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Electricity is one of the most important parts in our daily life.Now there are different types of electricity power stations. They could be classified by different energy sources such as thermal, hydroelectricity, solar, wind and nuclear power station. Because of the significant energy nuclear power station released, it tends to replace the conventional power plants. There are two kinds of nuclear reactions, fission and fusion. However, now nuclear power station only makes electricity via fission reaction because of fusion is still under intense theoretical and experimental research. Dunlap, R.A. (2004). In the following content, this essay will introduce how electricity made by nuclear power station and having a overview of the whole process including Reactor vessel, turbine (generator) and cooling system (cooling tower) and fission reactions in more detail.
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A nuclear power station could be divided into two parts, nuclear reactor and electricity generator. It is necessary to have an overview before looking at the detail of each component. After immersing the fuel rods which are a kind of material (the most common elements are U235 or Pu239) consumed by fission to produce energy into the reactor, the plant starts working. Meanwhile, the control rods control the rate of the nuclear fission. In term of moderator, it is a kind of medium in the fission reaction and produce steam. The steam which generates by the nuclear fission goes through the tube connected to the turbine and spins it. The generator which connected to the turbine rotates at same time to produce electricity. The steam is generated continually when nuclear fission occurs. Then the steam flows into condenser which is a kind of device used to condense a substance from its gaseous to its liquid state and converts into water again. At the same time, some water will flow into river and process circulates until nuclear reaction stops.(Brain., Lamb. (date not given))
Fission is the only nuclear reaction used in nuclear plant so far. It is a nuclear reaction in which the nuclear splits into lighter atoms, often producing free neutrons and gamma rays. The principles of the plants are depended on the large amount energy the fission of heavy elements released.Even there are many kinds of fissions, but only two elements' fissions are most common used in plants nowadays (U and Pu). Uranium-235 is important for nuclear reactors, because it is the only uranium isotope existing in nature which is fissile inherently. However, such as U238 could be fissionable but it could not keep the chain fission occurring continually. In term of the U239, exposing U238 to neutron radiation in nuclear reactor to make a U239, and then decaying into neptunium-239 in a beta decay which absorbed a total energy of about 1.29 Mev. Neptunium-239 further decays to Pu-239 which is fissile inherent as well. (Dunlap. (2004)) (why only use U and Pu. Talk U238 as well)
For an U235, figure 1 shows a U235 absorb a slow-moving neutron, it splits into two fast-moving daughter nuclei fragments which are as fast as 3% of the speed of lighter, three free neutrons which will be absorbed by another U235 to keep reaction sustained, an energy of ~202.5MeV is released. Typically ~169MeV are released as kinetic energy of daughter nuclei. The fission reaction also releases ~7 MeV in prompt gamma rayphotons which heat serve to raise the temperature of the bomb core to 100 million kelvins. Since the significant heat ray photons released, the water in the steam generator is changed into steam.
A nuclear reactor includes fuel rods, control rods and moderator.The fuel rods mainly include Uranium-235 or Plutonium-239 pellets. It can start or stop the process by lowering or raising the height from the moderator. Moderator is a kind a medium which can reduce the speed of fast moving neutrons, thereby converting them into thermal neutrons to keep the chain reaction occurring continually involving U235. In terms of control rods, they are made to maintain the speed of the fission which is made from some chemical elements which could absorb many neutrons without fission. (Such as silver, indium and cadmium)This process is sat up inside the reactor. If, on average, exactly one of the free neutrons from each fission hits another U-235 nucleus and causes it to split, then the mass of uranium is said to be critical and mass will exist at a stable temperature. However, if there is less than 1 neutron hit the U235, the chain will stop and this condition is called Subcritical mass. As for Supercritical mass, it means there are more than one of the free neutrons hits another U-235 atom, then the mass is supercritical. This will cause the reactor to heat up. So nuclear reactor controls rods inside the vessel to control the number of neutrons hitting the U235. In term of moderator which take the part of a medium in the fission reaction and produce steam. Water is the most common moderator used in nuclear power station. (Fishbare (2005)) There are two vessels which show in the figure3 work together to produce steam. The fission which is happened in the first one heats the water. The first is pressurized to control the temperature (about 400 degree) of water not boil (because if you pressurized water, the boiling point of the water will increase proportional to the pressure) and pass water into second one by pipes. The heated water comes from first one pass the heat to second one and produce steam.
Turbine and generator:
Turbine and generator is connected by a shaft. The turbine is spun by the steam made by two vessels. Generator is a device that converts mechanical energy to electrical energy when it rotates.(Brain, Lamb. (date not given))
Cooling off the water:
Considering the large amount of energy, the plant needs a cooling system to control its temperature of the turbine and steam generator. The cooling system lowers the temperature of the plant by its water circulation system. After the steam flows through turbine, it will flow into a water circulation which link to the cooling tower by a heat exchange tube. The steam meets the cold water in condenser, then converts into liquid again.The source of cold is cooling tower which is always near river or coast. The cooling tower transfer water to condenser by a strong pump. After mixing the cold and hot water together, the rest of them will propose by the cooling tower as warm vapour. (Brain., Lamb. (date not given))