UNDERGROUND BURST

An underground burst (fig. 8-6) is a burst where the point of detonation is below the ground's surface. An underground burst produces a severe earth shock, especially near the point of detonation. Thermal radiation, air blast, initial nuclear radiation, and fallout will be negligible or absent if the burst is confined below the earth's surface. For shallow underground bursts, the air blast, thermal radiation and initial nuclear radiation will be less than for a ground surface burst. Ground shock will cause damage within about three crater radii, but little beyond. Early fallout can be significant, and at distances near the explosion, base surge (evidenced by a dust cloud) will be an important hazard.

WEAPON EFFECTS AND THEIR MODIFICATION BY THE ENVIRONMENT

Specific effects depend on the type of nuclear weapon and the type of burst. These effects are influenced considerably by the environment in which the weapon is detonated. A description of the weapon effects and the modification caused by the environment is provided in the following paragraphs.

AIR BLAST

Air blast is the shock wave that is produced in the air by an explosion. The shock wave initially travels outward at a velocity of approximately seven times the speed of sound at high overpressures (overpressure is any pressure greater than normal atmospheric pressure). It will then gradually slow down to sonic speed (about 1,000 feet per second [ft/s]) at low overpressures. An air blast produces a rapid increase in the normal atmospheric (static) pressure and causes high wind (dynamic) overpressures. High static overpressures cause damage by squeezing and crushing targets. Dynamic overpressures cause damage by bending or dragging targets. Ship structures and

buildings are primarily vulnerable to static overpressures, whereas aircraft, masts, antennas, and exposed personnel are vulnerable to dynamic overpressures.

Air blast is measured in pounds per square inch (psi) over the ambient or atmospheric pressure. The damage produced by an air blast is related to the peak static and dynamic overpressures.

UNDERWATER SHOCK

Underwater shock is the shock wave that is produced in the water by an explosion. The shock wave initially travels several times the speed of

sound in the water but quickly slows down to hypersonic speed (approximately 5,000 ft/s). Underwater shock produces rapid accelerations that can disarrange equipment and machinery, rupture hulls, and/or injure personnel. Both the directly transmitted shock wave and the shock wave reflected from the sea bottom can be damaging. An underwater explosion produces a shock wave similar to that of an airburst. However, underwater shock damage is measured by the peak vertical velocity (for surfaced ships) and by the peak translational velocity (for submerged submarines) rather than by the water overpressures produced by the shock front. Figure 8-7 shows the direct and reflected shock waves.

Figure 8-7.-Direct and reflected shock waves for an underwater burst. 8-5

Four factors determine whether the greater damage will be caused by the direct wave or the reflected wave: . Distance from burst . Depth of burst . Depth of water . Bottom configuration and structure

When the point of detonation is above the bottom, the shock wave reflected from the bottom can produce more severe damage to weapondelivery equipment at a given range than the direct shock wave. Even though the peak pressure of the reflected wave is less than that of the direct wave, the reflected wave will disperse in a more nearly vertical direction. It is, therefore, more effective in producing the vertical motions that control the degree of damage.

The time separation between direct and reflected shock waves decreases as the point of detonation approaches the bottom. When the point of detonation is directly on the bottom, the two waves overlap. For such a burst, the water depth has a direct effect on the range at which the weapon-delivery capability of surface ships will be impaired. However, the water depth has no significant effect on the ranges at which their mobility and seaworthiness will be impaired. Where the sea bottom is sloped, a ship downslope from the point of detonation will tend to receive less damage than a ship an equal distance upslope from the point of detonation. Where the sea bottom is essentially flat, the strength of a reflected wave will depend on the bottom structure. It will be less for mud than for sand, but greater for rock than for sand.