Scaling Law - A mathematical relationship which permits the effects of a nuclear (or atomic) explosion of given energy yield to be determined as a function of distance from the explosion (or from ground zero), provided the corresponding effect is known as a function of distance for a reference explosion (e.g., of 1-kiloton energy yield). See Blast scaling law, Cube root law.
Scattering - The diversion of radiation, including radio, radar, thermal and nuclear, from its original path as a result of interactions (or collisions) with atoms, molecules or larger particles in the atmosphere or other medium between the source of the radiations (i.e., a nuclear explosion) and a point at some distance away. As a result of scattering, radiations (especially gamma rays and neutrons) will be received at such a point from many directions instead of only from the direction of the source.
Scavenging - The selective removal of material from the radioactive cloud from a nuclear explosion by inert substances, such as earth and water, introduced into the fireball. The term is also applied to the process of removal of fallout particles from the atmosphere by precipitation. See Rainout, Snowout, Washout.
Shear (Wind) - Unless the term "velocity shear" is used, wind shear refers to differences in direction (directional shear) of the wind at different altitudes.
Shear Wall - A wall, or partition designed to take a load in the direction of the plane of the wall, as distinct from lateral loads perpendicular to the wall. Shear walls may be designed to take lateral loads as well. See Bearing wall.
Shielding - Any material or obstruction which absorbs (or attenuates) radiation and thus tends to protect people or materials from the effects of a nuclear (or atomic) explosion. A moderate thick layer of opaque material will provide satisfactory shielding from thermal radiation, but a considerable thickness of material of high density may be needed for nuclear radiation shielding. Electrically continuous housing for a facility, area, or component, attenuates impinging electric and magnetic fields.
Shock Front (or Pressure Front) - The fairly sharp boundary between the pressure disturbance created by an explosion (in air, water, or earth) and the ambient atmosphere, water or earth respectively. It constitutes the front of the shock (or blast) wave. See Shock wave.
Shock Wave - A continuously propagated pressure pulse (or wave) in the surrounding medium which may be air, water, or earth, initiated by the expansion of the hot gases produced in an explosion. A shock wave in air is generally referred to as blast wave, because it resembles and is accompanied by strong, but transient, winds. The duration of a shock (or blast) wave is distinguished by two phases: First there is the positive (compression) phase during which the pressure rises very sharply to a value that is higher than ambient and then decreases rapidly to the ambient pressure. The positive phase for the dynamic pressure is somewhat longer that for overpressure, due to the momentum of the moving air behind the shock front. The duration of the positive phase increases and the maximum (peak) pressure decreases with increasing distance from an explosion of given energy yield. In the second phase, the negative (suction, rarefaction, or tension) phase, the pressure falls below ambient and then returns to ambient value. The duration of the negative phase may be several times the duration of the positive phase. Deviations from the ambient pressure during the negative phase are never large and they decrease with increasing distance from the explosion. See Dynamic pressure, Overpressure.
Skyshine - Radiation, particularly gamma rays from a nuclear explosion, reaching a target from many directions as a result of scattering by the oxygen and nitrogen in the intervening atmosphere.
Slant Range - The distance from a given location, usually on the earth's surface, to the point at which the explosion occurred.
Slick - The trace of an advancing shock wave seen on the surface of reasonably calm water as a circle of rapidly increasing size apparently darker that the surrounding water. It is observed, in particular, following an underwater explosion. See Crack.
Snowout - The removal of radioactive particles from a nuclear cloud by precipitation when this cloud is within the snow cloud. See Rainout.
Spray Dome - See Dome.
Stopping Altitude - The altitude in the vicinity of which a specified ionizing radiation coming from above (i.e., from an high altitude nuclear explosion) deposits most of its energy by absorption in the atmosphere. The stopping altitude varies with the nature of the ionizing radiation.
Stratosphere - A relatively stable layer of the atmosphere between the tropopause and a height of about 30 miles in which temperature changes very little (in polar and temperate zones) or increases (in the tropics) with increasing altitude. In the stratosphere clouds of water never form and there is practically no convection. See Tropopause, Troposphere.
Subsurface Burst - See Underground burst, Underwater burst.
Supercritical - A term used to describe the state of a given fission system when the quantity of fissionable material is greater than the critical mass under the existing conditions. A highly supercritical system is essential for the production of energy at a very rapid rate so that an explosion may occur.
