A fission reaction is a type of nuclear reaction in which the nucleus of an atom splits into smaller parts, releasing a large amount of energy.
U235, or uranium-235, is a naturally occurring isotope of uranium that is used as a fuel in nuclear reactors.
During a fission reaction, U235 absorbs a neutron, becoming unstable and splitting into two smaller nuclei. This process releases more neutrons and a large amount of energy.
The mass of U235 decreases during a fission reaction due to the conversion of some of its mass into energy according to Einstein's famous equation, E=mc².
Fission reactions and U235 have several applications, including producing electricity in nuclear power plants, powering submarines and aircraft carriers, and producing medical isotopes for cancer treatment.