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An underwater explosion, also known as an UNDEX, is an explosion beneath the surface of water. The type of explosion may be chemical or nuclear. They are categorized in accordance with their depth beneath the water's surface, because this has a strong influence on their effects.

Contents

Effects

The effects of an underwater explosion depend on a number of things, including distance from the explosion, the energy of the explosion, the depth of the explosion, and the depth of the water.[1]

Underwater explosions are categorized by the depth of the explosion. Shallow underwater explosions are those where a crater formed at the water's surface is large in comparison with the depth of the explosion. Deep underwater explosions are those where the crater is small in comparison with the depth of the explosion.[1]

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Shallow underwater explosion

The BAKER test, just after the chimney had broken through the cloud, and the crack had formed on the water's surface.

An example of a shallow underwater explosion is the BAKER nuclear test at Bikini Atoll in July 1946, which was part of Operation Crossroads. A 20 kiloton warhead was detonated in a lagoon which was approximately 200 ft (61 m) deep. The first effect was illumination of the water because of the underwater fireball. A rapidly expanding gas bubble created a shock wave that caused an expanding ring of apparently dark water at the surface, called the slick, followed by an expanding ring of apparently white water, called the crack. A mound of water and spray, called the spray dome, formed at the water's surface which became more columnar as it rose. When the rising gas bubble broke the surface, it created a shock wave in the air as well. Water vapor in the air condensed as a result of a Prandtl-Glauert singularity, making a spherical cloud that marked the location of the shock wave. Water filling the cavity formed by the bubble caused a hollow column of water, called the chimney or plume, to rise 6,000 ft (1,800 m) in the air and break through the top of the cloud. A series of surface waves moved outwards from the center. The first wave was about 94 ft (29 m) high at 1,000 ft (300 m) from the center. Other waves followed, and at further distances some of these were higher than the first wave. For example, at 22,000 ft (6,700 m) from the center, the ninth wave was the highest at 6 ft (1.8 m). Gravity caused the column to fall to the surface and caused a cloud of mist to move outwards rapidly from the base of the column, called the base surge. The ultimate size of the base surge was 3.5 mi (5.6 km) in diameter and 1,800 ft (550 m) high. The base surge rose from the surface and merged with other products of the explosion, to form clouds which produced moderate to heavy rainfall for nearly one hour.[2]

Deep underwater explosion

The WIGWAM test, at the time of the formation of the spray dome

An example of a deep underwater explosion is the WAHOO test, which was carried out in 1958 as part of Operation Hardtack. The nuclear device was detonated at a depth of 500 ft (150 m) in deep water. There was little evidence of a fireball. The spray dome rose to a height of 900 ft (270 m). Gas from the bubble broke through the spray dome to form jets which shot out in all directions and reached heights of up to 1,700 ft (520 m). The base surge at its maximum size was 2.5 mi (4.0 km) in diameter and 1,000 ft (300 m) high.[2]

The heights of surface waves generated by deep underwater explosions are greater because more energy is delivered to the water. Deep underwater explosions are thus particularly able to damage coastal areas, because surface waves increase in height as they move over shallow water, and can flood the land beyond the shoreline.[3] Many of the theories and concepts about these waves are similar to those that are applicable to other types of surface waves, in particular, tsunamis, and waves generated by the fall of a meteor.[1]

If a deep underwater explosion occurs at a sufficient depth, the rising gas bubble can over expand because the gas pressure falls below the pressure of the surrounding water. This causes the bubble to collapse, which causes a second shock wave and bubble expansion. This may be repeated, though there are unlikely to be more than three expansions. An example is the WIGWAM test, which was carried out in 1955. The nuclear device was detonated at a depth of 2,000 ft (610 m).[2]

Other effects

The detonation of an explosive charge underwater results in an initial high-velocity shockwave through the water, in movement or displacement of the water itself and in the formation of a high-pressure bubble of high-temperature gas. This bubble expands rapidly until it either vents to the surface or until its internal pressure is exceeded by that of the water surrounding it. (The volumetric expansion of the bubble also leads to a drop in internal temperature in accordance with Charles’ Law.) At this point, as noted above, the overexpanded bubble collapses into itself, leading again to a rise in bubble pressure and internal temperature until such time as the bubble pressure exceeds water pressure. The bubble again expands, although to a rather smaller size. A second shockwave is produced by this expansion, although it will be less intense and of rather greater duration than the first. With each cycle, the bubble moves upwards until it eventually vents or dissipates into a mass of smaller bubbles. The number of cycles, while generally low, is difficult to predict; they and the overall effects, depend on explosion depth (and thus water pressure), the size and nature of the explosive charge and the presence, composition and distance of reflecting surfaces such as the seabed, surface, thermoclines, etc. This phenomenon has been extensively used in antiship warhead design since an underwater explosion (particularly one underneath a hull) can produce greater damage than an above-surface one of the same explosive size. Initial damage to a target will be caused by the first shockwave; this damage will be amplified by the subsequent physical movement of water and by the repeated secondary shockwaves or bubble pulse. Additionally, charge detonation away from the target can result in damage over a larger hull area.[4]

See also

Sources

  1. ^ a b c Le Méhauté, Bernard; Wang, Shen (1995). Water waves generated by underwater explosion. World Scientific Publishing. ISBN 981-02-2083-9.  
  2. ^ a b c Glasstone, Samuel; Dolan, Philip (1977). "Descriptions of nuclear explosions". The effects of nuclear weapons (Third Edition ed.). Washington: U.S. Department of Defense; Energy Research and Development Administration. http://www.princeton.edu/~globsec/publications/effects/effects.shtml.  
  3. ^ Glasstone, Samuel; Dolan, Philip (1977). "Shock effects of surface and subsurface bursts". The effects of nuclear weapons (Third Edition ed.). Washington: U.S. Department of Defense; Energy Research and Development Administration. http://www.princeton.edu/~globsec/publications/effects/effects.shtml.  
  4. ^ RMCS Precis on Naval Ammunition, Jan 91

Further reading


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