Nuclear power stations require significant investment to construct, but their relatively low running costs over a long operational life make them one of the most cost-effective low-carbon technologies.
Nuclear power has one of the smallest carbon footprints of any energy source. The vast majority of carbon dioxide emissions associated with nuclear power stations arise during construction and fuel processing, not during electricity generation.
A nuclear power station turns the nuclear energy in uranium atoms into electrical energy that can be used in homes and businesses.
The reactor vessel (1) is a tough steel capsule that houses the fuel rods: sealed metal cylinders containing pellets of uranium oxide. When a neutron – a neutrally charged subatomic particle – hits a uranium atom, the atom sometimes splits, releasing two or three more neutrons. This process converts the nuclear energy that binds the atom together into heat energy.
The fuel assemblies are arranged in such a way that when atoms in the fuel split, the neutrons they release are likely to hit other atoms and make them split as well. This chain reaction produces large quantities of heat.
Water flows through the reactor vessel where the chain reaction heats it to around 300°C. The water needs to stay in liquid form for the power station to work, so the pressuriser (2) subjects it to around 155 times atmospheric pressure, which stops it boiling.
The reactor coolant pump (3) circulates the hot pressurised water from the reactor vessel to the steam generator (4). Here the water flows through thousands of looped pipes before circulating back to the reactor vessel. A second stream of water flows through the steam generator around the outside of the pipes. This water is under much less pressure, so the heat from the pipes turns it into steam.
The steam then passes through a series of turbines (5), causing them to spin and converting the heat energy produced in the reactor into mechanical energy. A shaft connects the turbines to a generator – so when the turbines spin, so does the generator. The generator uses an electromagnetic field to convert this mechanical energy into electrical energy.
A transformer converts the electrical energy from the generator to a high voltage. The national grid uses high voltages to transmit electricity efficiently through the power lines (6) to the homes and businesses that need it (7). Here other transformers reduce the voltage to a usable level.
After passing through the turbines, the steam comes into contact with pipes full of cold water pumped in from the sea (8). The cold pipes cool the steam so that it condenses back into water. It is then piped back to the steam generator, where it can be heated up again, turn into steam again, and keep the turbines turning.
Radiation is energy in the form of high-speed particles or waves. Radiation can be thought of as forming a continuous energy spectrum: ranging from low-frequency radio waves to high-frequency cosmic rays. Sunshine, a form of natural radiation, is essential for life on Earth – it is the energy that sustains all plants and animals. The visible light from the sun is medium-frequency radiation.
Exposure to high doses of radiation can cause damage to living tissues, including the human body. Anyone who has suffered from sunburn, for example, has experienced the damaging effects of ultra-violet (UV) radiation on the skin. The high-frequency end of the radiation spectrum is sometimes called ionising radiation, and includes x-rays and gamma rays.
What is radioactivity?
Some atoms are naturally unstable – that is, they continually change or 'decay' until they become entirely new, stable atoms. As unstable atoms decay, they emit ionising radiation. These unstable atoms are said to be radioactive. Radioactive materials may emit different types of ionising radiation, including alpha particles, beta particles and gamma rays.
Nuclear reactors contain a radioactive form of the element uranium. When a neutron – a type of subatomic particle – collides with a uranium atom, the atom can split. This process, known as nuclear fission, releases more neutrons. These neutrons collide with other uranium atoms, triggering a chain reaction, releasing enormous amounts of energy. Nuclear power stations use this energy to create high-pressure steam for generating electricity. The neutrons emitted during nuclear fission are another form of ionising radiation.
Millions of people have benefited from the use of radioactivity – for example, in medicine (as x-rays for diagnosis or radiotherapy) and in agriculture, as well as in nuclear energy.
How powerful is radiation?
Different types of radiation vary a great deal in their ability to penetrate materials. X-rays are powerful enough to pass straight through parts of the human body – a property doctors and dentists regularly exploit.
The main forms of ionising radiation are:
- Gamma rays, which are high-energy beams similar to x-rays. They travel at the speed of light and only a very dense material (such as lead) can slow them down or stop them.
- Alpha particles, which are high-energy, positively charged particles much slower than gamma rays. They also have far less penetrating power: a sheet of paper can stop them.
- Beta particles, which are negatively charged, much lighter than alpha particles, and much faster moving. They are also more penetrating than alpha particles, but can still be stopped by a layer of aluminium a few millimetres thick.
- Neutron particles, which are not electrically charged, allowing them to pass through a wide variety of materials. When neutron particles come into contact with living tissue and other matter, beta and gamma radiation is emitted.
Several layers of different materials are needed to effectively shield against all forms of ionising radiation.
The raw material used to create fuel for nuclear power stations is uranium ore. Uranium is relatively abundant, and can even be found in seawater.
More than half of the uranium mined in the world is taken from mines in Australia and Canada. After it has been mined, uranium ore is crushed and ground in a mill. The ground ore is then mixed with strong acid to dissolve the waste rock, leaving concentrated uranium oxide, also known as yellowcake.
