Event frequency

Fault Tree Analysis Faiilt tree analysis permits the hazardous incident (called the top event) frequency to be estimated from a logic model of the failure mechanisms of a system. The top event is traced downward to more basic failures using logic gates to determine its causes and hkelihood. The model is based on the combinations of fail-... 

Zaoh specific initiating event frequency history. 

Flood Event Frequency Estimates were developed from flooding events in nuclear power plants with adjustments for plant-specific features and data. The data were from the IPE Surry flood analysis, industry sources, and licensing event reports (LERs). Some plant specific models were developed for the circulating water (CW) and service water (SW) lines... 

Event frequencies estimated from historical data failure rates from System Reliability Service... 

NUMBER AND TYPE OF RECORDS Event frequencies estimated from historical... 

The Millstone Unit 1 PRA contains component failure and maintenance unavailability data, and initiating event frequency data, including typical BWR anticipated operational occurrences and LOSP. 

Event frequency data were developed from a detailed review of plant trip reports and shift supervisor s logbook entries. The first two years of plant experience were discarded as they appear to represent experience typical of early plant operation and tests that are not typical of operation in later years. The plant experience was used to perform a Bayesian update of EPRI NP-2230 reactor trip experience. 

CFR 514.80 (b) 4 New non-clinical and clinical data Adverse events not previously reported Summary of increased adverse event frequency Minor supplementary changes... 

In using any risk measure, it should be remembered that risk measures, at best, are only estimates of possible event frequency and consequences All risk measurements have uncertainties. In some situations, the uncertainties can be highly significant. The fact that risk measurement is imprecise should be a consideration in any risk-based decision-making process. Chapter 5 of Reference 4 provides further discussion of uncertainty in risk decision making. 

Document the hazard evaluation process for later use in determining event frequency and consequences. 

All these events, along with the others included in Reference 4, have occurred in the chemical industry. Factors to consider when determining event frequency include the following ... 

In evaluating event frequency, past incidents can provide invaluable guidance. For example, if a pump seal failure is identified as having the potential to lead to a vapor cloud release, and if previous pump seal failures have occurred frequently, it might be reasonable to conclude that such a scenario is likely. This conclusion may be valid even though previous pump seal failures did not result in a vapor cloud. 

Factors such as those above may act independently of one another. An evaluation of event frequency should consider all such factors applicable to a specific site. 

Risk screening relies upon generic frequency data to provide conservative approximations of individual and aggregate risks. Table 4.9 offers an example of generic frequencies for major explosions for certain types of refinery process units. A methodology is also offered to predict expected event frequencies where generic data are not available. 

Qualitative frequency evaluation involves defining broad categories of event frequency, which can be used to assess the likelihood of occurrence of a specific incident outcome (consequence). These categories cover a full spectmm of frequencies, from those representing events that are likely to those that are highly unlikely. Definitions of likelihood categories vary, but Table 5.4 presents a typical list and definitions. 

Most of the screening tools and assessment methods discussed in this book rely upon estimates of event consequences, event frequencies, or both. These estimates, in turn, are based upon interpretations of site-specific conditions as well as a determination of which events (of many possible events) are to be considered evaluation-case events. Each step in the evaluation process has the potential to introduce uncertainty into the overall building assessment. As discussed in Reference 87 ... 

Crawley and Grant (1997) have developed a risk assessment tool for new offshore installations. They have examined typical leak frequencies of equipment items and the ignition probability of these leaks in four pressure bands. With this information it was possible to define leak size and frequency for any piece of equipment and the ignited leak frequency. In off-shore installations gas separation vessels were found to have ten times higher ignited event frequency than oil pumps. 

Event trees are used to perform postrelease frequency analysis. Event trees are pictorial representations of logic models or truth tables. Their foundation is based on logic theory. The frequency of n outcomes is defined as the product of the initiating event frequency and all succeeding conditional event probabilities leading to that outcome. The process is similar to fault tree analysis, but in reverse. 

Fault Tree Analysis (ETA)—Estimation of the hazardous incident (top event) frequency from a logic model of the failure mechanisms of a system. 

Risk Analysis—The development of a quantitative estimate of risk based on engineering evaluation and mathematical techniques for combining estimates of initiating event frequency and independent protection layers and consequences. 

Consider the of the clinical event frequency related to drug versus the background frequency of... 

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