tree analysis begins widi a specific initiating event and works forward to evaluate potential accident outcomes. 

Event tree analysis is a teclmique for evaluating potential accident outcomes resulting from a specific initiating event. Tlie results of the event tree analysis are clironological sets of failures or errors that may define an accident. 

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. 

Tliis cliapter is concerned willi special probability distributions and tecliniques used in calculations of reliability and risk. Tlieorems and basic concepts of probability presented in Cliapter 19 are applied to llie determination of llie reliability of complex systems in terms of tlie reliabilities of their components. Tlie relationship between reliability and failure rate is explored in detail. Special probability distributions for failure time are discussed. Tlie chapter concludes with a consideration of fault tree analysis and event tree analysis, two special teclmiques lliat figure prominently in hazard analysis and llie evaluation of risk. 

Three hazard evaluation procedures using logic diagrams are (1) fault-tree analysis (FTA), (2) event-tree analysis (ETA), and (3) cause-consequence analysis (CCA). Appropriate references are [2,3,251,261]. 

Layer-of-protection analysis (LOPA) A method, based on event tree analysis, of evaluating the effectiveness of independent protection layers in reducing the likelihood or severity of an undesired event. 

The starting point in event tree analysis is the initiating event. The quantitative evaluation of the event tree requires condition probabilities. These may be based on reliability data, historical records, experience, or from fault trees. 

Other examples of inductive tools that have limited application in incident investigation include failure mode and effects analysis (FMEA), hazard and operability study (HAZOP), and event tree analysis (ETA). These are detailed in the CCPS book, Guidelines for Hazard Evaluation Procedures... 

HAZAN, on the other hand, is a process to assess the probability of occurrence of such accidents and to evaluate quantitatively the consequences of such happenings, together with value judgments, in order to decide the level of acceptable risk. HAZAN is also sometimes referred to as Probabilistic Risk Assessment (PRA) and its study uses the well-established techniques of Fault Tree Analysis and/or Event Tree Analysis ... 

A systems hazards analysis (SHA) is a systematic and comprehensive search for and evaluation of all significant failure modes of facility systems components that can be identified by an experienced team. The hazards assessment often includes failure modes and effects analysis, fault tree analysis, event tree analysis, and hazards and operability studies. Generally, the SHA does not include external factors (e.g., natural disasters) or an integrated assessment of systems interactions. However, the tools of SHA are valuable for examining the causes and the effects of chemical events. They provide the basis for the integrated analysis known as quantitative risk assessment. For an example SHA see the TOCDF Functional Analysis Workbook (U.S. Army, 1993-1995). 

A method that applies in particular to fixed installations is one that is based on experimental feedback. Starting from an analysis of incidents by the method of event trees, it evaluates the probability that such events may degenerate into a serious accident, and by adding these probabilities over a period of time and referring the results to the number of installations of the same type, it can sometimes be shown that a serious accident becomes probable after a certain number of period trees (Savkovic-Stevanovic 2007, 2009, 2010 Savkovic-Stevanovic and Krstic 2006 Savkovic-Stevanovic et al. 2006). The importance of this method is that it is able to show that the safety of a system has become compromised even though no accident has occurred, and those responsible may believe that the system is operating satisfactory. 

Ereignisablaufanalyse Verfahren, graphische Symbole und Auswertung (Event tree analysis method, graphical symbols and evaluation) DIN 25419 1985-11... 

In all of the SCB fire scenarios, the fire was assumed to envelop the liquid dissolution cocktail in the process containers during or following UO2 dissolution and release of radioactive material to the SCB and Zone 1 ventilation systems. While dilution of combustion products is expected to preclude damage to the ventilation system, an unmitigated release bounds the scenario where the filters or the ventilation system itself have been degraded or compromised as a consequence of a fire. The frequencies per year for such an accident developed in the event tree analysis shown in Appendix 3E.3 agree with the frequency for an extraction SCB fire as assessed in event CP-7 in the hazard evaluation (Appendix 3C). 

The potential initiating events, preventive features, and mitigating features were evaluated using event tree analysis methodology, which is detailed in Appendix 3E. The analysis showed that the accident risk could be conservatively bounded by a worst case crash of the forklift with target into fixed, unyielding features at the entrance to the HCF. The expected orientation in a collision impact would be a side impact of the cask. Structural analysis of the isotope transfer cask indicates that the cask will not be broken or breached in a conservatively worst case collision. 

The frequency of the energetic forklift collision after a roil down the truck ramp, as developed by the event tree analysis, is consistent with the hazard evaluation in most cases vwth the event tree analysis assessing frequency the same or one level less often. The collimated radiation beam, high dose rate, and airborne release outcomes are produced by the cask lid opening or coming off in a forklift crash. 

This internally initiated DBA is a potential fire in a radioactive material storage area that is associated with the HCF. The potential for a radioactive material storage area fire was examined by using event tree analysis methodology, which is detailed in Appendix 3E. The frequencies per year for such an accident developed In the event tree analysis agreed with the frequency for a radioactive material storage area fire as assessed in the hazard evaluation. 

Event tree analysis was used to analyze the accident sequence and to evaluate the accident frequency as suggested in (DOE 1997) and (Mahn et al. 1995). Figure 3E.3-1 presents the event tree using the two accident progression sequences developed previously. 

The frequency of each operational outcome or sequence for the event tree of Figure 3E-4 has been calculated as shown in Table 3E.4-1 below. The hazard analysis event corresponding to hydrogen combustion in the elevator pit is hazard event WE-4 of Appendix 3C. Event WE-4 frequency of occurrence was assessed as frequency bin IV. Sequence D also has a calculated frequency bin of IV so the event tree analysis is consistent with the hazard evaluation in assessed frequency of occurrence. 

The frequency of each operational outcome or sequence for the event tree of Rgure 3E-7 has been calculated as shown in Table 3E.5-1. The frequencies calculated in the event tree analysis compare well with the assessed frequency bins of hazard event CP-12 for a ventilation system confinement failure. The frequency bin for event CP-12 was III as corripated to frequency bins of II to V. CP-12 did not assess differing prior conditions or coincident volatile material releases as evaluated in the event tree analysis. 

There exist different methods like Fault Tree Analysis (FTA), Event Tree Analysis (ETA) and Monte Carlo Simulation (MCS) that can be applied and combined for the purpose of evaluating the frequency and probability of initiating events. However, the MCS can be handled much easier in order to accoimt for bormdary conditions like stochastic dependence, time dependence and physical impact (Hauschild Meyna 2007). The MCS has been apphed successfully for PSA in order to assess the safety of nuclear power plants (Devooght Smidts 1996, Woltereck2001) and especially for taking into account uncertain input data (BfS 2005). 

The predominant method of evaluation in these studies has been fault tree analysis. The Reactor Safety Study also utilized event tree analysis to conveniently document accident sequences and to link the subsystem fault trees into a plant analysis. Failure Mode and Effects Analysis, used extensively in fast reactor safety, is the recommended method for preliminary analysis. All of these methods have application to the analysis of the fuel cycle Including the problems of safe arding special nuclear material. ... 

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