1: less or lower than critical in respect to a specified factor
2 a: of insufficient size to sustain a chain reaction subcritical mass of fissionable material
b: designed for use with fissionable material of subcritical mass subcritical reactor
"Anti-nuclear peace activists claim that subcritical tests violate the spirit of the Comprehensive Test Ban Treaty (CTBT) and could introduce a new round of the nuclear arms race."- from nuclearfiles.org
"Many of the important tests at the Nevada site, including the one named Unicorn, are called "subcritical experiments." In a "subcrit" experiment, plutonium, the explosive ingredient in a nuclear weapon, is detonated with high explosives so scientists can observe how the materials interact and respond to the blast. The experiments take place in the U1a Complex at the site, an underground laboratory composed of roughly a mile of mined tunnels first excavated during the 1960s. In 1997, "Rebound," the first subcrit, was conducted in a 10-by-15-by-30-foot room. Once the scientists capture the blast data with multibillion-dollar, state-of-the-art supercomputers, they seal the radioactive experiment in layers of concrete 960 feet underground, presumably for all eternity.
"Subcritical" refers to the fact that the tests do not reach "criticality"; that is, they don't sustain a nuclear chain reaction, the perpetual explosion of energy that unleashes radioactive destruction. For that reason, subcrits are not banned under the Comprehensive Test Ban Treaty, the international agreement that President Clinton signed in 1996. The treaty forbids any nuclear test explosions that cause a chain reaction -- as well as the improvement and development of nuclear weapons.
Bob Peurifoy, an engineer for 39 years at New Mexico's Sandia National Laboratory before retiring in 1991, says that subcrits "are perhaps not necessary but are highly desirable" for maintaining the stockpile. Because they can't reach criticality, he says, "these experiments could be conducted in the open air, except for the fear of spreading plutonium around.
Read how the labs have kept themselves happy and busy with their subcritical projects here.