Enrichment is the physical process by which uranium isotopes are separated. Most commercial nuclear reactors worldwide run on uranium fuel which has been enriched to the level of approximately 4.5% (by weight) in the isotope U235, which is the fissionable isotope that enables a chain reaction to occur. In nature, about 99% of uranium atoms exist as the isotope U238, while only about 0.7% exist as U235. In order to achieve this, it is necessary to physically separate the U235 and U238 isotopes, utilizing the tiny differences in mass between the two isotopes. This process is done through one of two commercial-scale methods: gaseous diffusion or centrifuge. In the diffusion process, which is still used in the United States and France, uranium hexafluoride (UF6) gas is bombarded against a semi-permeable membrane under high pressure, so that the slightly lighter U235 atoms pass through the barrier at a greater rate than the heavier U238 atoms. In the centrifuge process, UF6 gas is spun at high speeds in specially designed centrifuges so that the heavier U238 atoms migrate to the outside of the cylinder. Centrifuge technology is employed in Russia and several European countries. The enrichment process is typically very energy-intensive and expensive (about 35% of the cost of the finished fuel), although the centrifuge process uses only about 5% of the energy of the diffusion process. It is also a heavily restricted technology because of the ability to produce weapons-grade uranium. Enrichment programs are monitored by the International Atomic Energy Agency in Vienna. Most of the uranium enrichment for commercial nuclear fuel is conducted in Russia, France, the U.K., Germany, the Netherlandx, and the United States, with smaller programs in China and Japan.