Current Research in Nuclear Fusion Power and Its Place in Future Electricity Production.
Current Research in Nuclear Fusion Power and Its Place in Future Electricity Production
Introduction
Nuclear fusion is a process where two lighter atomic nuclei fuse, forming a heavier nucleus and releasing energy.
The type of nuclear fusion currently being researched most vigorously in the context of forming a future source of power is the fusion of two isotopes of hydrogen - deuterium and tritium, forming helium and a neutron and releasing large amounts of energy.
= Neutron = Proton
Deuterium Nucleus Neutron (+14.1 MeV)
Tritium Nucleus Helium Nucleus (+3.5 MeV)
Figure 1
This can be described by the equation H + H ==> He + n, yielding 17.6 MeV per fusion. Another fusion reaction which is possible is the fusion of two deuterium nuclei, though this yields less energy and is not generally being pursued as a source of power in future reactors. It is described by the equation H + H ==> He + n, yielding 3.3 MeV per fusion.
A central concept in understanding the origin of this extra energy is the equivalence of mass and energy.
The strong nuclear force binds together protons and neutrons in the nucleus, it is very strong over short distances hence it overcomes the electromagnetic repulsion of protons in the nucleus. The energy that is required to hold to hold the nucleus together can be thought of as adding to its mass. Hence the mass of a deuterium nucleus is higher than the mass of its individual components when isolated - one proton and one neutron, and hence energy must be put in for them to bind together by the strong nuclear force. Hence also energy is released when they are taken apart. When considering the fusion reaction, the total mass of the deuterium and the tritium nuclei before the reaction is more than that of the helium nucleus and neutron formed. This is because the neutron that is emitted is no longer held by the strong nuclear force, the forces have been rearranged at the end of the reaction ending up with a lower potential energy. This energy that once held the neutron is carried away as translational kinetic energy, which does not affect the particles' mass, largely by the neutron. The typical values of this extra kinetic energy are shown on the diagram above, 14.1 MeV on the neutron, 3.5 MeV on the helium.
However the energy required for this to occur is staggeringly high, and the deuterium and tritium gas must be heated to such high temperatures that they forms a plasma - the fourth state of matter. It is only at such high energy states that the deuterium and tritium will collide with enough energy to overcome the strong force binding the protons and neutrons together in the nucleus. A plasma state occurs when atoms become so energetic that the nucleus and the electrons overcome the electromagnetic forces holding them together and separate and a 'sea' of charged particles, the negative electrons and the positive nuclei, is formed. Temperatures around and above 100 million degrees Celsius are required for fusion to occur.
Fusion and Current World Energy Production
This is the reaction which powers stars, and international research has been progressing for around fifty years to try to make it possible to perform this reaction in a controlled and sustainable manner to provide electricity in a clean and practically limitless way.
Fossil fuels, by far the main source of electricity globally, cause incredible damage to the environment and could lead to extreme climate change unless action is taken in the immediate future. More alarming for governments worldwide, fossil fuels are running out.
At current consumption levels reserves of oil are likely to be exhausted within 50 years, natural gas within 80 years and coal lasting around 250 years. Oil especially is used in the production of other industrial products such as plastics, and the exhaustion of its reserves is certainly an uncomfortable thought.
Nuclear fission, involving the splitting of uranium-235 into smaller elements and releasing large amounts of energy, has been used in power plants since 1954. However it suffers many drawbacks including the production of radioactive waste which must then be disposed of, the waste remains radioactive for thousands of years and exposure can cause damage to cells as radiation sickness, cancer, and eventually death. There is also the risk of meltdown should the reactor become unstable, this occurred in 1986 in Chernobyl in the Ukraine. Dozens of people died instantly or within days, countless others have been affected by the movement of radioactive particles in the wind causing increased cases of cancer in the region. The region is still irradiated and will be for thousands of years. These factors have led to poor public opinion of fission, and whilst many hundreds of reactors around the world remain functional, the construction of new reactors has slowed to a virtual standstill since Chernobyl.
Power from renewable resources often requires large areas of land to be dedicated to it, solar and wind power for example need large areas to produce the same power as one fossil fuel power plant, and there are few areas suitable for large-scale geothermal power or hydroelectric dams.
