1958 US–UK Mutual Defence Agreement
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.
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 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 tonnes (11,800 lb) of UK-produced plutonium was sent to the US in return for 6.7 kilograms (15 lb) of tritium and 7.5 tonnes (17,000 lb) of highly enriched uranium over the period 1960–79. A further 470 kilograms (1,000 lb) 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.
See also 
- Atomic Weapons Establishment
- Nassau agreement
- Nuclear weapons and the United Kingdom
- Nuclear weapons and the United States
- Project E
- Quebec Agreement
- Special relationship
- UKUSA Agreement
- "Amendment to the 1958 US-UK Mutual Defence Agreement (on nuclear weapons' cooperation)". British American Security Information Council. June 2004. Archived from the original on 30 November 2006. Retrieved 15 March 2007.
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- "Yellow Sun MK.2 Enters Service". Atomic Weapons Establishment. Archived from the original on 18 January 2008. Retrieved 15 March 2007.
- Prospects for Further Proliferation of Nuclear Weapons, Special National Intelligence Estimate, CIA, 23 August 1974, p. 40, SNIE 4-1-74, archived from the original on 13 February 2008, retrieved 20 January 2008
- "National Security Decision Memorandum 276". United States National Security Council. 15 October 1974. Retrieved 15 March 2007.
- "National Security Directive 61" (PDF). The White House. 2 July 1991. Retrieved 15 March 2007.
- Dan Plesch (March 2006). The Future of Britain’s WMD (PDF). Foreign Policy Centre. p. 15. Archived from the original on 21 June 2006. Retrieved 15 March 2007.
- Ministry of Defence and Property Services Agency: Control and Management of the Trident Programme. National Audit Office. 29 June 1987. pp. ara. 1.1, 3.27, A4.4. ISBN 978-0-10-202788-4.
- "Britain's Next Nuclear Era". Federation of American Scientists. 7 December 2006. Archived from the original on 6 February 2007. Retrieved 15 March 2007.
- "Stockpile Stewardship Plan: Second Annual Update (FY 1999)" (PDF). United States Department of Energy. April 1998. Retrieved 15 March 2007.
- "Plutonium and Aldermaston – an historical account" (PDF). UK Ministry of Defence. 4 September 2001. Archived from the original on 13 December 2006. Retrieved 15 March 2007.
- "Additional Information Concerning Underground Nuclear Weapon Test of Reactor-Grade Plutonium". US Department of Energy. June 1994. Retrieved 15 March 2007.
- David Lowry (29 April 2004). "Obituary: Ross Hesketh". The Guardian. Retrieved 23 March 2012.
- Full text of treaty
- Negotiations for extension to treaty
- US-UK Agreement, Atomic Weapons Establishment
- Mutual Defence Agreement and the Nuclear Non-Proliferation Treaty – Legal Opinion