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Nuclear Reactors
The majority of the electricity produced in the world is generated in power stations which are based on the production of heat. Most of these produce the heat by burning fossil fuels, such as coal, oil or gas. In contrast nuclear power stations capture the heat released from splitting atoms in a process called nuclear fission.
A thermal nuclear reactor is a device for sustaining a fission chain reaction in a controlled environment. Fission takes place in the reactor core which is contained within a pressure vessel and a biological shield. Within the core is a moderator, which is usually graphite or water. The moderator acts to slow down the neutrons so that a chain reaction occurs. Control rods made of a material which absorbs neutrons are inserted into the core, thereby controlling or stopping the reaction. Fuel rods containing fissile material are also placed in the core, interspersed with the control rods. The control rods are withdrawn to start the reaction and reinserted to shut it down. A coolant such as water or gas, passes through the reactor and conveys the heat generated to a boiler. Thereafter the production of electricity at a nuclear power station is similar to a fossil-fuelled station.
In the UK, a nuclear power station has many safety features, including automatic safety systems which monitor reactor performance and can shut down the reactor in the event of an operating anomaly and also activate independent auxiliary systems including back-up power supplies. The reactor can also be shut down manually.
Nuclear Power currently provides about 16% of the world's electricity generation. There are around 440 nuclear reactors currently in operation, with a further 28 reactors under construction and another 62 planned. Over 30% of the electricity in the European Union is generated by nuclear power. In France and Belgium the proportion is about 75% and 55% respectively. Details about the UK's nuclear power plants can be found on the Nuclear Power Generation Development page.
There are several key differences between nuclear and fossil-fuelled power generation in the UK. A nuclear station consumes much smaller amounts of fuel per unit of electricity generated than a fossil fuel station, some 40 tonnes of uranium fuel per annum as opposed to some 3 million tonnes of coal per annum at stations of comparable generation capacity. Uranium has limited alternative uses, is relatively inexpensive and a large proportion (96%) of the original uranium remaining in spent fuel can be recovered from reprocessing and recycled for use in new fuel. Unlike a fossil fuelled station, a nuclear power station produces virtually no carbon dioxide, the main greenhouse gas thought to contribute to global warming, or sulphur dioxide and oxides of nitrogen, which cause acid rain. However, a nuclear station produces radioactive wastes, which require careful handling and disposal. Over time, parts of the station become radioactive and therefore the station is more complex and expensive to decommission than a fossil fuelled station. These characteristics raise issues of safety, including the protection of station staff and the public, and also issues of radioactive waste management and disposal and decommissioning, which add to the costs of nuclear generation and create long term liabilities.
Magnox Reactors
The first commercial nuclear stations in the UK were of the Magnox type named after the magnesium alloy used to make the fuel can containing the uranium fuel. Magnox reactors use natural uranium metal as the fuel, have a graphite moderator and use pressurised CO2 as the coolant. Early designs have the core contained within a steel pressure vessel surrounded by a steel and concrete biological shield over one metre thick and have the boilers located outside the shield. Later designs have a steel lined pre-stressed concrete pressure vessel which also acts as the biological shield, with the boilers contained inside.
Pressurised Water Reactors (PWRs)
The PWR design is based on US technology and is the most common reactor type used in the world. The reactor is contained in a steel pressure vessel. Pressurised water, which acts as both moderator and the coolant, is pumped around the reactor and through the boilers. The pressure vessel, boilers and connecting pipe-work form a sealed primary pressurised circuit, which is contained within a steel-lined pre-stressed concrete containment building, which also acts as a biological shield. The remainder of the generation process is similar to that for other power stations.
Advanced Gas-cooled Reactors (AGRs)
Successor to the Magnox reactors and unique to the UK. AGRs use enriched uranium clad stainless steel cans and also have a graphite moderator and use pressurised CO2 as the coolant. These allow the AGRs to operate at a higher temperature than the Magnox reactor. The AGR Reactor is encased in a steel-lined pre-stressed concrete pressure vessel several metres thick which acts as the biological shield, with the boilers inside. The coolant conveys heat from the reactor to the boilers which, in turn, heats water in an isolated steam circuit which is then used to turn the turbines, just as in coal, oil or gas-fired stations.