SEMINAR REPORT ON
NUCLEAR TECHNOLOGY FUSION REACTORS
By NAVIN SHUKLA Roll no.-0616540031 MECHANICAL 3rd YEAR
Submitted to the department of Mechanical Engineering In partial fulfillment of the requirement For the degree of Bachelor of technology In Mechanical Engineering
Kanpur Institute of Technology, A-1 Rooma, Kanpur U.P.Technical University, Lucknow April 2009, 14
Table of Contents 1) 2) 3) 4) 5)
Int Introdu roduct ctio ion n Basic nuclear fusion technology Magnet Magnetic ic confin confineme ement nt (MFE) (MFE) Inertial confinement (ICF) Plasma Heating 5.1 Ohmic Heating 5.2 Neutral-Beam Injection 5.3 Radio-frequency Heating 5.3.1) Induction 5.3.2) Dielectric 5.3.3) Microwave 6) Cold fusion 7) Fusi Fusion on hist histor ory y 8) Fusion power plant 9) Joint European Torus (JET) 9.1) Machine information 9.2) Current status 9.3) Remote handling 10) How to start a fire 10.1) Produce plasma 10.2) feed the coils 10.3) Produce pulses 11) International Thermonuclear Experimental Reactor (ITER) 11.1) Objective 11.2) Reactor overview 11.3) Technical design 11.4) Assessment of vacuum vessel 12) HiPER 12.1) Background 12.2) Description 12.3) Fast ignition and HiPER 12.4) Current status 12.5) Assessing fusion power 13)Fusion advantages 14) Conclusion 14.1) How fusion is safe? 14.2) fusion for the near future
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CERTIFICATE “NUCLEAR TECHNOLOGY ATOMIC FUSION ” It is to certify that that the seminar seminar report report entitled entitled “NUCLEAR TECHNOLOGY which is submitted by by NAVIN SHUKLA in partial fulfilment of the requirement for the award of degree B.Tech in department of Mechanical Engineering Of U.P. Technical University, is a record of candidate own work carried out by him under my/our supervision. The matter embodied in this thesis is original and has not been submitted for the award of any other degree.
DATE:
Engineering)
Head of Department (Mechanical
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ACKNOWLEDGEMENT It gives us a great sense of pleasure to present the report of the seminar undertaken during B.Tech third year. We owe special debt of gratitude to Mr. S.K.SONKAR , Department of mechanical engineering, Kanpur Institute of Technology; Kanpur for his constant support and guidance throughout the course of our work. His sincerity, thoroughness and perseverance have been a constant source of inspiration for us. It is only his cognizant efforts that our endeavours have seen light of the day. We also take the opportunity to acknowledge to contribution of Mr. A.P.Mishra , Department of Mechanical Engineering, Kanpur Institute of Technology; Kanpur for his full support and assistance during the preparation of seminar report. We also don’t like to miss the opportunity to acknowledge the contribution of all faculty member of the department for their kind assistance and cooperation during preparation of seminar report but not the least, we acknowledge our friends for their contribution in the completion of the project.
Name: NAVIN SHUKLA Roll NO.-0616540031 Date: Signature:
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ABSTRACT Nuclear reactions are capable of releasing huge quantities of energy. Such reactions can be achieved either by the nuclear fission (splitting) of elements of high atomic number or by the nuclear fusion (joining) of elements with low atomic number. In astrophysics, fusion reactions power the stars and produce all but the lightest elements. The most efficient reaction to utilise fusion on earth is the DT fusion reaction in which nuclei of the two Hydrogen isotopes Deuterium (D) and Tritium (T) are forced together to overcome the rejection due to their electric charge and to allow them to fuse due to the strong nuclear binding force between them. The product of this reaction is a Helium nucleus and a neutron, both with very high kinetic energy. Research in controlled nuclear fusion and its associated field plasma physics has progressed steadily for several decades and is now at a crossroad. The construction of a new international experimental machine ITER, to be built in worldwide international co-operation, has been decided. ITER aims to prove the scientific and technological feasibility of fusion energy. With this machine and its goals, controlled nuclear fusion makes the decisive transition to a new area: from a time where plasma physics and nuclear engineering were separate disciplines to a time where plasma physics and nuclear engineering will be intimately intertwined. With a foreseen power of 400 MW, ITER will produce 1.5 x 1020 neutrons/s; equivalent to the number of neutrons/s produced by a 2.2 GWth fission reactor. Fusion will need the nuclear engineering expertise. The paper introduces nuclear fusion from basic principles common to fusion and fission. The differences between fission and fusion, the reasons for them and the consequences are pointed out. Different research lines were followed to achieve the conditions for a self-sustaining controlled thermonuclear burn. Examples of major hurdles, which have been overcome, highlight the progress of research in magnetic confinement. Though challenges remain, ITER is likely to show the feasibility of fusion energy. The promise of fusion energy opens up new perspectives and opportunities for the development of fission energy and could lead to better boundary conditions for fission energy in the near future.
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1) Introduction Even though renewable resources will probably be able to meet a greater proportion of the World's energy requirements than they do at present, experts agree that they will not be able to satisfy the total demand. New energy options must therefore be developed - systems which are optimally safe, environment-friendly and economical. Controlled thermonuclear fusion is one of these rare options . JET and ITER are fusion devices of the "tokamak" type. The JET Tokamak of the European Community, based in Abingdon (UK), is the largest and most powerful in the World. Worldwide cooperation involving Europe, Japan, Russia, USA, China, South Korea and India have agreed to site ITER (International Thermonuclear Experimental Reactor) at Cadarache in France. The start of construction will be in 2007 with construction time of about 10 years. Around 600 scientists, engineers, technicians and other personnel will work on the device for approximately twenty years. Fusion devices of "stellarator" type: TJ-II is being operated at Madrid and Wendelstein 7-X is being built at Griswold. Upon completion in 2012 the latter will be the world's largest experiment of the stellarator type. Fusion powers the sun and stars as hydrogen atoms fuse together to form helium, and matter is converted into energy. Hydrogen, heated to very high temperatures, changes from a gas to plasma in which the negatively charged electrons are separated from the positively charged atomic nuclei (ions). Normally, fusion is not possible because the positively charged nuclei naturally repel each other. But as the temperature increases the ions move faster, and they collide at speeds high enough to overcome the normal repulsion. The nuclei can then fuse, causing a release of energy.
The overall reaction in the sun is "burning" hydrogen to make helium: 4 1H + 2 e --> 4He + 2 neutrinos + 6 photons Each time this reaction occurs, 26 million electronvolts (MeV) of energy are released.
