Lecture 14

Destructive Effects of Nuclear Weapons

1. Blast Damage-  The generation of a mechanical shock through the sudden increase of pressure causes mechanical damages.
2. Thermal Damage-  The generation of a heat wave expanding with the shock causes incineration.
3. Radiation Damage-  The distribution of radiation through short range and atmospheric fallout causes short term and long term radiation sickness effects.
4. EM pulse-  Electromagnetic shock leads to a break-down of communication systems.

Mechanical Shock

Shock characteristics

Shock expansion

1. Stages of Shock Expansion
   
   1. Shock expands radially from explosion center.
   2. Amplitude decreases, but after time t=5 seconds the pressure behind the shock front falls below atmospheric pressure.
   3. Under-pressure causes the air to be sucked in.
2. Pressure conditions with transversing shock
3. 1. Shock hits
   2. Generates strong wind
   3. Shock decreases
   4. Underpressure
   5. Wind direction changes
   6. Normal air pressure
   7. Wind calms down.

Scaling laws for blast effects

1. Shock/blast expands over volume (d^3), the following scaling law can be applied for estimating distance effects between different blast strengths
   
   1. 
      
2. Normalized to a standard 1kT blast, the following expression can be applied:
   
   1.  
      
3. Problems:
4. 1. At a distance of 2500 ft the over pressure of a 1 kT bomb surface explosion has a value of 1 psi. Calculate the distance for the overpressure of a 200 kT and a 500 kT surface explosion using the scaling laws.
   2. Suppose you have a 15 kT bomb detonation at a height of 2320 ft, determine the peak overpressure at a distance of 1860 ft from ground zero.

Distance effects of airburst 

Scaling in Altitude

Thermal effects

Approximately 35 percent of the energy from a nuclear explosion is an intense burst of thermal radiation (i.e., heat). Thermal effects are mainly due to originated heat from the blast which expands with wind velocity and incinerates everything within the expansion radius. The thermal radiation from a nuclear explosion can directly ignite kindling materials.  Ignitable materials outside the house, such as leaves, are not surrounded by enough combustible material to generate a self-sustaining fire. Fires more likely to spread are those caused by thermal radiation passing through windows to ignite beds and overstuffed furniture inside houses.

Firestorms

1. Fires can result from combustion of dry, flammable debris set loose by the blast or from electrical short circuits, broken gas lines, etc. These fires can combine to form a terrible firestorm similar to those accompanying large forest fires. The intense heat of the fire causes a strong updraft, producing strong inward drawn winds which fan the flame, take away oxygen so it is difficult to breathe, and destroy everything in their path (Chimney Effect).
2. The expansion of firestorms
   Different scaling laws apply for calculating the heat and incinerating effects from bomb yield. Fire advances by wind driven heat propagation. In a uniform atmosphere without turbulent or convective processes the expansion would follow an exponential law with the radiation absorption parameter. The heat exposure at distance d would be:
   

   

Effects of thermal radiation on materials

1. Heat exposure in bright sunlight:    2 cal/cm2.
2. Ignition
   
   1. White paper ignites at ~ 5 cal/cm2, (magnifying glass)
   2. Fabric ignites at ~20-40 cal/cm2 depending on color & material.
3. Burns
   
   1. 1st degree burn ~ 3 cal/cm2  (sunburn)
   2. 2nd degree burn ~5 cal/cm2  (skin loss, no scars - if burns are over 25% of a victim's body, he/she must be hospitalized)
   3. 3rd degree burn ~8 cal/cm2   (destroys skin nerves, scarring, no cell regeneration; if 50% of body has these burns, they are typically fatal).
4. Fire expansion is driven by shock driven winds which develop rapidly. Turbulences arise due to temperature differences. Fire spreads with rapid speed, leaving no chance to escape.