Threshold Test Ban Treaty

The Threshold Test Ban Treaty (TTBT), formally the Treaty on the Limitation of Underground Nuclear Weapon Tests, was signed in Moscow on 3 July 1974 by US President Richard Nixon and Soviet General Secretary Leonid Brezhnev. It capped the yield of any underground nuclear weapon test conducted by either party at 150 kilotons, and committed both sides to restrict the number of tests to a minimum and to exchange detailed data on test sites and geology to aid verification.

The treaty was negotiated as a follow-on to the 1963 Partial Test Ban Treaty, which had already driven testing underground, and as a step toward the long-sought goal of a comprehensive ban. Because it left lower-yield testing untouched and contained no on-site inspection regime in its original form, arms control advocates criticised it as a modest measure. The Carter and Reagan administrations declined to submit the TTBT for Senate advice and consent, citing verification concerns, although both superpowers stated they would observe the 150-kt threshold.

A companion agreement, the Peaceful Nuclear Explosions Treaty (PNET) of 1976, extended the same 150-kt limit to explosions conducted outside declared weapons test sites for peaceful purposes (e.g., excavation, gas-field stimulation).

The verification impasse was resolved by a 1990 Protocol negotiated by Presidents George H. W. Bush and Mikhail Gorbachev, which added hydrodynamic yield-measurement, seismic monitoring, and on-site inspection procedures. The US Senate gave its advice and consent in September 1990, and the TTBT and PNET entered into force on 11 December 1990.

The TTBT remained in force between the United States and the Russian Federation (as the USSR's successor state) and was effectively superseded in practice by the testing moratoria of the 1990s and the still-not-in-force Comprehensive Nuclear-Test-Ban Treaty (CTBT) opened for signature in 1996.