HAZARD ASSESSMENT METHODOLOGIES

3HAZARD ASSESSMENT METHODOLOGIES

3.1Introduction

3.1.1External fire and explosion

1. 
   For man-made hazards like fires, explosions and releases of toxic substances a kind of risk assessment should be performed (like QRA). The input parameters are related to the frequency of the event, while the output contains the result of the consequence analysis, typically in the form of the frequency (or probability) of the occurrence of different consequences for each event. In the first phase of this analysis, screening is performed in order to eliminate events or sources, which are not relevant for the considered plant, for example, due to the distance or the very low probability. In fact screening criteria are usually based on these two factors. In the second phase, detailed evaluation should be made for the events that are not screened out. ASAMPSA_E Deliverable D30.3 “Methodology for Selecting Initiating Events and Hazards for Consideration in an Extended PSA” [3] describes the basic methodology to be applied for the screening process.
   
2. 
   The consequences of man-made hazards should be transformed into initiating events of the NPP (or in some cases common conditioning events incorporated in the model structure on higher level), for example, by fragility curves, which represent the probability of exceeding a given damage state as a function of an engineering demand parameter. This step corresponds to the analysis of vulnerability of the plant to the external events.
   
3. 
   These initiating events are further evaluated in PSA model for a given NPP.
   

Methodologies for the assessment of man-made hazards have to take into account a number of various factors and usually utilize different approaches and methods in order to make full image of the real processes. In general, man-made hazards are related to the physical properties of the material and these properties determine the inherent risk and the potential consequences in case of accident with the release of dangerous substances. Various processes have to be taken into consideration and various characteristics of the material should be examined, like: flammability, combustibility, toxicity, corrosiveness, reactivity, explosiveness, radioactivity, etc. In case of transportation different initiating events have to be considered, like the situations where there are immediate fires, explosions (or the releases of toxic substance) or conditions for such events have happened with probability that cannot be negligible. This depends on the type of transportation, in general one can identify the ones shown in Table 3 , below.

3.1.2Aircraft crash

A number of guidance documents, standards as well as technical publications exist on hazard assessment methodology for aircraft crash. In general, the main analysis steps to quantify the crash rates can be considered identical in the different commonly applied methods, although there are some differences in the formulas and factors to be taken into account for quantification. The authors consider [4] and [5] as the most relevant ones with respect to site level hazard characterization. The aim of this section is to provide generic information, while the specificities of the state-of-the-art concerning site level hazard characterization of aircraft crash are included in the Appendix 2. The approach discussed there takes the widely used methodology of [4] and [5] as a basis and supplements it with other related documents as well as with additional considerations to ensure the applicability of the method in a PSA context.
The primary objective of site level hazard assessment for aircraft crash is to determine aircraft crash frequencies for different aircraft categories. This should be based on a statistical evaluation of accidents and air traffic information and on data applicable to the vicinity of a specific site. The results should be presented in accident frequencies specific to a unit ground area (event/year/unit area). If the evaluation concludes that aircraft crash hazard cannot be screened out on a probabilistic basis, the aircraft crash potential effects on the nuclear facility located at the site have to be characterised.

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