32 © The Containment of Underground Nuclear Explosions the testing program.* Testing was suspended for 7 months while a detailed examination of testing practices was conducted by the Atomic Energy Commission. The examination resulted in new testing procedures and specific recommendations for review of test containment. The procedures initiated as a consequence of Baneberry are the basis of present-day testing practices. Today, safety is an overriding concern throughout every step in the planning and execution of an underground nuclear test. Underground nuclear test explosions are designed to be contained, reviewed for containment, and conducted to minimize even the most remote chance of an accidental release of radioactive material. Each step of the testing authorization procedure is concerned with safety, and conservatism and redundancy are built into the system.> WHAT HAPPENS DURING AN UNDERGROUND NUCLEAR EXPLOSION Tenths of a Second As the cavity continues to expand. the internal pressure decreases. Within a few tenths of a second. the pressure has dropped to a level roughly comparable to the weight of the overlying rock. At this point, the cavity has reached its largest size and can no longer grow.® Meanwhile, the shock wave created by the explosion has traveled outward from the cavity, crushing and fracturing rock. Eventually, the shock wave weakens to the point where the rock is no longer crushed, but is merely compressed and then retums to its original state. This compression and relaxation phase becomesseismic waves that travel through the Earth in the same manner as seismic waves formed by an earthquake. A Few Seconds After a few seconds, the molten rock begins to collect and solidify in a puddle at the bottom of the cavity.’ Eventually, cooling causes the gas pressure within the cavity to decrease. The detonation of a nuclear explosion under- ground creates phenomena that occur within the following time frames: Microseconds Within a microsecond (one-millionth of a sec- ond), the billions of atoms involved in a nuclear explosion release their energy. Pressures within the exploding nuclear weapon reach several million poundsper square inch; and temperaturesare as high as 100 million degrees Centigrade. A strong shock waveis created by the explosion and moves outward from the point of detonation. Milliseconds Within tens of milliseconds (thousandths of a second), the metal canister and surrounding rock are vaporized, creating a bubble of high pressure steam and gas. A cavity is then formedboth bythe pressure of the gas bubble and by the explosive momentum imparted to the surrounding rock. Minutes to Days Whenthe gas pressure in the cavity declinesto the point where it is no longer able to support the overlying rock, the cavity may collapse. The collapse occurs as overlying rock breaks into rubble and falls into the cavity void. As the process continues, the void region moves upward as rubble falls downward. The ‘‘chimneying’’ continues until: e the void volumewithin the chimney completely fills with loose rubble, e the chimneyreaches a level where the shape of the void region and the strength of the rock can support the overburden material. or e the chimney reaches the surface. If the chimney reachesthe surface, the ground sinks forming a saucer-like subsidence crater. Cavity collapse and chimney formation typically occur within a few hours of the detonation but sometimes take days or months. 4See for example, Bruce A. Bolt, Nuclear Explosions and Earthquakes San Francisco, CA. (W.H. Freeman & Co., 1976). 5See ‘Detonation Authority and Procedures’’ (ch. 2). 6See the next section, '‘How explosions remain contained,’* for a detailed explanauon of cavity formation. The solidified rock contains most of the radioactive products from the explosion. The performanceof the nuclear weapon is analyzed when samples of this material are recovered by dniling back into the cavity.