Accident probability categories

Traffic accidents involve collisions between two or more vehicles and collisions with fixed objects. Traffic accidents rate depends onroad category, traffic density and road location (urban areas/ out of urban areas). Traffic accident probability is usually measured per one kilometre (2nd task category). 

This leads to an accident probability of 0.0000001 (10 ) per hour for technical cause factors. Therefore, for transport category aircraft, most civil airworthiness authorities require that aircraft systems and associated components (considered separately and in relation to other systems) be designed in a manner such that the occurrence of any failure condition which would prevent the continued safe flight and landing of the aircraft should virtually never occur in the life of an aircraft type. 

The UK MOD base their acceptance of hazards on a risk classification scheme, which is based on the combination of the severity, probability and time of exposure for each particular hazard. For the purposes of the accident risk classification scheme, accidents are considered single events (Table B.9). These classifications can be combined to determine a hazard risk index (HRI), which is a numerical risk factor that can be used to prioritise the need for corrective action or resolution. The HRI matrix in Table B. 10 is an example showing how the hazard severity and the hazard probability categories combine to yield the HRI. 

The risk for other on-site workers (outside the TOCDF and DCD storage area) is evaluated in the same manner as the risk to the public. The probability of one or more fatalities for other on-site workers during the 7.1 years of disposal processing is 5 x 10 (1 in 2,000). With about 100 workers in this category, and assuming that most accidents cause a single fatality, the individual annual risk is 1 x 10 (1 in 1 million per year) for other on-site workers. 

Category C personnel with minimal probability of exposure to nerve agents, even under accident conditions, but whose activities may place them in close proximity to agent areas. 

IV Very Unlikely 10 >F>10 Accidents that will probably not occur during the life c cle of the facility. This category includes the design basis accidents. 

A frequency measure is used for the initiating event, i.e., the accident scenario. Conditional probabilities are used for the remaining events. These probabilities are assessed from the previous events in e chain of events . In order to assess the conditional probabilities, the analyst uses the combination of extent and duration of the previous event. Table 3 presents the categories (Cat) used for F, P, E, D. 

The consequences are cormected with determined losses. They may pertain to people, artefacts and the natural environment. They are expressed in units of a physical and/or financial character. Detailed data on losses are very difficult to obtain, particularly those related to rare events, e.g. consequences of the Cl and C2 category accidents. The data cannot be obtained from experts, as in great majority they have not experienced events where such losses occur. We have to relate the risk only to the ICF type propulsion function loss event consequences. We define the PR as a probability of occurrence of the ICF event consequences during one year in a population of specific type ships operating on a given shipping line. 

A study of the event trees allows one to determine the various containment failure modes and the characteristics of the radioactivity release to the environment following the possible accident sequences. By using the fault tree analysis we can then link each release to its associated probability of occurrence. It is found that many of the possible sequences in the event trees lead to similar types of radioactivity release, with the result that it is possible to summarize the consequences of the PWR accident in terms of as few as nine release categories, each with its corresponding probability. The corresponding figure for the BWR is six categories. 

Using these probability and severity categories, a risk analysis matrix can be developed for any type of event and used to identify imacceptable risks for the operation. It can also be used to prioritize which risks wiU be addressed—where action is taken to eliminate or reduce them—and in which order. An example risk-analysis matrix is given in Figure 18.1, with high priority cells identified. The highest priority cells are located in the upper left part of the matrix, while the lowest priority cells are in the lower right corner. The approach could be used to compare the impact of many different events, such as roof faU accidents, rock burst events, and mine fires. It can also be used to combine both quantitative and quahtative risks. 

No. Hazard present YES/NO Describe the hazards and obvious control or protective measures necessary Likely consequences of an accident (a) Number of workers exposed to hazard (b) Probability of harm (c) Risk rating and n sk category Extra control measures necessary... 

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