34 © The Containment of Underground Nuclear Explosions return (rebound) to its original position ( 3-1(c)). The rebound creates a large compr stress field, called a stress “‘containment c around the cavity (figure 3-1(d)). The physics stress containment cage is somewhat analoge how stone archways support themselves. In th of a stone archway, the weight of each stone p against the others and supports the archway. | case of an underground explosion, the rebo rock locks around the cavity forming Stress that is stronger than the pressure inside the c The stress **containment cage”’ closes any fra that may have begun and prevents new fra from forming. Photo cree: Harold E. Edgerton Early phase of firebail 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 cavity, @ the porosity of the rock, e the depth of burial. e the strength of the rock, and * the stemming of the emplacementhole. Counter to intuition, only minimal rock strength is required for containment. At first, 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 even collapses forming a chimney. Whencollapse o the steam in the cavity is condensed through c: with the cold rock falling into the cavity noncondensible gases remain within the chimney at low pressure. Once collapse o high-pressure steam is no longer present to gases from the cavity region to the surface. If the test is conducted in porous material. si alluvium or tuff, the porosity of the mediun provide volume to absorb gases produced t explosion. For example, all of the steam gen by a 150 kiloton explosion beneath the water can be contained in a condensed state with volume of pore space that exists in a hemispt pile of alluvium 200 to 300 feet high. Althougt steam condenses before leaving the cavity rn the porosity helps to contain noncondensible such as carbon dioxide (CO,) and hydrogen The gas diffuses into the interconnected pore and the pressure is reduced to a level thatis tc to drive the fractures. The deep water table an porosity of rocks at the Nevada Test Site fac containment. Containment also occurs because of the pr of overlying rock. The depth of burial prov stress that limits fracture growth. For exampl fracture initiated from the cavity grows, gas from the fracture into the surrounding m: Eventually, the pressure within the fractu creases below what is needed to extend thefri Atthis point, growth of the fracture stops and| simply leaks into the surrounding matenal. Rock strength is also an important asp containment, but only in the sense that an ext weak rock (such as water-saturated clay) °