A nuclear power plant is a facility for obtaining electrical energy using nuclear energy.
Its operation is similar to that of a thermal power plant or that of a solar thermal plant: from a source of energy thermodynamics is used to obtain heat, with the heat to get steam and with the steam to drive a turbine that will generate electricity.
The difference between the different types of electrical installations is in the energy source: a nuclear power plant uses the heat released in the nuclear fission reactions of certain atoms, in a thermal power station the heat source (thermal energy) comes from the combustion of one or more fossil fuels (coal, natural gas, fuel. Finally in thermoelectric solar plants, the source of energy is solar radiation.
As in a conventional thermal power plant the heat is used to generate steam that drives a steam turbine connected to a generator that produces electricity.
The thermal energy used by the nuclear power plant to generate electrical energy is generated by a nuclear reactor. Within the reactor, chain fission reactions occur in a controlled manner. The element that fission, the nuclear fuel, is natural uranium or enriched uranium. Enriched uranium is natural uranium with a higher uranium-235 isotope ratio.
In addition to the reactor, a nuclear power plant always consists of a steam turbine, an alternator, two or three circuits – primary, secondary and tertiary – and one or more cooling towers of condensing fluid, which is usually water. The total efficiency is between 30% and 40%.
The operating lifetime of a nuclear power plant is usually considered in about thirty years. The main problem they present is the management of the nuclear waste they generate.
Basic operation of a nuclear power plant
In a nuclear power plant, energy is extracted from the nucleus of atoms by means of its division (nuclear fission). The atoms have internal bonds that unite their sub particles (electrons, neutrons and protons). When divided, these bonds break and release the inner bonding energy inside the atom that bound the separated particles.
Nuclear fission is generated artificially and in a controlled manner. In this, a neutron is fired to an atom of a large chemical element, a small particle at a certain speed, that breaks the atom (breaking its nucleus, formed by neutrons and protons linked by very energetic bonds) in an exothermic nuclear reaction, so a lot of energy is released in the form of heat (thermal energy).
Nuclear chain reaction in the core
To get that the energy obtained be greater than the energy used, it is necessary that the nuclear reactions are chain reactions. Thus, in spite of it takes a lot of energy to start the nuclear chain reaction, once the reaction is initiated, it does not take as much energy to maintain it, and there comes a time when the energy obtained is higher than the energy used. To achieve this, it is necessary that the large chemical element (we call heavy, with a high atomic mass) is also radioactive. The element that is usually used is a rare isotope of uranium.
Chain reactions occur in the core of the nuclear reactor. Subsequently, a circuit of tubes through which a fluid called refrigerant flows will be in charge of transporting the heat (thermal energy) out of the tank by cooling it.
Driving turbines
With the heat obtained from the nuclear reactions, water is heated until it boils; The water is converted to steam at a very high pressure. With steam under pressure the blades of a turbine are made to move. In this way we have converted thermal energy into mechanical energy. The turbine will connect to an electric generator (or alternator) that allows us to transform mechanical energy into electrical energy (or electricity).
This process is explained in more detail in the section: operation of a nuclear power plant.
Nuclear reactor
The nuclear reactor is the most important part inside a nuclear power plant. The nuclear reactor is the site where nuclear fission reactions are generated. In short, it is a device that produces heat as shown in below fig. (heat energy or thermal energy).
In the reactor of the plant the chain reaction begins by bombarding an atom with a neutron. Once initiated the reaction can be controlled in the same reactor by the control rods and the neutronmoderator.
Types of nuclear reactors used in nuclear power plants
In the world there are different types of nuclear reactors, all of fission, installed in the different nuclear power plants. The UN classifies them as follows:
- Pressurized water reactor (PWR and VVER). They use water at high pressure to produce steam to steam generators. They have three circuits. It is the most used in nuclear power plants.
- Boiling water reactor (BWR): The second type most common in the world. The water boils, generating steam directly into the reactor core. The steam generated drives directly the turbines. They have only two circuits.
- Pressurized heavy water reactor (PHWR): Uses high pressure heavy water as a neutronmoderator and as refrigerant.
- Gas reactor (GCR: AGR and Magnox): this reactor type uses graphite as a moderator of neutrons and carbon dioxide in the gaseous state as refrigerant.
- Graphite moderated by light water cooled (LGR and RBMK): Models of Russian origin. “Light water” is normal water.
- Rapid reactor (LBR, or LMFBR): It does not decelerate neutrons from the chain reaction and cools with liquid sodium. They are in prototype and research phase.