Fusion power on the other hand, has only helium as a by-product and cannot suffer any kind of meltdown as if the plasma becomes unstable it will collapse and the nuclear reaction will cease. Any future fusion power plants would occupy roughly the same area as a fossil fuel ...
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Power from renewable resources often requires large areas of land to be dedicated to it, solar and wind power for example need large areas to produce the same power as one fossil fuel power plant, and there are few areas suitable for large-scale geothermal power or hydroelectric dams.
Fusion power on the other hand, has only helium as a by-product and cannot suffer any kind of meltdown as if the plasma becomes unstable it will collapse and the nuclear reaction will cease. Any future fusion power plants would occupy roughly the same area as a fossil fuel plant and would produce similar or higher power. Fusion has received much attention for research, however progress has been slow.
Worldwide, the production of electricity from different sources breaks down as follows (figures for 2000):
Figure 2
**Other includes solar, wind, tidal and geothermal power
Current Research
The method of fusion which is currently being researched by far the most extensively is magnetic containment using a tokamak reactor. A tokamak reactor consists of a torus-shaped chamber in which the plasma is formed and travels round in a circular path. The plasma is contained in the chamber on this path, and is first raised to the temperatures required for a plasma state, by large magnets surrounding the chamber (thus it employs magnetic containment). It is ensured that the plasma does not ever touch the walls of the chamber by these magnets, as this would lead to a rapid dispersal of the energy of the plasma into the walls of the chamber and the almost instantaneous collapse of the plasma. This manipulation of the plasma by magnets can occur as the plasma itself has large currents induced through it, which then carry a magnetic field.
Just how to extract the energy provided by the fusion reaction has been a major area of research and the most favoured method of extracting this energy currently being researched is a method involving coating the outer walls of the reactor in a lithium 'blanket'. The high energy neutrons created in the reaction pass through the walls of the chamber and meet this lithium wall, then strike lithium atoms releasing their energy and exciting the lithium atoms, which then heats a layer of water that surrounds the lithium. This water is turned to steam by this heating and is used to turn turbines as in fossil fuel power plants; this is discussed further under 'harnessing the energy'.
There is currently only one operational tokamak fusion reactor in the world which has achieved plasma states and fusion. This is the JET (Joint European Torus) reactor in Oxfordshire, until recently operated by a European Union coalition. The facility is still funded by the EFDA (European Fusion Development Agreement), which draws funding from every EU country as well as Switzerland, but is now operated by the UKAEA (UK Atomic Energy Authority). Other reactors have been largely halted research in preparation of the next generation reactor which it is hoped will provide the stepping stone to commercial fusion power on a large scale - ITER (International Thermonuclear Experimental Reactor).
For the moment I will focus largely on JET and the methods it uses, as it has made many advances during its twenty year lifetime. Indeed as JET is currently the largest tokamak reactor ever built, ITER will be two to three times as large when it is completed, it is closest to ITER and it is hoped that smaller scale experiments will continue to be run there alongside ITER - which should become operational around 2020.
Other projects worldwide, which have now largely ceased, include TFTR in America, JT-60 in Japan as well as smaller projects around the world in Brazil, Australia, India and many other countries.
The two other ways of containing the plasma and thus making fusion possible are gravitational and inertial. Gravitational containment takes place in stars and is not currently possible on earth nor is it likely ever to be due to the large gravitational fields that would need to be created. Inertial containment uses intense lasers to compress a solid pellet of deuterium and tritium to around a thousand times normal solid density and heating it at the same time, thus forcing fusion by the extreme compression and heat. However this method is inherently unstable and difficult to predict and control, and many predict that only around a 10% efficiency would be achievable, thus little research has been conducted in this area.
JET has been operational since 1983 and is the largest tokamak reactor ever built. Below to the left is a cutaway drawing of the JET reactor showing the magnets and the chamber. To the right is a picture of the inside of the chamber, taken in 2001.
Figure 3 Figure 4
The apparatus is approximately 12 metres high and 15 metres in diameter. The proposed lithium blanket is not in place on JET or indeed any other tokamak reactor. Detectors have been placed around the chamber, and show the emission of neutrons and separate experiments have shown that firing neutrons at a layer of lithium will raise the temperature of a surrounding layer of water. Thus the proposed method of extracting energy has not yet been directly tested - that will be performed at ITER.
