Nuclear Weapon Underground Effects Tests
Underground nuclear weapons effects tests (UGWETs) provide nuclear environments for demonstrating the hardness and surviv-ability of military equipment and materials as well as for studying basic nuclear effects phenomenology. Full-yield nuclear tests are the only way to produce all relevant nuclear weapon effects simultaneously. Underground nuclear weapons effects tests can provide insight into weapon performance, nuclear radiation effects, shock and blast, thermal effects, and source region EMP (SREMP). Even when it was allowed, underground testing was a very expensive way to garner the needed information. It was used by countries with significant economic bases and which were also committed to the development of nuclear offensive and defensive capabilities.
The UGWET-specific technologies include horizontal emplacement of the device, the provision of evacuated horizontal line-of-sight (HLOS) tubes for viewing the detonation, and mechanical closures to prevent debris from traveling through the HLOS tube to the experiment station that measures the radiation and shock environment and the response of systems. Also included are scattering station design and the computer codes necessary to understand the results of the experiments. Technologies to contain the release of radiation are only covered to the extent that they differ from those used in nuclear weapon development tests.
For effects testing, horizontal emplacement tests (HET) are preferred over vertical emplacement tests because the emplacement of device and test equipment is simplified. Horizontal tunnels provide greater experiment flexibility and access. Vertical shaft tests are less expensive but only provide limited exposure area because of the risk associated with containment when the crater is formed. The need to excavate large cavities for the placing of "test samples" and the construction of appropriate environments for those samples (for example, a vacuum for reentry bodies) drives the conductor of HLOS tests to seek suitable terrain such as a mesa or mountainside. Effects tests could also be conducted inside a deep mine.
HETs can incorporate large cavities so that shock and SREMP from a low-yield device actually have space to develop to the point where they are representative of similar effects in the open air from a large-yield weapon. The minimum burial depth is:
and the radius of the cavity formed by the detonation is:
where linear dimensions are measured in feet and yield in kilotons.
The object of an HET is often to allow nuclear radiation to reach the test object while preventing it from being destroyed by the other effects. Indeed, scientists expect to be able to recover the test instrumentation. Such a test requires redundant containment vessels: the first around the device, a second around all of the experiment to protect the tunnel system if the inner vessel fails and the experimental equipment is lost, and a third to ensure that no radiation escapes into the atmosphere even if the experimental equipment is lost and the tunnel system contaminated.
The HET-HLOS configuration is most often used for radiation effects tests, but the HLOS configuration must withstand the blast and shock waves produced by the device. The HLOS pipe is tapered from about 6 inches in diameter at the “zero room” (the device emplacement cavity) to about 30 feet in diameter at the experimental area 1,500 to 1,800 feet away and provides a clear line of sight to the device for those test subjects which need to see direct radiation.
Not all experiments require "direct" nuclear radiation; many are suitable for use with a scattered (lower intensity) beam produced in a scatter station-typically made with appropriate nuclear and atomic properties to deflect the correct wavelength and intensity of radiation. The design of these scatter stations requires both technical skill and experience so that the scattered radiation is properly tailored for its intended use. An incorrectly designed station could mean that the test object is exposed to incorrect radiation types or intensities, which could significantly reduce the value of the test.
Complete containment of radioactive debris is probably essential if a nation wishes to conduct a clandestine nuclear test. In any underground nuclear weapons effects test (UGWET), fast-acting mechanical closures to prevent debris from reaching the test objects are unique and critical equipment. A number of techniques are used in parallel to ensure that the HLOS pipe is closed before nuclear debris reaches the experiment. X- and gamma-rays travel at the speed of light, and electrons (beta particles) and neutrons are not much slower. The debris, however, moves much more slowly, at hydrodynamic velocities. [A "modified auxiliary closure" (MAC) or, when lower-yield weapons are used, a "fast acting closure" (FAC), positioned close to the device location-the working point-is able to shut the pipe in about 1 ms and to withstand pressures of about 30,000 psi.] A gas seal auxiliary closure (GSAC) farther along the HLOS pipe can close in less than 30 ms, and the tunnel and pipe seal (TAPS) will shut the pipe off in 300-700 ms. The TAPS is considerably farther from the working point than the FAC and therefore (a) has more time to function and (b) must close a larger aperture due to the taper of the HLOS pipe. These closure technologies are likely to require significant experience to develop to the point of reliable operation.
Other instrumentation to measure device performance, delivered shock, thermal pulse, electromagnetic pulse, and radiation is essentially similar to that used in a device development test.
Emplacement canisters, fast-acting closures for HLOS tunnels, and containment technology are the keys to preventing the release of radioactive debris into the atmosphere, allowing UGWET tests to be conducted without their being detected off-site. Mechanical closure designs and materials unique to underground tests in general and UGWET in particular include mechanical and cable gas-flow blocking designs and techniques that operate up to a pressure difference of 1,000 psi for up to an hour and specialized explosive and/or mechanically driven devices capable of isolating portions of the HLOS pipe during or within the first 100 ms after exposure to radiation.
Because the experimental area is often quite large and is at a considerable distance from the working point, the vacuum systems needed to evacuate air from them to simulate a space environment are unusual. Required are specially designed diffusion or cryogenic pumps capable of maintaining a pressure much less than 10 -3 Torr over a pipe system as long as 1,800 feet and varying in diameter from as small as 1 inch to as large as 30 feet. The crystals used to determine the energy spectrum of the radiation are unusual as well, and must be specially designed and fabricated to measure x-ray fluences at levels >0.1 cal/cm 2 in a time <50 ns and to operate in the UGT environment.
Some foreign vendors can manufacture digitizers, measurement systems, and fiber-optic equipment comparable to those used in U.S. UGWET. France manufactures digitizing oscilloscopes; Japan, South Korea, and Taiwan manufacture the electronic components for measurement and recording systems; and Germany manufactures cryogenic vacuum pumps of the large size required for HLOS events.
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