Yellowcake contains two different types (or isotopes) of uranium: uranium-238 and uranium-235. Only uranium-235 can be used to produce electricity in a modern nuclear power reactor, so the yellowcake next needs to be processed until its uranium-235 content reaches at least 4%.
The result is a black powder, which is compressed and baked into ceramic fuel pellets. These pellets are encased in metal tubes called fuel rods. A collection of fuel rods is called a fuel assembly. A typical 1.3 million kilowatt (kW) reactor contains 200 fuel assemblies, each containing 289 fuel rods – a total of about 15 million pellets.
Nuclear power stations generate electricity by harnessing the energy from a nuclear fission chain reaction. This reaction produces ionising radiation, which can be harmful to living organisms. Nuclear power stations are therefore designed with robust containment measures to prevent uncontrolled release of ionising radiation or radioactive material into the environment. These include measures for controlling the rate of the chain reaction in the fuel, and physical barriers intended to keep radioactive material contained within the reactor.
Controlling the chain reaction
Nuclear fission or 'splitting the atom' releases energy and neutrons: a type of sub-atomic particle. When a neutron released by fission strikes another atom, that atom can also split, releasing more neutrons. In a nuclear reactor, the uranium fuel is arranged so that neutrons released through fission continually cause more fission in adjacent atoms, releasing more neutrons which cause more fission – a chain reaction.
The rate of this reaction can be regulated using control rods. These rods are made of a material that absorbs neutrons. Inserting control rods into the reactor slows the rate of the reaction, because the rods absorb neutrons that might otherwise split atoms. Withdrawing control rods leaves more neutrons free to split atoms, accelerating the rate of the reaction.
Fully inserting a number of the control rods halts the reaction, although heat will still be generated by radioactive decay. In UK nuclear power stations, control rods are designed to drop into the reactor automatically in emergencies as part of the shutdown process.
Nuclear power stations use multiple barriers to prevent the uncontrolled release of radioactive material from the reactor into the environment.
The uranium fuel for modern nuclear power stations comes as small ceramic pellets. Most of the radioactive material produced by the fission reaction stays bound within these pellets. They are placed into metal tubes known as fuel rods, which are welded shut at both ends.
In UK advanced gas cooled reactors (AGRs), these fuel rods are made of stainless steel. In pressurised water nuclear reactors (PWRs) the fuel rods are made of zirconium alloy (zircaloy). Neutrons can pass through stainless steel or zircaloy, so the chain reaction can still take place, but radioactive material cannot pass through and so stays trapped inside.
In a PWR the fuel rods are kept in a steel pressure vessel with walls some 25 centimetres thick. This pressure vessel is usually surrounded by a concrete containment building with walls at least one metre thick. The PWR at Sizewell B in Suffolk is built in this way, as are the latest European pressurised reactors (EPR) proposed for the next generation of UK nuclear power stations.
Other than the Sizewell B PWR, there is one Magnox power station and seven AGR power stations currently operating in the UK. These power stations have massive concrete pressure vessels that provide a very robust containment structure. The structural integrity of containment structures is subject to thorough monitoring to ensure they are able to withstand significant stresses.
Containment in the Fukushima nuclear accident
In March 2011, an earthquake and tsunami hit the Fukushima Dai-Ichi nuclear power station in Japan. Their combined effect knocked out the station’s connection to Japan’s electricity grid and flooded its backup diesel generators. Deprived of power, cooling systems failed, and without cooling the reactors overheated.
Fukushima Dai-Ichi used boiling water reactors (BWRs), a design never used in the UK. Our existing nuclear fleet consists of Magnox reactors, AGRs and the PWR at Sizewell.
In the Fukushima accident, some of the overheating reactors boiled away enough of their coolant water to expose the fuel rods. The hot fuel rods reacted with steam to produce hydrogen, which caused explosions when it was vented and mixed with oxygen. These explosions damaged containment.
The fuel rods in UK Magnox and AGR power stations are cooled using CO2 (not water), so the risk of a similar accident occurring at these sites is very small. The Sizewell B reactor is cooled with water, but has safety systems specifically designed to prevent hydrogen forming.
Nuclear power stations work by starting a chain reaction in the uranium fuel. This reaction causes the splitting or ‘fission’ of uranium atoms into other radioactive elements, and gives off very large amounts of heat. This heat needs to be continually transferred away from the reactor.
It is not physically possible to convert all the heat generated in a nuclear power reactor into electricity. The safe operation of a nuclear power station depends on its cooling systems, which remove the heat from the reactor during normal operation and when the reactor has been shut down.
Eight out of the nine UK nuclear power stations use carbon dioxide (CO2) to cool the reactor core. Sizewell B, our only pressurised water reactor (PWR), uses pressurised water as the coolant. In each case, the coolant picks up heat from the reactor core and carries it away. In a PWR, the coolant leaves the reactor at about 300°C.
Multiple cooling systems
Once it has picked up enough heat from the reactor, the coolant itself needs to be cooled. This is the job of the secondary cooling system (sometimes known as the steam circuit) where water is the coolant.
