Frequency Assessment Methodology

3.3Frequency Assessment Methodology

3.3.1External fire and explosion

• 
  estimation of the frequency of external fires and explosions that can have impact on NPP;
  
• 
  analysis of the consequences of these fires and explosions.
  

where:

The frequency of the events (N_i) can be estimated basing on the archive data and current activities. For the transportation, it should be mentioned that there are various regulations applying for the safe and secured transportation of hazardous materials. In Europe, the following regulations are obligatory:

• 
  ADR (Accord européen sur le transport des marchandises Dangereuses par Route) for Road Transport) — The European Agreement concerning the International Carriage of Dangerous Goods by Road;
  
• 
  RID (Règlement International concernant le transport des marchandises Dangereuses par chemin de fer) for Rail Transport — International Regulations concerning the Transport of Dangerous Goods by Rail;
  
• 
  The European Agreement Concerning the International Carriage of Dangerous Goods by Inland Waterways (ADN).
  

Data concerning both actual events and near-misses (i.e. events that could have happened with high probability) should be taken into account, as this can improve understanding of the development of accident scenarios and provide a wider spectrum of possible events which should be analysed. It should be added, however, that availability of such information (in particular on near-misses events) can be weak. For accident initiating events, among the variety of causes or factors having essential impact on the development of the accident scenarios, one can mention the following ones: equipment failures, road or rail defects, human factors, control system failures, navigational errors, improper balancing or ballasting and external events. In case of non-accident type of initiating events, the most typical elements that should be considered are: incorrect securement, corrosion or metallurgical failure, over- or under-filling, overpressure, component failures (valves, rupture disk, etc.), activation of relief device, control system failures and contamination.

It should be added that the simplest and mostly used approach is to use accident frequencies and random data failure of equipment for non-accident situations.
According to the combustion mode, an explosion can take the form of a deflagration, which generates moderate pressures, heat or fire, or a detonation, which generates very high near field pressures and associated drag loading: usually thermal effects are present only in the case of special fuel–air mixtures. Whether or not the ignition of a particular chemical vapour or gas behaves as a deflagration or detonation in air depends primarily on the concentration of the chemical vapour or gas present. At concentrations two to three times the deflagration limit, detonation can occur. A possibility of direct and delayed ignition should be considered. In case of delayed ignition a gas cloud can for instance enter the ventilation system and explode inside the NPP buildings.

For identification of initiating events induced by explosions, the following methods could be used:

• 
  analysis of operating experience involving functional degradation or unavailability of systems due to external events (such data are usually limited);
  
• 
  analysis of possible consequences and failures caused by occurrence of the external event.
  

The estimation of the frequency of external events is more or less related to the observation of phenomena occurrences, but this could be difficult, since the explosions are quite rare phenomena. The frequencies estimates can be based on analysis of local and worldwide industrial statistics, but in case of extremely scarce data for these events, the frequency and distributions are usually estimated by expert opinion, taking into account insights gained from analysis and operating experiences. The consequence distribution of external explosion pressure waves can be successfully assessed by means of the Monte Carlo simulation.

3.3.2Aircraft crash

For each aviation category, the following three aircraft crash quantities based on flight phase should be determined and then summed up to quantify the overall aircraft crash frequency:

• 
  background crash rate (caused by free air traffic);
  
• 
  airport related crash rate (caused by take-off and landing);
  
• 
  airway related crash rate (caused by route and waiting loop traffic);
  

Eventually one also considers intentional crash rates (for example caused by hijacker or pilot).
The standard on accident analysis for aircraft crash into hazardous facilities [5] developed by the U. S. Department of Energy proposes the quantification of all the three types of flight phase based crash rates defined above by using one general formula. After minor modifications, the formula can be applied to determine the aircraft crash frequency that is appropriate for use in PSA:

where:

F_i,j annual aircraft crash frequency specific to a unit ground area for each aircraft category (j) and flight phase based crash rate type (i) [event/year/unit area],

N_i,j,k annual number of site-specific aircraft operations (i.e., take-offs, landings and in-flights) for each applicable parameter (i.e. aircraft category (j), flight phase based crash rate type (i), flight sources as runways, non-airport operations, etc. (k)) [event/year],

P_i,j,k aircraft crash probability per operation in the vicinity of the site for each applicable parameter (i.e. i, j, k, as discussed above) [-],

f_i,j,k(x,y) aircraft crash location conditional probability (per unit ground area) given a crash evaluated at the facility location for each applicable parameter (i.e. i,j,k, as discussed above) [1/unit area].
Because of the limited number of historical in-flight crashes, particularly for commercial and large military aircraft, frequency calculations for non-airport operations are based on modelling directly the number of crashes per unit ground area per year, i.e., the product N_i,k*P_i,k*f_i,k(x,y).The document [5] defines a detailed quantification method to assess f_i,j,k(x,y) in the vicinity of airports as well as a table for the product of N_i,k*P_i,k*f_i,k(x,y) for non-airport operations relevant to different areas in the United States. However, this enables a simple application of the approach in the United States; it does not include a methodology to quantify uncertainty. Although the above methodology is widely used in the U.S. and in some other countries (e.g. Switzerland), an alternative method is presented in Appendix 2, that provides empirical formulas to assess the different types of flight phase based crash rates. It also addresses, to some extent, uncertainties using a traditional approach based on frequencies.
In order to enable plant risk quantification characterization of impact for each aircraft category to the extent of the parameters described in Sec.3.2.2 should be included for aircraft categorization, calculation of aircraft crash frequency thereof, as well as trend analysis and hazard assessment.

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