Chapter 3—Containing Underground Nuclear Explosions © 35 SOO Figure 3-1—Formation of Stress “Containment Cage” Compressive residual stress A B ¢ 0 1) Cavity expands outward and deforms surrounding rock. 2) Natural resistance to deformation stops expansion. 3) Cavity contract (rebounds) from elastic unloading of distant rock. 4) Rebound locks in compressive residual stress around cavity. SOURCE: Modified from Lawrence Livermore Nationa) Laboratory. support a stress containment cage. 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 emplacement hole. Impermeable plugs, located at various distances along the stemming column, force the gases into the surrounding rock whereit is ‘‘sponged up’’ in the pore spaces. How the various containment features perform depends on many variables: the size of the explosion, 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, remainin the chimney if there is available pore space. If the quantity of noncondensible gases is 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 water in the rock near the explosion and the amount of ror available for reaction are considered when evaluat. ing containment.!° SELECTING LOCATION, DEPTH, AND SPACING The site for conducting a nuclear test 1s. al first selected only on a tentative basis. The final decisiot is made after various site charactenstics have beet reviewed. The location, depth of bunal. and spaciny are based on the maximum expected yield tor th nuclear device, the required geometryof the fest. ani the practical considerations of scheduling. conven ience, and available holes. If none of the inventor holes are suitable, a site is selected and a hol drilled.!! The first scale for determining how deep a explosion should be buried was denved trom th Rainier test in 1957. The depth, based on the cub root of the yield, was originally: Depth = 300 (yield) where depth was measured in feet and yield i 5Lack of a stress ‘‘containment cage" may notbe a serious problem if the medium is sufficently porous or if the depth of bunal 1s sufficent The CO,is formed from the vaporization of carbonate material; while the H, is formed when water reacts with the iron in the nuclear device at diagnostics equipment. '0The carbonate material in Frenchman Flat created CO, that is thought to have caused a se¢p during the Diagonal Line test (Nov 24. 1971) Diagon Line was the last test on Frenchman Flat. the area is currently considered impractical for underground testing largely because of the carbonate materti 'See ch. 2, ‘The Nevada Test Site,'’ for a description of the areas each Laboratory uses for testing.