Limitations on nuclear testing have been on the international agenda since 1954. The United States ratified one such treaty in 1963 and two in 1990 that together bar all but underground nuclear tests with an explosive yield of 150 kilotons or less. The United States has observed a unilateral moratorium on nuclear tests since 1992. In 1996, this nation signed the Comprehensive Nuclear-Test-Ban Treaty (CTBT), which would ban all nuclear explosions. The Senate rejected the CTBT in 1999.