Project 639 in the context of Nuclear test

The Teller–Ulam design is the technical concept behind thermonuclear weapons, also known as hydrogen bombs. The design relies on the radiation implosion principle, using thermal X-rays released from a fission nuclear primary to compress and ignite nuclear fusion in a secondary. This is in contrast to the simpler design and usage of nuclear fusion in boosted fission weapons.

The design is named for scientists Edward Teller and Stanisław Ulam, who originally devised the concept in January 1951 for the United States nuclear weapons program, though their individual roles have been subsequently debated. The US Greenhouse George test in May 1951, the world's first artificial thermonuclear fusion, validated the radiation implosion principle. The US first tested the "true" Teller-Ulam design with the very high-yield Ivy Mike test in 1952. The design was independently devised and then tested by teams of nuclear weapons scientists working for at least four more governments: the Soviet Union in 1955 (RDS-37), the United Kingdom in 1957 (Operation Grapple), China in 1966 (Project 639), and France in 1968 (Canopus). There is not enough public information to determine whether India, Israel, or North Korea possess multi-stage weapons. Pakistan is not considered to have developed them. The Teller-Ulam design is the basis for all nuclear weapons tests above one megaton yield.

China was the fifth country to develop and test nuclear weapons. China's tested its first nuclear weapon in 1964 and its first full-scale thermonuclear weapon in 1967. It is one of the five nuclear-weapon states recognized by the Nuclear Non-Proliferation Treaty (NPT), to which it acceded in 1992. It carried out 45 nuclear tests before signing the Comprehensive Nuclear-Test-Ban Treaty in 1996.

The Federation of American Scientists and the Stockholm International Peace Research Institute estimate China's stockpile at approximately 600 nuclear warheads as of 2025, making it the third-largest in the world. It is the only NPT nuclear-weapon state significantly expanding its arsenal, which has doubled since 2019, and is projected to reach between 750 and 1,500 warheads by 2035, although it has not restarted fissile material production. Unlike the US and Russia, nearly all Chinese warheads are believed to be in central storage, separate from their delivery system.

The People's Republic of China has possessed nuclear weapons since the 1960s. It was the last to develop them of the five nuclear-weapon states recognized by the Nuclear Non-Proliferation Treaty (NPT). China acceded to the Biological Weapons Convention (BWC) in 1984, acceded to the NPT in 1992, and ratified the Chemical Weapons Convention (CWC) in 1997.

China tested its first nuclear bomb in 1964 and its first full-scale thermonuclear bomb in 1967. It carried out 45 nuclear tests before signing the Comprehensive Nuclear-Test-Ban Treaty in 1996.