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The extreme heat and pressure of an underground nuclear explosion causes changes in the surrounding rock. The rock closest to the location of the test is vaporised, forming a cavity. Farther away, there are zones of crushed, cracked, and irreversibly strained rock. Following the explosion, the rock above the cavity may collapse, forming a rubble chimney. If this chimney reaches the surface, a bowl-shaped subsidence crater may form.
The first underground test took place in 1951; further tests provided information that eventually led to the signing of the Limited Test Ban Treaty in 1963, which banned all nuclear tests except for those performed underground. From then until the signing of the Comprehensive Test Ban Treaty in 1996, most nuclear tests were performed underground, in order to prevent nuclear fallout from entering into the atmosphere.
Contents
Background
Although public concern about fallout from nuclear testing grew in the early 1950s,[1][2] fallout was discovered after the Trinity test in 1945.[2] Photographic film manufacturers would later report 'fogged' films, these were traced both to Trinity and later tests at the Nevada Test Site.[2] Intense fallout from the 1953 Simon test was documented as far as Albany, New York.[2]The fallout from the March 1954 Bravo test in the Pacific would have "scientific, political and social implications that have continued for more than 40 years."[3] The multi-megaton test caused fallout to occur on the islands of Rongerik and Rongelap, and a Japanese fishing boat known as the Daigo Fukuryƫ Maru (Lucky Dragon).[3] Prior to this test, there was "insufficient" appreciation of the dangers of fallout.[3]
The test became an international incident. In a PBS interview, the historian Martha Smith argued: "In Japan, it becomes a huge issue in terms of not just the government and its protest against the United States, but all different groups and all different peoples in Japan start to protest. It becomes a big issue in the media. There are all kinds of letters and protests that come from, not surprisingly, Japanese fishermen, the fishermen's wives; there are student groups, all different types of people; that protest against the Americans' use of the Pacific for nuclear testing. They're very concerned about, first of all, why the United States even has the right to be carrying out those kinds of tests in the Pacific. They're also concerned about the health and environmental impact."[4] The Prime Minister of India "voiced the heightened international concern" when he called for the elimination of all nuclear testing worldwide.[1]
Knowledge about fallout and its effects grew, and with it concern about the global environment and long-term genetic damage.[5] Talks between the United States, the United Kingdom, Canada, France, and the Soviet Union began in May 1955 on the subject of an international agreement to end nuclear tests.[5] On August 5, 1963, representatives of the United States, the Soviet Union, and the United Kingdom signed the Limited Test Ban Treaty, forbidding testing of nuclear weapons in the atmosphere, in space, and underwater.[6] Agreement was facilitated by the decision to allow underground testing, eliminating the need for on-site inspections that concerned the Soviets.[6] Underground testing was allowed, provided that it does not cause "radioactive debris to be present outside the territorial limits of the State under whose jurisdiction or control such explosion is conducted."[5]
Early history of underground testing
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The examples and perspective in this article deal primarily with the United States and do not represent a worldwide view of the subject. (December 2010) |
On 26 July 1957, Plumbbob Pascal-A was detonated at the bottom of a 485-foot shaft.[17][18] According to one description, it "ushered in the era of underground testing with a magnificent pyrotechnic Roman candle!"[19] As compared with an above-ground test, the radioactive debris released to the atmosphere was reduced by a factor of ten.[19] Theoretical work began on possible containment schemes.[19]
Cannikin, the last test at the Amchitka facility was detonated on 6 November 1971. At approximately 5 megatons, it was the largest underground test in US history.[24]
Effects
| Name | Radius[26] |
|---|---|
| Melt cavity | 4 – 12 m/kt1/3 |
| Crushed zone | 30 – 40 m/kt1/3 |
| Cracked zone | 80 – 120 m/kt1/3 |
| Zone of irreversible strain | 800 – 1100 m/kt1/3 |