The Sun In the sun, massive gravitational forces create the right conditions for this, but on Earth they are much harder to achieve. Fusion fuel—different isotopes of hydrogen—must be heated to extreme temperatures of some 100 million degrees Celsius, and must be kept dense enough, and confined for long enough (at least one second), to trigger the energy release. The aim of the controlled fusion research program is to achieve "ignition", which occurs when enough fusion reactions take place for the process to become self-sustaining, with fresh fuel then being added to continue it.
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In principle, fusion has some extremely attractive features. The big advantage of fusion compared with fossilfuel-based energy production is its relatively small fuel requirements. For the same amount of energy, fusion requires about six orders of magnitude (~106) less fuel compared with chemical energy sources (coal, oil, etc.). A convenient way to think about this is to consider that the hydrogen in an ordinary cup of tap water contains the energy equivalent of a full tank of motor gasoline in an automobile. That is, the approximately one drop of heavy water in that cup could, through fusion, provide as much energy as 20 gallons of motor gasoline.
2) Nuclear fusion The aim of fusion research is to utilize the energy source of the sun and stars here on earth: A fusion power plant is to derive energy from fusion of atomic nuclei. Under terrestrial conditions this can most rapidly be achieved with the two hydrogen isotopes, deuterium and tritium. These fuse to form helium, thus releasing neutrons and large quantities of energy: One gram of fuel could yield in a power plant 90 000 kilowatt-hours of energy, i. e. the combustion heat derived from 11 tons of coal. The basic substances needed for the fusion process, viz. deuterium and lithium, from which tritium is produced in the power plant, are available throughout the world in almost inexhaustible quantities. A cubic meter of water contains 34 grams of deuterium the energy equivalent of 300000 litters of oil. The oceans, the seas and lakes could supply enough deuterium for a 1000 reactors over a millions of years. With special conditions fusion needs an ignition temperature of 100 million degrees.
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With current technology, the reaction most readily feasible is between the nuclei of the two heavy forms (isotopes) of hydrogen—deuterium (D) and tritium (T). Each D-T fusion event releases 17.6 MeV (2.8 x 10-12 joule, compared with 200 MeV for uranium-235 (235U) fission). Deuterium occurs naturally in seawater (30 grams per cubic meter), which makes it very abundant relative to other energy resources. Tritium does not occur naturally and is radioactive, with a half-life of around 12 years. It can be made in a conventional nuclear reactor, or in the present context, bred in a fusion system from lithium. Lithium is found in large quantities (30 parts per million) in the Earth's crust and in weaker concentrations in the sea. While the D-T reaction is the main focus of attention, long-term hopes are for a D-D reaction, but this requires much higher temperatures.
In a fusion reactor, the concept is that neutrons will be absorbed in a blanket containing lithium which surrounds the core. The lithium is then transformed into tritium and helium. The blanket must be thick enough (about 1 meter) to slow down the neutrons. This heats the blanket, and a coolant flowing through it then transfers the heat away to produce steam which can be used to generate electricity by conventional methods. The difficulty has been to develop a device that can heat the D-T fuel to a high enough temperature and confine it long enough so that more energy is released through fusion reactions than is used to get the reaction going. At present, two different experimental approaches are being studied: fusion energy by magnetic confinement (MFE) and fusion by inertial confinement (ICF). The first method uses strong magnetic fields to trap the hot plasma. The second involves compressing a hydrogen pellet by smashing it with strong lasers or particle beams.
3) Magnetic confinement (MFE) In magnetic confinement (MFE), hundreds of cubic meters of D-T plasma at a density of less than a milligram per cubic meter are confined by a magnetic field at a few atmospheres pressure and heated to fusion temperature. Magnetic fields are ideal for confining plasma because the electrical charges on the separated ions and electrons mean that they follow the magnetic field lines. The aim is to prevent the particles from coming into contact with the reactor walls as this will dissipate their heat and slow them down. The most effective magnetic configuration is toroidal, shaped like a thin doughnut, in which the magnetic field is curved around to form a closed loop. For proper confinement, this toroidal field must have superimposed upon it a perpendicular field component (a poloidal field). The result is a magnetic field with force lines following spiral (helical) paths, along and around which the plasma particles are guided. There are several types of toroidal confinement systems, the most important being tokamaks, stellarators and reversed field pinch (RFP) devices. Scheme of the tokamak principle: arrangement of magnetic field coils and the resulting magnetic field that confines the plasma The word tokamak means "toroidal chamber" in Russian. It is a magnetic fusion device that is in a shape of a torus (e.g., a doughnut). In a tokamak, the toroidal field is created by a series of coils evenly spaced around the torus-shaped reactor, and the poloidal field is created by a strong electric current flowing through the plasma. In a stellarator, the helical lines of force are produced by a series of coils which may themselves be helical in shape. But no current is induced in the plasma. RFP devices have the same toroidal and poloidal components as a tokamak, but the current flowing through the plasma is much stronger and the direction of the toroidal field within the plasma is reversed.
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In tokamaks and RFP devices, the current flowing through the plasma also serves to heat it to a temperature of about 10 million degrees Celsius. Beyond that, additional heating systems are needed to achieve the temperatures necessary for fusion. In stellarators, these heating systems have to supply all the energy needed. The tokamak (toroidalnya kamera ee magnetnaya katushka—torus-shaped magnetic chamber) was designed in 1951 by Soviet physicists Andrei Sakharov and Igor Tamm. Tokamaks operate within limited parameters outside which sudden losses of energy confinement (disruptions) can occur, causing major thermal and mechanical stresses to the structure and walls. Nevertheless, it is considered the most promising design, and research is continuing on various tokamaks around the world, the two largest being the Joint European Torus (JET) in the UK and the tokamak fusion test reactor (TFTR) at Princeton in the USA. Research is also being carried out on several types of stellarators. The biggest of these, the Large Helical Device at Japan's National Institute of Fusion Research, began operating in 1998. It is being used to study of the best magnetic configuration for plasma confinement. At Garching in Germany, plasma is created and heated by electromagnetic waves, and this work will be progressed in the W7-X stellerator, to be built at the new German research center in Greifswald. Another stellarator, TJ-II, is under construction in Madrid, Spain. Because stellarators have no toroidal current, there are no disruptions and they can be operated continuously. The disadvantage is that, despite the stability, they do not confine the plasma so well. RFP devices differ from tokamaks mainly in the spatial distribution of the toroidal magnetic field, which changes sign at the edge of the plasma. The RFX machine in Padua, Italy is used to study the physical problems arising from the spontaneous reorganization of the magnetic field, an intrinsic feature of this configuration.
