Nuclear explosion

(nucleonics) An explosion for which the energy is produced by a nuclear transformation, either fission or fusion.

An explosion whose energy is produced by a nuclear transformation, either fission or fusion. See also Nuclear fission; Nuclear fusion.

The energy of a nuclear explosion is usually stated in terms of the mass of trinitrotoluene (TNT) which would provide the same energy. The complete fissioning of 1 kg of uranium or plutonium would be equivalent to 17,000 metric tons of TNT (17 kilotons); 2 lb would be equivalent to 17,000 short tons. The indicated yield-to-mass ratio of 1.7 × 10⁷ cannot be realized, largely because of the ancillary equipment necessary to assemble the nuclear components into an explosive configuration in the very short time required.

Though the size of a typical nuclear explosion is appalling, the most significant feature of a nuclear device derives from its yield-to-weight ratio. The first nuclear weapons (1945) weighed about 5 tons, but with yields of 15 to 20 kT their yield-to-weight ratio was, nevertheless, close to 4000 times larger than that of previous weapons. By 1960 the United States had developed a weapon with a yield of about 1 megaton (MT) in a weight of about 1 ton for use in an intercontinental missile. With this, the yield-to-weight ratio was raised to 10⁶.

Although weapons with yields of up to 15 and 60 MT were fired by the United States and Soviet Union respectively, about 1960 the main interest of the major nuclear powers focused on adapting nuclear devices for delivery by missile carriers, and this called for smaller weapons with so-called moderate yields of tens or hundreds of kilotons, up to a few megatons. See also Missile.

The damage mechanism of a conventional explosion is blast—the pressure, or shock, wave transmitted in the surrounding medium. The blast wave from a nuclear explosion is similar, except for the great difference in scale. A nuclear explosion also produces several kinds of effects not experienced with ordinary explosives.

About one-third of the energy of the explosion is distributed as thermal radiation on line-of-sight trajectories. Exposure to 5–10 cal/cm² (2–4 × 10⁵ J/m²) of thermal radiation energy in the short time (a second, or so) during which the thermal pulse is delivered will ignite many combustible materials (fabrics, paper, dry leaves, and so forth). It will cause serious flash burns on exposed skin. Such energy levels will be delivered in clear air to 0.3–0.4 mi (0.5–0.6 km) by an explosion of 1 kT. At Hiroshima, burn injuries alone would have been fatal to almost all persons in the open without protection out to a little over 1 mi (1.6 km), and burns serious enough to require treatment were experienced at distances greater than 2 mi (3.2 km).

During the few tens of seconds before the fireball rises away from the point at which the explosion occurred, it provides an intense source of gamma rays. The radiation emitted during this interval is referred to as prompt radiation. The remaining radioactivity, which is swept upward from the scene of the explosion to the altitude at which the fireball stops rising, and some of which ultimately returns to the surface, constitutes the residual radiation. See also Alpha particles; Neutron; Radioactivity.

At Hiroshima a dose equal to or greater than 450 rads (4.5 grays) extended to almost 1 mi (1.6 km). About half of the persons exposed to this dose in a short time will die within a few weeks, so that persons in this area who were not protected by heavy building walls experienced severe hazard from radiation. This is about the same distance for severe hazards from blast and thermal effects. The prompt radiation exposure falls off more rapidly with distance than the blast effect which, in turn, falls more rapidly than the intensity of thermal radiation. For an explosion much larger than 15 or 20 kT, the hazard range from thermal radiation or blast will be larger than that from prompt radiation, and prompt radiation will be a relatively unimportant effect. For much smaller yields, this order of importance will be reversed.