1958 US–UK Mutual Defence Agreement

The 1958 US–UK Mutual Defence Agreement is a bilateral treaty between the United States and the United Kingdom on nuclear weapons cooperation.

It was signed after the UK successfully tested its first hydrogen bomb during Operation Grapple. While the US has nuclear cooperation agreements with other countries, including France and some NATO countries, this agreement is by far the most comprehensive.

The treaty is renewed every ten years, most recently extending the treaty to 2014.

Details of the agreement
The agreement enables the US and the UK to exchange classified information with the objective of improving each party's "atomic weapon design, development, and fabrication capability".

This includes development of defence plans; training personnel in the use and defence against nuclear weapons; evaluation of enemy capabilities; development of nuclear delivery systems; and research, development and design of military reactors. The agreement also provides for the transfer of special nuclear material (e.g. plutonium, highly enriched uranium, tritium), components, and equipment between the two countries, and the transfer of "non-nuclear parts of atomic weapons" to the UK.

The agreement also covered the export of one complete US submarine nuclear propulsion plant and its enriched uranium fuel which was installed in the UK's first nuclear-powered submarine, HMS Dreadnought.

The UK was able to carry out underground nuclear tests at the US Nevada Test Site, the first taking place on 1 March 1962, following this agreement.

There are also confidential intelligence matters covered by the agreement. The UK government has not published these sections "because of the necessity for great confidentiality and because ... it might well assist proliferation".

This agreement replaced the earlier "Agreement for Cooperation Regarding Atomic Information for Mutual Defense Purposes" of 1955. A separate Polaris Sales Agreement was signed on 6 April 1963.

Assistance to UK nuclear weapons development
An early benefit of the agreement was to allow the UK to "Anglicise" the US W28 nuclear warhead as the Red Snow thermonuclear weapon for the Blue Steel missile by 1961. In 1974 a CIA proliferation assessment noted that "In many cases [Britain's sensitive technology in nuclear and missile fields] is based on technology received from the US and could not legitimately be passed on without US permission."

The US President authorised the transfer of "nuclear weapon parts" to the UK between at least the years 1975 to 1996.

The UK National Audit Office noted that most of the UK Trident warhead development and production expenditure was incurred in the US who would supply "certain warhead-related components". Some of the fissile materials for the UK Trident warhead were purchased from the US There is evidence that the warhead design of the British Trident system is similar to, or even based on, the US W76 warhead fitted in some US Navy Trident missiles, with design and blast model data supplied to the UK.

Special nuclear materials barter
Under the agreement 5.37 t of UK-produced plutonium was sent to the US in return for 6.7 kg of tritium and 7.5 t of highly enriched uranium over the period 1960–79. A further 470 kg of plutonium was swapped between the US and the UK for reasons that remain classified. Some of the UK produced plutonium was used in 1962 by the US for the only known nuclear weapon test of reactor-grade plutonium.

The plutonium sent to the US included some produced in UK civil Magnox reactors, and the US gave assurances that this civil plutonium was not used in the US nuclear weapons program. It was used in civil programmes which included californium production and reactor research. However, the UK did obtain military nuclear material in return, so via this barter UK civil power stations probably provided weapons material.