Non-Proliferation Experiment (NPE)
The detonation known as the Non-Proliferation Experiment occurred on September 22, 1993, in the rocky Rainier Mesa of the Nevada Test Site, where some of the nation's nuclear tests were conducted until a testing moratorium went into effect in 1992. There are a number of tunnels mined into Rainier Mesa, within which most DoD horizontal line-of-sight exposure experiments were conducted. In particular, N-, P-, and T-Tunnel complexes were extensively developed during the past several decades. N-Tunnel was also the location for a non-proliferation experiment, detonated in September 1993; this experiment involved 1,300,000 kg (2,900,000 lb) of conventional high explosives.
The chemical explosion simulated a 1-kiloton underground nuclear detonation, which, as expected, did not produce any visible new cracks in the Earth. The explosive was in a cylindrical cavity centered at 389 m depth, and was 7.7 m in radius (horizontally) and 5.2 m in height (vertically). The test revealed that seismic signals from an underground chemical blast closely mimic the signals that would be expected from an underground nuclear test.
One example of Los Alamos modeling capabilities is given by the simulation of the DOE Non-Proliferation Experiment. The NPE source consisted of 1.29x106 kg with the approximate energy of 1.1 kilotons. A cross-section of the modeled geology of the NPE site reveals the highly layered nature of that location. The material geology at the NPE site is based on that of the nearby Misty Echo event and consists of 4 layers. Layers 1 and 4 are nonporous, and layers 2 and 3 have porosities of 3% and 0.5% respectively.The region of nonlinear deformation near the explosion is approximately spherical, but elongated and slightly offset vertically. The region of cracking is confined to near the free surface. This explosion was deeply overburied, so there is less asymmetry than in a normally buried explosion. Because this is a very low velocity structure, the crust effectively traps P->S converted waves from the explosion, so in this case the full waveform and in particular the Lg phase from the complex explosion source is modeled quite well by a point explosion source.
Potential treaty violators might be tempted to detonate a nuclear device in the center of a large underground cavity, a technique called decoupling. The seismic signal from such a test is reduced by a factor of up to 70 through a muffling effect that reduces the amplitude of the signal. A 1-kiloton nuclear explosion, for example, would produce a magnitude in the range of approximately 2.5 to 3 on the Richter scale when tested in a large underground cavity. Seismic signals of the lower magnitude are produced frequently in a large number of mine explosions worldwide, and many thousands of earthquakes are in this range.
President Clinton and other world leaders signed the landmark Comprehensive Nuclear Test Ban Treaty in September 1996, they served notice that any signatory nation trying to conceal an underground nuclear test would have to elude a vigorous international verification program armed with the latest monitoring technologies. The CTBT forbids all nuclear tests, including those intended for peaceful purposes, and creates an international monitoring network to search for evidence of clandestine nuclear explosions.
Low-frequency aftershocks associated with nuclear explosions may also be caused by mining operations. Livermore experts compared aftershocks from the 1993 Non-Proliferation Experiment at the Nevada Test Site with those from routine operations at the Henderson Mine in Colorado. Although the events from both sources are similar, there are subtle differences in the aftershock signals. They were interested in the Henderson Mine because the caving operation is similar to the chimney formation following an underground nuclear event.
Also as part of the Non-Proliferation Experiment, Livermore experts found that very small amounts of rare radioactive gases such as xenon-133 and argon-37 generated in underground nuclear detonations can migrate toward the surface along natural fault lines and earth fissures in a time frame consistent with an on-site inspection. The technology used in these tests can be an extremely sensitive way to detect nearby underground nuclear explosions that do not fracture the surface.
Minute amounts of rare, radioactive gases generated in underground nuclear detonations will migrate toward the surface along natural fault lines and earth fissures. With the help of results from earlier studies, a team at Livermore theorized that highly sensitive instruments might detect telltale radioactive gases rising during periods of barometric low pressure through natural fissures in the ground above the blast. To test the hypothesis, the team obtained two gases, 0.2 kilograms (7 ounces) of helium-3 and 50 kilograms (110 pounds) of sulfur hexafluoride, as tracers. These nonradioactive gases are ideal tracers because they are present in very low quantities in the natural environment.
The bottles containing the gases were placed with the 1.3-kiloton charge of chemical explosives into a mined cavity that was 15 meters (50 feet) in diameter and 5 meters (17 feet) high. The cavity was located 400 meters (1,300 feet) below the surface, two to three times deeper than that required for a similar sized underground nuclear test. A somewhat shallower detonation might have produced a collapse crater or extensive fractures connecting the cavity with the surface, both telltale signs of an underground explosion. Hence, clandestine tests would very likely be conducted at the greater depth to avoid easy detection of treaty violations.
Over the year and a half following the blast, team members, including technical support personnel from Test Site contractors EG&G and REECo, collected nearly 200 samples of subsoil gases for measurement. At some sampling stations, sampling tubes were driven into the ground to depths of 1.5 to 5 meters (5 to 16 feet) along fractures and faults. At other stations, tubes were simply placed beneath plastic sheeting that was spread on the ground to trap rising soil gases and to limit atmospheric infiltration (see photo below).
The first positive finding came 50 days after the explosion, when sulfur hexafluoride was detected in fractures along a fault. Interestingly, the much lighter helium-3 showed up 375 days--more than a year--following the explosion. Both gases were first detected along the same natural fissure within 550 meters (1,800 feet) of the blast site.
Over the course of the extended sampling period, virtually all the samples yielding concentrations of the two tracers appeared along natural faults and fractures in the mesa during periods of low atmospheric pressure, mainly at the beginning of storms. The low pressure accompanying storms makes it possible for the gases to move toward the surface along the faults.
|Join the GlobalSecurity.org mailing list|