Plasma states and indeed fusion have been achieved at JET, one 1997 reaction sustained fusion for 5 seconds, producing 5MW of fusion power. Also in 1997 over 10MW was sustained for just over 0.5 seconds, with a peak of 16.1MW. For a more powerful reaction, the plasma becomes unstable very quickly. It is hoped that ITER will be able to sustain fusion for 5-10 minutes, paving the way for future reactors which many predict could be left running indefinitely, with the reaction slowed at times of low demand by reducing the input of deuterium and tritium atoms.
Several methods are employed at JET to heat the deuterium-tritium plasma to the 100 million degrees Celsius required for it to 'burn' - for fusion to occur. Alternating current is passed through the powerful electromagnets surrounding the chamber, creating changing magnetic fields. These accelerate the charged particles in the plasma as charge moves under the influence of a moving electromagnetic field, this constantly changing magnetic field accelerates the plasma, heating it. This induces a strong current in the plasma due to the moving charge, up to 7 million amps. This current generates its own electromagnetic field, and this interacts with the magnetic field of the magnets, ensuring that the plasma never touches the walls of the chamber - which would rapidly disperse the heat of the plasma and cause it to collapse, and also ensuring the plasma keeps moving around the chamber in a uniform direction.
Further methods must be employed to raise the plasma to the 100 million degrees needed for fusion. Powerful microwaves injected in high frequency ranges (MHz to GHz range) can surrender their energy to ions or electrons in the plasma, when conditions and frequencies are carefully selected. Another method involves injecting beams of highly accelerated neutral particles into the plasma, these then collide with the ions and release their energy. Once the particles have released most of their energy they sink towards the bottom of the chamber and are removed with precise extractors.
Once fusion has been achieved the plasma can also self-heat by the fusion reaction. Earlier I noted that most of the energy of the fusion reaction is carried by the neutron as kinetic energy, around 80% (14.1MeV). Around 20% is carried by the helium nucleus, also as kinetic energy, and this is still a substantial amount (3.5MeV). The helium nucleus then goes on to collide with deuterium and tritium nuclei, transferring its energy, until its excess energy has all been transferred. It then sinks to the bottom of the chamber where it is removed using precise collectors. These collectors must be employed so that the helium nuclei do not build up and 'pollute' the plasma, obstructing deuterium and tritium nuclei. In this way, and employing accelerators which inject high energy deuterium and tritium in regular pulses, the fusion reaction could be sustained indefinitely if the plasma could be kept stable.
Such a sustained reaction may seem so close, however it likely to be around 2050 before large-scale fusion reactors are providing our electricity.
Harnessing the energy
As I discussed earlier, the most favoured method to convert the fusion energy to electricity in the future is the lithium blanket. This blanket surrounds the reactor, and the neutrons emitted in the fusion reaction collide with the lithium, exciting it and this excites the water molecules which surround the blanket. This water is boiled by this energy and, turning to steam, rises. This steam builds up in a chamber which would be designed to maximise the pressure produced. The pressure grows until is enough to force a turbine to rotate. This turbine is attached to the rotor in a generator, and this causes the rotation of the rotor which induces an alternating current in the stator. The diagram below illustrates the basic mechanism.
The rotor is an electromagnet and has a direct current input to activate it. The rotor rotates due to the rotation of the turbine it is attached to, the wires wound round the stator cut the moving magnetic field of the rotor and this induces an alternating current in the stator.
This may seem a rather indirect method of extracting the energy of the fusion reaction, however it is currently the most practical and simplest method. Possible other methods would require large amounts of research and currently most funding is geared towards actually creating stable and sustainable fusion. Even if efficiency was only around 50%, this would still create substantial energy. And due to the small amounts of deuterium and tritium required and the large deposits available, there would be no worry of reserves running out. Indeed current efficiency in fossil fuel power plants is around this figure.
