Nuclear fission


Limitations and gaps in existing methods



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3.7Limitations and gaps in existing methods

3.7.1External Fires and explosions


Several issues should be mentioned here:

  • in principle a full QRA study should be performed in order to estimate the frequency of initiating events for PSA ; First of all, this can be a complex and time consuming task ; secondly the uncertainty is propagated from the initiating events of the QRA, through physical and chemical phenomena descriptions, to the consequence analysis, which is also burdened with some uncertainties ; this means that finally, the estimation of the frequency of initiating events for the PSA can be quite rough ; therefore a deterministic approach can be as useful as the QRA, for example in the consequence analysis ; hence an overestimation of the frequency of initiating events for a PSA can be expected;

  • understanding of underlying physical processes for some types of fires and explosions (like vapour cloud evaporating expansion) still needs research ; this is, of course, a main source of uncertainty;

  • identification of the impact on SSCs of NPP must include not only combinations of different hazards, but also take into account that some events in the plant can happen simultaneously and independently ; this is taken into account by using the internal events model as a basis for the modelling of the external hazards.



3.7.2Aircraft crash


The methodology for assessing the aircraft crash hazard is reasonably well covered in state–of-the-art guidance documents, standards as well as technical publications. This step of the aircraft crash PSA is more mature than most of the other steps, such as plant response analysis, HRA (which in fact is still under development for extreme situations), etc.. However, some specific analysis tasks need further developmental efforts in hazard assessment too, including the following in particular:

  • the methodology for the assessment of impact mass and velocity distributions for each aircraft type is not complete at all points ; appropriate input data with a description of distribution types and values of distribution parameters should be developed, and applicable data sources need to be identified and described in detail ; also, the methods of determining correlation between mass and velocity should be presented in detail in updated guides ;similarly, comprehensive international databases are needed on characterizing the impact parameters for each aircraft type to ensure adequate input data for hazard assessment.

  • there is no definite consensus on how to practically model the secondary effects of an aircraft crash in PSA ; available guides suggest an exhaustive listing of all possible secondary effects, however, guidance on applicable modelling assumptions are not given and detailed evaluation methods are not elaborated in detail either.

  • the identification of potential impact zones is usually based on the similarities in primary impacts of different aircraft crashes ; consequently, safety related buildings are usually considered impact zones ; further developmental work is needed to give appropriate considerations to secondary effects in defining impact zones ; moreover, the existing methodology should be refined to enable the identification of those impact zones that are hit by an aircraft having only secondary (hence no primary) effects on safety related SSCs ; specifically for each representative aircraft or aviation categories the following tasks need to be done:

    • estimation of fire effects distances based on the amount of fuel and other combustibles loads (cable, seats, luggage, etc.) of aircraft.

    • estimation of fuel quantity penetrating into a building after an aircraft impact.

    • estimation of the effects distances of missiles based to statistical analysis of past accidents.

  • state-of-the-art guidance documents do not propose any methodology on how to avoid the double counting of crashes when both the background crash rate and the airway related crash rates are assessed and summed up ; this analysis area also needs further development.

  • more detailed guidance is needed on how to perform trend analysis and qualitative evaluation of future changes.

  • uncertainty issues mentioned in previous sections.


4HAZARDS COMBINATIONS

4.1External explosions

4.1.1Explosion hazard correlations


External man-made hazards are generally characterized by a relatively large number of cross-correlated phenomena. The correlations between hazards can lead to the following categories:

  • Causally connected hazards where one hazard may cause another hazard; or where one hazard is a prerequisite for a correlated hazard;

  • Associated hazards which are probable to occur at the same time due to a common root cause.

In case of explosion, the causality dependence can be divided into 2 categories:



  • Causality dependence between explosion and external natural hazards;

  • Causality dependence of explosions and other man-made hazards.

A correlation map for the external hazards was developed in D21.2 (List of external hazards to be considered in ASAMPSA_E) WP21 of the ASAMPSA_E project [15].The analysis of the map has led to the following remarks.

As already mentioned the combination of external fire and explosion hazards can be classified in the following categories:


  • industrial fire/explosion;

  • military fire/explosion;

  • transportation fire/explosion;

  • pipeline fire/explosion.

All the above categories of fires and explosions can be induced by the following hazards: vibratory ground motion; induced vibratory ground motion; fault capability; liquefaction; tsunami; slope instability; meteorite fall; volcanic hazard. Industrial and military explosions can be induced also by lightning, tornado, windblown debris and snow avalanche. Transportation explosions can be induced also by tornado and snow avalanche. Industrial explosion, transportation explosion and pipeline explosion may induce the occurrence of forest fire.


Regarding the dependence between explosions and other man-made hazards, the following aspects may be specified [15]:

  • chemical (toxic) releases can be induced in case of explosions;

  • ground transportation – direct impact may induce transportation and pipeline explosions;

  • transportation explosions can induce industrial pipeline explosions;

  • pipeline explosion can induce industrial explosions, and vice versa;

  • military activities can induce transportation explosions, industrial and pipeline explosions.

Comment : the remarks above shall be taken into account in final version of [15].

