The Partial Test Ban Treaty (PTBT), formally known as the 1963 Treaty Banning Nuclear Weapon Tests in the Atmosphere, in Outer Space and Under Water, prohibited all test detonations of nuclear weapons except for those conducted underground. It is also abbreviated as the Limited Test Ban Treaty (LTBT) and Nuclear Test Ban Treaty (NTBT), though the latter may also refer to the Comprehensive Nuclear-Test-Ban Treaty (CTBT), which succeeded the PTBT for ratifying parties.
Negotiations initially focused on a comprehensive ban, but that was abandoned because of technical questions surrounding the detection of underground tests and Soviet concerns over the intrusiveness of proposed verification methods. The impetus for the test ban was provided by rising public anxiety over the magnitude of nuclear tests, particularly tests of new thermonuclear weapons (hydrogen bombs), and the resulting nuclear fallout. A test ban was also seen as a means of slowing nuclear proliferation and the nuclear arms race. Though the PTBT did not halt proliferation or the arms race, its enactment did coincide with a substantial decline in the concentration of radioactive particles in the atmosphere.
The PTBT was signed by the governments of the Soviet Union, the United Kingdom, and the United States in Moscow on 5 August 1963 before it was opened for signature by other countries. The treaty formally went into effect on 10 October 1963. Since then, 123 other states have become party to the treaty. Ten states have signed but not ratified the treaty.
The treaty contributed to a lasting taboo on non-underground tests. Non-signatories France and China continued atmospheric testing until 1974 and 1980. Signatories Israel and South Africa may have violated it with the 1979 Vela incident. Since 1980, all declared nuclear weapons states have made underground tests, and there have been no suspected non-underground tests.
Background
The primary motivation for the treaty stemmed from growing public concern over radioactive fallout from above-ground and underwater nuclear testing, especially as the power of nuclear devices continued to increase. Additionally, there was widespread apprehension about the broader environmental damage caused by such testing. In 1952–53, both the United States and the Soviet Union detonated their first thermonuclear weapons (hydrogen bombs), which were significantly more powerful than the atomic bombs that had been tested and deployed since the Trinity test in 1945. In 1954, the U.S. conducted the Castle Bravo test at Bikini Atoll as part of Operation Castle, which resulted in a yield of 15 megatons of TNT, more than double the expected yield. This unexpectedly high yield caused widespread radioactive fallout, leading to severe environmental and human consequences. The Castle Bravo test resulted in the worst radiological event in US history as radioactive particles spread over more than , affected inhabited areas (including Rongelap Atoll and Utirik Atoll), and sickened Japanese fishermen aboard the Lucky Dragon upon whom "ashes of death" had rained. In the same year, a Soviet test sent radioactive particles over Japan. Around the same time, victims of the atomic bombing of Hiroshima visited the US for medical care, which attracted significant public attention. In 1961, the Soviet Union tested the Tsar Bomba, which had a yield of 50 megatons and remains the most powerful human-made explosion in history, though due to use of a lead tamper, fallout was relatively limited. Between 1951 and 1958, the US conducted 166 atmospheric tests, the Soviet Union conducted 82, and Britain conducted 21; only 22 underground tests were conducted in this period (all by the US).
Negotiations
Early efforts In 1945, Britain and Canada made an early call for international discussions on the control of atomic power, anticipating the global implications of nuclear technology. At the time, the U.S. had not yet developed a cohesive policy or strategy regarding nuclear weapons. Vannevar Bush, who had been instrumental in initiating and overseeing the Manhattan Project, sought to leverage this moment to advance his long-term goal of banning nuclear weapons production, despite the project's role in developing them. As a first step in this direction, Bush proposed an international agency dedicated to nuclear control. Bush unsuccessfully argued in 1952 that the US pursue a test ban agreement with the Soviet Union before testing its first thermonuclear weapon, but his interest in international controls was echoed in the 1946 Acheson–Lilienthal Report, which had been commissioned by President Harry S. Truman to help construct US nuclear weapons policy. J. Robert Oppenheimer, who had led Los Alamos National Laboratory during the Manhattan Project, exerted significant influence over the report, particularly in its recommendation of an international body that would control production of and research on the world's supply of uranium and thorium. A version of the Acheson-Lilienthal plan was presented to the United Nations Atomic Energy Commission (UNAEC) as the Baruch Plan in June 1946. The Baruch Plan proposed that an International Atomic Development Authority would control all research on and material and equipment involved in the production of atomic energy. Though Dwight D. Eisenhower, then the Chief of Staff of the United States Army, was not a significant figure in the Truman administration on nuclear questions, he did support Truman's nuclear control policy, including the Baruch Plan's provision for an international control agency, provided that the control system was accompanied by "a system of free and complete inspection."
