Nuclear fusion is a chain reaction

Nuclear fission and nuclear fusion

Otto HAHN (1879-1968), Fritz STRASSMANN (1902-1980) and Lise MEITNER (1878-1968) were the first to discover that the heavy uranium-235 nuclei can be bombarded with slow neutrons (typical speed approx. \ frac {\ rm {km}} {\ rm {s}} \)) can split. The uranium-235 core picks up the slow neutron and an unstable intermediate core (uranium-236) is formed for a short time, which "bursts" into two medium-weight core fragments. This fission process produces 2 to 3 very fast neutrons (typical speed approx. \ (10000 \, \ frac {\ rm {km}} {\ rm {s}} \)), which can trigger further nuclear fission after deceleration (chain reaction ). The core fragments produced during the fission are radioactive and continue to decay.

Part of the binding energy that held the uranium nucleus together and is no longer needed is taken by the nuclear fragments as kinetic energy. Furthermore, the fragments are positively charged, repel each other electrically and fly apart. However, since they are embedded in a crystal lattice, they cannot fly away freely, but are slowed down very quickly. During the braking process, the kinetic energy is converted into internal energy.

Critical mass

In order for a chain reaction to take place, some of the neutrons released must in turn trigger nuclear fission. The smallest mass of a fissile material with which such a chain reaction can be maintained is calledcritical mass. If the critical mass is not reached, too many neutrons leave the material on its surface before they have caused a fission. Therefore, the critical mass also depends on the geometric arrangement of the fissile material. In the favorable case of a spherical arrangement, the critical mass of U ‑ 235 is about \ (50 \, \ rm {kg} \). If this amount of U-235 were rolled out flat, the critical mass would, however, be undershot and a chain reaction would not be possible.