Accident source models

The next part of the procedure involves risk assessment. This includes a deterrnination of the accident probabiUty and the consequence of the accident and is done for each of the scenarios identified in the previous step. The probabiUty is deterrnined using a number of statistical models generally used to represent failures. The consequence is deterrnined using mostiy fundamentally based models, called source models, to describe how material is ejected from process equipment. These source models are coupled with a suitable dispersion model and/or an explosion model to estimate the area affected and predict the damage. The consequence is thus determined. 

Once the scenario has been identified, a source model is used to determine the quantitative effect of an accident. This includes either the release rate of material, if it is a continuous release, or the total amount of material released, if it is an instantaneous release. Eor instance, if the scenario is the mpture of a 10-cm pipe, the source model would describe the rate of flow of material from the broken pipe. 

Chlorine is used in a particular chemical process. A source model study indicates that for a particular accident scenario 1.0 kg of chlorine will be released instantaneously. The release will occur at ground level. A residential area is 500 m away from the chlorine source. Determine... 

Symptom based procedures can be directly used as the best information source for building of accident scenarios models and accident sequences logic covering the phase of control room crew response to initiating event. According to current requirements, the symptom based procedures set developed for specific plant should cover response to all initiating events defined in plant PSA. 

Lind S (2009) Accident sources in industrial maintenance. Proposals for identification, modelling and the management of accident risks. VTT Publications 710. Edita Prima Oy, Helsinki... 

Figure 13.6 The Finnish accident-analysis model. Source Tuominen and Saari, 1982.

A typical example is the so-called Finnish model for accident analysis. Figure 13.6. It displays accidents in a standard format in terms of two separate event sequences. One of these considers the build-up of hazardous energy and the second the movements of the victim in relation to this energy source. The accident occurs when the person and the released hazard come in contact. We recognise the energy model presented in Section 5.4 in this accident-analysis model. 

A logic model that graphically portrays the range of outcomes from the combinations of events and circumstances in an accident sequence. For example, a flammable vapor release may result in a fire, an explosion, or in no consequence depending on meteorological conditions, the degree of confinement, the presence of ignition sources, etc. These trees are often shown with the probability of each outcome at each branch of the pathway... 

Fauske, H. K. and M. Epstein, 1988, Source Term Considerations in Connection with Chemical Accidents and Vapor Cloud Modeling, Proceedings of the International Conference on Vapor Cloud Modeling, CCSP, 251-273. 

There are also several possibilities for the temporal distribution of releases. Although some releases, such as those stemming from accidents, are best described as instantaneous release of a total amount of material (kg per event), most releases are described as rates kg/sec (point source), kg/sec-m (line source), kg/sec-m (area source). (Note here that a little dimensional analysis will often indicate whether a factor or constant in a fate model has been inadvertently omitted.) The patterns of rates over time can be quite diverse (see Figure 3). Many releases are more or less continuous and more or less uniform, such as stack emissions from a base-load power plant. Others are intermittent but fairly regular, or at least predictable, as when a coke oven is opened or a chemical vat... 

The authorities must also allow suitable and full-scope information to the impacted population about the possible risk sources, their impact in case of accident, methods of warning and recommended models of behaviour under the occurrence of such extraordinary situation. 

Simple switches that can be manually activated can be considered a fire alarm device. Models are used which normally require the use of positive force, i.e., to avoid accident and fraudulent trips. Fire alarm switches normally can only be reset by special tools in order to trace the source of the alarm, however sophisticated data reporting systems with addressable data collection may make this requirement obsolete. 

In most tests, molten aluminum was poured into a vessel containing water. In a few instances, however, water was injected onto or below a pool of aluminum. The first type of experiment would model one of the more familiar types of industrial accidents, i.e., one in which an ingot break or a furnace leak allowed molten aluminum to contact a source of... 

Weapons-grade plutonium, dispersed at military accidents such as Thule in 1968 or as non-fissioned weapon particles after detonation of a Pu-bomb can be characterized by high Pu content relative to the other Pu-isotopes, while accidentally dispersed Pu from the previously widely used nuclear-powered satellites are characterized by high Pu content." The ratio of americium-241 to plutonium isotopes (as " Am is formed by the decay of Pu) is proportional to the initial " Pu concentration, thus it can also be used as an indicator to assess the origin of contamination. However, in most cases, as several sources may contribute to the transuranics content in environmental samples, mixing models applying several isotope ratios are required to assess the origin of possible contamination sources. 

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