Underground nuclear testing is the test detonation of nuclear weapons that is performed underground. When the device being tested is buried at sufficient depth, the nuclear explosion may be contained, with no release of radioactive materials to the atmosphere.
The extreme heat and pressure of an underground nuclear explosion cause 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 soon led scientists to conclude that even notwithstanding environmental and diplomatic considerations, underground testing was of far greater scientific value than all other forms of testing. This understanding strongly influenced the governments of the first three nuclear powers to sign 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 Nuclear-Test-Ban Treaty in 1996, most nuclear tests were performed underground, which prevented additional nuclear fallout from entering into the atmosphere.
Background Public concern about fallout from nuclear testing grew in the early 1950s. Fallout was discovered after the Trinity test, the first ever atomic bomb test, in 1945. Photographic film manufacturers later reported 'fogged' films; this was traced to packaging materials sourced from Indiana crops, contaminated by Trinity and later tests at the Nevada Test Site, over away. 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 Ocean had "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 the Rongerik and Rongelap atolls, 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 Public Broadcasting Service (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."
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 US Joint Chiefs of Staff thus obtained the agreement of the United States Atomic Energy Commission (AEC) to perform experiments on both surface and sub-surface detonations. The Alaskan island of Amchitka was initially selected for these tests in 1950, but the site was later deemed unsuitable and the tests were moved to the Nevada Test Site.
The first underground nuclear test was conducted on 29 November 1951.Some sources identify later tests as the "first". Adushkin (2001) defines such a test as "the near-simultaneous detonation of one or more nuclear charges inside one underground excavation (a tunnel, shaft or borehole)", and identifies Uncle as the first. This was the 1.2 kiloton Buster-Jangle Uncle,Some sources refer to the test as Jangle Uncle (e.g., Adushkin, 2001) or Project Windstorm (e.g., DOE/NV-526, 1998). Operation Buster and Operation Jangle were initially conceived as separate operations, and Jangle was at first known as Windstorm, but the AEC merged the plans into a single operation on 19 June 1951. See Gladeck, 1986. which detonated beneath ground level. The test was designed as a scaled-down investigation of the effects of a 23-kiloton ground-penetrating gun-type fission weapon that was then being considered for use as a cratering and bunker-buster weapon. The explosion resulted in a cloud that rose to , and deposited fallout to the north and north-northeast. The resulting crater was wide and deep.
The next underground test was Teapot Ess, on 23 March 1955. The one-kiloton explosion was an operational test of an 'Atomic Demolition Munition' (ADM). It was detonated 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 wide and deep. The resulting mushroom cloud rose to a height of and subsequent radioactive fallout drifted in an easterly direction, travelling as far as from ground zero.
On 26 July 1957, Plumbbob Pascal-A was detonated at the bottom of a shaft.According to the Nuclear Weapons Archive, the yield is described as "slight", but was approximately 55 tons. 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 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 to 2,000 ft (500 to 600 m) 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 facility on Amchitka, was detonated on 6 November 1971. At approximately five 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 subsidence 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 yards in diameter and depth).
One figure used in determining how deeply the device should be buried is the scaled depth of burial, or -burst (SDOB) This figure is calculated as the burial depth divided by the cube root of the yield. It is estimated that, in order to ensure containment, this figure should be greater than per kiloton1/3.Hawkins and Wohletz specify a figure of 90–125.
+ Zones in surrounding rock Name Radius Melt cavity per kt1/3 Crushed zone per kt1/3 Cracked zone per kt1/3 Zone of irreversible strain per 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. Within milliseconds, a bubble of high-pressure gas and steam is formed. The heat and expanding shock wave cause the surrounding rock to vaporise, or be melted further away, creating a melt cavity. The shock-induced motion and high internal pressure cause this cavity to expand outwards, which continues over several tenths of a second until the pressure has fallen sufficiently, to a level roughly comparable with the weight of the rock above, and can no longer grow. Although not observed in every explosion, four distinct zones (including the melt cavity) have been described in the surrounding rock. The crushed zone, about two times the radius of the cavity, consists of rock that has lost all of its former integrity. The cracked zone, about three times the cavity radius, consists of rock with radial and concentric fissures. Finally, the zone of irreversible strain consists of rock deformed by the pressure. The following layer undergoes only an elastic deformation; the strain and subsequent release then forms a seismic wave. A few seconds later the molten rock starts collecting on the bottom of the cavity and the cavity content begins cooling. The rebound after the shock wave causes compressive forces to build up around the cavity, called a stress containment cage, sealing the cracks.The Containment of Underground Nuclear Explosions. (PDF) . Retrieved on 2010-02-08.
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. 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 around 100 feet to over half a mile 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. The radius r (in feet) of the cavity is proportional to the cube root of the yield y (in kilotons), r = 55 * ; an 8 kiloton explosion will create a cavity with radius of .