Such nuclear batteries have been used for decades in spacecraft by all major spacefaring nations and some have even been implanted in humans as power sources for medical devices such as pacemakers, meaning their behavior and tradeoffs are well understood. The Japanese spacecraft Kaguya confirmed the existence of both Uranium and Thorium near the lunar surface, which might in the future allow for the local production of nuclear fission fuel from lunar resources. However, concentrations are relatively low and unless the thorium fuel cycle is used, uranium would likely have to be enriched to produce fuel usable in light water reactors. The isotopic composition of lunar uranium is not known, but there is little reason to assume it would differ much from that found on earth (99% , ~0.72% , 55 ppm ). Nuclear reprocessing in situ would reduce the need for enrichment or imported fuel from earth. Pyroprocessing, which has been demonstrated at the Integral Fast Reactor prototype operated by Argonne National Laboratory, could be used instead of the de facto standard PUREX which requires large amounts of organic solvents. One specific development program in the 2000s was the Fission Surface Power (FSP) project of NASA and DOE, a fission power system focused on "developing and demonstrating a nominal 40 kWe power system to support human exploration missions. The FSP system concept uses conventional low-temperature stainless steel, liquid metal-cooled reactor technology coupled with Stirling power conversion." , significant component hardware testing had been successfully completed, and a non-nuclear system demonstration test was being fabricated. In 2017 NASA started the Kilopower project that tested the KRUSTY reactor. Japan has the RAPID-L conceptual design. Helium-3 mining could be used to provide a substitute for tritium for potential production of fusion power in the future. Solar energy Solar energy is a possible source of power for a lunar base. Many of the raw materials needed for solar panel production can be extracted on site. The long lunar night (354 hours or 14.75 Earth days) is a drawback for solar power on the Moon's surface. This might be solved by building several power plants, so that at least one of them is always in daylight. Another possibility would be to build such a power plant where there is constant or near-constant sunlight, such as at the Malapert mountain near the lunar south pole, or on the rim of Peary crater near the north