Fission

Fission means the splitting of the heavy atomic nuclei of uranium, for example, into medium-mass nuclei. In addition to these fission product nuclei, the fission releases neutrons that can cause new fissions, thus maintaining a continuous chain reaction. The combined mass of the fission products and neutrons is slightly smaller than the mass of the original heavy nucleus, the ´missing´ matter being converted into energy.

The energy that has been created appears initially in the form of the kinetic energy of fission products and neutrons, but converts gradually into heat when particles and the atoms of the surrounding matter collide. This thermal energy is exploited for producing electricity at nuclear power plants by means of a steam turbine and electricity generator.

The thermal energy released by a single uranium nucleus splitting is naturally a tiny amount; it is, after all, taking place in the world of atoms. To maintain one watt of power requires 31 billion fission reactions per second. On the other hand, the amount of energy released is great compared with the amount of mass associated with the event: 1 g of fissile matter is equivalent to 24,000 kWh of energy.

History

Fission was discovered by two German chemists in 1939, Otto Hahn and Fritz Strassman. They bombed a solution containing uranium with neutrons and observed that it created substances not originally in the solution, e.g barium. The mystery was solved a little later by two physicists, Lise Meitner and Otto Frisch, who concluded that a uranium nucleus can split into two parts of roughly equal size when a neutron hits it.

The existence of neutrons had been discovered in 1932 by James Chadwick, an English physicist, and it was soon observed that neutrons held a special position among elementary particles. Electrically neutral, they can be used as missiles that can penetrate an atomic nucleus, thereby causing different reactions.