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 material s 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 of UK - produced plutonium was sent to the US in return for of tritium and of highly enriched uranium over the period 1960 – 79 . A further 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 .