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SOLUTION FOR THE MODELLING OF MAN_MADE HAZARDS AND AIRCRAFT CRASH FOR L1 PSA
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- Navigate this page:
- State of the Art Methodology for PSA Model Development
- 7.1.1Consequence analysis of PSA Initiating Events
- 7.1.1.2External Explosions and Fires
- 7.1.2Accident Sequences
- 7.1.3Development of Accident Sequence Models
7SOLUTION FOR THE MODELLING OF MAN_MADE HAZARDS AND AIRCRAFT CRASH FOR L1 PSA
Similarly to most external hazards (as discussed in [18]), the level 1 PSA model for internal initiating events is practically always used as a basis for the accident sequence development in aircraft crash, external fire and external explosion PSA. Consequently, the availability of the Level 1 PSA model for internal events and hazards is a prerequisite for performing a detailed analysis of the man-made hazards. The detailed analysis should be based on realistic models and data, including a comprehensive Level 1 PSA model that provides the possibility of modelling all phenomena associated with man-made hazards. In accordance with good practices, preference is given to developing an integrated model for internal and external events (including aircraft crash) in contrast to building separate stand-alone models for different categories of events. In order to properly address the impact of a man-made hazard, integrated models should also incorporate aspects that are different from internal initiating events. The major impacts of a man-made hazard that could lead to various types of internal initiating events or to core damage directly should be assessed in the selection of the appropriate event sequences from the PSA model for internal initiating events. The probabilities of recoveries and post-initiator human errors should be revised by assessing the impact of a man-made hazard on the credited recoveries and human actions modelled in the Level 1 PSA for internal initiating events. Also, it may be necessary to include and analyse recovery actions over and above those included in the internal events PSA model.
This section presents the specificities of PSA model development for man-made hazards by going through the general PSA model development process and the associated analysis steps: characterization of PSA initiating events, development of accident sequence models, fault tree development, human reliability analysis, analysis of input reliability data. Some of these analysis areas are presented in detail in other parts of this document, respectively this section focuses on issues that are not discussed elsewhere. To avoid unnecessary overlaps, only the most important aspects are summarized here, and reference is made to the relevant section for more details on a given issue. 7.1.1Consequence analysis of PSA Initiating Events7.1.1.1Aircraft crashThe first step of PSA model development for external events is the unambiguous definition of PSA initiating events. The identification and characterization of PSA initiating events is performed during hazard assessment, i.e. the output of hazard assessment is the list of PSA initiating events and the relevant characteristics thereof (amongst others their occurrence frequency). Section 3 presents hazards assessment for man-made hazards in detail, consequently only PSA model development is discussed hereby. For the purposes of defining PSA initiating events, aircrafts are classified into different categories relevant to the vicinity of a specific site, because of the different flying characteristics and in the reliability of different aircraft categories. The direct impact of an aircraft crash depends on the descent angle, mass and velocity of an aircraft that differ significantly among aircraft categories. As an example of such detailed analysis of aircraft induced impact on the structure because of vibratory loading can be found in [37]. Similarly safety assessment of reactor building for large commercial aircraft crash is presented in [38]. For initiating event characterization, the impact mass and velocity distributions are also determined as primary information on the hazard. The state of the art methodology does not consider the distribution of descent angle; rather it applies conservative values to assess the effective target areas. As it has been already stated in Sec. 3.6.1 primary and secondary impact areas and related effects have to be analysed basing on the description of initiating events according to aircraft categories and the affected impact zone for each category as well as the calculation of crash frequency for each of these events. 7.1.1.2External Explosions and FiresRegardless of the origin of the explosion, its effect can be expressed in terms of the following parameters: impact loads, impulsive loads, thermal loads and vibratory loads. The number of missiles which may be generated and may affect different parts of the plant is as important as their size and velocity. Similarly the effects of fires are mostly expressed by thermal loads. The analyses that consider externally induced initiating events in the frame of internal events PSA (e.g., explosions or fires induced losses of off-site power) may not always consider important dependencies (e.g., vibrations induced failures or the smoke caused by fire). Both direct effect of the explosions (e.g., shock-induced collapses or fire destroying elements of electrical system) and the indirect effects (e.g., explosion induced missiles or fire causing blast) are required to be analysed as part of the EEF