The "dual use" capability of the Magnox design led to the UK building up a large stockpile of fuel grade/"reactor grade" plutonium, with the aid of the B205 reprocessing facility. The low-to-interim burnup feature of the reactor design would become responsible for changes to US regulatory classifications after the US-UK "Reactor-grade" plutonium detonation test of the 1960s. Despite improving its electricity generating capabilities in later decades, marked by the transition to electric-power becoming the primary operational aim, magnox reactors were never capable of consistently generating high efficiency/high fuel "burn-ups" due to the handicap of its design and natural uranium heritage, compared with pressurized water reactors, the most widespread power-reactor design.