Some of the fissile materials for the UK Trident warhead were purchased from the US, but much of the HEU supplied by the US was used not for weapons but as fuel for the growing fleet of UK nuclear submarines. Under the treaty, the US supplied the UK with not only nuclear submarine propulsion technology but also a complete S5W pressurised water reactor of the kind used to power the US submarines. That was used in the Royal Navy's first nuclear-powered submarine, , which was launched in 1960 and commissioned in 1963. The S5W was fuelled by uranium enriched to between 93 and 97 per cent uranium-235. Reactor technology was transferred from Westinghouse to Rolls Royce, which used it as the basis for its PWR1 reactor used in the UK's , , , and submarines.