Nuclear fission

4.2.1Causally connected fire hazards – correlated fire hazards

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  Forest fires;
  
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  Industrial fires in neighbourhood plants to the NPP site;
  
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  Transport fires in the roads located nearby to the NPP site, etc.;
  
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  Possibly agriculture (stubble, bale of hay, feeds, etc.) fires.
  

These types of fire should be categorized depending on the source and cause of the fire as follows:

1. 
   Fires without human intervention:
   

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  natural occurring fires – extremely high/sharp increased ambient temperatures, lightning;
  
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  technological reasons, accidents caused by failure of equipment, industrial accidents, failures of vehicles, fuel leaks, fuel spills, etc.
  

2. 
   Fires with human intervention:
   

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  fires due to human negligence, lack of knowledge, failure to comply with rules and instructions, etc.
  
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  intentional fires - related sabotages, because of terrorist motives or other – these are in general not a part of a PSA.
  

All of these subcategories of external fire should be considered in the L1 PSA correlated with different internal fire(s) depending on their locations on the NPP’ site. Even further, a matrix “External-Internal fire correlated hazards” could be defined at least for quality assessment of the correlated hazards, and maybe to quantify the probability of their occurrence.

There are a lot of examples in almost all European countries, especially in the south part of Europe. For instance in Bulgaria selected 27 external fires (19 of which are human-induced external fires) - significant as hazards for different types of installations, have been registered by specialized institutions in the last 20 years. They occurred in regions distant from Kozloduy NPP and did not induce any internal fires on the plant site. But for an extended PSA, such cases of external fire should be considered by answering the following question - what would happen if they arise nearby, close to some NPP site? This is a concern for the safety of NPPs and potential external-Internal fire correlated hazards have to be considered probabilistically.
Apart from the above mentioned events, one can also consider some internal events arising as consequences of the “external and internal fire events”. As they occur as consequences of fires, they are not treated as associated hazards but causally connected - they can last further simultaneously with fire. In this respect the following internal events could be listed:

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  endangered buildings and constructions;
  
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  damages to the main and auxiliary equipment;
  
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  failure of interrupted power supply and/or water supply;
  
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  interrupted communications and partial impaired technological control and management;
  
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  hampered or impaired the possibility of applying SAM procedures where necessary;
  
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  outbreak of hazardous gases;
  
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  radioactive releases.
  

4.2.2Associated fire hazards

In this section, the associated external and internal fire hazards and internal events are discussed.

Events associated to external fire due to a common cause and occurring at the same time as the external fire, or due to the superposition of the consequences of events should be discussed.

An example of associated events that could be considered in an extended PSA is an external fire hazard (forest or industrial or other) due the impact of lightning combined with an induced overvoltage of the switchgear of the NPP also due to lightning.
Possibly the following events can be also considered:

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  accidents and/or local fires as results of technological reasons or damaged equipment, for instance in the turbine hall, accumulator stations, charging systems and compartments, diesel-generator stations, cable corridors, channels and shafts, circulation pump stations, heavy oil and oil farms, chemical departments/workshops, nitrogen-oxygen stations, auto fleets of the NPP, etc.;
  
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  accidents in heating, ventilation and air conditioning installations, departmental petrol and gas stations;
  
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  local fires in warehouses due to improper storage of inflammable materials, welding, etc.
  

All of the listed internal events are considered as potential events in previous studies and analysis as EIA of the Kozloduy NPP [24] and PSA level 1 for Units 5 and 6 of the NPP Kozloduy [25]. Also, the experience in other countries showed the necessity to have in-depth discussions on associated events with external and internal fire hazards in the extended PSA.
Finally one should mentioned combination of independent hazards (i.e. coincident hazards) – in most cases they can be screened out because of low frequency.

4.2.3Screening of fire

Comment : screening approach will be updated in the final report to get consistency between all ASAMPSA_E reports.
A general approach to screening process is described in D30.3 Methodology for Selecting Initiating Events and Hazards for Consideration in an Extended PSA [3]. In this section the approach for fire hazards is described.

In principle this approach can be similar to the screening process used for explosions described in Sec. 4.1.3. This means that fire can be screened out from further analysis basing on the criteria related to: distance, taking into account however, possibility of the spread of the fire caused by meteorological conditions; inclusion and severity. For combination of hazards additionally criteria are the same as for the explosions.

In the frame of extended PSA, external fire hazards might have a significant impact on the plant and have to be considered further if they are connected to other correlated or associated hazards, like internal fire, consequences of radiation heat (high temperatures), smoke effects, toxic effects, explosions (both external and internal) and their consequences. Thus the external fire can lead to a number of sequences with various events, where each event can have impact either on the same or different SSC, simultaneously or happening one after another. Hence various effects should be examined like loss of off-site power, for example due to the damage of external power grid caused either directly by external fire or as a fire consequence (for instance explosion). Such situations can be often modelled as a combined event.