4) Inertial confinement Fusion (ICF)
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In inertial confinement fusion (ICF), a newer line of research, laser or ion beams are focused very precisely onto the surface of a target—a sphere of D-T ice, a few millimeters in diameter. This evaporates or ionizes the outer layer of the material to form a plasma crown that expands, generating an inward-moving compression front or implosion that heats up the inner layers of material. The core or central hot spot of the fuel may be compressed to one thousand times its liquid density, and ignition occurs when the core temperature reaches about 100 million degrees Celsius. Thermonuclear combustion then spreads rapidly through the compressed fuel, producing several times more energy than was originally used to bombard the capsule. The time required for these reactions to occur is limited by the inertia of the fuel (hence the name), but is less than a microsecond. The aim is to produce repeated microexplosions. Recent work at Osaka, Japan suggests that 'fast ignition' may be achieved at lower temperature with a second very intense laser pulse through a millimeter-high gold cone inside the compressed fuel, and timed to coincide with the peak compression. This technique means that fuel compression is separated from hot spot generation with ignition, making the process more practical. So far, most inertial confinement work has involved lasers, although their low energy makes it unlikely that they would be used in an actual fusion reactor. The world's most powerful laser fusion facility is the NOVA at Lawrence Livermore Laboratory in the US, and declassified results show compressions to densities of up to 600 times that of the D-T liquid. Various light and heavy ion accelerator systems are also being studied, with a view to obtaining high particle densities.
5) Plasma Heating In an operating fusion reactor, part of the energy generated will serve to maintain the plasma temperature as fresh deuterium and tritium are introduced. However, in the startup of a reactor, either initially or after a temporary shutdown, the plasma will have to be heated to 100 million degrees Celsius. In current tokamak (and other) magnetic fusion experiments, insufficient fusion energy is produced to maintain the plasma temperature. Consequently, the devices operate in short pulses and the plasma must be heated afresh in every pulse.
Glowing plasma inside a tokamak fusion test reactor
5.1) Ohmic Heating Since the plasma is an electrical conductor, it is possible to heat the plasma by passing a current through it; in fact, the current that generates the poloidal field also heats the plasma. This is called ohmic (or resistive) heating; it is the same kind of heating that occurs in an electric light bulb or in an electric heater.
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The heat generated depends on the resistance of the plasma and the current. But as the temperature of heated plasma rises, the resistance decreases and the ohmic heating becomes less effective. It appears that the maximum plasma temperature attainable by ohmic heating in a tokamak is 20-30 million degrees Celsius. To obtain still higher temperatures, additional heating methods must be used.
5.2) Neutral-Beam Injection Neutral-beam injection involves the introduction of high-energy (neutral) atoms into the ohmically – heated, magnetically – confined plasma. The atoms are immediately ionized and are trapped by the magnetic field. The high-energy ions then transfer part of their energy to the plasma particles in repeated collisions, thus increasing the plasma temperature.
5.3) Radio-frequency heating Radio frequency heating is the heating of materials by radio frequency (otherwise called electromagnetic) energy. This can be divided into 3 general categories as below. The term "radio frequency" is misleading - electromagnetic energy of any frequency is absorbed (and reflected) to a greater or lesser degree by all materials. In radio-frequency heating, high-frequency waves are generated by oscillators outside the torus. If the waves have a particular frequency (or wavelength), their energy can be transferred to the charged particles in the plasma, which in turn collide with other plasma particles, thus increasing the temperature of the bulk plasma .The frequency used for any particular purpose will depend on many things and this is shown below. In general, any material may accept electromagnetic energy but the degree to which that happens is dependent on;
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• • • • •
Frequency of the electromagnetic energy, Intensity of the electromagnetic energy, Proximity to the source of the electromagnetic energy, Conducting or non conducting material, Nature of the material (ie how lossy).
5.3.1) Induction Heating Induction heating involves the heating of electrically conducting materials by electromagnetic induction. Currents are induced in the material and these currents cause heating. The frequency used may vary from as low as mains frequency (50/60 Hz) to more than 10 MHz. Heating also occurs by hysteresis loss if the material has significant relative permeability (eg. Steel). Induction heating is generally a non-contact process and usually consists of a coil in close proximity to but not touching the material to be heated (usually a metal).
5.3.2) Dielectric Heating Dielectric heating involves the heating of electrically insulating materials by dielectric loss. Voltage across the material causes energy to be dissipated as the molecules attempt to line up with the continuously changing electric field. A common perception is that the molecules rub together, with the friction causing heat. This is not so. Friction is a macroscopic process and does not exist at the molecular level. The heat is generated solely by the inability of the molecules to line up with the electric field. Frequencies in the range of 10-100 MHz are necessary to perform dielectric heating. Dielectric heating is generally a contact process and usually consists of the material to be heated (usually a non-metal) sandwiched between metal plates forming a capacitor .
5.3.3) Microwave Heating Microwave heating is actually a sub-category of dielectric heating in that insulating materials are heated primarily by dielectric loss. The difference is that of frequency. At frequencies above 100 MHz an electromagnetic wave can be launched from a small dimension emitter and c onveyed through space. The material to be heated (a non-metal) can therefore be simply placed in the path of the waves and heating takes place. It is a non-contact process. Typical domestic microwave ovens operate at 2.45 GHz.
6) Cold fusion In 1989, spectacular claims were made for another approach, when two researchers, in USA and UK, claimed to have achieved fusion in a simple tabletop apparatus working at room temperature. Other experimenters failed to replicate this "cold fusion", however, and most of the scientific community no longer considers it a real phenomenon. Nevertheless, research continues. Cold fusion involves the electrolysis of heavy water using palladium electrodes on which deuterium nuclei are said to concentrate at very high densities.
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7) Fusion history Today, many countries take part in fusion research to some extent, led by the European Union, the USA, Russia and Japan, with vigorous programs also under way in China, Brazil, Canada, and Korea. Initially, fusion research in the USA and USSR was linked to atomic weapons development, and it remained classified until the 1958 Atoms for Peace conference in Geneva. Following a breakthrough with the Soviet tokamak design, fusion research became big science in the 1970s. But the cost and complexity of the devices involved increased to the point where international co-operation was the only way forward. In 1978, the European Community (with Sweden and Switzerland) launched the JET project in the UK. JET produced its first plasma in 1983, and saw successful experiments using a D-T fuel mix in 1991. In the USA, the PLT tokamak at Princeton produced a plasma temperature of more than 60 million degrees in 1978 and D-T experiments began on the Tokamak Fusion Test Reactor (TFTR) there in 1993. In Japan, experiments have been carried out since 1988 on the JT-60 Tokamak.
