42 @ The Containment of Underground Nuclear Explosions Figure 3-5—Three Redundant Containment Vessels (Pian View) Tunnel entrance Three containment vessels for the Mighty Oak Test conducted in the T-Tunnel Complex. SOURCE: Modified from Defense Nuclear Agency. between it and the vessel III plug is pressurized to approximately 10 pounds per square inch. The plugsthat separate the vessels are constructed of high strength grout or concrete 10 to 30 feet thick. The sides of the vessel II plugs facing the working point are constructed of steel. Vessel II plugs are designed to withstand pressures up to 1,000 pounds per square inch and temperatures up to 1,000 °F. Vessel III plugs are constructed of massive concrete and are designed to withstand pressures up to 500 pounds per square inch and temperatures up to 500 *F. - Before eachtest, the runnel system is checked for leaks. The entire system is closed off and pressurized to 2 pounds per square inch with a gas containing tracers in it. The surrounding area is then monitored for the presence of the tracer gas. Frequently. the chimney formedby the explosion is also subjected to a post-shot pressurization test to ensure that no radioactive material could leak through the chimney to the surface. The structure of vessel I, as shown in figure 3-6, is designed to withstand the effects of ground shock and contain the pressure, temperatures, and radiation of the explosion. The nuclear explosive is located at the working point, also known as the "zero room.”” A long, tapered, horizontal line-of-sight (HLOS) pipe extends 1,000 feet or more from the working point to the test chamber where the experimental equipmentis located. The diameter of the pipe may only be a few inches at the working point, but typically increases to about 10 feet before it reaches