"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. Since 1997, the United States has held 21 'subcritical experiments' at the Nevada Test Site, most recently on May 25, 2004, to study how plutonium behaves under pressures generated by explosives. It asserts these experiments do not violate the CTBT because they cannot produce a self-sustaining chain reaction. Russia has reportedly held some since 1998, including several in 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. […] Congress continues to consider the Stockpile Stewardship Program, which seeks to maintain nuclear weapons without testing."
CRS Issue Brief for Congress, IB92099