8) Fusion power plants
In the most likely scenario for a fusion power plant, a deuterium-tritium (D-T) mixture is admitted to the evacuated reactor chamber and there ionized and heated to thermonuclear temperatures. The fuel is held away from the chamber walls by magnetic forces long enough for a useful number of reactions to take place. The charged helium nuclei which are formed give up energy of motion by colliding with newly injected cold fuel atoms which are then ionized and heated, thus sustaining the fusion reaction. The neutrons, having no charge, move in straight lines through the thin walls of the vacuum chamber with little loss of energy. The neutrons and their 14 MeV of energy are absorbed in a "blanket" containing lithium which surrounds the fusion chamber. The neutrons' energy of motion is given up through many collisions with lithium nuclei, thus creating heat that is removed by a heat exchanger which conveys it to a conventional steam electric plant. The neutrons themselves ultimately enter into nuclear reactions with lithium to generate tritium which is separated and fed back into the reactor as a fuel.
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The successful operation of a fusion power plant will require the use of materials resistant to energetic neutron bombardment, thermal stress, and magnetic forces. Additional work also needs to be done on the design of systems for the removal of spent gas.
9) Joint European Torus (JET) The Joint European Torus (JET) is the largest tokamak operating in the world today. Up to 16 MW of fusion power for one second has been achieved in D-T plasmas using the device and many experiments are conducted to study different heating schemes and other techniques. JET has been very successful in operating remote handling techniques in a radioactive environment to modify the interior of the device, and has shown that the remote handling maintenance of fusion devices is realistic. JET, the Joint European Torus, is the largest nuclear fusion experimental reactor yet built.The reactor is situated on an old Navy airfield near Culham, Oxfordshire, in the UK : the construction of the buildings which house the project was undertaken by Tarmac Construction,[1] starting in 1978 with the Torus Hall being completed in January 1982. Construction of the experiment itself began immediately after the completion of the Torus Hall, with the first experiments beginning in 1983. The components for the JET experiment came from manufacturers all over Europe, with these components transported to the site. Because of the extremely high power requirements for the tokamak, and the fact that power draw from the main grid is limited, two large flywheel generators were constructed to provide this necessary power. One generator provides power for the 32 toroidal field coils, the other for inner poloidal field coils. The outer field coils draw their power from the grid. Equipment capability
JET is equipped with remote handling facilities to cope with the radioactivity produced by Deuterium-Tritium (D-T) fuel, which is the fuel proposed for the first generation of fusion power plants. Pending construction of ITER, JET remains the only large fusion reactor with facilities dedicated to handling the radioactivity released from D-T fusion. The power production record breaking runs from JET and TFTR used 50-50 D-T fuel mixes. During a full D-T experimental campaign in 1997 JET achieved a world record peak fusion power of 16 MW which equates to a measured Q of approximately 0.7. Q is the ratio of fusion alpha heating power to input heating power. In order to achieve burning plasma, a Q value greater than 1 is required. This figure does not include other power requirements for operation, most notably confinement. A commercial fusion reactor would probably need a Q value somewhere between 15 and 22. As of 1998, a higher Q of 1.25 is claimed for the JT-60 tokamak; however, this was not achieved under real D-T conditions but estimated from experiments performed with a pure Deuterium (D-D) plasma. Similar extrapolations have not been made for JET, but it is likely that increases in Q over the 1997 measurements could now be achieved if permission to run another full D-T campaign was granted. Work has now begun on ITER to further develop fusion power.
9.1) Machine information
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The inside of the thermo-nuclear fusion research reactor at JET is an environment hostile to human beings. In order to maintain and repair the reactor, as well as reconfigure it with new components before any new series of fusion experiments, a bespoke Remote Handling system was developed at JET. The design and operation of the Remote Handling system requires a myriad of different technologies, all of which are also applicable to ITER, the next generation fusion research project .
1) Weight of the vacuum vessel: 100 tonnes 2) Weight of the toroidal field coils: 384 tonnes
3) Weight of the Iron Core: 2700 tonnes 4) Wall material: Primarily carbon fibre composite, Beryllium coated. 5) Plasma major radius: 2.96 m 6) Plasma minor radius: 2.10 m (vertical), 1.25 m (horizontal) 7) Flat top pulse length: 20 s 8) Toroidal magnetic field (on plasma axis): 3.45 T 9) Plasma current: 3.2 MA (circular plasma), 4.8 MA (D-shape plasma) 10) Lifetime of the plasma: 20–60 s 11) Auxiliary heating: 12) Neutral beam injection heating ≤23 MW 12) Radio frequency heating ≤15 MW
Major diagnostics: 1) Visible/infrared video cameras 2) Numerous magnetic coils – provide magnetic field, current and energy measurements 3) Thomson scattering spectroscopy – provides electron temperature and electron density profiles of the plasma 4) Charge exchange spectroscopy – provides impurity ion temperature, density and rotation profiles 5) Interferometers – measure line integrated plasma density 6) Electron cyclotron emission antennas – fast, high resolution electron temperature profiles 7) Visible/UV/X-ray spectrometers – temperatures and densities 8) Neutron spectroscopy – Number of neutrons leaving the plasma relates directly to the fusion power. 9) Neutron energy relates to the ion velocity distribution and hence the fuel reactivity. 10) Bolometers – energy loss from the plasma 11) Various material probes – inserted into the plasma to take direct measurements of flow rates and temperatures 12) Soft X-ray cameras to examine MHD properties of plasmas 13) Time resolved neutron yield monitor 14) Hard X-ray monitors 15) Electron Cyclotron Emission Spatial Scanners
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9.2) Current status JET was originally set up by EURATOM with a discriminatory employment system that allowed non-British staff to be employed with more than twice the salaries of their British equivalents. The British staff eventually had this practice declared illegal, and substantial damages were paid at the end of 1999 to UKAEA staff, and later to contractors. This was the immediate cause of the ending of EURATOM’s operation of the facility. In December 1999 JET's international contract ended and the United Kingdom Atomic Energy Authority (UKAEA) then took over managing the safety and operation of the JET facilities on behalf of its European partners. From that time (2000), JET's experimental programme was then co-ordinated by the European Fusion Development Agreement (EFDA) Close Support Unit. JET operated throughout 2003 culminating in experiments using small amounts of tritium. For most of 2004 it was shut down for a series of major upgrades increasing total available heating power to over 40 MW, enabling further studies relevant to the development of ITER to be undertaken. In the future it is possible that JET-EP (Enhanced Performance) will further increase the record for fusion power. In late September 2006, experimental campaign C16 was started. Its objective is to study ITER-like operation scenarios.