Surface Burst - The explosion of a nuclear (or atomic) weapon at the surface of the land or water at a height above the surface less that the radius of the fireball at maximum luminosity (in the second thermal pulse). An explosion in which a weapon is detonated actually on the surface (or within 5W0.3 feet, where W is the explosion yield in kilotons, above or below the surface) is called a contact surface burst or a true surface burst. See Air burst.
Surface Zero - See Ground zero.
Surge (or Surge Phenomena) - See Base surge.
Survey Meter - A portable instrument, such as a Geiger counter or ionization chamber, used to detect nuclear radiation and to measure the dose rate. See Monitoring.
Syndrome, Radiation - The complex of symptoms characterizing the disease known as radiation injury, resulting from excessive exposure of the whole (or large part) of the body to ionizing radiation. The earliest of these symptoms are nausea, vomiting and diarrhea, which may be followed by loss of hair (epilation), hemorrhage, inflammation of the mouth and throat, and general loss of energy. In severe cases, where the radiation exposure has been relatively large, death may occur within 2 to 4 weeks. Those who survive 6 weeks after the receipt of a single dose of radiation may generally be expected to recover.
Tenth-Value Thickness - The thickness of a given material which will decrease the intensity (or dose) of gamma radiation to one-tenth the amount incident on it. Two tenth value-thickness will reduce the dose received by a factor of 10x10, i.e., 100, and so on. The tenth-value thickness of a given material depends on the gamma-ray energy, but for radiation of a particular energy it is roughly inversely proportional to the density of the material.
Tests - See Nuclear Tests.
Thermal Energy - The energy emitted from the fireball (or other heated region) as thermal radiation. The total amount of thermal energy received per unit area at a specified distance from a nuclear (or atomic) explosion is generally expressed in terms of calories per square centimeter . See Radiant exposure, Thermal radiation, Transmittance, X-ray pancake.
Thermal Energy Yield (or Thermal Yield) - The part of the total energy yield of the nuclear (or atomic) explosion which is received as thermal energy usually within a minute or less. In an air burst, the thermal partition,( i.e., the fraction of the total explosion energy emitted as thermal radiation) ranges from about 0.35 to 0.45. The trend is toward the smaller fraction for low yields or low burst height and toward the higher fraction at high yields or high bursts. Above 100,000 feet burst height, the fraction increases from about 0.45 to 0.6, and then decreases to about 0.25 at burst altitudes of 160,000 to 260,000 feet. At still greater burst height, the fraction decreases rapidly with increasing altitude.
Thermal Radiation - Electromagnetic radiation emitted (in two pulses from an air burst) from the fireball as a consequence of its very high temperature; it consists essentially of ultraviolet, visible, and infrared radiations. In the early stages (first pulse of an air burst), when the temperature of the fireball is extremely high, the ultraviolet radiation predominates; in the second pulse, the temperatures are lower and most of the thermal radiation lies in the visible and infrared regions of the spectrum. For high-altitude bursts (above 100,000 feet), the thermal radiation is emitted as a single pulse, which is of short duration below about 270,000 feet. but increases at greater burst heights.
Thermal X-Rays - The electromagnetic radiation, mainly in the soft ( (low energy) X-ray region, emitted by the extremely hot weapon residue in virtue of its extremely high temperature; it is also referred to as the primary thermal radiation. It is the absorption of this radiation by the ambient medium, accompanied by an increase in temperature, which results in the formation of the fireball (or other heated region) which then emits thermal radiation. See Weapon residue, X-ray pancake, X-rays.
Thermonuclear - An adjective referring to the process (or processes) in which very high temperatures are used to bring about the fusion of light nuclei, such as those of the hydrogen isotopes (deuterium and tritium), with the accompanying liberation of energy. A thermonuclear bomb is a weapon in which part of the explosion energy results from thermonuclear fusion reactions. The high temperatures required are obtained by means of a fission explosion. See Fusion.
TNT Equivalent - A measure of the energy released in the detonation of a nuclear (or atomic) weapon, or in the explosion of a given quantity of fissionable material, expressed in terms of the mass of TNT (trinitrotoluene) which would release the same amount of energy when exploded. The TNT equivalent is usually stated in kilotons or megatons. The basis of the TNT equivalence is that the explosion of one ton of TNT is assumed to release 109 calories of energy. See Kiloton, Megaton. Yield.
Transmittance (Atmospheric) -The fraction (or percentage) of the thermal energy received at a given location after passage through the atmosphere relative to that which would have been received at the same location if no atmosphere were present.
- Having an atomic number grater than that of uranium (The known elements belonging to the actinide series).