analysis. It must be decided whether or not the fire or explosion will cause an initiating event in the plant, and which initiating event is the most probable to occur. In most cases the initiating event will be a transient. The explosions are primarily affecting the structural integrity of buildings or structures. An important consideration in EEF PSA is whether the explosion can (depending on design and site-specific details), in addition to disturbing the operation of the plant, also disable or degrade one or more safety functions needed to cope with the initiating event. Similarly fires can affect electrical or power supply systems and the question of degradation of safety functions should be raised. The results of external events PSAs are sensitive to the modelling of dependencies between initiating events and safety system failures as well as between failures of different safety systems. To reflect the degree of protection against the impact by the pressure waves or heat the important areas of the plant could be divided into three classes (A, B and C), the same as for the consideration of aircraft crashes [39]:
Depending on the impact zone, and based on the above classification, the occurrence or not of an initiating event in case of explosions or fires, and the situations where an external influence can cause an initiating event and simultaneously degrade safety systems can be estimated. The identification of dependencies is based on operating experience, plant walk-downs, interviews of designers and operating and maintenance personnel and systematic analysis of plant systems and components and their design basis.
Usually, events such as aircraft crashes and missile strikes have limited impact areas (even when more than one missile is considered), while explosions, fires, ground motions and gas clouds can have plant-wide effects. If the affected area is plant-wide, items important to safety located anywhere in the plant could be affected coincidentally, and necessary safety functions might be affected. Fires resulting from deflagration shall be dealt with on the same basis as fires due to other man-induced impacts [40]. An analysis of the ability of plant structures to resist the effects of a gas cloud explosion can normally be limited to an examination of their capacity to withstand the overpressure (direct and drag) loading. In general, the effects of explosions which are generally of concern when analysing the structural response are [29]:
The relative importance of these effects depends mainly on the quantity and type of the explosive substances, the distance of the structure under consideration from the source of the explosion, and details of the geometry and spatial arrangements of the structures and the explosive. In case of fire the main concern relates to the duration, the velocity and direction of fire spread (which depends on meteorological conditions) and the location of the source. The extent of the fire and the distance to the structures plays important role.
If the design of the plant takes into account natural fires (wildfires) then the effects of man-made external fires may have been already incorporated into PSA. The response of the structures (and auxiliary systems) depends on their capabilities for resisting heat load. Analogously the resistance of electrical systems on internal fires is a part of the design, therefore the effects of the heat may be already taken into account – however one should keep in mind that significance of external fire may be higher due to the possibility of additional effects like explosions. In general the heat or fire load from a detonation is not considered a part of the design basis for a target structure (as is considered for a deflagration), this effect should be dealt with on the same basis as fires due to other human induced events. However, particularly in the case of fuel–air mixtures, fire effects associated with a detonation may be significant, and the same provisions should be applied as for deflagrating media. 7.1.2Accident SequencesIdeally the fragilities are used to calculate the frequencies of different event scenarios – and this depends on the hazard intensity. Therefore in order to determine frequency (or probability) of the core melt and/or radionuclide release, caused by a sequence of events initiated by a human induced external event, integration over whole range of hazard intensities (or response parameter, in general) has to be performed. Depending on the type of the hazard, various impacts on the plant have to be considered which is related to different sets of parameters to be analysed. The most important impacts and associated parameters are as follows (according to [41]):
7.1.3Development of Accident Sequence ModelsThe main objective of developing the accident sequence models is to construct an event tree structure that integrates event sequences developed in the internal events PSA and distinctive man-made hazards induced transients into a generic model that reflects the specifics of man-made hazards initiating events (for details see section 7.1.1). There are several approaches appropriate to fulfil this objective. In section 8, a series of analysis steps applied by a state of the art methodology is presented, however several, slightly different methods are used in recent PSA studies. According to the presented method, accident sequence models for man-made hazards PSA are developed in the following major steps:
This method is presented in more details in section 8.
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