9.3) Remote handling 9.3.1) Why do Remote Handling at JET? JET is the world’s largest experiment on thermo-nuclear fusion, the energy producing process which takes place in the sun. Over time, high-energy neutrons render all components and support structures of the reactor radioactive. Furthermore many plasma facing tiles are covered in Beryllium, which, if breathed in as dust, poses a further hazard to anyone working inside the reactor. Therefore, JET always placed great emphasis on its Remote Handling group, to ensure a maximum of tasks can be carried out fully remotely.
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The JET machine is a complex device whose detailed configuration changes as the physics e xperimental requirements dictate. The Remote Handling system is required to fulfill two functions: • •
Repair of any system whose failure stops the experiment Modification of Torus components for new experiments
Experience shows that remote handling interventions achieve higher prec ision and introduce less impurities than sending men inside the torus did in the past .
The basic remote maintenance work is undertaken by a dexterous, force-reflecting master-slave servomanipulator (called the Mascot). The Mascot Slave unit is transported on the end of a 10 metre long articulated robot. The Mascot master station is driven by experienced operators situated in the Remote Handling Control Room .
To gain access to the inside of the torus, two of the eight main horizontal ports are reserved for Remote Handling. A second articulated Boom works in parallel with the first to transfer components and tools between storage facilities outside the torus and the workplace within the torus. Both Booms are hyperredundant multi-joint devices to allow them to “snake” their way through the narrow ports and around the torus. Other robots are designed for Ex-Vessel work, like the Telescopic Articulated Remote Mast (TARM), which is suspended from the main 150 ton gantry crane .
9.3.2) Remote Handling shutdown Life Cycle
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The JET experimental reactor undergoes a planned refurbishment period (called a shutdown) typically every 1 to 3 years. During this time plasma experiments are suspended, the torus is vented (i.e. the vacuum is ended) and brought to normal atmospheric pressure just above room temperature. This is the time when the Remote Handling equipment enters the torus to do planned refurbishment work. However, the work to prepare a Remote Handling shutdown starts several years before then. First the plasma physicists think up a new series of fusion experiments and begin to design the new components required. Already at this stage the Remote Handling engineers are involved, to ensure that the design of all new components is compatible with Remote Handling tooling. Then the Remote Handling design engineers at JET start to design tools and (if needed) robotic equipment such as dedicated end-effectors, to assist in the installation of the new torus components. At the same time Remote Handling operation engineers work out strategies of how to best achieve certain installation tasks and develop the task logistics, procedures and teach-files. The new work procedures are derived at in a Virtual Reality simulation, and later tested and fine-tuned in a physical full-scale mockup facility, using the real robotic equipment and the real Remote Handling tools, but mostly using dummy components.
10) How to start a fire? Nowadays we don’t use controlled fire directly in our houses, but we still take advantage of it: Fossil fuels are burned in power plants to produce the energy we need in our daily life. The predictions how long the reserves of fossil fuels will last differ, depending on who analyses the resources. One thing is certain: some day fossil fuels will be exhausted. Therefore scientists and decision makers are looking for new energy resources. One of them could be the use of the fusion process that has been happening in the Sun f or 4.5 billion years with temperatures of 15 million degrees. To use this energy source on Earth physicists and engineers aim to reach temperatures ten times higher than in the Sun. JET as a European fusion experiment can achieve these extreme conditions under which the process is investigated in detail to use it in future power plants.
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10.1) Firstly: Produce plasma To investigate the plasma state and later to harness energy with the man-made technology, the plasma has to have a temperature of hundreds of millions of degrees Celsius. “Plasma” is composed of nuclei and electrons moving independently from each other. JET is capable of producing a completely ionised gas. Obtaining these extraordinary high temperatures requires extraordinarily powerful heating. This is done by Neutral Beam Injection Heating (NBI) and Radio Frequency Heating (RF). The total input power to these systems can be up to 250 megawatts. The installed output power is 55 megawatts of the radio frequency power. Powerful heating is also needed to sustain this temperature, otherwise the p lasma would rapidly cool down due to inevitable heat losses via radiation and heat convection or conduction.
Plasma and its heating
10.2) secondly: Feed the coils Plasma heating is not the biggest consumer of energy at JET. At the hundreds of million degrees Celsius needed, standard thermal insulation methods are totally inadequate. The reason tungsten is the material with the highest known melting point of 3,422 degree Celsius, which isn’t at all sufficient to resist the high temperature of the plasma. So to confine the plasma JET uses a magnetic confinement system to keep the charged particles of the plasma away from the vessel wall and to protect it from the hot plasma. Unless the plasma is well insulated in the magnetic field, it can lose energy due to a temperature gradient from the vessel wall to the plasma centre of about one million degrees per centimetre. The well-defined magnetic coils producing the strong magnetic fields need a significant amount of power. Under the circumstances high currents normally required, electrical resistance of the coils causes significant losses of energy in form of heat. As a consequence they need to be water-cooled. The energy to do so is mostly dissipated to the atmosphere via special cooling towers. Some fusion experiments, like Tore Supra in France, LHD in Japan, EAST in China, KSTAR in South Korea,Wendelstein 7-X (under construction) in Germany, or the future experiment ITER use superconducting coils that avoid energy losses at the expense of running them at very low temperatures, around -270 degree Celsius, using liquid helium. These experiments will run with higher energy efficiency by using superconducting coils
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JET's coils and plasma
10.3) thirdly: Produce pulses Every individual experiment at JET lasts several tens of seconds. During experimental campaigns there are some 30 experiments daily, which physicists call a “pulse”. Most of the JET power consumption is concentrated in short bursts, which is quite demanding on the electricity grid and on electrical engineering in general. Moreover, even during a single pulse, the power requirements are not constant – the start-up needs more power than the “plateau”, the sustaining phase. On one hand, the toroidal field coils are the largest single load on JET. On the other hand, the poloidal field system has complex switching and control requirements. The plasma is always in the move and after it has been created, its position and shape is feedback-controlled. The magnetic field is continuously measured, and additional power is supplied to the vertical and horizontal polemical field amplifiers according to plasma behaviour. Running a JET pulse requires around 500 megawatts of power, of which more than a half is fed to the toroidal field coils. Around 100 megawatts of power is needed to run the poloidal field sys tem. The rest of nearly 150 megawatts runs the additional heating sources. The energy conversion efficiencies of all heating systems limit the power the plasma receives. However, in most JET pulses only part of these installed capacities is exploited, depending on experimental scenarios. Last but not least, the plasma also gets a few megawatts of power from ohmic heating. Ohmic heating means electric current induced in the plasma by the inner poloidal coils. In total, JET plasmas usually consume a few tens of megawatts and accumulate only a fraction of the consumed energy. The difference between input and output disappear via radiation, heat conduction and particle losses. When the fire is lit in this way the real work has just begun: physicists explore the conditions under which the plasma will be able to produce energy in a future fusion power plant.
