Uranium enrichment is the process by which one isotope of uranium (U-235) is concentrated relative to another (U-238). The U-235 isotope is used for generation of nuclear energy but constitutes only 0.7% of natural uranium. The remaining 99.3% is U-238 which does not directly contribute to the fission process required for energy generation. The Iranian government has started enriching uranium with 20% purity at its underground Fordo Nuclear Facility. This brings Tehran just steps away from obtaining weapons grade uranium.