From the earliest days of the development of peaceful nuclear power, it has been recognized that efficient utilization of nuclear fuel resources requires a closed fuel cycle (recycle). With a closed cycle, essentially all the energy content of mined uranium can be used, whereas a once-through light water reactor (LWR) cycle uses only [approximately]0.5%. Since weapons programs have used the PUREX process to extract plutonium, it has further been assumed that this is the appropriate technology for closing the uranium fuel cycle. In the United States, these assumptions were put into question by concerns over commerce in separated plutonium and the threat of diversion of this material for weapons use.