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JET's power loads
11) International Thermonuclear Experimental Reactor (ITER) ITER (International Thermonuclear Experimental Reactor ) is an international tokamak (magnetic confinement fusion) research/engineering proposal for an experimental project that will help to make the transition from today's studies of plasma physics to future electricity-producing fusion power plants. It will build on research done with devices such as DIII-D, EAST, KSTAR, TFTR, ASDEX Upgrade, Joint European Torus, JT-60, Tore Supra and T-15, and will be considerably larger than any of them. ITER is designed to produce approximately 500 MW (500,000,000 watts) of fusion power sustained for up to 1000 seconds (compared to JET's peak of 16 MW for less than a second) by the fusion of about 0.5 g of deuterium/tritium mixture in its approximately 840 m 3 reactor chamber. Although ITER is expected to produce (in the form of heat) 5-10 times more energy than the amount consumed to heat up the plasma to fusion temperatures, the generated heat will not be used to generate any electricity. According to the ITER consortium, fusion power offers the potential of "environmentally benign, widely applicable and essentially inexhaustible" electricity, properties that they believe will be needed as world energy demands increase while simultaneously greenhouse gas emissions must be r educed, justifying the expensive research project. ITER was originally an acronym for I nternational Thermonuclear E xperimental Reactor , but that title was dropped due to the negative popular connotation of "thermonuclear," especially when in conjunction with "experimental". "Iter" also means "journey", "direction" or "way" in Latinand this double meaning reflects ITER's role in harnessing nuclear fusion as a peaceful power source.
11.1)
Objectives
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The official objective of ITER is to "demonstrate the scientific and technological feasibility of fusion energy for peaceful purposes". ITER has a number of specific objectives, all concerned with developing a viable fusion power reactor: •
• • • •
•
To momentarily produce ten times more thermal energy from fusion heating than is supplied by auxiliary heating (a Q value of 10). To produce steady-state plasma with a Q value greater than 5. To maintain a fusion pulse for up to eight minutes. To ignite a 'burning' (self-sustaining) plasma. To develop technologies and processes needed for a fusion power plant — including superconducting magnets and remote handling (maintenance by robot). To verify tritium breeding concepts.
To refine neutron shield/heat conversion technology (most of energy in the D+T fusion reaction is released in the form of fast neutrons).
11.2) Reactor overview When deuterium and tritium fuse, two nuclei come together to form a helium nucleus (an alpha particle), and a high-energy neutron. While in fact nearly all stable isotopes lighter on the periodic table than iron will fuse with some other isotope and release energy, deuterium and tritium are by far the most attractive for energy generation as they require the lowest activation energy (thus lowest temperature) to do so. All proto- and mid-life stars radiate enormous amounts of energy generated by fusion processes. Mass for mass, the deuterium-tritium fusion process releases roughly three times as much energy as uranium 235 fission and millions of times more energy than a chemical reaction such as the burning of coal. It is the goal of a fusion power plant to harness this energy to produce electricity. The activation energy for fusion is so high because the protons in each nucleus will tend to strongly repel one another, as they each have the same positive charge. A heuristic for estimating reaction rates is that nuclei must be able to get within 100 femtometer (1 × 10 −13 meter) of each other, where the nuclei are increasingly likely to undergo quantum tunnelling past the electrostatic barrier and the turning point where the strong nuclear force and the electrostatic force are equally balanced, allowing them to fuse. In ITER, this distance of approach is made possible by high temperatures and magnetic confinement. High temperatures give the nuclei enough energy to overcome their electrostatic repulsion. For deuterium and tritium, the optimal reaction rates occur at temperatures on the order of 100,000,000 K. The plasma is heated to a high temperature by ohmic heating (running a current through the plasma). Additional heating is applied using neutral beam injection (which cross magnetic field lines without a net deflection and will not cause a large electromagnetic disruption) and radio frequency (RF) or microwave heating. At such high temperatures, particles have a vast kinetic energy, and hence velocity. If unconfined, the particles will rapidly escape, taking the energy with them, cooling the plasma to the point where net energy is no longer produced. A successful reactor would need to contain the particles in a small enough volume for a long enough time for much of the plasma to fuse. In ITER and many other magnetic confinement reactors, the plasma, a gas of charged particles, is confined using magnetic fields. A charged particle moving through
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a magnetic field experiences a force perpendicular to the direction of travel, resulting in centripetal acceleration, thereby confining it to move in a circle. A solid confinement vessel is also needed, both to shield the magnets and other equipment from high temperatures and energetic photons and particles, and to maintain a near-vacuum for the plasma to populate. The containment vessel is subjected to a barrage of very energetic particles, where electrons, ions, photons, alpha particles, and neutrons constantly bombard it and degrade the structure. The material must be designed to endure this environment so that a powerplant would be economical. Tests of such materials will be carried out both at ITER and at IFMIF (International Fusion Materials Irradiation Facility). Once fusion has begun, high energy neutrons will radiate from the reactive regions of the plasma, crossing magnetic field lines easily due to charge neutrality. Since it is the neutrons that receive the majority of the energy, they will be ITER's primary source of energy output. Ideally, alpha particles will expend their energy in the plasma, further heating it. Beyond the inner wall of the containment vessel one of several test blanket modules will be placed. These are designed to slow and absorb neutrons in a reliable and efficient manner, limiting damage to the rest of the structure, and breeding tritium from lithium and the incoming neutrons for fuel. Energy absorbed from the fast neutrons is extracted and passed into the primary coolant. This heat energy would then be used to power an electricity-generating turbine in a real power plant; however, in ITER this heat is not of scientific interest, and will be extracted and disposed.
