46 © The Containment of Underground Nuclear Explosions locks in the residual stress field, thereby forming a containment cage. The shock wave passes the test chamber. e After 0.75 seconds(e), the final mechanicalseal (TAPS) closes, preventing late-time explosive and radioactive gases from entering the test chamber. The entire closure process for containment takes less than 3/4 of a second. Because the tests are typically buried at a depth greater than necessary for containment, the chimney does not reach the surface and a collapse crater normally does not form. A typical post-shot chimney configuration with its approximate boundaries is shownin figure 3-9. Figure 3-9—Typical Post-Shot Configuration hy — Approximate chimney boundary 400 m LOS pipe In loweryield tests, such as those conductedin the P-tunnel complex, the first mechanical closure is a The FAC(figure 3-7(c)) closes in 0.001 seconds and TYPES OF RADIATION RELEASES Terms describing the release or containment of underground nuclear explosions have been refined to account for the volume of the material and the conditions of the release. The commonly used terms are described below. Containment Failure Containment failures are releases of radioactive material that do notfall within the strict definition of successful containment, which is described by the Department of Energy as: Containmentsuchthat a test results in no radioactivity detectable off site as measured by normal monitoring equipment and no unanticipated release of radioactivity onsite. Detection of noble gases that appear onsite long after an event, due to changing atmospheric conditions, is not unanticipated. Anticipated releases will be designed to conform to specific guidance from DOE/HQ.*6 Containmentfailures are commonly describedas: Test chamoer Tunnel com oiex Stemming plug Fast Acting Closure (FAC) rather than a MAC.*5 can withstand pressures of 30,000 pounds per square inch. The FAC acts like a cork, blocking off the HLOSpipe early, and preventing debris and stemming material from flying down the pipe. A similar closure is currently being developed for larger yield tunnel tests. te Tunnel shots are typically overouried and the collapse cnimney rarely extends to the surface. SOURCE: Modified from Defense Nuclear Agency. Ventings Ventings are prompt, massive, uncontrolled releases of radioactive material. They are characterized as active releases under pressure. such as when radioactive material is driven out of the yround hy steam or gas. “‘Baneberry,’” in 1970. 1s the !ast example of an explosion that *‘vented.” Seeps Seeps, which are not visible, can only be detected by measuring for radiation. Seeps are charactenzed as uncontrolled slow releases of radioactive matenal with little or no energy. Late-Time Seep Late-time seeps are small releases of nonconden- sable gases that usually occur days or weeks after a vertical drill hole test. The noncondensable vases diffuse up through the pore spaces ofthe overlying rock and are thought to be drawn to the surface by a decrease in atmospheric pressure (called “‘atmospheric pumping"’). 25The P-tunnel complex is mined in Aqueduct Mesa and has less overburden than the N-tunnel complex in Rasnier Mesa. Theretore. P :unnel is generally used for lower yield tests. 26Section VIII.F, Containment Evaluation Panel Charter.