44 © The Containment of Underground Nuclear Explostons Figure 3-7—Vessel | Closures Mechanical closures (MAC/GSAC) “F~ 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 Nuciear Agency the pipe ahead of the shock wave to prevent enoughto squeezethe pipe shut. The stemming e After 0.05 seconds (c), the ground shock moves past the second closure andis no longer strong ® After 0.2 seconds (d), the cavity growth 1s complete. The rebound from the explosion early flow of high-velocity gas and debris into the experiment chamber. plug stops forming at about the distance where the first mechanical pipe closure is located.