Fission Reaction

Nuclear fission reaction, which is the source of thermal energy, takes place in nuclei of isotope of uranium U 235 in the active zone of nuclear reactor. Uranium occurs in nature as a mixture of isotopes of uranium U 235 and U 238, which differ mutually by the number of neutrons in their nuclei. The possibility of fission is high only in isotopes of uranium U 235. After fission of the nucleus, two to three neutrons are usually released, which can, under suitable conditions, fission surrounding nuclei of uranium and release other neutrons in this way. This phenomenon is called the chain fission reaction. It is necessary for practical utilization to regulate the whole process, so that number of free neutrons is the same all the time. Abundant neutrons are absorbed in suitable materials — absorbers. For example, chemical element boron is such a suitable material. However, not every neutron is suitable for fission of nucleus of uranium U 235, but only such one, which has a small velocity. That is why a suitable obstacle is put into the path of neutrons, which is able to slow them down quickly and efficiently. Neutrons are slowed down by moderators to so called thermal neutrons, which have a higher probability to generate the next fission of nuclei of uranium 235. The best moderator is pure hydrogen. In practice common pure water is used for this purpose most frequently.