The secondary coolant picks up heat from the primary coolant – the CO2 or pressurised water – in heat exchangers or boilers. The primary and secondary coolants do not mix directly in the boilers. The secondary coolant exits the boilers as high-pressure steam, and the primary coolant circulates back through the reactor core.
This steam is used to drive the power station’s turbines. The turbines convert much of the steam's heat energy into mechanical energy, which is then converted into electrical energy by a generator.
Before it is pumped back around the steam circuit to the boilers, the steam needs to be cooled further and condensed back into water. To achieve this, the steam from the turbine passes through another heat exchanger (the condenser) where it comes into contact with cold, water-filled pipes.
Most UK nuclear power stations are situated on the coast, so that the water for the condenser can be pumped in directly from the sea. It passes through the condenser just once before being discharged back into the sea, at a slightly higher temperature than when it entered.
None of the water discharged into the sea from the condenser has ever entered the reactor, come into direct contact with the radioactive fuel, or mixed with any coolant that has.
Even after the reactor is shut down, the radioactive by-products of fission continue to give off some residual heat (known as decay heat). Back-up cooling systems remove this heat from reactor to prevent it overheating.
Nuclear power is one of the most comprehensively regulated industries, and operates within a culture of continuous improvement. In the UK, the industry is regulated by the Independent Office for Nuclear Regulation and the Environment Agency or the Scottish Environment Protection Agency (SEPA).
Specific laws govern the storage, transport and use of nuclear materials. Whenever accidents have occurred at nuclear facilities, increased national and international regulation and cooperation have followed.
Generating electricity – whether by burning fossil fuels or harnessing nuclear power – inevitably produces waste materials.
Nuclear power stations produce waste in relatively low volumes compared to fossil fuel power stations, but the used nuclear fuel and a small amount of other waste is highly radioactive. Waste produced by the nuclear industry is more heavily regulated than waste from any other energy sector. Decades of industry experience, however, have yielded proven techniques for disposing of all levels of nuclear waste.
Ten UK nuclear power stations have ceased operation and are at various stages of decommissioning. While there are still radioactive hazards associated with the site, this must be carefully managed and (depending on the strategy used) can take decades.
The International Atomic Energy Agency (IAEA) has identified three options for decommissioning:
The station is defuelled, dismantled and decontaminated as soon as possible after shutdown. The site is quickly cleared and made available for reuse.
Any high-level waste is removed and most of the installations are dismantled except for the reactor itself which is put in a safe state. The defueled and safe power station is monitored for 40-80 years while its radioactivity levels decay. Then the final deconstruction of the reactor can be completed..
All radioactive material is placed in one part of the power station. This is then encased in thick concrete. While this is an option that is being pursued in some parts of the world it is not one that is actively promoted within the international nuclear community.
EDF Energy believes nuclear power has a key role in providing safe, reliable and affordable energy to the UK. And with its small carbon footprint, nuclear will also be integral to our response to climate change and greenhouse gas emissions.
Nuclear power should contribute as much as possible to the need for new capacity – and to avoid the threatened energy gap.
In July 2011, the UK Government’s National Policy Statement for Nuclear Power Generation called for new nuclear power stations to be built and to start generating as soon as possible. Nuclear power stations require major investment to construct, but their relatively low running costs over a long operational life make them one of the most cost-effective low-carbon technologies.
Nuclear power has one of the smallest carbon footprints of any energy source. The vast majority of carbon dioxide (CO2) emissions associated with nuclear power stations arise during construction and fuel processing, not during electricity generation.
No nuclear power station generates electricity all of the time. There are periods when it will operate at reduced levels or will be shut down for refuelling and maintenance. Most shutdowns are planned, and because of this can happen when demand is expected to be lower.
Experience & expertise
EDF Energy is part of the EDF Group. With 58 nuclear reactors in France and a total of 78 reactors across the world, EDF is by far the leading global nuclear power operator. Our 58 French reactors are divided among 19 power station sites and have a total installed capacity of around 63GW
In France, EDF has acquired expertise in the design, maintenance, operation and decommissioning of nuclear plants during almost 50 years within the nuclear industry. We are committed to ensuring the safety of facilities and continue to advance our technology to that end.
Our nuclear sites
EDF Energy owns and operates 15 nuclear reactors at eight sites in the UK and our current plans include building four new reactors : two at Hinkley Point in Somerset, and two at Sizewell in Suffolk.
We are working with modern reactor technology as part of our Nuclear New Build programme. New-generation plants are designed to be more reliable, longer-lasting, safer and more efficient – yet more powerful and flexible in their output.
For our four new reactors, we will use a design based on the European Pressurised Water Reactor (EPR) , the type we are also building in France and China. In 2012, the EPR completed the Generic Design Assessment (GDA) – operated by the UK’s joint nuclear safety, security and environmental regulators to ensure new nuclear power stations meet regulatory expectations.
Building on Government policy for new nuclear, and drawing on our extensive expertise, we are confident that our plans for UK EPRs will help to realise our vision of low-carbon and affordable energy for all in the UK.