Overview of Risk and Safety Analyses

In modem technology, which is characterized amongst others by the use of nuclear power, the operation of complex process plants and air and space travel, failure can hardly be tolerated because its consequences may be large. They can be economic, such as unplanned shut-downs or destraction of plants, or endanger humans and environment. Accidents do happen despite the numerous safety measures described in the preceding chapters. The reason is that technical measures and human interventions for avoiding accidents can never be perfect. They fail with a certain probability. This is the rationale for the barrier concept presented in Sect. 4.2. 

The second step characteristics comprises the initial and boundary conditions for assessing the consequences of the event sequence for employees and the population at large (Example leak cross section 10 cm, elevation of release point 10 m, pressure difference 500 kPa). It makes sense to assign the different sequences to categories (e.g. small leaks, medium leaks, large leaks, fires, explosions etc.), each of them representing several event sequences by one set of initial and boundary conditions. This set must lead to the most severe consequences of all the event sequences covered by the category and hence be conservative. The boundary conditions usually are stochastic, i.e. at most the probability of occurrence, for example for the above mentioned leak, may be indicated. It is normally not equal to 1, as supposed in the deterministic approach. Other leak sizes and locations are, of course, possible. 

The two steps described above represent the analysis of engineered systems of the plant. In the first place the analysis is done qualitatively, for which methods like HAZOP (Sect. 9.1.2.3) and FMEA (Sect. 9.1.2.4) can be used. The result of the investigation is then represented by means of event and/or fault trees. Chains of events leading to the different end points of the accident sequences shown in the event tree (e.g. leaks of different sizes) are systematically arranged. Thus probabilities can be assigned to them on the basis of reliability data for the failure of 

If the investigation ends here we speak of a probabilistic safety analysis (PSA). Its main objective is to identify possibly existing weak points in the plant design and to show how they are efficiently removed. 

We arrive at a probabilistic risk assessment (PRA) if the accident consequences are assessed as well and frequency and consequence are combined. This is done in the following steps. 

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