Dangerous accident phenomena

• Fire: Self-sustaining, uncontrolled combustion that may last for a long time and spread to other locations. The study of this phenomenon is based on the fire triangle: the simultaneous presence of a combustive (oxygen), a combustible product and a source of inflammation. The type of fire depends on the nature of the combustible, which may be in solid (warehouse or forest fire), liquid (oil slick fire) or gas form (torch fire caused by a pipe leak). Although related, fireballs are treated separately due to their distinctive characteristics: very short duration, flame projections, etc.
• Explosion: Sudden oxidation or decomposition producing an increase in temperature, pressure, or both simultaneously. There are two main types of explosion: physical (a burst tire, a change in the physical state of material) and chemical (combustion, substance decomposition reactions). For explosions involving powder or gas combustion, a distinction is made between deflagration (the flame produced by the explosion spreads at a speed lower than the speed of sound, around several meters per second) and detonation (the flame produced by the combustion spreads at a speed higher than the speed of sound, in the range of several kilometres per second). An explosion can transition from a deflagration to a detonation.
• Atmospheric dispersion as a dangerous phenomenon: the path in time and space of a cloud of particles emitted into the atmosphere (aerosols, gases, dust) which are harmful to humans or to the environment. The dispersed cloud may be composed of smoke from a fire or substances with flammable and toxic properties (ammonia, chlorine, etc.) Dispersion depends on the release conditions (mode of emission, type of substance, etc.), meteorological conditions (wind, temperature, etc.), and the surrounding environment (topography, presence of obstacles, etc.).

The effects of dangerous phenomena fall into 4 categories.

• Thermal effects, linked to the combustion of a flammable product or an explosion
• Mechanical effects, caused by the high-pressure shock wave from an explosion
• Projection effects, which are indirect effects of the formation and movement of debris and fragments from an explosive shock wave
• Toxic effects which result in the inhalation of a toxic chemical substance following a leak at a site.