44 « The Containment of Underground Nuclear Explosions Figure 3-7—Vessel | Closures Mechanical closures (MAC/GSAC) ~*~ Mechanical closure (TAPS) Pre-fire geometry Approximate closed FAC geometry Fast acting closure (FAC) A) Mechanical Closures (MAC/GSAC) B) Tunnel and Pipe Seal (TAPS) C) Fast Acting Closure (FAC) SOURCE: Modified from Defense Nuclear Agency the pipe ahead of the shock wave to prevent early flow of high-velocity gas and debris into the experiment chamber. enoughto squeeze the pipe shut. The stemming plug stops forming at about the distance where the first mechanical pipe closure is located. e After 0.05 seconds(c), the ground shock moves e After 0.2 seconds (d), the cavity growth is past the second closure andis no longer strong complete. The rebound from the explosion