A nuclear chain reaction is a string of reactions that occurs when a given nuclear reaction causes, on average, at least one more nuclear reaction to take place. Such chain reactions are important for the generation of nuclear power and also for nuclear weapons. Uranium-235 is responsible for the chain reaction that generates power in nuclear reactors and in some bombs as well. Uranium-238 is the more common isotope, so it is typically necessary to first enrich the uranium to be used in the 235 isotope. When a neutron collides with uranium-235 it generates uranium-236, which then undergoes fission to release energy and further neutrons that can collide with other uranium-235 atoms, causing the chain reaction to continue.