Editing Nuclear fallout (section)

==Factors affecting fallout==
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===Location===
There are two main considerations for the location of an explosion: height and surface composition. A nuclear weapon detonated in the air, called an [[air burst]], produces less fallout than a comparable explosion near the ground. A nuclear explosion in which the fireball touches the ground pulls soil and other materials into the cloud and neutron activates it before it falls back to the ground.  An air burst produces a relatively small amount of the highly radioactive heavy metal components of the device itself.

In case of water surface bursts, the particles tend to be rather lighter and smaller, producing less local fallout but extending over a greater area. The particles contain mostly [[sea salt]]s with some water; these can have a [[cloud seeding]] effect causing local [[Rainout (radioactivity)|rainout]] and areas of high local fallout. Fallout from a [[seawater]] burst is difficult to remove once it has soaked into [[porous]] surfaces because the fission products are present as metallic [[ion]]s that chemically bond to many surfaces. Water and detergent washing effectively removes less than 50% of this chemically bonded activity from [[concrete]] or [[steel]]. Complete decontamination requires aggressive treatment like [[sandblasting]], or acidic treatment. After the ''Crossroads'' underwater test, it was found that wet fallout must be immediately removed from ships by continuous water washdown (such as from the [[fire sprinkler]] system on the decks).

Parts of the sea bottom may become fallout. After the [[Castle Bravo]] test, white dust—contaminated [[calcium oxide]] particles originating from pulverized and [[calcination|calcined]] [[coral]]s—fell for several hours, causing [[beta burn]]s and radiation exposure to the inhabitants of the nearby atolls and the crew of the ''[[Daigo Fukuryū Maru]]'' fishing boat. The scientists called the fallout ''Bikini snow''.

For subsurface bursts, there is an additional phenomenon present called "[[base surge]]". The base surge is a cloud that rolls outward from the bottom of the subsiding column, which is caused by an excessive density of dust or water droplets in the air. For underwater bursts, the visible surge is, in effect, a cloud of liquid (usually water) droplets with the property of flowing almost as if it were a homogeneous fluid. After the water evaporates, an invisible base surge of small radioactive particles may persist.

For subsurface land bursts, the surge is made up of small solid particles, but it still behaves like a [[fluid]]. A soil earth medium favors base surge formation in an underground burst. Although the base surge typically contains only about 10% of the total bomb debris in a subsurface burst, it can create larger radiation doses than fallout near the detonation, because it arrives sooner than fallout, before much radioactive decay has occurred.

===Meteorological===
[[Image:Fallout G&D77.JPG|right|300px|thumb|Comparison of fallout gamma dose and dose rate contours for a 1 Mt fission land surface burst, based on DELFIC calculations. Because of radioactive decay, the dose rate contours contract after fallout has arrived, but dose contours continue to grow]]

[[Meteorology|Meteorological]] conditions greatly influence fallout, particularly local fallout. Atmospheric winds are able to bring fallout over large areas.<ref>{{cite web|title= Continental US Fallout Pattern for Prevailing Winds (FEMA-196/September 1990) |url= http://ocw.nd.edu/physics/nuclear-warfare/notes/lecture-17 |website= [[University of Notre Dame]] |url-status= dead |archive-url= https://web.archive.org/web/20110315084044/http://ocw.nd.edu/physics/nuclear-warfare/notes/lecture-17 |archive-date= March 15, 2011}}</ref> For example, as a result of a ''[[Castle Bravo]]'' surface burst of a 15 Mt thermonuclear device at [[Bikini Atoll]] on 1 March 1954, a roughly cigar-shaped area of the [[Pacific]] extending over 500&nbsp;km downwind and varying in width to a maximum of 100&nbsp;km was severely contaminated. There are three very different versions of the fallout pattern from this test, because the fallout was measured only on a small number of widely spaced Pacific Atolls. The two alternative versions both ascribe the high radiation levels at north [[Rongelap]] to a downwind hot spot caused by the large amount of radioactivity carried on fallout particles of about 50–100 micrometres size.<ref>{{cite journal|publisher=General Electric Company|editor=Howard A. Hawthorne|title=COMPILATION OF LOCAL FALLOUT DATA FROM TEST DETONATIONS 1945–1962 – EXTRACTED FROM DASA 1251 – Volume II – Oceanic U.S. Tests|date=May 1979| url=http://worf.eh.doe.gov/data/ihp1d/1362e.pdf| archive-url=https://web.archive.org/web/20080410131321/http://worf.eh.doe.gov/data/ihp1d/1362e.pdf| archive-date=2008-04-10}}
</ref>

After ''Bravo'', it was discovered that fallout landing on the ocean disperses in the top water layer (above the [[thermocline]] at 100 m depth), and the land equivalent dose rate can be calculated by multiplying the ocean dose rate at two days after burst by a factor of about 530. In other 1954 tests, including ''Yankee'' and ''Nectar,'' hot spots were mapped out by ships with submersible probes, and similar hot spots occurred in 1956 tests such as ''Zuni'' and ''Tewa''.
<ref>{{cite journal|publisher=US Naval Radiological Defense Laboratory|author=Project Officer T. Triffet, P. D. LaRiviere|title=OPERATION REDWING – Project 2.63, Characterization of Fallout – Pacific Proving Grounds, May–July 1956|date=March 1961|url=http://worf.eh.doe.gov/data/ihp1c/0881_a.pdf|archive-url=https://web.archive.org/web/20080410131256/http://worf.eh.doe.gov/data/ihp1c/0881_a.pdf|archive-date=2008-04-10}}
</ref> However, the major U.S. "[[DELFIC]]" (Defence Land Fallout Interpretive Code) computer calculations use the natural size distributions of particles in soil instead of the [[afterwind]] sweep-up spectrum, and this results in more straightforward fallout patterns lacking the downwind hot spot.

[[Snow]] and [[rain]], especially if they come from considerable heights, accelerate local fallout. Under special meteorological conditions, such as a local rain shower that originates above the radioactive cloud, limited areas of heavy contamination just downwind of a nuclear blast may be formed.