Underground Nuclear Weapons Testing
Since 1963, the United States has conducted all of its nuclear weapons tests underground in accordance with the terms of the Limited Test Ban Treaty. Hence, complete containment of all nuclear weapons tests is a dominant consideration in nuclear test operations.
Various methods are used for emplacing nuclear test devices so that the ensuing explosion is contained. The most common method is to emplace a test device at the bottom of a vertically drilled hole. Another method is to emplace a test device within a tunnel that has been mined horizontally to a location that is sufficiently deep to provide containment.
Emplacement of a test device in a drill hole or tunnel is not accomplished until the containment design has been reviewed by the Containment Evaluation Panel. The Containment Evaluation Panel is composed of individuals who have extensive experience in nuclear testing and associated phenomenology. The Containment Evaluation Panel assists the Manager, DOE/NV, in the review of proposed nuclear tests to ensure that each containment design is one that will provide reasonable assurance of satisfactory containment of radioactivity or release radioactivity only under controlled conditions in compliance with all treaty constraints and under health and safety guidelines established by the Secretary of Energy.
Panel membership include scientists and engineers from the Los Alamos National Laboratory, Lawrence Livermore National Laboratory, Sandia National Laboratories, the Defense Nuclear Agency, the U.S. Geological Survey, the Desert Research Institute, and up to four independent consultants. The Panel examines each factor that might contribute to the unwanted escape of radionuclides into the atmosphere during or after the detonation. Such reviews consider in detail the device yield, depth of burial, geology, hydrology, characteristics of the soil and rock, location of the emplacement site (including the proximity to and the success of previous test locations), closure methods, stemming design, and drilling and construction history.
TESTS IN VERTICAL DRILL HOLES
Tests in vertical drill holes are of two types: smaller-yield devices in relatively shallow holes in the Yucca Flat area (Areas 1, 2, 3, 4, 5, 6, 7, 8, 9, and 10) and higher-yield devices in deeper holes on Pahute Mesa (Areas 18, 19, and 20). Tests at the Yucca Flat and Pahute Mesa event sites have the same general requirements, but differ in the magnitude of the operations. Deeper-hole operations disturb a larger area, require more on-site equipment, and have a higher requirement for electrical power and utilities. The distance from the core of the infrastructure is also a factor; Pahute Mesa operations are 48 to 81 kilometers (km) (30 to 50 miles [mi]) farther away than Yucca Flat.
The following description of a vertical drill-hole test breaks down the operation into seven individual steps:
Step1. Site Selection and Drilling.
There are two subsets of site selection as it applies to nuclear tests, namely: selection of an existing drill hole for a specific event, and selection of a new drill site from the Nuclear Test Zone for a specific event because the stockpile does not contain a suitable site. The goal of siting is to optimize the various parameters so that operational feasibility and successful containment of yields of interest to device designers can be attained at a suitably low cost.
Many factors are considered. Some of these are: (1) scheduling of field resources; (2) event schedules; (3) shock sensitivity of a given experiment and possible interactions with other experiments; (4) depth range required for a suitable device emplacement; (5) geologic structure; (6) geologic material properties; (7) depth of standing water; (8) potential drilling problems; (9) adjacent expended sites, craters, chimneys, subsurface collapses; (10) adjacent open emplacement holes or unplugged post-shot or exploratory holes; and (11) non-test program constraints such as groundwater concerns, roads, and power lines (Olsen, 1993).
When drilling is required after a test location is chosen by the sponsoring national laboratory, a drilling program outlining the requirements of the specific hole is completed. The event site is surveyed, staked, and checked for cultural and biological resources. When all environmental clearances are completed, the site is graded and leveled, and a drilling-fluid sump is constructed to contain drilling fluid and cuttings. A drill rig, usually with its own power and utilities, is moved onto the site. Water is brought in by truck, or piped in, and mixed with drilling compounds to fill the sump. The hole is then drilled using standard NTS big-hole drilling techniques. A normal hole is from 1 to 3 meters (m) (48 to 120 inches [in.]) diameter and from 213 to 762 meters (m) (600 to 2,500 feet [ft]) deep. During drilling, samples of drill cuttings are collected at 3-m (10-ft) intervals, and rock cores are taken as required. After drilling is complete,geophysical logs are run into the hole to evaluate the condition of the hole and gain a more thorough understanding of the geology. The drill site is then secured by filling the sump and installing specially designed covers over the hole.
Step 2. Event-Site Engineering and Construction.
When a hole is selected as a location for a nuclear test, the area around the hole is surveyed and staked according to the criteria set forth by the sponsoring national laboratory. The cultural and biological surveys are then rerun to determine if the status of the area has changed. The hole is also uncovered, and selected geophysical logs are refed in the hole to reconfirm its condition.