Triple Point - The intersection of the incident, reflected, and merged (or Mach) shock fronts accompanying an air burst. The height of the triple point above the surface (i.e., the height of the Mach stem) increases with increasing distance from a given explosion. See Mach stem.
Tritium - A radioactive isotope of hydrogen having a mass of 3 units; it is produced in nuclear reactors by the action of neutrons on lithium nuclei. Tritium is a beta emitter.
Tropopause - The imaginary boundary layer dividing the stratosphere from the lower part of the atmosphere, the troposphere. The tropopause normally occurs at an altitude of about 25,000 to 45,000 feet in polar and temperate zones, and at 55,000 feet in the tropics. See Stratosphere, Troposphere.
Troposphere - The region of the atmosphere, immediately above the earth's surface and up to the tropopause, in which the temperature falls fairly regularly with increasing altitude, clouds form, convection is active, and mixing is continuous and more or less complete.
True Surface Burst - See Surface burst.
Tuballoy (TU) - A term, of British origin, for uranium metal containing U-238 and U-235 in natural proportions, therefore, the term is considered ambiguous and its use is discouraged. This term is sometimes applied to depleted uranium.See uranium.
2W Concept - The concept that the explosion of a weapon of energy yield W on the earth's surface produces (as a result of reflection) blast phenomena identical to those produced by a weapon twice the yield (i.e., 2W) burst in free air (i.e., away from any reflecting surface).
Ultraviolet - Electromagnetic radiation of wave length between the shortest visible violet (about 3,850 Angstroms) and soft X-rays (about 100 Angstroms).
Underground Burst - The explosion of a nuclear (or atomic) weapon with its center more than 5W0.3 feet, where W is the explosion yield in kilotons, beneath the surface of the ground. See also Contained underground burst.
Underwater Burst - The explosion of a nuclear (or atomic) weapon with its center beneath the surface of the water.
Uranium - Uranium is a heavy, silvery white, radioactive metal. In air, the metal becomes coated with a layer of oxide that will make it appear from a golden-yellow color to almost black. Uranium is an alpha emitter.
Visibility Range (or Visibility) - The horizontal distance (in kilometers or miles) at which a large dark object can just be seen against the horizon sky in daylight. The visibility is related to the clarity of the atmosphere ranging from 170 miles (280 kilometers) for an exceptionally clear atmosphere to 0.6 mile (1.0 kilometer) or less for dense haze or fog. The visibility on an average clear day is taken to be 12 miles (19 kilometers).
Warhead - The part of a missile, projectile, torpedo, rocket or other munition which contains either the nuclear or thermonuclear system, high explosive system, chemical or biological agents, or inert materials intended to inflict damage.
Washout - The removal of radioactive particles from a nuclear cloud by precipitation when this cloud is below a rain (or snow) cloud. See Rainout, Snowout.
Weapon, Atomic (or Nuclear) - See Nuclear weapon.
Weapon Debris -The highly radioactive material, consisting of fission products, various products of neutron capture, and uranium and plutonium that have escaped fission, remaining after the explosion.
Weapon Residue - The extremely hot, compressed gaseous residues formed at the instant of the explosion of a nuclear weapon. The temperature is several tens of million degrees (Kelvin) and the pressure is many millions of atmospheres.
Wilson Cloud Chamber - See Condensation cloud.
Worldwide Fallout - See Fallout.
X-Ray Pancake - A layer of air, about 30,000 feet thick at a mean altitude of roughly 270,000 feet, which becomes incandescent by absorption of the thermal X rays from explosions above 270,000 feet altitude. The heated air emits thermal radiation (of longer wavelengths) in a single pulse of several seconds duration. See Thermal radiation, Thermal X rays.
X Rays - Electromagnetic radiations of high energy having wavelengths shorter than those in the ultraviolet region, i.e., less than 10-6 centimeters or 100 Angstroms. Materials at very high temperatures (millions of degrees) emit such radiations; they are the called thermal X rays. As generally produced by X-ray machines, they are bremsstrahlung resulting from the interaction of electrons of 1 kilo-electron volt or more energy with a metallic target. See Bremsstrahlung, Electromagnetic radiation, Thermal X-rays.
Yield (or Energy Yield) - The total effective energy released in a nuclear (or atomic) explosion. It is usually expressed in terms of the equivalent tonnage of TNT required to produce the same energy release in an explosion. The total energy yield is manifested as nuclear radiation, thermal radiation, and shock (and blast) energy, the actual distribution being dependent upon the medium in which the explosion occurs (primarily) and also upon the type of weapon and the time after detonation. See TNT equivalent.

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