11.3) Technical design Selected facts: The central solenoid coil will use superconducting niobium-tin, to carry 46 kA and produce a field of 13.5 teslas. The 18 toroidal field coils will also use niobium-tin. At maximum field of 11.8 T they will store 41 GJ (total?). They have been tested at a record 80 kA. Other lower field ITER magnets (PF and CC) will use niobium-titanium.
11.4) Assessment of the vacuum vessel
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ITER has decided to ask AIB-Vinçotte International (an inspection organisation located in Belgium and accredited by the French Nuclear Authorities ASN) to assess the confinement (vacuum) vessel, heart of the project, following the French Nuclear Regulatory requirements. The Vacuum Vessel is the central part of the ITER machine: a double walled steel container in which the plasma is contained by means of magnetic fields. The ITER Vacuum Vessel will be the biggest fusion furnace ever built. It will be twice as large and 16 times as heavy as any previously manufactured fusion vessel: each of the nine torus shaped sectors will weigh about 450 tons. When all the shielding and port structures are included, this adds up to a total of 5,116 tons. Its external diameter will measure 19.4 m, the internal 6.5 m. Once assembled, the whole structure will be 11.3 m high. The primary function of the Vacuum Vessel is to provide a hermetically sealed plasma container. Its main components are the main vessel, the port structures and the supporting system. The main vessel is a double walled structure with poloidal and toroidal stiffening ribs between 60 mm thick shells to reinforce the vessel structure. These ribs also form the flow passages for the cooling water. The space between the double walls will be filled with shield structures made of austenitic stainless steel which is corrosion resistant and does not conduct heat well. The inner surfaces of the vessel will be covered with blanket modules. These modules will provide shielding from the high-energy neutrons produced by the fusion reactions and some will also be used for tritium breeding concepts. The Vacuum Vessel has 18 upper, 17 equatorial and 9 lower ports that will be used for remote handling operations, diagnostic systems, neutral beam injections and vacuum pumping.
12)
HiPER
The High Power laser Energy Research facility (HiPER ) is an experimental laser-driven inertial confinement fusion (ICF) device undergoing preliminary design for possible construction in the European Union starting around 2010. HiPER is the first experiment designed specifically to study the "fast ignition" approach to generating nuclear fusion, which uses much smaller lasers than conventional designs, yet produces fusion power outputs of about the same magnitude. This offers a total "fusion gain" that is much higher than devices like the National Ignition Facility (NIF), and a reduction in construction costs of about ten times
12.1)
Background
Inertial confinement fusion (ICF) devices use "drivers" to rapidly heat the outer layers of a "target" in order to compress it. The target is a small spherical pellet containing a few milligrams of fusion fuel, typically a mix of deuterium and tritium. The heat of the laser burns the surface of the pellet into plasma, which explodes off the surface. The remaining portion of the target is driven inwards due to Newton's Third Law, eventually collapsing into a small point of very high density. The rapid blow off also creates a shock wave that travels towards the center of the compressed fuel. When it reaches the center of the fuel and meets the shock from the other side of the target, the energy in the shock wave further heats and compresses the tiny volume around it. If the temperature and density of that small spot can be raised high enough, fusion reactions will occur.
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The fusion reactions release high-energy particles, some of which (primarily alpha particles) collide with the high density fuel around it and slow down. This heats the fuel further, and can potentially cause that fuel to undergo fusion as well. Given the right overall conditions of the compressed fuel—high enough density and temperature—this heating process can result in a chain reaction, burning outward from the center where the shock wave started the reaction. This is a condition known as "ignition", which can lead to a significant portion of the fuel in the target undergoing fusion, and the release of significant amounts of energy. To date most ICF experiments have used lasers to heat the targets. Calculations show that the energy must be delivered quickly in order to compress the core before it disassembles, as well as creating a suitable shock wave. The energy must also be focused extremely evenly across the target's outer surface in order to collapse the fuel into a symmetric core. Although other "drivers" have been suggested, notably heavy ions driven in particle accelerators, lasers are currently the only devices with the right combination of features.
12.2) Description In the case of HiPER, the driver laser system is similar to existing systems like NIF, but considerably smaller and less powerful. The driver consists of a number of "beamlines" containing Nd:glass laser amplifiers at one end of the building. Just prior to firing, the glass is "pumped" to a high-energy state with a series of xenon flash tubes, causing a population inversion of the neodymium (Nd) atoms in the glass. This readies them for amplification via stimulated emission when a small amount of laser light, generated externally in a fibre optic, is fed into the beamlines. The glass is not particularly effective at transferring power into the beam, so in order to get as much power as possible back out the beam is reflected through the glass four times in a mirrored cavity, each time gaining more power. When this process is complete, a Pockels cell "switches" the light out of the cavity. One problem for the HiPER project is that Nd: glass is no longer being produced commercially, so a number of options need to be studied to ensure supply of the estimated 1,300 disks
. From there, the laser light is fed into a very long spatial filter to clean up the resulting pulse. The filter is essentially a telescope that focuses the beam into a spot some distance away, where a small pinhole located at the focal point cuts off any "stray" light caused by inhomogeneities in the laser beam. The beam then widens out until a second lens returns it to a straight beam again. It is the use of spatial filters that lead to the long
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beamlines seen in ICF laser devices. In the case of HiPER, the filters take up about 50% of the overall length. The beam width at exit of the driver system is about 40 cm × 40 cm. One of the problems encountered in previous experiments, notably the Shiva laser, was that the infrared light provided by the Nd: glass lasers (at ~1054 nm in vaco) couples strongly with the electrons around the target, losing a considerable amount of energy that would otherwise heat the target itself. This is typically addressed through the use of an optical frequency multiplier, which can double or triple the frequency of the light, into the green or ultraviolet, respectively. These higher frequencies interact less strongly with the electrons, putting more power into the target. HiPER will use frequency tripling on the drivers. When the amplification process is complete the laser light enters the experimental chamber, lying at one end of the building. Here it is reflected off of a series of deformable mirrors that help correct remaining imperfections in the wavefront, and then feeds them into the target chamber from all angles. Since the overall distances from the ends of the beamlines to different points on the target chamber are different, delays are introduced on the individual paths to ensure they all reach the center of the chamber at the same time, within about 10 ps. The target, a fusion fuel pellet about 1 mm in diameter in the case of HiPER, lies at the center of the chamber. HiPER differs from most ICF devices in that it also includes a second set of lasers for directly heating the compressed fuel. The heating pulse needs to be very short, about 10 to 20 ps long, but this is too short a time for the amplifiers to work well. To solve this problem HiPER uses a technique known as chirped pulse amplification (CPA). CPA starts with a short pulse from a wide-bandwidth (multi-frequency) laser source, as opposed to the driver which uses a monochromatic (single-frequency) source. Light from this initial pulse is split into different colors using a pair of diffraction gratings and optical delays. This "stretches" the pulse into a chain several nanoseconds long. The pulse is then sent into the amplifiers as normal. When it exits the beamlines it is recombined in a similar set of gratings to produce a single very short pulse. But because the pulse now has very high power, the gratings have to be large (approx 1 m) and sit in a vacuum. Additionally the individual beams must be lower in power overall; the compression side of the system uses 40 beamlines of about 5 kJ each to generate a total of 200 kJ, whereas the ignition side requires 24 beamlines of just under 3 kJ to generate a total of 70 kJ. The precise number and power of the beamlines are currently a subject of research. Frequency multiplication will also be used on the heaters, but it has not yet been decided whether to use doubling or tripling; the latter puts more power into the target, but is less efficient converting the light. As of 2007, the baseline design is based on doubling into the green.
