"A comprehensive test ban treaty, or CTBT, is the oldest item on the nuclear arms control agenda. Three treaties currently limit testing to underground only, with a maximum force equal to 150,000 tons of TNT. According to the Natural Resources Defense Council, the United States conducted 1,030 nuclear tests, the Soviet Union 715, the United Kingdom 45, France 210, and China 45. The last U.S. test was held in 1992; the last U.K. test, in 1991. Russia claims it has not conducted nuclear tests since 1991, though some suspect it may have conducted several low-yield tests since then. Since 1997, the United States has held 16 'subcritical experiments' at the Nevada Test Site to study the behavior of plutonium under pressures generated by explosives. It asserts they do not violate the CTBT because they cannot produce a self-sustaining nuclear fission chain reaction. The most recent was held February 14, 2002. Russia has reportedly conducted some since 1998, including several in fall 2000. In May 1998, India and Pakistan each announced several nuclear tests and declared themselves nuclear weapons states. Each declared a moratorium on further tests, but separately stated, in the summer of 2000, that the time was not right to sign the CTBT."
CRS Issue Brief for Congress, IB92099