10 U.S.C. § 6121

1. (a)No underground test of nuclear weapons may be conducted by the United States after September 30, 1996, unless a foreign state conducts a nuclear test after this date, at which time the prohibition on United States nuclear testing is lifted.
2. (b)None of the funds appropriated pursuant to the National Defense Authorization Act for Fiscal Year 1994 (Public Law 103–160; 107 Stat. 1547) or any other Act for any fiscal year may be available to maintain the capability of the United States to conduct atmospheric testing of a nuclear weapon.