Nuclear Weapon Underground Testing
A deep underground explosion is one occurring at such a depth that the effects are essentially fully contained. The surface above the detonation point may be disturbed, e.g., by the formation of a shallow subsidence crater or a mound, and ground tremors may be detected at a distance. There is no significant venting of the weapon residues to the atmosphere, although some of the noncondensable gases present may seep out gradually through the surface.
An underground nuclear explosion generates quasi-instantaneously an amount of energy, e, which is then converted into other forms of energy through a series of processes. The explosion energy is released in less than a millionth of a second. The pressure in the hot gas bubble formed will rise to several million atmospheres and the temperature will reach about a million degrees within a few microseconds. Within a few microseconds after the detonation, the device and some surrounding rock and water are vaporised. A fraction of the initial energy released by the explosion is expended in this process. Within a few tens of microseconds after the explosion, the cavity expands to a final radius of rc under the influence of the extremely high temperature and pressure of the gas in the cavity. The expansion of the gas into the surrounding rock generates a shock wave, while some of the energy of the gas is dissipated in melting some of the rock surrounding the cavity. The volume of the final cavity is proportional to the yield, e, so that the final cavity radius may be expressed as
rc = r'c e1/3
For typical test conditions, r'c ~ 10 -12 m/kt1/3, so a 1 kt explosion produces a cavity of radius 10-12 m, depending on the depth of burial. A deep 150 kt explosion would produce a cavity with a radius of approximately 55 m.
Soon after the detonation, the molten rock around the cavity periphery begins to solidify and accumulate at the bottom of the cavity. As the gas inside the cavity cools and some gas seeps into the surrounding rock, the gas pressure in the cavity decreases to the point when it can no longer support the overburden. Consequently the crushed and sheared rock above the cavity will collapse progressively, especially when the horizontal in situ stresses are low. Over a period of a few minutes to a few hours after the detonation, a tall cylinder, commonly referred to as a "chimney," form. The chimney will propagate upwards until it naturally stabilises. The blocky rubble that accumulates in the chimney void occupies a greater volume (in the range of 20-30% more) than it did in situ. This causes the eventual arrest of the upward propagation of the chimney. The chimney height can be in the range 4 -10 rc , with values near the lower end of this range (5 - 6 rc ) being most common. If the collapse of the chimney material should reach the surface, the ground will sink into to the empty space thereby forming a subsidence crater. Some deeply buried explosions of low yield form cavities that do not collapse to the surface and, consequently, do not create subsidence craters. If the top of the chimney does not reach the ground surface, an empty space, roughly equivalent to the cavity volume, will remain at the top of the chimney.
Most nuclear weapon states have constructed underground testing facilities similar to the U.S. Nevada Test Site. That is, weapons development and proof tests are usually carried out in vertical shafts stemmed to prevent the escape of radioactive debris. The development of the fireball and the propagation of a shock wave proceed quite differently when the device is tightly tamped at the bottom of a borehole than when it is detonated in free air. However, when the borehole or mine shaft have been properly stemmed, underground experiments have the advantage of not releasing significant amounts of radioactive debris. It is also simpler to place large masses of experimental apparatus close to an underground shot than to locate the same hardware next to a balloon gondola or on the platform of a slender tower, either of which has a limited carrying capacity. In any event, very few atmospheric tests have been carried out during the last three decades, and even the French and Chinese abandoned their atmospheric test programs.
Power and signal cables for the device are routed up the shaft and fanned out to several instrumentation trailers outside the probable cratering zone. Nuclear weapons effects tests are primarily carried out in horizontal mine shafts sealed to prevent the escape of debris; instrumentation cables are connected to the surface through a vertical bore hole. In both cases, the tests are characterized by the large amount of electronic instrumentation used to study the details of the functioning of the implosion assembly and of the nuclear phases of the explosion. A beginning nuclear power opting for simpler weapons may well choose not to employ sophisticated diagnostic instrumentation, selecting instead to determine the approximate yield with seismographs.
It appears likely that the drilling technology needed to emplace nuclear devices and instruments at the bottom of a deep borehole is the most difficult for a proliferator to acquire and use. Such boreholes are frequently a kilometer or more deep and 2 meters or more in diameter. The specialized drilling machinery required for such construction is not commonly available and exceeds what is found in the oil industry.
The most accurate measurement of yield is through the radio-chemistry studies of device debris -- the radioactive isotopes produced in the detonation. No electronics are used to gather the data for such analyses; it is only necessary to drill back into the device chamber and to extract samples for lab examination. A faster but less accurate yield determination can be done using seismographs to measure ground motion, but such a test would not collect a large quantity of data usually considered desirable by US weapon designers and testers. Radioactive debris from an atmospheric test or from an underground shot which vents can be analyzed by other nations. Much information about the design and performance of the test device can be inferred from the debris.
Only with a large collection of data derived from yield tests of different types of devices can a weapons designer be confident that he understands the behavior of different possible designs within what is termed the nuclear weapons "design space," and only then can he be confident that the computer programs used to predict device performance deliver reliable results. This may be the strongest motivation for a proliferator to test at full yield. However, even a series of full-yield tests may not provide all of the information needed for weapons design.
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