Uranium 235 is an isotope of uranium, accounting for 0.72% of natural uranium, the source of raw materials in the production of nuclear electricity by nuclear fission. Uranium-235 has a half-life of 703.8 million years. It was discovered by Arthur Jeffrey Dempster in 1935. Its slow neutron fission is 584,994 barn, fast neutron is about 1 barn.
10n + 23592U → 14156Ba + 9236Kr + 3 10n
Heavy water reactors, and some graphite-based slow-moving reactors, can use natural uranium, but light water reactors must use low-enriched uranium because higher neutrons absorb light water. Uranium enrichment removes some uranium-238 and increases the proportion of uranium-235. Highly enriched uranium (HEU), containing an even greater proportion of uranium-235, is sometimes used in nuclear submarines and nuclear weapons.
If at least one neutron from uranium-235 fission strikes another nucleus and causes it to fission, the chain reaction will continue. If the reaction will be self-sustaining, it is said to be critical, and the mass of U-235 required to create the critical condition is said to be critical. An important chain reaction can be achieved at low concentrations of U-235 if the neutrons from fission are moderated to reduce their speed, because the probability of fission with slow neutrons is greater. The fission chain reaction produces highly radioactive bulk fragments and further energy generation by radioactive decay. Some of them produce neutrons, known as delayed neutrons, which contribute to the fission chain reaction. A nuclear reactor's power output is regulated by the position of a control rod containing elements that strongly absorb neutrons, e.g. boron, cadmium or hafnium, in the reactor core. In a nuclear bomb, the reaction is not controlled and a large amount of energy is released creating a nuclear explosion
The Little Boy type atomic bomb dropped on Hiroshima on August 6, 1945 was made of highly enriched uranium with a large amount of tamper. The nominal spherical critical mass for an unguided 235 U nuclear weapon is 56 kg (123 lb), a sphere with a diameter of 17.32 cm (6.8"). Must have 85% or more of 235 U and be called uranium grade weapons, although for a crude, 20% effective weapon is enough (called weapon(s) -usable). can be used, but is then required to rapidly increase critical mass.Using a large tamper, geometric explosion, trigger tube, activated polonium, enhanced tritium and neutron reflection may allow more compact, more economical weapons using a quarter or less of the nominal critical mass, although this is only possible in a country with extensive experience in nuclear weapons engineering. Most modern nuclear weapon designs use plutonium-239 as the primary stage fission component, however HEU is commonly used in the secondary stage.
Natural decay sequence