The Soviet Union dismissed the Baruch Plan as a US attempt to secure its nuclear dominance, and called for the US to halt weapons production and release technical information on its program. The Acheson–Lilienthal paper and Baruch Plan would serve as the basis for US policy into the 1950s. Between 1947 and 1954, the US and Soviet Union discussed their demands within the United Nations Commission for Conventional Disarmament. A series of events in 1954, including the Castle Bravo test and spread of fallout from a Soviet test over Japan, redirected the international discussion on nuclear policy. Additionally, by 1954, both US and Soviet Union had assembled large nuclear stockpiles, reducing hopes of complete disarmament. In the early years of the Cold War, the US approach to nuclear control reflected a strain between an interest in controlling nuclear weapons and a belief that dominance in the nuclear arena, particularly given the size of Soviet conventional forces, was critical to US security. Interest in nuclear control and efforts to stall proliferation of weapons to other states grew as the Soviet Union's nuclear capabilities increased.
After Castle Bravo: 1954–1958 In 1954, just weeks after the Castle Bravo test, Indian Prime Minister Jawaharlal Nehru made the first public call for a "standstill agreement" on nuclear testing. Nehru viewed a testing moratorium as a crucial first step toward broader and more comprehensive arms control agreements, reflecting his commitment to nuclear disarmament and global peace. In the same year, the British Labour Party, then led by Clement Attlee, called on the UN to ban testing of thermonuclear weapons. 1955 marks the beginning of test-ban negotiations, as Soviet leader Nikita Khrushchev first proposed talks on the subject in February 1955. On 10 May 1955, the Soviet Union proposed a test ban before the UN Disarmament Commission's "Committee of Five" (Britain, Canada, France, the Soviet Union, and the US). This proposal, which closely reflected a prior Anglo-French proposal, was initially part of a comprehensive disarmament proposal meant to reduce conventional arms levels and eliminate nuclear weapons. Despite the closeness of the Soviet proposal to earlier Western proposals, the US reversed its position on the provisions and rejected the Soviet offer "in the absence of more general control agreements," including limits on the production of fissionable material and protections against a surprise nuclear strike. The May 1955 proposal is now seen as evidence of Khrushchev's "new approach" to foreign policy, as Khrushchev sought to mend relations with the West. The proposal would serve as the basis of the Soviet negotiating position through 1957.
Eisenhower had supported nuclear testing after World War II. In 1947, he rejected arguments by Stafford L. Warren, the Manhattan Project's chief physician, concerning the detrimental health effects of atmospheric testing, agreeing instead with James Bryant Conant, a chemist and participant in the Manhattan Project, who was skeptical of Warren's then-theoretical claims. Warren's arguments were lent credence in the scientific community and public by the Castle Bravo test of 1954. Eisenhower, as president, first explicitly expressed interest in a comprehensive test ban that year, arguing before the National Security Council, "We could put [the Russians] on the spot if we accepted a moratorium ... Everybody seems to think that we're skunks, saber-rattlers and warmongers. We ought not miss any chance to make clear our peaceful objectives." Then-Secretary of State John Foster Dulles had responded skeptically to the limited arms-control suggestion of Nehru, whose proposal for a test ban was discarded by the National Security Council for being "not practical." Harold Stassen, Eisenhower's special assistant for disarmament, argued that the US should prioritize a test ban as a first step towards comprehensive arms control, conditional on the Soviet Union accepting on-site inspections, over full disarmament. Stassen's suggestion was dismissed by others in the administration over fears that the Soviet Union would be able to conduct secret tests. On the advice of Dulles, Atomic Energy Commission (AEC) chairman Lewis Strauss, and Secretary of Defense Charles Erwin Wilson, Eisenhower rejected the idea of considering a test ban outside general disarmament efforts. During the 1952 and 1956 presidential elections, Eisenhower fended off challenger Adlai Stevenson, who ran in large part on support for a test ban.
The British governments of 1954–58 (under Conservatives Winston Churchill, Anthony Eden, and Harold Macmillan) also quietly resisted a test ban, despite the British public favoring a deal, until the US Congress approved expanded nuclear collaboration in 1958 and until after Britain had tested its first hydrogen bombs. In their view, testing was necessary if the UK nuclear program were to continue to develop. This opposition was tempered by concern that resistance to a test ban might lead the US and Soviet Union to pursue an agreement without Britain having any say in the matter.
Members of the Soviet military–industrial complex also opposed a test ban, though some scientists, including Igor Kurchatov, were supportive of antinuclear efforts. France, which was in the midst of developing its own nuclear weapon, also firmly opposed a test ban in the late 1950s.