![]() These are ones in which moderators are used to decreasing the quick neutrons to thermal energies so that they can produce more fission. Nuclear materials cannot be dumped into oceans or left anywhere as they will contaminate the environment through soil and water. The best place for the preservation is in the bottom of old salt mines which are countless meters below the surface area of Earth. Proper arrangements need to preserve nuclear disposal. The radiations from these used fuels are very hazardous and harmful for livings. Their half-life is about thousands of years. In the utilized fuel, there is a large number of radioactive materials. The utilized fuel is then removed from the reactor and fresh fuel is fed into the reactor. The nuclear fuel once used for charging the reactor works for a few months. In this case, refueling is at periods of 12, 18, or 24 months, when a quarter to a third of the fuel assemblies are replaced with fresh ones. Many reactors need to be closed down for refueling so that the reactor vessel can be opened. It is usually a meter-thick concrete and steel structure. The structure around the reactor and associated steam generators are developed to protect it from outdoor intrusion and to safeguard that outside from the effects of radiation in case of any major breakdown inside. Usually, a robust steel vessel containing the reactor core and moderator/coolant, but it may be a series of tubes holding the fuel and conveying the coolant through the surrounding moderator. To cool this steam generally water from river or sea is utilized. The temperature of the steam coming out of the turbine is 300 ᵒC. When this very hot water is traveled through the heat exchanger where water is converted into steam to run the turbine and produce electrical power with the help of a generator. Water is pumped into the reactor under high pressure to gather heat. When fission takes place in the core, heat is produced and the temperature is as high as 1200 ᵒC. In some reactors, unique control rods are utilized to allow the core to sustain a low level of power effectively. Within one year after having been unloaded from the reactor, the activity contained in the irradiated fuel decreases to about 1/100 of the original level and slowly decreases further in the following years.These are made with neutron-absorbing material such as cadmium, hafnium or boron, and are inserted or withdrawn from the core to control the rate of reaction, or to stop it. The water largely shields the radiation and at the same time absorbs the generated residual heat. Therefore the spent fuel elements are initially stored in a water-filled pool inside the nuclear power plant ( spent fuel pool). If the heat would not be removed, this so-called residual heat would increase the temperature far beyond the melting point of the fuel elements. ![]() The fission products generating inside the fuel elements are radioactive and generate large amounts of heat, even after the reactor has been shut down. In the case of a pressurised water reactor, water with added boron is additionally fed into the reactor for permanent subcriticality. The control rods catch the neutrons generated in the reactor and thus end the nuclear chain reaction. Subcriticality is achieved by lowering the neutron-absorbing control rods between the fuel elements in the reactor core. To shut down a nuclear power plant, the reactor must be brought into a permanently uncritical state (subcriticality) and the heat that continuous to generate must be discharged safely. This chain reaction is maintained by the fact that more neutrons are produced in the fission process than are used in the absorption process. During the power operation of a nuclear power plant, a self-sustaining chain reaction occurs in the reactor core.
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