U-235 is a radioactive isotope of uranium that is used in nuclear reactors and certain nuclear weapons. The value 235 is the isotope’s mass number, which indicates the total number of neutrons and protons in the nucleus of the atom. U-235 occurs naturally in such minerals as carnotite and pitchblende. It is always combined with two other uranium isotopes, U-234 and U-238.
The nucleus of U-235 decays (breaks apart) by giving off high-energy radiation in the form of alpha particles. U-235 emits these particles for a long period of time. It has a half-life of about 700 million years—that is, due to radioactive decay, half the atoms in a sample of U—235 would still be atoms of that isotope after 700 million years.
U-235 is used as a nuclear fuel because it readily undergoes fission when struck by a neutron. Fission is a nuclear reaction in which a nucleus splits into two smaller nuclei of nearly equal size and energy is released. Before U-235 can be used in reactors and atomic bombs, it must be separated from the more abundant U-238, which does not fission easily. This separation produces enriched uranium, which contains a higher percentage of U-235 than natural uranium does. U-235 makes up less than 1 percent of natural uranium but about 2 to 4 percent of the enriched uranium that is used in most reactors. See Uranium (Separating uranium isotopes) .
The fissioning of U-235 produces hundreds of other radioactive isotopes. Many of them, such as cesium 137 and strontium 90, are very hazardous. Large amounts of these fission products are generated during nuclear power production. Their safe disposal has become a serious problem, which experts are working to solve (see Nuclear energy (Wastes and waste disposal) ).
U-235 was discovered in 1935 by Arthur J. Dempster, a Canadian-born physicist. In 1942, a group of physicists at the University of Chicago used U-235 to produce the first self-sustaining nuclear chain reaction (series of fissions). The first atomic bomb used in warfare contained U-235. This bomb was dropped by the United States armed forces on Hiroshima, Japan, in 1945, and helped bring an end to World War II.
