Chapter 3—Containing Underground Nuclear Explosions ¢ 37 “‘maximum credible yield’’ that the nuclear device is thought physically capable of producing, not to the design yield or mostlikely yield.'¢ Whethera test will be conducted on Pahute Mesa or Yucca Flat depends on the maximum credible yield. Yucca Flat is closer to support facilities and therefore more convenient, while the deep water table at Pahute Mesa is more economical for large yield tests that need deep, large diameter emplacementholes. Large yield tests in small diameter holes (less than 7 feet) can be conducted in Yucca Flat. A test area may also be chosen to avoid scheduling conflicts that might result in a test damagingthe hole or diagnostic equipmentof another nearbytest. Once the area has been chosen,several candidatesites are selected based on such features as: proximity to previous tests or existing drill holes; geologic features such as faults, depth to basement rock, and the presence of clays or carbonate materials; and practical considerations such as proximity to power lines, roads, etc. In areas well suited for testing, an additional site selection restriction is the proximity to previous tests. For vertical drill hole tests, the minimum shot separation distance is about one-half the depth of burial for the new shot (figure 3-2). For shallow shots, this separation distance allows tests to be spaced so close together that in some cases, the surface collapse craters coalesce. The '/2 depth of burial distance is a convention of convenience, rather than a criteron for containment.'> It is, for example, difficult to safely place a drilling rig too close to an existing collapse crater. Horizontal tunnel tests are generally spaced with a minimum shot separation distance of twice the combined cavity radius plus 100 feet, measured from the point of detonation (called the ‘‘working point’’) (figure 3-3). In other words, two tests with 100 foot radius cavities would be separated by 300 feet between cavities, or 500 feet (center to center). The size of a cavity formed by an explosion is proportional to the cube rootof the yield and can be estimatedby: Radius = 55 (yield)'”, where the radius is measured in feet and the yield in kilotons. For example, an 8 kiloton explosion would be expected to produce an underground cavity with approximately a 110 foot radius. Two such test explosions would require a minimum separation distance of 320 feet between cavities or 540 feet between working points. Occasionally, a hole or tunnel is found to be unsuitable for the proposed test. Such a situation, however, is rare, occurring at a rate of about | out of 25 for a drill hole test and about | out of 15 for a tunnel test.'© Usually, a particular hole that is found unacceptable for onetest can be used for anothertest at a loweryield. REVIEWINGA TEST SITE LOCATION Once the general parameters for a drill-hole have been selected, the sponsoring laboratory requests a pre-drill Geologic Data Summary (GDS) from the U.S. Geological Survey. The GDS is a geologic interpretation of the area that reviews the three basic elements: the structures, the rock type, and the water content. The U.S. Geological Survey looks for features that have caused containment problems in the past. Of particular concern is the presence of any faults that might become pathways forthe release of radioactive material, and the close location of hard basementrock that mayreflect the energy created by the explosion. Review of the rock type checks for features such as clay content which would indicate a weak area where it may be difficult for the hole to remain intact, and the presence of carbonate rock that could produce CO,. Water content is also reviewedto predict the amountof steam and H,that might be produced.If the geology indicates less than ideal conditions, alternate locations may be suggested that vary from less than a few hundred feet from the proposedsite to an entirely different area of the test site. Whenthe final site location is drilled, data are collected and evaluated by the sponsoring laboratory. Samples and geophysical logs. including downhole photography, are collected and analyzed. The U.S. Geological Survey reviews the data, consults with the laboratory throughout the process, and reviews the accuracy of the geologic interpretations. '4In many cases the maximum credibleyield is significantly larger than the expected yield for a nuciear device. '5As discussed later, testing in previously fractured rock is not considered a containmentrisk in most instances. '6On three occasions tunnels have been abandoned because of unanticipated conditions such as the discovery of a fault or the presence of too much water.