Thermal Pulse Effects

One of the important differences between a nuclear and conventional weapon is the large proportion of a nuclear explosion's energy that is released in the form of thermal energy. This energy is emitted from the fireball in two pulses. The first is quite short, and carries only about 1 percent of the energy; the second pulse is more significant and is of longer duration (up to 20 seconds).

The thermal pulse charring the paint.
The thermal pulse charring the paint.

The energy from the thermal pulse can initiate fires in dry, flammable materials, such as dry leaves, grass, old newspaper, thin dark flammable fabrics, etc. The incendiary effect of the thermal pulse is also substantially affected by the later arrival of the blast wave, which usually blows out any flames that have already been kindled. However, smoldering material can reignite later.

Effects of the thermal pulse on clothing.
Effects of the thermal pulse on clothing.

The major incendiary effect of nuclear explosions is caused by the blast wave. Collapsed structures are much more vulnerable to fire than intact ones. The blast reduces many structures to piles of kindling, the many gaps opened in roofs and walls act as chimneys, gas lines are broken open, storage tanks for flammable materials are ruptured. The primary ignition sources appear to be flames and pilot lights in heating appliances (furnaces, water heaters, stoves, etc.). Smoldering material from the thermal pulse can be very effective at igniting leaking gas.

The shadow of the parapet is imprinted on the surface of the bridge, 880 meters south-southwest of the hypocenter.
Bridge across the Ota river, 880 meters from the hypocenter of the bomb burst above Hiroshima. Note where roadway is burned and the ghostly shadow imprints left where the surface was shielded by cement pillars.

Thermal radiation damage depends very strongly on weather conditions. Cloud cover, smoke, or other obscuring material in the air can considerably reduce effective damage ranges versus clear air conditions.

Thermal radiation also affects humans both directly - by flash burns on exposed skin - and indirectly - by fires started by the explosion.


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