Subcritical experiments obtain technical information about the U.S. nuclear weapons stockpile in the absence of nuclear testing. In subcritical experiments, chemical high explosives generate high pressures, which are applied to nuclear weapon materials, such as plutonium. The configuration and quantities of explosives and nuclear materials are such that no nuclear explosion will occur. Thus, the experiments are consistent with the existing U.S. nuclear testing moratorium. These experiments are called "subcritical" because there will be no critical mass formed, i.e., no self-sustaining nuclear fission chain reaction will occur. Scientific data is obtained on the behavior of nuclear weapon materials by the use of a wide variety of sophisticated, high-speed diagnostic instruments.
Subcritical experiments take place in the U1a Complex, an underground laboratory consisting of horizontal tunnels, each about one-half mile in length. The U1a Complex is mined at the base of a vertical shaft approximately 960-feet below ground surface. One vertical shaft is equipped with a mechanical hoist for personnel and equipment access while another vertical shaft, about 1,000 feet away, provides cross ventilation, instrumentation and utility access, and emergency exits.
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