Several minutes to days later, once the heat dissipates enough, the steam condenses, and the pressure in the cavity falls below the level needed to support the overburden, the rock above the void falls into the cavity, creating a rubble chimney. Depending on various factors, including the yield and characteristics of the burial, this collapse may extend to the surface. If it does, a subsidence crater is created.[26] Such a crater is usually bowl-shaped, and ranges in size from a few tens of metres to over a kilometre in diameter.[26] At the Nevada Test Site, 95 percent of tests conducted at a scaled depth of burial (SDOB) of less than 150 caused surface collapse, compared with about half of tests conducted at a SDOB of less than 180.[26] The radius r (in feet) of the cavity is proportional to the cube root of the yield y (in kilotons), r = 55 *
![\sqrt[3]{y}](http://upload.wikimedia.org/math/f/6/1/f618f2509ef261b8ef52b213392a7f43.png)
; a 8 kiloton explosion will create a cavity with radius of 110 feet.[28]
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The released nuclides can undergo bioaccumulation. Iodine-131, strontium-90 and caesium-137 are concentrated in milk of grazing cows; cow milk is therefore a convenient, sensitive fallout indicator. Soft tissues of animals can be analyzed for gamma emitters, bones and liver for strontium and plutonium, and blood, urine and soft tissues are analyzed for tritium.[28]
Although there were early concerns about earthquakes arising as a result of underground tests, there is no evidence that this has occurred.[25] However, fault movements and ground fractures have been reported, and explosions often precede a series of aftershocks, thought to be a result of cavity collapse and chimney formation. In a few cases, seismic energy released by fault movements has exceeded that of the explosion itself.[25]
International treaties
Signed in Moscow on August 5, 1963 by representatives of the United States, the Soviet Union, and the United Kingdom, the Limited Test Ban Treaty agreed to ban nuclear testing in the atmosphere, in space, and underwater.[6] Due to the Soviet government's concern about the need for the on-site inspections, underground tests were excluded from the ban.[6] 108 countries would eventually sign the treaty, with the significant exception of China.[29]In 1974, the United States and the Soviet Union signed the Threshold Test Ban Treaty, which banned underground tests with yields greater than 150 kilotons.[30] By the 1990s, technologies to monitor and detect underground tests had matured to the point that tests of one kiloton or over could be detected with high probability, and in 1996 negotiations began under the auspices of the United Nations to develop a comprehensive test ban.[29] The resulting Comprehensive Nuclear-Test-Ban Treaty was signed in 1996 by the United States, Russia, United Kingdom, France, and China.[29] However, following the United States Senate decision not to ratify the treaty in 1999, it is still yet to be ratified by 8 of the required 44 'Annex 2' states and so has not entered into force as United Nations law.
Monitoring
In the late 1940s, the United States began to develop the capability to detect atmospheric testing using air sampling; this system was able to detect the first Soviet test in 1949.[30] Over the next decade, this system was improved, and network of seismic monitoring stations was established to detect underground tests.[30] Development of the Threshold Test Ban Treaty in the mid-1970s led to an improved understanding of the relationship between test yield and resulting seismic magnitude.[30]When negotiations began in the mid-1990s to develop a comprehensive test ban, the international community was reluctant to rely upon the detection capabilities of individual nuclear weapons states (especially the United States), and instead wanted an international detection system.[30] The resulting International Monitoring System consists of a network of a total of 321 monitoring stations and 16 radionuclide laboratories.[31] Fifty "primary" seismic stations send data continuously to the International Data Center, along with 120 "auxiliary" stations which send data on request. The resulting data is used to locate the epicentre, and distinguish between the seismic signatures of an underground nuclear explosion and an earthquake.[30][32] Additionally, eighty radionuclide stations detect radioactive particles vented by underground explosions. Certain radionuclides constitute clear evidence of nuclear tests; the presence of noble gases can indicate whether an underground explosion has taken place.[33] Finally, eleven hydroacoustic stations[34] and sixty infrasound stations[35] monitor underwater and atmospheric tests.
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