He also eventually came up with the idea of using more powerful shaped charges and less fissionable material to greatly increase the speed of assembly (compression).
This is the reason that nuclear reprocessing is a useful activity: solid spent nuclear fuel contains about 97% of the original fissionable material present in newly manufactured nuclear fuel.
Fast reactors have the potential to produce less transuranic waste because all actinides are fissionable with fast neutrons, but they are more difficult to build and more expensive to operate.
This mined uranium is used to fuel energy-generating nuclear reactors with fissionable uranium-235 which generates heat that is ultimately used to power turbines to generate electricity.