Nuclear-Weapon Effects

The devastation from the Hiroshima bomb might be replicated by terrorists.

The immense energy released in a nuclear explosion does the bulk of its damage in five ways, listed here in ascending order of the distance, measured from the epicenter of a 10-kiloton explosion, to which fatal effects are likely: [1]

• the vaporization of matter by the tremendous temperatures within the fireball;
• the shock wave propagating outward from the blast, carrying a pulse of crushing overpressure followed by powerful winds;
• the prompt radiation dose carried by neutrons and gamma rays from the fission reactions;
• heating, burning, and charring by the thermal energy radiated by the fireball; and
• delayed radiation doses from radioactive fallout in the form of fission products and neutron-activation products deposited downwind of the explosion.

In what follows, we treat each of these in turn and then consider their combined effects. We give the damage distances that correspond to nuclear explosions taking place on the ground, rather than at some altitude above the target, because most of the easiest modes of delivery for a low-capability adversary—boat, truck, car—would lead to detonation at ground level. A more sophisticated adversary might detonate a weapon near the "optimum" altitude for widespread damage (which varies with the explosive yield of the weapon) and do even greater harm.

Fireball

The fireball from a 10-kiloton explosion at ground level would reach a radius of 200 meters, hence would have a diameter of 400 meters or about a quarter of a mile. Everything within this radius would be completely destroyed. In the case of a badly designed or badly implemented bomb that yielded only 1 kiloton, the fireball would have a diameter about 2.5 times smaller, hence about 150 meters. The surface explosion of a 100 kiloton weapon, as conceivably might be stolen from the arsenal of an advanced nuclear-weapon state, would produce a fireball 1 kilometer in diameter.

Overpressure and Wind

A blast-wave overpressure of 5 pounds per square inch, which is associated with winds around 150 miles per hour, is enough to destroy wood-frame buildings and cause severe damage to brick apartment buildings; overpressure of this magnitude would be experienced at about 1,000 meters from a 10-kiloton surface explosion and 500 meters from a 1-kiloton surface explosion. A 100-kiloton surface explosion would generate this overpressure at 2,000 meters. The distance for 15 pounds per square inch overpressure – which is associated with winds around 400 miles per hour and is enough to destroy office buildings constructed of steel-reinforced concrete—is some 500 meters for a 10-kiloton surface explosion.

Prompt Ionizing Radiation

The distance from a 10-kiloton surface explosion at which a person in the open could receive a prompt dose of 500 rem from neutrons and gamma rays (the dose that will prove fatal within 30 days to about half the people receiving it) is around 1,500 meters. For a 1-kiloton surface explosion, this distance would be around 1,100 meters, and for a 100-kiloton thermonuclear explosion about 1,800 meters. Doses received by people shielded from the explosion by buildings would be lower.

Thermal Radiation

A nuclear fireball radiates energy in infrared, visible, and ultraviolet wavelengths with enough intensity to burn exposed skin and char or ignite flammable materials at substantial distances. For a 10-kiloton surface explosion, the radiant intensities on a clear day are sufficient to ignite clothing at a distance of 1,100 meters and to cause second-degree burns on exposed skin at 1,700 meters. For a 1-kiloton surface explosion, the distance for second-degree burns would be about 600 meters; for 100 kilotons it would be about 4,700 meters.[2]

Fallout

"Local" or "early" fallout occurs when a nuclear weapon explodes close enough to the ground to vaporize and entrain in the fireball much larger amounts of solid material than that from which the weapon is made. When this vaporized material later cools and condenses into liquid and solid particles, many of these end up heavy enough to fall to the ground rather than remaining suspended in the atmosphere, and they carry with them a substantial fraction of the radioactive fission products and activation products produced in the explosion.

The spatial pattern over which such fallout is deposited depends strongly on the wind patterns and atmospheric stability prevailing at the time. It is possible to make a rough estimate for the area over which a given dose from fallout will typically be equaled or exceeded, however. If the dose at the boundary of the area of interest is taken to be 500 rem within 48 hours to unprotected persons who do not leave the area, the answer comes out around 3 square kilometers per kiloton of fission energy release, hence 30 square kilometers for a 10-kiloton explosion.

Combined effects

Table 3 summarizes the damage potential of these phenomena by showing the areas over which they would be likely to cause significant numbers of fatalities. These figures are only indicative. Estimating actual casualties from a nuclear explosion requires taking many factors into account, including population density, weather, the types of buildings present, how many people are outdoors as opposed to indoors at the time of the blast, and so on.

Table 3. Areas of Potentially Lethal Effects from Surface Nuclear Explosions (square kilometers)

Notes:

Areas obtained from radii in text using A = pi * r², except for fallout contour (which is presented as an area in the text). The fallout contour for 100 kilotons is not proportional because at this yield we assume a thermonuclear weapon, wherein only half of the energy release comes from fission. Numbers have been rounded. One square mile is 2.59 square kilometers.

For purposes of very rough estimation, it is sometimes assumed that the radius of 5 pounds per square inch overpressure defines the circle for which the number of survivors inside would equal the number of fatalities outside, taking into account all the listed effects other than fallout. This would mean that the total number of early fatalities, other than from fallout, would be estimated as the population density multiplied by the area of this circle.

Manhattan has a population density of about 70,000 (residents) per square mile (27,000 per square kilometer), so by the indicated rule of thumb a 10-kiloton nuclear explosion in Manhattan late at night (when the residents and overnight tourists are there, but commuting workers are not) would be estimated to kill outright more than 80,000 people.[3] The toll could be increased by detonating the weapon in the heart of one of the high-rise business districts during working hours. This figure does not include fallout, nor does it include deaths from fires that could be ignited at considerable distances from the blast as a result of broken gas mains and the like, or other "indirect" effects.

Obviously, such an explosion would be a catastrophe far beyond contemporary experience, exceeding by perhaps 30-fold even the terrible attack on the World Trade Center towers on September 11, 2001.

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FOOTNOTES

[1] The classic reference on nuclear weapons effects is Samuel Glasstone and Phillip J. Dolan, The Effects of Nuclear Weapons (Washington, D.C.: Government Printing Office, 1977). Ten kilotons is used here as a typical yield to be expected from a first-generation fission weapon developed by a newly proliferating country or by a highly sophisticated terrorist group. It also corresponds to the yield of a "tactical" nuclear weapon of modest size from the arsenal of one of the major nuclear powers, as might conceivably be stolen by terrorists or by agents acting for a proliferant nation. A typical strategic thermonuclear weapon from the US arsenal would have a yield 30 to 50 times greater, and the largest strategic weapons have yields 200 to 1,000 times greater. The ranking of effects according to lethal radius changes somewhat for yields much larger (or much smaller) than 10 kilotons. The epicenter is the location of an explosion on the ground, or the location on the ground directly under an explosion in the air.

[2] The deposit of 4 to 5 calories per square centimeter of exposed skin in the short time associated with the pulse of thermal radiation from a moderate-yield weapon is enough to produce second-degree burns. Third-degree burns are produced by 6-7 calories per square centimeter. Igniting common clothing fabrics takes 10-12 calories per square centimeter. The delivered intensities diminish a bit faster than the square of the distance from the explosion because of attenuation passing through the air.

[3] This is obtained by taking the 3.1 square kilometer area from Table 3 (for the 5 psi overpressure contour for a 10-kiloton surface explosion) and multiplying by the 27,000 residents per square kilometer in Manhattan. The "more than" in the result depends on the assumption that the number of tourists staying in the blast area is greater than the number of residents who are absent.