The energy changes in the generation of electricity from fusion power are summarised below:
Potential Energy (Strong Force) ==> Kinetic (Neutron) ==> Kinetic (Water Molecules) ==>Kinetic (Turbine and Rotor in Generator) ==> Electrical (Induced in Stator)
Conclusions
Nuclear fusion has the potential to replace all current fossil fuel power plants, it is predicted that plants producing around 1-2 GW are feasible - this is the same as a typical fossil fuel power plant. 3GW is around the largest so far considered. Once the fusion reaction has started the reaction can be maintained with very little human effort. The deuterium and tritium injections must be controlled, the plasma monitored and the reaction kept under control however much of this could be controlled with computers as there would be little consequence if something were to go wrong - the plasma would collapse. Then it would simply have to be restarted after any repairs were effected, most of which would be performed by robots due to the precise nature of the equipment.
The eventual successful creation of stable and sustainable fusion reactions in such reactors as I have discussed, and the use of these reactions to generate electricity in the ways I have discussed, seems to be a foregone conclusion in the minds of the majority of the scientific community. Given that fusion has been sustained for several seconds already, and given the precise and promising theory behind it, it seems only a matter of time and money before fusion is providing our electricity.
It is not as simple as solving the problems of nuclear fusion to implement it, there are many economic and political factors to account for. Many countries rely on the export of oil and coal and the world economy would alter significantly with the introduction of fusion. Indeed one could argue that funding for fusion research, whilst significant, is not as high as one would have thought given the benefits it would bring because of these changes fusion would bring to the world order, possibly destabilising economies and governments.
However one would hope that regardless of such worries fusion would allow the world to have cheap, clean and limitless power within 50 years. Indeed, the world would seem to be a better place without the stranglehold oil exports and imports have over the world economy and global politics.
I feel I have produced a good and concise report which explains nuclear fusion, how it is achieved and how it is hoped electricity will be produced from it. I have summarised the current level of research and what is hoped from the future. I have analysed other sources of electricity, and the merits of fusion in comparison. I believe I have satisfied the requirements of the coursework, and have crossed different areas of physics in my report from particle physics, electromagnets and the general topic of electromagnetic machines such as generators, and have discussed energy changes and the equivalence of mass and energy.
Bibliography
The following sources were used in my research:
Figure 1: adapted from an animation at www.fusion.org.uk
Figure 2: courtesy of www.iea.org.
Figures 3 and 4: courtesy of www.jet.edfa.org
www.jet.efda.org
Information on the JET reactor, methods used at JET to achieve plasma states and fusion, proposed methods to generate electricity from fusion. This is a scientific website and its data is based on experimental results and accepted theoretical ideas, its information is very reliable and has been confirmed by some of the other sources listed here.
www.fusion.org.uk
Information on the JET reactor, methods used at JET to achieve plasma states and fusion, proposed methods to generate electricity from fusion. This is a scientific website and its data is based on experimental results and accepted theoretical ideas, its information is very reliable and has been confirmed by some of the other sources listed here.
www.ippex.pppl.gov
Information on US fusion program, general information on plasma and fusion. This is a scientific website and its data is based on experimental results and accepted theoretical ideas, its information is very reliable and has been confirmed by some of the other sources listed here.
www.iter.org
Information on ITER reactor, general information on plasma and fusion. This is a scientific website and its data is based on experimental results and accepted theoretical ideas, its information is very reliable and has been confirmed by some of the other sources listed here.
www.iea.org
Information on current worldwide electricity usage and sources of power. The information on this website is based on thorough and accepted research by the International Energy Agency - an independent body which monitors energy usage and sources internationally. The information is very reliable and has been confirmed by other sources I have read on the internet which were less significant.
Advanced Physics (fifth edition) - Tom Duncan
Information on exact value of energy gained in fusion reaction. This is an accepted textbook, and its data has been verified at websites listed here, its information is reliable.
Professor Walter Gear, Department of Physics and Astronomy, Cardiff University
Information on tokamak reactors, methods of obtaining and sustaining plasma state and fusion. Information on methods of generating electricity from fusion. He is well-read in the theory and practice of fusion and his information has been verified in the websites listed here, his information is reliable. Much of his information was used to clarify points raised in the other sources.
Predictions on when fossil fuel reserves will be exhausted - various sources on internet and in books, averages taken. Care was taken to consult a wide variety to ensure unbiased values arrived at, and I was far more sceptical when dealing with this data than with the other data I collected from various sources.
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