Processes
Uranium Mining, Milling, and Refining
EXPLOSION
Processes > Bomb Testing and Weapon Effects
A fission explosion occurs when the fissionable material (such as uranium-235 or plutonium) within the core of a nuclear bomb reaches critical mass and a fission chain reaction begins. This releases a large amount of energy, which vaporizes the bomb assembly and heats the gases trapped within. Energy is released in the form of x-rays. These x-rays travel a short distance and heat the air around them, which in turn emits less powerful x-rays. The mass of gas that had been initially trapped in the bomb assembly rapidly expands into a roughly spherical fireball, brighter than the sun. At Trinity, where the bomb had been detonated at the top of a 100-foot tower, the fireball's radius reached the ground in about two milliseconds. After about 30 milliseconds the fireball approached a diameter of 1000 feet. After about 100 milliseconds, or one ten-thousandth of a second, a shock wave formed and traveled out from the center of the explosion. One second after the detonation at Trinity the shock wave had traveled about 3,000 feet from the center, and after two seconds the fireball measured 2,000 feet across. 
The cooling of the fireball from a nuclear explosion is accompanied by condensation of the vapors and the formation of the mushroom cloud. This cloud initially contains solid particles of bomb debris together with small drops of moisture condensed from the air. At first it takes on a red or reddish brown color due to the formation of nitrous acid and nitrogen oxides at the surface of the fireball, but as the cloud cools the color changes to white. Depending upon the height of the explosion and the nature of the terrain below, a strong updraft with inflowing winds may be produced, causing varying amounts of dirt and debris from the ground to be sucked up into the cloud, accounting for the stem shape of the familiar mushroom cloud. At first the particulate matter (now consisting of dirt and ground debris as well as bomb debris) continues upward with the cloud, but in due course it begins to settle out. The heavier particles settle to the ground as "early" radioactive fallout. The lighter radioactive particles behave as smoke and, following the prevailing air currents, extend the base of the cloud into a drifting lateral column, from which dust-like particles gradually settle to the ground ("tropospheric" fallout). The mushroom generally remains visible for about an hour before it is dispersed by currents in the atmosphere. The particulate matter still contained can be widely distributed in the wind ("stratospheric" radioactive fallout).
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The text for this page is original to the Department of Energy's Office of History and Heritage Resources. Major sources consulted include the following. John F. Hogerton, ed., "Weapons Phenomenology," The Atomic Energy Deskbook (New York: Reinhold Publishing Corporation, 1963; prepared under the auspices of the Division of Technical Information, U.S. Atomic Energy Commission), pages 604-608. For a detailed discussion of the first milliseconds of a nuclear explosion, see Richard Rhodes, The Making of the Atomic Bomb (New York: Simon and Schuster, 1986), 670-672. For information about the photograph of the first fractions of a second of an explosion, click here. The photograph of Able is courtesy of the Federation of American Scientists.