Nuclear power plants work on the same principle as conventional power plants except for the fact that the heat energy required to convert the working medium into steam is not attained by burning fossil fuels but rather through the process of nuclear fission. During this reaction, a large amount of energy is released which the power plant uses to produce electrical current.

Nuclear fission is attained by a nuclear reaction between heavy atomic nuclei and neutrons, which are elementary particles without electrical charge. In this nuclear reaction, the nuclei are broken down into two, releasing more neutrons in the process. These released neutrons then bombard into other atoms, resulting in further fragmentation. This process is referred to as a nuclear fission chain reaction. A condition for this chain reaction to occur is when neutrons freed during fission react with other atomic nuclei to create further nuclear fission. This resulting chain reaction continues until all available material has been broken down. Nuclear fuel, usually a mixture of various uranium isotopes, is cleaned during production and processed, usually into fuel rods.

The actual nuclear reaction takes place in microseconds, which can be demonstrated with the effect of an atom bomb. To be able to use this form of energy safely, it is necessary for the chain reaction not to occur like an avalanche or like an explosion but rather that the release of energy and its use take place under full control so that the entire reaction can be maintained under control. This is the type of reaction which takes place in a nuclear reactor.

An atomic reactor is made of several enclosed systems which work together to generate energy. The fission material, which is the fissionable isotope in the fuel rods, is placed where the nuclei get broken down. To initiate this reaction, the fuel rods can either be placed into or removed from the reactor chamber. However, the output is controlled using rods made out of material, such as cadmium or metal containing boron, which are able to strongly absorb neutrons. The more this regulating rod is pushed into the reactor, the more the flow of neutrons are slowed. This regulation takes place rather automatically. In case of a danger of fallout, so-called safety rods are dropped into the reactor which slow down the flow of neutrons to the point where the reaction stops outright.

The heat energy released during nuclear fission is drawn away by a working, highly radioactive medium within the primary circuit (such as water heated to a temperature of as high as 290°C under a high pressure in the order of 107 Pa or 100 bar, which is around 100 atmospheres) and diverted in the exchanger to the secondary circuit, where steam is generated which drives an electric generator. A generator produces electrical energy which, with the help of a transformer, is converted into the required voltage and current values, in turn used for long distant transfer.

Substances suitable for slowing down fast moving neutrons are referred to as moderators (from the Latin word modero, which means to moderate). Such substances contain, for example, hydrogen (paraffin) because the hydrogen nucleus has practically the same mass as a neutron. However, not only does normal water break down neutrons but it also absorbs them. For this reason, a moderator tends to use heavy water (D2O) or graphite.

During nuclear fission, radioactive substances are created which radiate alpha, beta and gamma rays. These nuclear rays are very dangerous and may cause cancer (leukaemia) and other serious illnesses to humans. While alpha and beta rays can be successfully blocked using various measures, gamma ray penetration can be a serious problem. Gamma rays can be sufficiently blocked using barriers several meters thick made out of concrete, heavy spar concrete or water. Nuclear radiation is also emitted from exhausted, fission material and radioactive substances can emit radiation over a period of several millions of years.

Another method how to generate energy on nuclear principles is with thermonuclear fusion reactors. This nuclear process (synthesis) also creates energy within the sun. Using this nuclear fusion (thermonuclear reaction), the sun gains the necessary energy it needs to emit its radiation (nuclei synthesis). To a certain degree, nuclear fusion is the opposite of fission because it consists of a fusion of two light atomic nuclei into a heavier atomic nucleus (or atomic nuclei), a process which, once again, releases energy. In technical practice, no form of nuclear fusion has been successfully applied yet, even in light of decades of intensive research. To take advantage of nuclear fusion, a sufficient speed of atoms would need to be attained under temperatures exceeding 100 million °C and the repulsive force of atoms would have to be overcome. An uncontrolled example of using nuclear fusion (nuclear synthesis, thermonuclear reaction) has been attained with hydrogen bombs.