34 @ The Containment of Underground Nuclear Explosions return (rebound) to its original position (figure 3-1(c)). The rebound creates a large compressive stress field, called a stress ‘‘containment cage’’, aroundthe cavity (figure 3-1(d)). The physics of the stress containment cage is somewhat analogous to how stone archways support themselves. In the case of a stone archway, the weight of each stone pushes against the others and supports the archway. In the case of an underground explosion, the rebounded rock locks around the cavity forming a stress field that is stronger than the pressure inside the cavity. The stress *‘containment cage’’ closes any fractures that may have begun and prevents new fractures from forming. Photo credit: Harold E. Edgerton Early phase offireball from nuclear explosion. WHY NUCLEAR EXPLOSIONS REMAIN CONTAINED Radioactive material produced by a nuclear explosion remains underground due to the combined efforts of: e the sealing nature of compressed rock around the the the the the cavity, porosity of the rock, depth ofburial, strength of the rock, and stemming of the emplacementhole. Counter to intuition, only minimal rock strength is required for containment. Atfirst, the explosion creates a pressurized cavity filled with gas that is mostly steam. As the cavity pushes outward, the surrounding rock is compressed (figure 3-1(a)). Because there is essentially a fixed quantity of gas within the cavity, the pressure decreases as the cavity expands. Eventually the pressure drops below the level required to deform the surrounding material (figure 3-1(b)). Mean- while, the shock wave has imparted outward motion to the material around the cavity. Once the shock wave has passed, however, the material tries to The predominantly steam-filled cavity eventually collapses forming a chimney. Whencollapse occurs, the steam in the cavity is condensed through contact with the cold rock falling into the cavity. The noncondensible gases remain within the lower chimney at low pressure. Once collapse occurs, high-pressure steam is no longer present to drive gases from the cavity region to the surface. If the test is conducted in porous material, such as alluvium or tuff, the porosity of the medium will provide volume to absorb gases produced by the explosion. For example, all of the steam generated by a 150 kiloton explosion beneath the watertable can be contained in a condensed state within the volume of pore space that exists in a hemispherical pile of alluvium 200 to 300 feet high. Although most steam condenses before leaving the cavity region. the porosity helps to contain noncondensible gases such as carbon dioxide (CO,) and hydrogen (H,). The gas diffuses into the interconnected pore space and the pressure is reducedto a levelthat is too low to drive the fractures. The deep water table and high porosity of rocks at the Nevada Test Site facilitate containment. Containment also occurs because of the pressure of overlying rock. The depth of burial provides a stress that limits fracture growth. For example. as a fracture initiated from the cavity grows, gas seeps from the fracture into the surrounding material. Eventually, the pressure within the fracture decreases below whatis needed to extendthe fracture. Atthis point, growth ofthe fracture stops and the gas simply leaks into the surrounding material. Rock strength is also an important aspect of containment, but only in the sensethat an extremely weak rock (such as water-saturated clay) cannot