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Introduction to Nuclear Energy
The use of nuclear energy as a source of electrical power has diminished mainly due to the problem of long-term, safe disposal of medium and high radioactive waste. The public’s perception of risk of meltdown, which occurred at the Chernobyl Nuclear Power Plant, has also made nuclear power unpopular.
However CO2 emissions from fossil fuelled power plants are still on increase, especially from coal-fired ones, adding further to global warming. An alternative method of producing electricity without the CO2 emission is required to mitigate these CO2 emissions.
Renewable energy is gradually filling this essential role but the larger output capacity devices to capture this energy are still being tested.
Nuclear power being a non producer of CO2 is another option for producing electricity, and it is this method of producing power that this article is based, and we shall start with the mining and processing of the ore.
We go on to explore the different types of nuclear reactors and have a look inside a typical nuclear power plant, seeing how it operates.
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Mining and Processing Uranium Ore
Uranium is found in deposits in various locations throughout the world, Canada, Australia and Kazakhstan being at the forefront of mining operations.
Once deposits of uranium ore have been located using surveys such as geophysical and geochemical analyses, it is mined in the conventional manner from the seams of uranium ore. Despite the uranium ore mining being perpetrated to be a low risk, in the early days it was positively linked to lung cancer in the workforce.
The lung cancer was caused by the inhalation of carcinogenic radon gas and, dust from the silicon contained in sandstone host rock.
Today the underground mines are well ventilated by vertical shafts to reduce radon gas concentrations and all underground workers now wear personal protection such as face masks and respirators.
Once the ore is brought to the surface it is conveyed to the milling area where it undergoes the next process where the ore is extracted from the sandstone by crushers and grinders. It is then sent to a leaching plant as uranium oxide (U308) where it is mixed with sulphuric acid into a slurry.
The slurry is passed through a system of water-wash tanks ensuring all available uranium ore is extracted and, just as important, the separation of uranium bearing liquid within the leached solids. The later process is very important as the waste solids are pumped into an open tailing pond which is susceptible to ground leaching.
The uranium solution is now processed further by subjection to an organic compound or ion exchange system. This produces a saline solution to which peroxide or ammonia are added turning the solution into slurry known as yellowcake. This is washed and dried in a rotating kiln after which the cake is packed into drums ready for the next process which is conversion to uranium hexafluoride (UF6) before being enriched and shipped to the nuclear reactor.
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Enrichment of Uranium and Formation of Fuel Assemblies
The UF6 is converted to a gas and is ready for enrichment by one of three methods, gaseous diffusion, gas centrifuge or laser enrichment.
We shall examine the gas centrifuge method of enrichment.
The principle of centrifuge is to rotate at high revolutions, which throw heavier components of a substance to the outside of a bowl, the lighter components remaining close to the centre. This type of gas centrifuge contains numerous tiers of interconnected cylinders, the UF6 gas is placed in the first stage. As the cylinders rotate the heavier molecules in the U238 gas are thrown towards the outside whilst the lighter U235 are held close to the centre of the cylinder. These lighter U235 molecules transfer into further cylinders where the process is continued until the optimum enrichment of the UF6 is achieved.
The enriched UF6 now undergoes the process of reactor fuel fabrication. This entails its conversion to uranium dioxide powder (UO2) which is compressed into pellets. After machining to uniform size these pellets are stacked into containers known as fuel rods which are bunched together in fuel assemblies to form the nuclear core.
We are now all set to go, but first we will have a wee look at the different types of reactors and describe a Pressurised Water Reactor (PWR) being the most popular type used in nuclear power stations.
Read on to see the types of reactors and the operation of the most popular type plus see the sketches associated with them.
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How Does a Pressurized Water Nuclear Reactor (PWR) Work? Assemblies of PWR There are various types of nuclear reactors used to produce electricity in a nuclear power station, the most popular of these being the Pressurized Water Reactor or PWR. Enriched uranium UO2 is placed in fuel rods and these packed together in a tube to form the fuel assemblies. The assemblies are loaded into the reactor core and fission takes place.During fission the speed and numbers of the free neutrons are controlled by the use of a moderator and control rods. The core rapidly heats up during the process of fission and is kept cool by the circulation of coolant such as water. The heat from the coolant is transferred to process water in a heat exchanger producing dry, high pressure steam which is used to drive turbo generators producing electricity for the national grid.
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Types of Nuclear Reactors
There are numerous types of nuclear reactors some of them still in use, the more common are listed below
- Pressurised Water Reactor (PWR)
- Boiling Water Reactor (BWR)
- Advanced Gas Cooled Reactor (AGCR)
- Pebble Bed Reactor (PBR)
- Liquid Metal Fast Breeder Reactor (LMFBR)
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Components of a Pressurised Water Reactor (PWR)
This is the preferred type of reactor used in today’s power plants, and is considered currently the most reliable and safest form of nuclear energy. The core where the nuclear reactions occur contains the fuel rods, inside the fuel assemblies, the control rods, the moderator and the components coolant.
The Nuclear Fuel
This consists of the fuel rods we looked at earlier which are bundled together to form the fuel assemblies and packed tightly into the reactor core.
The Control Rods
These are fabricated from neutron absorbing material such as cadmium. They are used to control the number of free neutrons allowed to collide with the U235 atoms in the core. This is achieved by the automatic lowering the rods by a mechanical device. This can also be operated manually by the operative in an emergency, by being fully lowered into the core absorbing all the free neutrons thus stopping the reaction.
This can be ordinary water, heavy water (modified hydrogen content) or graphite. They all serve the same purpose which is to moderate the speed of the neutrons following fission.
The Cooling Medium
The reactor gets very hot during the process of fission and it is kept cool by circulating the coolant through the core. Some types of reactors combine the functions of the cooling medium and moderator.
Reactor Core Pressure Vessel
This is a heavy steel fabricated pressure vessel which contains the core, moderator and cooling medium.
This consists of heat exchanger which is located inside the containment area. It utilises the core cooling water to produce steam from process water. Both systems are closed circuits to prevent radiation contamination of the process steam, and returning condensate.
The Outer Containment Module
This is fabricated from steel reinforced concrete and can be over a meter thick. It is designed to protect operatives from radiation emission from within the reactor.
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Operation of a Pressure Water Reactor
The core is loaded with the fuel assemblies and moderator and coolant system operating, in most PWR these two being the same component. In the core the control rods are raised and fission occurs through the splitting of the U235 atoms, neutrons break free colliding with other U235 atoms causing a chain reaction that produces intense heat. The rate of reaction of the neutrons is controlled by the control rods and the speed of the neutrons slowed down by the moderator.
Cooling water is used to keep the core from overheating and this is piped to a heat exchanger also within the containment area. The heat exchanger uses the core coolant to convert process water to high pressure steam to drive steam turbine generators, exiting the LP turbine to the condenser. From here the condensate is returned to the heat exchanger in the containment area, and is converted back to steam in the normal mode of power plant steam systems operation.