Therefore the screening process may be quite complex and a two-step approach can be used for screening (to some extent similar to the one used for internal fires):

1. 
   Qualitative screening.
   
2. 
   Quantitative screening.
   

First, the qualitative screening consists in screening fire events depending on their impact based on identifying those fire zones where the expected risk of fire and associated events as well as their consequences is relatively low or negligible compared to others. In these zones, an external fire (or its consequences) causing a transient/initiating event and stopping the unit is unlikely, i.e. the risk caused by external fire is low and can be controlled with planned shutdown for prevention and no other PSA components modelled in the PSA are affected.

Other indicators should be applied, as for instance:

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  Existence of safety related equipment and cables related in the areas addressed in the external fire scenario;
  
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  Identified external fire and associated event load areas;
  
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  Effectiveness of the barriers between the fire zones, and other.
  

Two different fire situations should be studied:

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  Fire within only one fire zone, and;
  
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  Fire in more than one area – “multi-zones” (similar to "multi-compartment fires" considered in case of internal fire).
  

It should be stressed that in case of large external fire, multi-zones occurrence can be more probable than one zone. Additionally indirect consequences of external fire described by the sequence of various events should be practically examined almost always assuming fire effects in many zones.
Following the qualitative screening, the zones should be divided in two groups: one group for screened-out zones for which the analysis is terminated and one group for fire zones subjected to further analysis and the next stages of screening.
Then, following the qualitative screening, the second phase is the quantitative screening which is performed by establishing frequencies of fire events and sequences of events initiated by external fire, occurring in fire zones and complexes remaining after the qualitative screening analysis. In the frame of an extended PSA, the conditional core damage probability determined in the existing internal events PSA model can be used.
For the preliminary quantitative screening, the frequencies of fire scenarios at power operation, low power and shutdown should be presented. In case of fires of the type of "multi-compartment" in the extended PSA, the frequency of occurrence of fire and possible damage to equipment in "multi-zones" should be based on probability of failure of fire barriers data recommended in IAEA-TECDOC-1134 [26]. The generalized probabilities of failure of barriers from NUREG/CR-6850 [27] can be used only if technical problems on fire barriers in the NPP are not identified from the fire protection staff. According to the procedure NUREG/CR-6850 [27], the criterion for preliminary analysis of frequency of core damage is based on the assumption that the overall frequency of core damage is usually, for most stations, in the range of 1.0 × 10^-5 per year or has a greater value. If another assumption is used for the analysis, the frequency of core damage should be redefined.

4.2.4Examples of hazard combinations

In this section of the report, the typical or most important hazard combinations with external fire are listed.
First, the link between fire and explosion hazard should be considered. Different studies, such as the retrospective explosion and fire risk management analysis concerning the transport of liquid fuels, have shown immediate connection between both hazards, requiring mutual consideration in assessing risk for sites, facilities and equipment [28]. Highly probable combinations can exist between external fire and all the categories of explosion listed in 4.1.1:

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  industrial explosion;
  
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  military explosion;
  
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  transportation explosion;
  
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  pipeline explosion.
  

The hazard combination of the correlated external fire and internal fire with explosion(s) on the NPP’ site is also possible.

Other combinations for considerations are:

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  lightning and fires;
  
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  fires and hazardous gases;
  
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  extreme high ambient temperature and fire;
  
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  wind and fires, and other.
  

As an example of such combination one can mention an external fire hazard (forest or industrial or other) due the impact of lightning combined with an induced overvoltage of the switchgear of the NPP also due to lightning, pointed out in 4.2.2.

4.2.5Assessment of coincident hazards based on estimates of duration time of hazard

Recommendation No. 8 of the End Users workshop [1] categorized B/C ² mentions that the combinations/correlations/dependencies of fire hazards should be discussed in an extended PSA depending on the time frame (for example, addition of independent fire hazards may be considered for a long lasting accident), and that, if appropriate, specific rules should be defined in the guidance. This seems reasonable. However, this recommendation should be applied depending on the frequency of the other hazards and on the differences of their impact, e.g. if the plant situation after the external fire is not worsened with an additional hazard impact, there is no need to analyse this combination further and in detail.

4.2.6Worst case hazard combinations

The worst case hazard combinations can be found, practically basing on the consequence analysis. Typical candidates are the following:

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  the combination of an external fire hazard (forest or industrial or other) due to the impact of lightning combined with an induced overvoltage of the switchgear of the NPP also due to lightning and/or loss of off-site power (i.e. station blackout combined with fire);
  
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  eeexternal fire, explosion and hazard gases;
  
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  lightning and fire depending on the objects affected by lightning.
  

Recommendation No. 29 of the end users [1] categorized A, mentions that the effects of climate changes should be considered in two aspects:

1. 
   The effects of climate changes (global warming) as reason for an increase of the probabilities/frequencies of occurrence of external fires, especially forest fires and agricultural fires.
   
2. 
   The influence of the external fires on the greenhouse gases, especially forest fires in very larger areas lasting days and weeks, contributing to the global warming.
   

The last point is not directly related to fire impact on NPP, and in general, is difficult to realize taking into account the current knowledge of climate modelling, lots of uncertainties and therefore not enough credible results of modelling.
The above mentioned potential worst case hazard combinations have to be considered in further detail in the extended PSA.

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