Nuclear fission

3.2Hazard Categorization

3.2.1External fire and explosion

In assessing the external fire hazard, distinction can be made between different types of fire. The main categories of the fire are:

1. 
   Boiling Liquid Expanding Vapour Explosions (BLEVE): this is sudden rupture of a vessel containing liquefied flammable gas under pressure. The primary cause is usually an external flame impinging on the shell of a vessel above the liquid level, weakening the container and leading to sudden shell rupture. The pressure burst and the flashing of the liquid to vapour creates a pressure wave and potential missile damage. The immediate ignition of the expanding mixture of fuel and air leads to intense combustion and the creation of a fireball. The majority of BLEVEs have occurred during the transport of pressurized liquefied gases but a number have occurred at fixed installations. As BLEVE reduces the consequences of the fires to heat only, it can be treated as explosion.
   
2. 
   Pool Fires: liquid is spilt onto a flat surface spreads out to form a pool. If the liquid is volatile, evaporation takes place and if the liquid is flammable then the atmosphere above the pool will be in the flammable range. If ignition takes place, then a fire will bum over the pool. The heat from this fire will vaporize more liquid and air will be drawn in from the sides of the pool to support combustion. The system will then consist of a solid cylinder of flame burning above the pool.
   
3. 
   Flash Fires: this occurs when a cloud of a mixture of flammable gas and air is ignited. The shape of the fire closely resembles the shape of the flammable cloud prior to ignition but it also depends upon where within the cloud ignition occurred. In many cases the cloud extends back to the original point of release and can then give rise to a torch or pool fire depending on the mode of release. When ignition occurs, the flame front races or 'flashes' through the cloud very quickly. It is also possible that the flame may accelerate to a sufficiently high velocity for an explosion to occur.
   
4. 
   Jet or Torch Fires: it usually occurs when a high pressure release from a relatively small opening (ruptured pipe, pressure relief valve, etc) ignites. This gives rise to a torch which can burn with flame lengths several meters long. The flame is a hazard to persons nearby but the main hazard is generally its effect on adjacent vessels which may contain flammable liquids.
   

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

The requirement for vapour cloud explosion is a large pre-mixed cloud of flammable vapour and air within the flammable range and the presence of some confinement or obstructions.

BLEVE described shortly above is an example of combined fire and explosion hazard.

Dust explosion is a hazard whenever combustible solids of small particle size are handled.

3.2.2Aircraft crash

As the first step of aircraft crash hazard assessment, it is necessary to classify all aircrafts relevant to the airfields of the region in the vicinity of the site into different categories. In general, country-specific data is taken into consideration to identify all aircraft types that should be covered in the assessment. The categorization should be based on the differences in flying characteristics, reliability as well as the damage potential on structures. The damage potential of an aircraft is influenced by many physical characteristics thereof, although in practise, the commonly applied approaches take only mass and velocity into consideration for categorization.
One example of aircraft categorization applicable to hazard assessment (based on [4]) is:

• 
  drones, remote controlled ultra-light aircraft, sailplanes, gliders and aircrafts carrying negligible amount of fuel (e.g. gliders with engines),
  
• 
  light civil aircraft - fixed wing civil and military aircrafts having a maximum take-off mass authorized less than 2.3 tons,
  
• 
  helicopters – all civil and military helicopters,
  
• 
  small transport aircraft – fixed wing civil and military aircrafts having a maximum take-off mass authorized between 2.3 to 20 tons,
  
• 
  large transport aircraft – fixed wing aircrafts not covered in any other categories,
  
• 
  military combat and jet trainers – all military fixed wing aircraft with a maximum take-off mass authorized up to 50 tons and capable of acrobatic style flying.
  

In contrast, the Swiss Federal Nuclear Safety Inspectorate (ENSI) defines the following categories that shall be analysed [6]:

• 
  commercial aircraft (maximum take-off mass > 5.7 tons),
  
• 
  military aircraft,
  
• 
  light aircraft (maximum take-off mass < 5.7 tons).
  

The same approach is also applied in France.
Subsequently, for structural response evaluation, one (or more) representative(s) or surrogate aircraft design is assigned to each category reflecting all damage potential characteristics relevant to the category. The following aircraft and flight parameters may influence the structural response:

• 
  aircraft mass,
  
• 
  impact velocity distribution,
  
• 
  descent angle (alternatively, a conservative value may also be sufficient),
  
• 
  mass and cross-sectional area of potential missiles,
  
• 
  mass distribution along the aircraft,
  
• 
  type, mass and location of fuel,
  
• 
  type and amount of fuel and other combustible loads (seats, cables, luggage…).
  

These parameters have also some impact on fires and explosions response evaluation, in particular the latter one.

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