The operating principle of a nuclear power station does not differ fundamentally from that of a fossil fuel-fired power station. The only difference is how the heat is produced. Nuclear power stations have a reactor, in which the fission of uranium atoms is the source of heat. 'Conventional' power stations produce heat by burning natural gas, coal or oil.
The general principle consists of converting the heat produced (from the fission process) into electricity.
The heat from the reactor core is released into water circulating in a closed circuit along the nuclear fuel rods. This is called the primary circuit. The water in the primary circuit has an average temperature of 300 °C. In a pressurised-water reactor, water cannot boil because it is under pressure. The pressure is created by the pressuriser. The hot water in the primary sector in turn releases heat to a second closed circuit known as the secondary circuit. They are hermetically sealed from each other. The heat exchange takes place in a steam generator, a large cylindrical heat exchanger consisting of thousands of tubes. The heat converts the water in the secondary circuit into steam.
The steam produced in the secondary circuit expands across multiple turbine components, forcing it to turn. A generator connected to the turbine converts this kinetic energy into electricity, which is then injected into the high-voltage grid.
The steam used by the turbines is cooled in a condenser, where it is once again converted into water after coming into contact with thousands of tubes containing cooling water from the third circuit (which, in turn, is isolated from the secondary circuit). The water can then return to the steam generator, where it is once again heated to the steam state.
Like large-scale conventional power stations, nuclear power stations have a cooling tower that uses natural air circulation to reduce the temperature of the cooling water. For instance, in nuclear power stations, the water in the third circuit is re-used to cool the steam in the condenser. Only 1.5% of this water evaporates, taking the form of a plume of steam rising from the cooling tower.
The fission of uranium atoms is harnessed to produce heat and steam.
The fission reaction is triggered by bombarding the uranium nuclei with neutrons. The nucleus splits in two when it is struck by a neutron moving at a suitable speed. Once split, the mass of particles and fission products obtained is a fraction lighter than the original mass of the heavy atom and the neutron. This mass is transformed into energy, in accordance with Einstein's law (E = mc²).
Fission also releases neutrons that trigger new fissions, resulting in a self-perpetuating chain reaction. In a nuclear power station, man is able to control the chain reaction: it can be started, stopped or controlled via the correct dilution of the primary circuit or by means of control rods in the reactor. Control rods are used to slow down the chain reaction in line with requirements.
