The Effects of Nuclear War
Chapter 2
GENERAL DESCRIPTION OF EFFECTS
Fires
The thermal radiation from a nuclear explosion can directly ignite kindling materials. In general, ignitable materials outside the house, such as leaves or newspapers, 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. A rather substantial amount of combustible material must burn vigorously for 10 to 20 minutes before the room, or whole house, becomes inflamed. The blast wave, which arrives after most thermal energy has been expended, will have some extinguishing effect on the fires. However, studies and tests of this effect have been very contradictory, so the extent to which blast can be counted on to extinguish fire starts remains quite uncertain.
Another possible source of fires, which might be more damaging in urban areas, is indirect. Blast damage to stores, water heaters, furnaces, electrical circuits, or gas lines would ignite fires where fuel is plentiful.
The best estimates are that at the 5-psi level about 10 percent of all buildings would sustain a serious fire, while at 2 psi about 2 percent would have serious fires, usually arising from secondary sources such as blast-damaged utilities rather than direct thermal radiation.
It is possible that individual fires, whether caused by thermal radiation or by blast damage to utilities, furnaces, etc., would coalesce into a mass fire that would consume all structures over a large area. This possibility has been intensely studied, but there remains no basis for estimating its probability. Mass fires could be of two kinds: a “firestorm,” in which violent inrushing winds create extremely high temperatures but prevent the fire from spreading radially outwards, and a “conflagration,” in which a fire spreads along a front. Hamburg, Tokyo, and Hiroshima experienced firestorms in World War II; the Great Chicago Fire and the San Francisco Earthquake Fire were conflagrations. A firestorm is likely to kill a high proportion of the people in the area of the fire, through heat and through asphyxiation of those in shelters. A conflagration spreads slowly enough so that people in its path can escape, though a conflagration caused by a nuclear attack might take a heavy toll of those too injured to walk. Some believe that firestorms in U.S. or Soviet cities are unlikely because the density of flammable materials (“fuel loading”) is too low–the ignition of a firestorm is thought to require a fuel loading of at least 8 lbs/ft2 (Hamburg had 32), compared to fuel loading of 2 lbs/ft2 in a typical U.S. suburb and 5 lbs/ft2 in a neighborhood of two-story brick rowhouses. The likelihood of a conflagration depends on the geography of the area, the speed and direction of the wind, and details of building construction. Another variable is whether people and equipment are available to fight fires before they can coalesce and spread.