Uranium-235 is a naturally occurring isotope of uranium, making up about 0.7% of natural uranium. It is significant because it can undergo nuclear fission, a process where the nucleus of an atom splits into smaller parts, releasing a large amount of energy. This property makes Uranium-235 essential for both nuclear power generation and nuclear weapons. In nuclear reactors, Uranium-235 is used as fuel to produce electricity. When it absorbs a neutron, it can split and release more neutrons, which can then cause further fission reactions. This chain reaction is carefully controlled to ensure a steady output of energy.