Underground nuclear testing refers to test detonations of nuclear weapons that are performed underground. When the device being tested is buried at sufficient depth, the explosion may be contained, with no release of radioactive materials to the atmosphere.

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. Further 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.

Background

Although public concern about fallout from nuclear testing grew in the early 1950s, fallout was discovered after the Trinity test in 1945. Photographic film manufacturers would later report 'fogged' films, these were traced both to Trinity and later tests at the Nevada Test Site. Intense fallout from the 1953 Simon test was documented as far as Albany, New York.

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." The multi-megaton test caused fallout to occur on the islands of Rongerik and Rongelap, and a Japanese fishing boat known as the Daigo Fukuryū Maru (Lucky Dragon). Prior to this test, there was "insufficient" appreciation of the dangers of fallout.

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; the 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." The Prime Minister of India "voiced the heightened international concern" when he called for the elimination of all nuclear testing worldwide.

Knowledge about fallout and its effects grew, and with it concern about the global environment and long-term genetic damage. 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. 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. Agreement was facilitated by the decision to allow underground testing, eliminating the need for on-site inspections that concerned the Soviets. 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."

Early history of underground testing

Following analysis of underwater detonations that were part of Operation Crossroads in 1946, inquiries were made regarding the possible military value of an underground explosion. The Joint Chiefs of Staff thus obtained the agreement of the Atomic Energy Commission to perform experiments on both surface and sub-surface detonations. The island of Amchitka was initially selected for these tests in 1950, but the site was later deemed unsuitable and the tests were moved the Nevada Test Site.

The first underground nuclear test was conducted on 29 November, 1951. This was the 1.2 kiloton Buster-Jangle Uncle, detonated 5.2m (17ft) beneath ground level. The test was designed as a scaled-down investigation of the effects of a 23 kiloton ground penetrating gun-type device that was then being considered for use as a cratering and bunker-buster weapon. The explosion resulted in a cloud that rose to 11,500 ft, and deposited fallout to the north and north-northeast. The resulting crater was 260 feet wide and 53 feet deep.

The next underground test was Teapot Ess, on 23 March, 1955. The 1 kiloton explosion was an operational test of an atomic demolition munition (ADM). It was detonated 67 feet underground, in a shaft lined with corrugated steel, which was then back-filled with sandbags and dirt. Because the ADM was buried underground, the explosion blew tons of earth upwards, creating a crater 300 feet wide and 128 feet deep. The resulting cloud rose to a height of 12,000 feet and subsequent fallout drifted in an easterly direction, travelling as far as 225 km from ground zero.

On 26 July, 1957, Plumbbob Pascal-A was detonated at the bottom of a 485-foot shaft. According to one description, it "ushered in the era of underground testing with a magnificent pyrotechnic Roman candle!" As compared with an above-ground test, the radioactive debris released to the atmosphere was reduced by a factor of ten. Theoretical work began on possible containment schemes.

Plumbbob Rainier was detonated at 899 ft underground on 19 September, 1957. The 1.7 kt explosion was the first to be entirely contained underground, producing no fallout. The test took place in a 1,600 – 2,000 ft horizontal tunnel in the shape of a hook. The hook "was designed so explosive force will seal off the non-curved portion of tunnel nearest the detonation before gases and fission fragments can be vented around the curve of the tunnel's hook." This test would become the prototype for larger, more powerful tests. Rainier was announced in advance, so that seismic stations could attempt to record a signal. Analysis of samples collected after the test enabled scientists to develop an understanding of underground explosions that "persists essentially unaltered today." The information would later provide a basis for subsequent decisions to agree to the Limited Test Ban Treaty.

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.

Effects

The effects of an underground nuclear test may vary according to factors including the depth and yield of the explosion, as well as the nature of the surrounding rock. If the test is conducted at sufficient depth, the test is said to be contained, with no venting of gases or other contaminants to the environment. In contrast, if the device is buried at insufficient depth ("underburied"), then rock may be expelled by the explosion, forming a crater surrounded by ejecta, and releasing high-pressure gases to the atmosphere (the resulting crater is usually conical in profile, circular, and may range between tens to hundreds of metres in diameter and depth). One figure used in determining how deeply the device should be buried is the scaled depth of burial, or -burst. This figure is calculated as the burial depth in metres divided by the cube root of the yield in kilotons. It is estimated that, in order to ensure containment, this figure should be greater than 100.

Zones in surrounding rock

Name	Radius
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

The energy of the nuclear explosion is released in one microsecond. In the following few microseconds, the test hardware and surrounding rock are vaporised, with temperatures of several million degrees and pressures of several million atmospheres. The heat and expanding shock wave cause the surrounding rock to vaporise, or being melted further away, creating a melt cavity. The shock-induced motion and high internal pressure cause this cavity to expand outwards, which continues until the pressure has fallen sufficiently. Although not observed in every explosion, four distinct zones (including the melt cavity) have been described in the surrounding rock. The crushed zone consists of rock that has lost all of its former integrity. The cracked zone consists of rock with radial and concentric fissures. Finally, the zone of irreversible strain consists of rock deformed by the pressure.

Once the pressure in the cavity has fallen 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. Such a crater is usually bowl-shaped, and ranges in size from a few tens of metres to over a kilometre in diameter. 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.

Other surface features may include disturbed ground, pressure ridges, faults, water movement (including changes to the water table level), rock falls, and ground slump.

Although there were early concerns about earthquakes arising as a result of underground tests, there is no evidence that this has occurred. 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.

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. Due to the Soviet government's concern about the need for the on-site inspections, underground tests were excluded from the ban. 108 countries would eventually sign the treaty, with the significant exceptions of France and China.

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. 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. The resulting Comprehensive Test Ban Treaty was signed in 1996 by the United States, Russia, United Kingdom, France, and China.

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. Over the next decade, this system was improved, and network of seismic monitoring stations was established to detect underground tests. 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.

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. The resulting International Monitoring System consists of a network of a total of 321 monitoring stations and 16 radionuclide laboratories. 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. 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. Finally, eleven hydroacoustic stations and sixty infrasound stations monitor underwater and atmospheric tests.

See also

• Subsidence crater

Notes and references

External links

• http://www.princeton.edu/~globsec/publications/pdf/3_3-4Adushkin.pdf
• Nuclear Pursuits, The Bulletin of the Atomic Scientists, September/October 2003
• http://www.unscear.org/unscear/en/publications.html
• http://www.ingv.it/~roma/SITOINGLESE/research_projects/CTBTO/explosions.html
• http://www.globalsecurity.org/wmd/intro/ugt.htm
• http://www.fas.org/nuke/intro/nuke/ugt-nts.htm
• http://www.lanl.gov/natlsecurity/nuclear/current/subcritical.shtml
• http://www.atomictraveler.com/UndergroundTestOTA.pdf
• http://www-pub.iaea.org/MTCD/publications/PDF/Pub1215_web.pdf
• http://www.osti.gov/bridge/servlets/purl/793554-ZAQEpq/native/793554.pdf
• http://www.osti.gov/bridge/servlets/purl/793554-ZAQEpq/native/793554.pdf
• http://www.princeton.edu/~globsec/publications/effects/effects.shtml