Under the MDA, 5.37 tonnes of UK-produced plutonium was sent to the US in exchange for 6.7 kg of tritium and 7.5 tonnes of HEU between 1960 and 1979. A further 470 kg of plutonium was swapped between the US and the UK for reasons that remain classified. 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 MDA, the US supplied the UK with not just nuclear submarine propulsion technology, but a complete S5W pressurised water reactor of the kind used to power the US submarines. This was used in the Royal Navy's first nuclear-powered submarine, , which was launched in 1960 and commissioned in 1963. The S5W had a Nuclear reactor core that used 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 nuclear submarines.