The U1a Complex is an underground laboratory used for subcritical experiments; physics experiments that obtain technical information about the U.S. nuclear weapons stockpile. These experiments support NNSA's Stockpile Stewardship Program, created to maintain the safety and reliability of the U.S. nuclear weapons stockpile.
Located 960-feet underground, the U1a laboratory serves as the site for sub-critical experiments. It includes horizontal tunnels, each about one half mile in length, and vertical shafts. The first shaft was U1a. The U1g shaft, which is located about 1,000 feet away, allows for cross ventilation, instrumentation and utility access, and emergency exit. Between 1999 and 2001, the facility was upgraded with the construction of the U1h shaft, to be the primary means of access and egress. It is equipped with a mechanical hoist for the movement of personnel and equipment. The hoist system was installed in 2004.
Subcritical experiments, which use chemical high explosives to generate high pressures on nuclear weapons materials, such as plutonium, create no critical mass, result in no nuclear explosion, and produce no self-sustaining nuclear fission chain reaction. These characteristics are used to justify that subcritical experiments do not violate the nuclear testing moratorium. The most recent subcritical experiment, termed Unicorn, was conducted at 11:00 a.m. on August 30, 2006 by Los Alamos National Laboratory. It was the 23rd subcritical experiment, following Krakatau, which was conducted February 23, 2006.