If at least one neutron from uranium-235 fission strikes another nucleus and causes it to fission, then the chain reaction will continue. If the reaction will sustain itself, it is said to be critical, and the mass of U-235 required to produce the critical condition is said to be a critical mass. A critical chain reaction can be achieved at low concentrations of U-235 if the neutrons from fission are moderated to lower their speed, since the probability for fission with slow neutrons is greater. A fission chain reaction produces intermediate mass fragments which are highly radioactive and produce further energy by their radioactive decay. Some of them produce neutrons, called delayed neutrons, which contribute to the fission chain reaction. In nuclear reactors, the reaction is slowed down by the addition of control rods which are made of elements such as boron, cadmium, and hafnium which can absorb a large number of neutrons. In nuclear bombs, the reaction is uncontrolled and the large amount of energy released creates a nuclear explosion.
Uranium-235 ( 235 U ) is an isotope of uranium making up about 0.72% of natural uranium . Unlike the predominant isotope uranium-238 , it is fissile , i.e., it can sustain a fission chain reaction . It is the only fissile isotope that is a primordial nuclide or found in significant quantity in nature.