Uranium-235

Uranium-235 is an isotope of uranium that differs from the element's other common isotope, uranium-238, by its ability to cause a rapidly expanding fission chain reaction, i.e., it is fissile. It is the only fissile isotope found in any economic quantity in nature. It was discovered in 1935 by Arthur Jeffrey Dempster.

If at least one neutron from U-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.



The fission of one atom of U-235 generates 200 MeV = 3.2 &times; 10-11 J, i.e. 18 TJ/mol = 77 TJ/kg. However, approximately 5% of this energy is carried away by virtually undetectable neutrinos. 

The nuclear cross section for slow thermal neutrons is about 1000 barns. For fast neutrons it is in the order of 1 barn.

Only around 0.72% of all natural uranium is uranium-235, the rest being mostly uranium-238. This concentration is insufficient for a self sustaining reaction in a light water reactor; enrichment, which just means separating out the uranium-238, must take place to get a usable concentration of uranium-235. Pressurised Heavy Water Reactors, other heavy water reactors, and some graphite moderated reactors are known for using unenriched uranium. Uranium which has been processed to boost its uranium-235 proportion is known as enriched uranium, different applications require unique levels of enrichment.

The fissile uranium in nuclear weapons usually contains 85% or more of 235U known as weapon(s)-grade, though for a crude, inefficient weapon 20% is sufficient (called weapon(s)-usable); even less is sufficient, but then the critical mass required rapidly increases. However, judicious use of implosion and neutron reflectors can enable construction of a weapon from a quantity of uranium below the usual critical mass for its level of enrichment, though this would likely only be possible in a country which already had extensive experience in developing nuclear weapons. The Little Boy atomic bomb was fueled by enriched uranium. Most modern nuclear arsenals use plutonium as the fissile component, however U-235 devices remain a nuclear proliferation concern due to the simplicity of this nuclear weapon design.

Uranium-235 has a half-life of 700 million years.