Operation of a nuclear power plant
A nuclear power plant is a thermal power plant in which water vapour drives a turbine and an electrical generator connected to it.
In a nuclear power plant, the thermal energy needed to steam water is generated by the fission or fission reaction of atomic nuclei, whereas in conventional steam power plants, the heat is generated by the combustion of a fuel such as oil or coal. In a nuclear power plant, heat is produced in a nuclear reactor, where a controlled chain reaction of fission products is generated. The chain reaction takes place in the reactor core, which is sealed in a reactor pressure vessel.
The nuclear fission occurs when a neutron hits a uranium nucleus, which splits into two lighter nuclei. This releases a few new neutrons and a large amount of energy. The neutrons produced by the fission of the nucleus can in turn cause new fissions, allowing a chain reaction to occur. The kinetic energy of the nuclei and neutrons that result from the fission is converted into thermal energy when they collide with other atoms.
The substances produced in the nuclear fission (fission products) are radioactive, i.e. they decay into other elements. When they decay, the nuclei emit radiation. Radioactive substances are also produced when neutrons released during fission collide with the nuclei of atoms in the reactor's structural materials or in the cooling water. In this case, the nucleus is said to be activated.
Under normal conditions, fission products remain inside fuel rods sealed in a gas-tight envelope. Small amounts of fission products may be released into the water cooling the fuel rods, either by leakage of the fuel rod cladding or by cracking of the uranium nuclei, which are a microscopic impurity on the outer surface of the fuel. The amount of radioactive material in the reactor cooling cycle is less than one hundred thousandth of the activity of the fission products contained in the fuel in the reactor.
The risks to the environment associated with nuclear power plant accidents arise almost entirely from the radioactive fission products that accumulate in the reactor. If a reactor is damaged in such a way that fission products are released into the environment, the radiation emitted by the reactor as it decays can cause damage to humans and wildlife. Therefore, the release of radioactive materials into the environment must be prevented by identifying and eliminating as far as possible all accidental possibilities that could lead to reactor damage.
The radiation from radioactive fission products causes the formation of so-called "afterheat" in the fuel. When the reactor is shut down, the heat generated by the fission reactions stops very quickly, but the radiation from the decay of the fission products continues to heat the fuel for a long time.
The power of the residual heat decreases over time as the amount of radioactive fission products resulting from the decay decreases. The removal of residual heat from the reactor core must be organised in a reliable way. The loss of cooling would rapidly lead to a meltdown of the fuel, which could release large quantities of radioactive fission products into the containment and, in the worst case, into the environment.