4.1.2Combinations of explosion and other external hazards


The external hazards combinations can threaten simultaneously diverse safety systems, and screening out external hazards without consideration of dependencies can lead to an underestimation of the associated risk. The international experience shows that the combinations of external hazards are considered only if the hazards are correlated [16]. In practice, the selected combinations of correlated external hazards are strongly dependent on local conditions.
The analysis of possible correlations (dependency) between events can be made by assessing the physical bases of the phenomena, observed data, actual events and general knowledge of local conditions. The identification of potential combined external events depends to some extent on engineering judgment, and there is no evident best method for performing the identification.
Correlated external events are modelled as combined events in an External Events PSA. For example, an explosion may damage the external power grid and can induce fires at the same time. This can be modelled as a combined event “loss of off-site power and fire”.
In any case, the initiating events should be identified by expert judgment taking into account insights gained from analysis and operating experience.

4.1.3Screening of explosions and hazards combinations with explosions


Comment : screening approach will be updated in the final report to get consistency between all ASAMPSA_E reports.
A general approach of the screening process is described in D30.3 “Methodology for Selecting Initiating Events and Hazards for Consideration in an Extended PSA” [3]. The following methodology, consisting of the four major steps, has been proposed:

  1. Comprehensive identification of events and hazards and their respective combinations applicable to the plant and site;

  2. Initial frequency claims for events and hazards and their respective combinations applicable to the plant and the site;

  3. Impact analysis and bounding assessment for all applicable events and scenarios. Events are either screened out from further more detailed analysis, or are assigned to a bounding event (group), or are retained for detailed analysis;

  4. Probabilistic analysis of all retained (bounding) events at the appropriate level of detail.

In this section specific elements related to explosions are described.


An explosion should be screened-out from further analysis, if at least, one of the following site related selection criteria is satisfied [17]:

  • Distance - the potential explosion cannot occur close enough to the plant to affect it.

  • Inclusion - the event is included into another (enveloping) event or is included in a combined event (it causes risk increase in connection with some other event).

  • Severity - the effects of the event are not severe enough to damage the plant, since it has been designed for other loads with similar or higher strength.

Since the explosions are untimely and unexpected phenomena, the warning criterion (time available to impact) is not applicable for screening.


For combinations of hazards, in addition to the above, the following criteria for screening in might be also used [17]:

  • Different plant safety functions affected

If two external events are dependent and one affects the offsite power while the other one affects the ultimate heat sink, this would be a relevant combination;

  • Degree of impact on plant safety functions

If two dependent external events affect the same safety function, they may still be a relevant combination, provided that their combined effect is greater that the effect from any of the single events involved. It should be noted that it is possible that two single hazards could be screened out on consequences, but their combination not. This means that for combinations, in principle, all non-screened list should be used. On the other hand random combinations can be almost always screened on frequency.
It should be mentioned that in the screening process the impact on the plant and the consequences of explosions should be assessed. This concerns the following aspects:

  • collapse of structures or components, disruptions of systems or equipment due to pressure waves (characterized by the local overpressure at the plant as a function of time);

  • penetration, perforation, spalling or collapse of structures or disruption of systems and components caused by projectiles; false signals in equipment induced by vibration;

  • impaired habitability of control room, disruption of systems or components, ignition of combustibles caused by heat (characterized by maximum heat flux and duration);

  • blockage of intake filters, impaired habitability of control room and other important plant areas due to smoke (characterized by concentration and quantity as a function of time).

The effects of explosions which are generally of concern when analysing structural response to blast are:



  • incident and reflected pressure;

  • time dependence of overpressure and drag pressure;

  • blast-generated missiles;

  • blast-induced ground motion.

The relative importance of these effects depends mainly on the quantity and type of the explosive materials, the distance of the structure being considered from the source of the explosion, and the details of the geometry and spatial arrangements of the structures.


For the overpressure range less than 0.5 bars, overpressures exceed drag pressures by a sufficient amount so that drag pressures can generally be neglected (the expected low levels of drag pressures are generally accommodated by the usual design for wind loads). However, for the case of a wall vulnerable to negative pressure, or for elastic response of walls subjected to detonation of solid substances with TNT equivalents less than about 500 kg at distances less than about 50 m, the negative pressure may also be important [18] (see also [2] for application of TNT equivalent mass method for calculating minimum safe distance).
It is unlikely that any considerable number of large, hard blown objects will be thrown away for significant distances as a result of an explosion. If the plant has been designed to accommodate the effects of externally generated missiles resulting from other events such as hurricanes, tornadoes or aircraft crash, the effects of missiles generated by an explosion may already have been accounted for [18].
The intensity of blast-induced ground motion to be expected from above-ground detonations at overpressures less than 0.5 bar can generally be accommodated [18].
When calculating distances required for protection by means of separation, use can be made of the attenuation of peak overpressure as a function of distance from the explosion source. The data available for TNT can reasonably be used for other solid substances.
The adequacy of the protection afforded should be evaluated carefully when the location of the explosion can vary, as is the case in transport accidents. Since explosions of gas clouds can affect the entire plant area, the postulated gas cloud should be the most severe credible gas cloud relevant to the site. An analysis of the ability of plant structures to resist the effects of gas cloud explosion can normally be limited to an examination of their capacity to withstand the overpressure loading (primary effect). The pressure developed is a function of the energy release rate, as well as of the total energy release.
It should be noted that the overpressure-time history for a particular structure is heavily dependent on the layout of the surrounding buildings.
A recommendation from ASAMPSA_E report D30.2 [19] is to use appropriate PSA Level 2 risk measures for events screening. This recommendation targets the identification of low frequency events with potentially large consequences, as in the case of explosion and aircraft crash.


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