Effects of nuclear explosions

The effects of a nuclear explosion on its immediate vicinity are typically much more destructive and multifaceted than those caused by conventional explosives. In most cases, the energy released from a nuclear weapon detonated within the lower atmosphere can be approximately divided into four basic categories:[1]

The 14-kiloton test shot Charlie of Operation Buster–Jangle at the Nevada Proving Grounds on October 30, 1951. The red/orange color seen here in the cap of the mushroom cloud is largely due to the fireball's intense heat in combination with the oxygen and nitrogen naturally found in air. Oxygen and nitrogen, though generally unreactive toward each other, form NOx species when heated to excess, specifically nitrogen dioxide, which is largely responsible for the color. There was concern in the 1970s and 1980s, later proven unfounded, regarding fireball NOx and ozone loss.

Depending on the design of the weapon and the location in which it is detonated, the energy distributed to any one of these categories may be significantly higher or lower. The physical blast effect is created by the coupling of immense amounts of energy, spanning the electromagnetic spectrum, with the surroundings. The environment of the explosion (e.g. submarine, ground burst, air burst, or exo-atmospheric) determines how much energy is distributed to the blast and how much to radiation. In general, surrounding a bomb with denser media, such as water, absorbs more energy and creates more powerful shockwaves while at the same time limiting the area of its effect. When a nuclear weapon is surrounded only by air, lethal blast and thermal effects proportionally scale much more rapidly than lethal radiation effects as explosive yield increases. This bubble is faster than the speed of sound.[2] The physical damage mechanisms of a nuclear weapon (blast and thermal radiation) are identical to those of conventional explosives, but the energy produced by a nuclear explosion is usually millions of times more powerful per unit mass and temperatures may briefly reach the tens of millions of degrees.

Energy from a nuclear explosion is initially released in several forms of penetrating radiation. When there is surrounding material such as air, rock, or water, this radiation interacts with and rapidly heats the material to an equilibrium temperature (i.e. so that the matter is at the same temperature as the fuel powering the explosion). This causes vaporization of the surrounding material, resulting in its rapid expansion. Kinetic energy created by this expansion contributes to the formation of a shockwave which expands spherically from the center. Intense thermal radiation at the hypocenter forms a nuclear fireball which, if the explosion is low enough in altitude, is often associated with a mushroom cloud. In a high-altitude burst, where the density of the atmosphere is low, more energy is released as ionizing gamma radiation and X-rays than as an atmosphere-displacing shockwave.

In 1942, there was some initial speculation among the scientists developing the first nuclear weapons in the Manhattan Project that a large enough nuclear explosion might ignite the Earth's atmosphere. This notion concerned the nuclear reaction of two atmospheric nitrogen atoms forming carbon and an oxygen atom, with an associated release of energy. The scientists hypothesized that this energy would heat up the remaining atmospheric nitrogen enough to keep the reaction going until all nitrogen atoms were consumed, thereby burning all of the Earth's atmosphere (which is composed of nearly 80% diatomic nitrogen) in one single massive combustion event. Hans Bethe was assigned the task of studying this hypothesis from the project's earliest days, and eventually concluded that combustion of the entire atmosphere was not possible: the cooling of the fireball due to an inverse Compton effect all but guaranteed that such a scenario would not become a reality.[3] Richard Hamming, a mathematician, was asked to make a similar calculation just before the first nuclear test, with the same result.[4] Nevertheless, the notion has persisted as a rumor for many years and was the source of apocalyptic gallows humor at the Trinity test.


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