Chapter 3~—Containing Underground Nuclear Explosions “‘maximum credible yield’* that the nuclear device is thought physically capable of producing, not to kilotons. For example, an 8 kiloton explosion w be expected to produce an underground cavity 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. Largeyield tests in small diameter holes distance of 320 feet between caviuies or 540 the design yield or mostlikely yield.'* (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 damaging the hole or diagnostic equipmentof another nearby test. Once the area has been chosen, several candidate sites 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.'5 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 root of the yield and can be estimated by: Radius = 55 (yield), where the radius is measured in feet and the yield in approximately a 110 foot radius. Two such explosions would require a minimum separ: between working points. Occasionally, a hole or tunnel is found t unsuitable for the proposed test. Such a situa however,is rare, occurringat a rate of about 1 o 25 for a drill hole test and about | out of 15. tunne! test.’ Usually, a particular holethatis f unacceptable for one test can be used for anothe at a loweryield. REVIEWINGA TESTSITE LOCATION Once the general parameters for a drill-hole been selected, the sponsoring laboratory reque pre-drill Geologic Data Summary (GDS) fron U.S. Geological Survey. The GDSis a geol interpretation of the area that reviews the three | elements: the structures, the rock type, andthe v content. The U.S. Geological Survey looks features that have caused containment problen the past. Of particular concern is the presenceo! faults that might become pathwaysfor the relea radioactive material, and the close location of basementrock that may reflect the energy creat the explosion. Review of the rock type check features such as clay content which would ind a weak area where it may be difficult for the hc remain intact, and the presence of carbonate that could produce CO,. Water content is reviewed to predict the amountof steam and H might be produced.If the geology indicates less ideal conditions, alternate locations may be gested that vary from less than a few hundrec from the proposedsite to an entirely different a the test site. Whenthe final site location is drilled, dat collected and evaluated by the sponsoring la tory. Samples and geophysical logs, including d hole photography, are collected and analyzed U.S. Geological Survey reviews the data, coi with the laboratory throughout the process reviews the accuracy of the geologic interpreta '4In many cases the maximum credible yield is significantly larger than the expected yield for a nuclear device. 'SAs discussed later, testing in previously fractured rock is not considered a containmentrisk in mostinstances. '6Qn three occasions tunnels have been abandoned because of unanticipated condiuons such as the discovery of a fault or the presenceof to water.