Once it is assured that the environmental clearances are complete, an area is cleared and leveled for the surface ground-zero equipment; another area close by the selected site is cleared and leveled for the recording trailer park. This is a typical earthmoving operation; native materials are used to top the pads or, if active material is unstable, decomposed granite fill is used. The on-site construction is temporary and is abandoned after the event is complete. Concrete pads are poured around the surface ground-zero to provide a stable platform for downhole operations and to provide a base for the assembly towers. Equipment is moved in to emplace the nuclear device in the hole, record the data produced, and provide radiological and seismic monitoring of the site. An extensive grounding system is used to establish baseline instrumentation grounds, which might include a pit containing salt water. The equipment to be left in position during the explosion is protected with an aluminum-foil hexcell-shaped shock-mounting material or dense foam. A circle of radiation detectors is placed back from the surface ground-zero to detect and assess any releases from the experiment. Finally, a perimeter fence is erected, and access is controlled both into and out of the event site.
Step 3. Device Delivery and Assembly.
For safety reasons, the nuclear device is delivered to the NTS unassembled. The device is assembled and inserted into a container at the Device Assembly Facility in Area 6 or in the Area 27 Assembly/Staging Facilities. The Device Assembly Facility is discussed at the end of this section. The device, now encased in the container, is delivered to the event site accompanied by armored convoy. It is then attached to the diagnostics canister in preparation for emplacement into the hole. Checks are run, and alignment is assured. Heavy security is maintained during all operations that involve the nuclear device.
Step 4. Diagnostic Assembly.
A diagnostic canister is assembled off site and transported to the test site. A typical diagnostic canister might be 2 m (8 ft) in diameter and 30 m (120 ft) long and contain all the instrumentation required to receive data at the time of the explosion (real time). The diagnostic canister might contain lead and other materials as shielding for the detectors. Upon arrival at the event site, the diagnostic canister is installed in the assembly tower to be mated with the device on site. Instrumentation cables are connected to the experiments and the recording trailer park. Slack in the cables allows the diagnostic canister to be lowered into the hole.
Step 5. Emplacement of the Experiment.
The nuclear explosive and special measurement devices are moved to the hole and lowered to the detonation position; all required diagnostic materials and instrumentation cables are also lowered into the hole at this time. Downhole operations are conducted according to a defined checklist and are monitored by independent inspectors. The whole assembly is placed on a set of fracture-safe beams that span the opening. Any auxiliary equipment is then lowered into the hole, and the area is secured. Emplacement equipment is removed from the area, and test runs are conducted on the downhole experiment.
The hole is stemmed to prevent radioactive materials from escaping during or after the experiment. Stemming materials used to backfill the hole are generally placed in alternating layers, according to the containment specification. Alternate layers of 1-centimeter (cm) (3/8-in.) pea gravel are combined with fine material to provide a barrier equal to or better than the undisturbed material. Sand, gypsum, grout, cold tar, or epoxy plugs are also placed in the hole to provide impenetrable zones. In these zones, the instrument cables are sealed to prevent a radioactive gas path to the surface. Once completed, the area is cleared of unnecessary equipment. A report is compiled for the Containment Evaluation Panel to show that the as-built condition reflects the containment design plan.
Step 6. Test Execution.
After the Containment Evaluation Panel accepts the as-built design of containment and all preliminary tests are successful, the nuclear device is ready for detonation. Security operations begin two days before the test to assure that all nonevent-related personnel are evacuated prior to the test for security and personal safety. The explosive is armed. Radiation monitors are activated, and aircraft with tracking capability circle the site in case gas and debris unexpectedly vent to the surface. Weather forecasts and fallout pattern predictions are reviewed. Then, detonation occurs.
When an underground nuclear device is detonated, the energy release almost instantaneously produces extremely high temperatures and pressure that vaporizes the nuclear device and the surrounding rock. Within a fraction of a second after detonation, a generally spherical cavity is formed at the emplacement position. As the hot gases cool, a lining of molten rock puddles at the cavity bottom.
After a period of minutes to hours, as the gases in the cavity cool, the pressure subsides and the weight of the overburden causes the cavity roof to collapse, producing a vertical, rubble-filled column known as a rubble chimney.
The rubble chimney commonly extends to the ground surface, forming a subsidence crater. Numerous subsidence craters are present at the test site. Subsidence craters generally are bowl-shaped depressions with a diameter ranging from about 60 to 600 m (200 to 2,000 ft) and a depth ranging from a few meters up to 60 m (200 ft), depending on the depth of burial and the explosive energy yield. Some deeply buried explosions of low yield form cavities that do not collapse to the surface and, consequently, do not create subsidence craters. Past underground nuclear tests in Yucca Flat and on Pahute Mesa have fractured the ground surface above the explosions, causing displacement of the surface along preexisting faults adjacent to explosion sites.
After the test is conducted, the event site remains secure until it can be assured that the event has been contained. After a suitable time, a reentry crew is dispatched to the site. Data are retrieved, and the condition of equipment is noted. After all is assured to be secure, normal NTS operations resume. The event site is roped off, outlining an exclusion zone where there is danger of potential cratering.
Step 7. Post-shot Operations.
After the temperature of the cavity has cooled, a post-shot hole is usually drilled into the point of the explosion in order to retrieve samples of the debris. These samples are highly radioactive, but provide important information on the test. The post-shot hole is as small in diameter as possible and is drilled at an angle to allow the drill rig to be positioned safely away from surface ground-zero. After drilling and sampling operations are complete, the drill rig and tools are decontaminated. Residual radiation is cleaned up at the site, and the hole is plugged back to the surface. This generally completes the event operation, and the site is turned back to the DOE.
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