2.4 The Principles of Operating a Nuclear Power Plant

Electricity production in a nuclear power plant

A nuclear power plant is a promising emission-free and reliable source of electricity. By its principle, it belongs to the group of thermal power plants, in which an electric generator is driven by a turbine  turned by the energy of flowing hot steam. The only difference, compared to fossil fuel power plants, is the heat source that creates this high-potential steam. In nuclear power plants, the heat for steam generation is released in the controlled process of fission of nuclei of heavy elements in fuel taking place in the core of the nuclear reactor.

Due to the potential danger of ionizing radiation arising during nuclear reactions in the reactor, the technology of the entire nuclear part of the power plant is enclosed in a hermetic concrete cover, called containment and its operation is governed by strict safety standards. Containment fulfils the function of protecting the facility from dangerous external influences and at the same time prevents radioactivity from escaping into the environment in case of an unforeseen event. The entire primary circuit is located in the containment, consisting of the nuclear reactor, circulation pumps, steam generators, connecting pipes and auxiliary systems, ensuring the reliable and, above all, safe operation of the power plant.

The primary circuit is immediately followed by the secondary circuit and most of its facilities are located in the auxiliary building right next to the containment. The auxiliary building belongs to the non-nuclear part of the power plant and its layout is essentially the same for all thermal power plants. The main device of the secondary circuit, a turbogenerator with a condenser, is complemented by various systems of regenerative heat exchangers, tanks and pumps in the auxiliary building, which improve the heat balance of the thermodynamic cycle and thus the efficiency of steam energy conversion into electricity.

The place of contact between the two circuits is the steam generator, where the heat is transferred from the medium of the primary circuit to the water of the secondary circuit, which then produces steam that drives the turbine. The hermetic separation of the two circuits  ensures that the radioactive water of the primary circuit does not come into direct contact with the turbine or any other non-nuclear part of the power plant. 

Heat source in the primary part of the nuclear power plant

The most important facility of the primary part of a nuclear power plant is the nuclear reactor. In the types of reactors that are most used, it consists of a massive steel container capable of withstanding high temperatures and pressures of the operating media. The reactor pressure vessel is connected by a circulation pipe to the other parts of the primary circuit, which are arranged in several cooling branches around the central reactor. In this technological heart of the power plant, fuel rods with nuclear fuel in the form of small cylindrical pellets, made mostly of enriched uranium, are stacked in the core. The controlled chain fission of fuel nuclei, taking place in the reactor with the help of neutrons, is the source of a large amount of heat.

The released heat is removed from the fuel rods in the reactor by a circulating cooling liquid. The most commonly used coolant in nuclear energy is ordinary water which, in this case, serves not only as a cooling medium, but also serves as a moderator or decelerator of neutrons, which are used to split uranium nuclei. It has been found that uranium nuclei split more willingly when they are hit by neutrons that have already lost a large part of their initial energy in collisions with moderator atoms. Every time the atomic nucleus of the fuel is split, a certain amount of thermal energy is scattered into space and on average, 2—3 neutrons are released and they are capable of splitting other fuel nuclei after slowing down.

The amount of fissile nuclei and thus the power of the reactor can be affected by the operator by inserting and pulling out control rods with neutron absorbers into the core area. This disturbs the overall neutron balance in the reactor. When the rods are inserted, there is an increasing shortage of fission neutrons, the number of fissions of other nuclei decreases, the power decreases until the fission reaction stops completely. When the rods are pulled out of the core, the situation is the opposite and the power of the reactor gradually increases with the increasing number of neutrons that are no longer absorbed. Adjusting the control rods to the correct position makes it possible to stabilize the amount of fission neutrons in the core of the reactor at a level corresponding to a certain constant power. 

Equipment of the secondary part of the nuclear power plant and the cooling circuit

The non-nuclear part of the power plant begins on the secondary side of the steam generator, where the water of the secondary circuit is heated by heat transferred from the primary circuit through a system of heat exchange pipes. Since this water has a lower pressure than the water in the primary circuit, it begins to boil and in the space between the tubes of the steam generator, saturated steam is produced from it.

The steam produced in the steam generator flows through the steam pipes to the turbine. Here, its thermal energy acting on the impeller blades is transformed into mechanical energy and spins the turbine rotor up to a respectable 3,000 revolutions per minute. Passing through the individual stages of the turbine, the steam expands, its pressure decreases and its specific volume increases. Therefore, the blades of the last stages must be significantly larger and longer. After the last stage of the turbine, the steam is directed to a condenser, usually located below the low-pressure parts of the turbine.

The rotating turbine rotor is directly connected to an electrical generator, in which mechanical energy is transformed into electrical energy based on electromagnetic induction. The constant speed of rotation of the rotor ensures a stable frequency of the produced alternating current. From the electric generator, the electricity is taken out of the turbine hall by means of encapsulated conductors. After the necessary adjustments to the voltage level in the output block transformer, the produced electricity travels to the distribution network.

In order to fully utilize the energy potential of the steam in the turbine, the steam must condense as it exits the last stage of the turbine. Condensation takes place on cold condenser tubes under high pressure. The cooling of these pipes and thus the removal of condensation heat is provided by the water of the separate tertiary circuit of the nuclear power plant. At the end of a series of energy transformations, the unusable residual heat from this tertiary cooling circuit is released through natural or forced draft cooling towers into the atmosphere. If the nuclear power plant is located by a river or on the sea coast, river or sea water can be used to cool the condenser directly instead of the tertiary circuit.