In relation to the fission of heavy nuclides they are subdivided into two groups. First, the fissile isotopes. They efficiently undergo the fission by neutrons of any energy (thermal, intermediate, fast). The probability to cause the fission is higher for thermal in comparison to fast eutrons. For the nuclei with atomic number more than 90 the fissile nucleus usually has an even number of protons and an odd number of nucleons. For example, uranium 235 has the atomic number 92 and the mass of 235 atomic mass units. Second, the fertile isotopes. Fertile isotopes are broken down efficiently only by the fast neutrons. They usually have an even number of protons and an even number of nucleons. But can be converted into fissile isotopes (after neutron absorption and the subsequent nuclear decay). This process called breeding. What fission fragments are formed and how many free neutrons are radiated in the course of fission, is defined by the probability of these processes. A description of "two-humped curve" characterizing the process of formation of fission fragments is given on the Figure. The average mass of a light fission fragment is 95 and of a heavy fission fragment it equals to 137 atomic mass unit. By the fission we can get approximately 200 MeV of energy per one nuclear reaction. It is million times more compared to the chemical reaction. The reason of this huge energy yield is the Mass defect. The Mass defect is the difference in mass between a nucleus and the sum of the masses of the individual protons and neutrons in the nucleus (AMU). The difference between these numbers is converted to energy. The defect of mass can be represented in terms of the energy according the Einstein’s formula. This energy is called the binding energy. The binding energy is defined as the energy equivalent to the mass defect and is measured in the units of MeV (mega electron volts). On the Figure the dependence of binding energy per one nucleon for nuclei of different weight is given. The binding energy per nucleon slowly decreases as the mass number increases. This occurs because the proton-to-proton repulsive force increases faster than the nuclear attractive forces. If we calculate the difference between the masses of an initial heavy nucleus and fission fragments, we will get by using the Einstein formula 200 MeV excess energy per one reaction of fission. Nuclear fission is the main process generating nuclear energy. In the course of a nuclear fission of a heavy nuclide about 200 Mev of energy is released. In the table the distribution of this energy between fission products is described. You can see that most of the energy about 85 % is released in the form of the kinetic energy of the split parts. As a result of interaction of nuclei with fission fragments in general this energy turns into the heat energy. The fuel temperature at the same time can reach more than 1000 C. For heat removal from the fuel the coolant (water, gas, liquid metal) is used. The nuclear chain reaction occurs when one single nuclear reaction causes an average of one or more subsequent nuclear reactions. The fission process may produce 2, 3 or more free neutrons that are capable of inducing further fissions and so on. This sequence of fission events is known as the fission chain reaction and it is of importance in the nuclear reactor physics. It is obvious, that if one neutron causes two further fissions, the number of neutrons in the multiplication system will increase in time and the reactor power (reaction rate) will also increase in time. In order to stabilize such multiplication environment, it is necessary to increase the non-fission neutron absorption in the system (e.g. to insert control rods). Moreover, this multiplication environment (means the nuclear reactor) behaves like the exponential system, which means that the power increase is not linear, but it is exponential. On the other hand, if one neutron causes less than one further fission, the number of neutrons of the multiplication system will decrease in time and the reactor power (reaction rate) will also decrease in time.