Chapter 3—Containing Underground Nuclear Explosions ¢ 35 we Figure 3-1—Formation of Stress “Containment Cage” Compressive residual stress 1) Cavity expands outward and deforms surrounding rock. 2) Natural resistance to deformation stops expansion. 3) Cavity contracts (rebounds) from elastic unloading of distant rock. 4) Rebound locks in compressive residual stress around cavity. SOURCE: Modified from Lawrence Livermore National Laboratory. support a stress containmentcage. Detonation within weak, saturated clay is thought to have been a factor in the release of the Baneberry test. As a result, sites containing large amounts of water-saturated clay are now avoided. The final aspect of containment is the stemming that is put in a vertical hole after the nuclear device has been emplaced. Stemmingis designed to prevent gas from traveling up the emplacementhole. Impermeable plugs, located at various distances along the stemming column, force the gasesinto the surrounding rock whereit is ‘“sponged up’”’ in the pore spaces. How the various containment features perform depends on manyvariables: the size of the explo- sion, the depth of burial, the water content of the rock, the geologic structure, etc. Problems may occur when the containment cage does not form completely and gas from the cavity flows either through the emplacement hole or the overburden material. When the cavity collapses, the steam condenses and only noncondensible gases such as carbon dioxide (CO,) and hydrogen (H,) remain in the cavity.? The CO, and H, remain in the chimney if there is available pore space. If the quantity of noncondensible gasesis large, however, they can act as a driving force to transport radioactivity through the chimney or the overlying rock. Consequently, the amount of carbonate material and wuter in the rock near the explosion and the amount ot :ron available for reaction are considered when ev auat- ing containment.!° SELECTING LOCATION, DEPTH, AND SPACING The site for conducting a nuclear test... at tint, selected only on a tentative basis. The tina! decision is made after various site characteristics have been reviewed. The location, depth of bunal. and spacing are based on the maximum expected vield tor the nuclear device, the required geometry ot the test. and the practical considerations of scheduling. Lunven- ience, and available holes. If none ot the inventory holes are suitable, a site is selected and 4 hole drilled.!! The first scale for determining how deep an explosion should be buried was denveu trom the Rainier test in 1957. The depth, based on the Jube root of the yield, was originally: Depth = 300 (yield) ° where depth was measured in feet and vield in 8Lack ofa stress ‘‘containment cage’’ may not be a serious problem if the medium is sufficently porous or if the depth of burial 1s wuifwent 9The CO,is formed from the vaporization of carbonate material; while the H, is formed when water reacts with the iron in the nuc‘car Jes ve and diagnostics equipment. ‘0The carbonate material in Frenchman Flat created CO,that is thought to have caused a seep during the Diagonal Linetest(Nov 23 197! Dragonal Linewas the lasi test on Frenchman Fiat; the area is currently considered impractical for undergroundtesting largely because of the carbonate ateral. ‘See ch. 2, ‘The Nevada Test Site,’’ for a description of the areas each Laboratory uses for testing.