12.3) Fast Ignition and HiPER In traditional ICF devices the driver laser is used to compress the target to very high densities. The shock wave created by this process further heats the compressed fuel when it collides in the center of the sphere. If the compression is symmetrical enough the increase in temperature can create conditions close to the Lawson criterion, leading to significant fusion energy production. If the resulting fusion rate is high enough, the energy released in these reactions will heat the surrounding fuel to similar temperatures, causing them to undergo fusion as well. In this case, known as "ignition", a significant portion of the fuel will undergo fusion and release large amounts of energy. Ignition is the basic goal of any fusion device. The amount of laser energy needed to effectively compress the targets to ignition conditions has grown rapidly from early estimates. In the "early days" of ICF research in the 1970s it was believed that as little as 1 kilojoules (kJ) would suffice, and a number of experimental lasers were built in order to reach these power levels. When they did, a series of problems, typically related to the homogeneity of the collapse, turned out to
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seriously disrupt the implosion symmetry and lead to much cooler core temperatures that originally expected. Through the 1980s the estimated energy required to reach ignition grew into the megajoule range, which appeared to make ICF impractical for fusion energy production. For instance, the National Ignition Facility (NIF) uses about 330 MJ of electrical power to pump the driver lasers, and in the best case is expected to produce about 20 MJ of fusion power output. Without dramatic gains in output, such a device would never be a practical energy source. The fast ignition approach attempts to avoid these problems. Instead of using the shock wave to create the conditions needed for fusion above the ignition range, this approach directly heats the fuel. This is far more efficient than the shock wave, which becomes less important. In HiPER, the compression provided by the driver is "good", but not nearly that created by larger devices like NIF; HiPER's driver is about 200 kJ and produces densities of about 300 g/cm³. That's about one-third that of NIF, and about the same as generated by the earlier NOVA laser of the 1980s. For comparison, lead is about 11 g/cm³, so this still represents a considerable amount of compression, notably when one considers the target's interior contained light D-T fuel around 0.1 g/cm³. Ignition is started by a very-short (~10 picoseconds) ultra-high-power (~70 kJ, 4 PW) laser pulse, aimed through a hole in the plasma at the core. The light from this pulse interacts with the fuel, generating a shower of high-energy (3.5 MeV) relativistic electrons that are driven into the fuel. The electrons heat a spot on one side of the dense core, and if this heating is localized enough it is expected to drive the area well beyond ignition energies. The overall efficiency of this approach is many times that of the conventional approach. In the case of NIF the laser generates about 4 MJ of infrared power to create ignition that releases about 20 MJ of energy. This corresponds to a "fusion gain" —the ratio of input laser power to output fusion power— of about 5. If one uses the baseline assumptions for the current HiPER design, the two lasers (driver and heater) produce about 270 kJ in total, yet generate 25 to 30 MJ, a gain of about 100. Considering a variety of losses, the actual gain is predicted to be around 72. Not only does this outperform NIF by a wide margin, the smaller lasers are much less expensive to build as well. In terms of power-for-cost, HiPER is expected to be about an order of magnitude less expensive than conventional devices like NIF. Compression is already a fairly well-understood problem, and HiPER is primarily interested in exploring the precise physics of the rapid heating process. It is not clear how quickly the electrons stop in the fuel load; while this is known for matter under normal pressures, it's not for the ultra-dense conditions of the compressed fuel. To work efficiently, the electrons should stop in as short a distance as possible, in order to release their energy into a small spot and thus raise the temperature (energy per unit volume) as high as possible. How to get the laser light onto that spot is also a matter for further research. One approach uses a short pulse from another laser to heat the plasma outside the dense "core", essentially burning a hole through it and exposing the dense fuel inside. This approach will be tested on the OMEGA-EP system in the US. Another approach, tested successfully on the GEKKO XII laser in Japan, uses a small gold cone that cuts though a small area of the target shell; on heating no plasma is created in this area, leaving a hole that can be aimed into by shining the laser into the inner surface of the cone. HiPER is currently planning on using the gold cone approach, but will likely study the burning solution as well.
12.4) Current Status
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In 2005 HiPER completed a preliminary study outlining possible approaches and arguments for its construction. The report received positive reviews from the EC in July 2007, and moved onto a preparatory design phase in early 2008 with detailed designs for construction beginning in 2011 or 2012. In parallel, the HiPER project also proposes to build smaller laser systems with higher repetition rates. The high powered flash lamps used to pump the laser amplifier glass causes it to deform, and it cannot be fired again until it cools off, which takes as long as a day. Additionally only a very small amount of the flash of white light generated by the tubes is of the right frequency to be absorbed by the Nd:glass and thus lead to amplification, in general only about 1 to 1.5% of the energy fed into the tubes ends up in the laser beam. Key to avoiding these problems is replacing the flash lamps with more efficient pumps, typically based on laser diodes. These are far more efficient at generating light from electricity, and thus run much cooler. More importantly, the light they do generate is fairly monochromatic and can be tuned to frequencies that can be easily absorbed. This means that much less power needs to be used to produce any particular amount of laser light, further reducing the overall amount of heat being generated. The improvement in efficiency can be dramatic; existing experimental devices operate at about 10% overall efficiency, and it is believed "near term" devices will improve this as high as 20%. HiPER proposes to build a demonstrator diode-pump system producing 10 kJ at 1 Hz or 1 kJ at 10 Hz depending on a design choice yet to be made. The best high-repetition lasers currently operating are much smaller; MERCURY at Livermore is about 70 J, HALNA in Japan at ~20 J, and LUCIA in France at ~100 J. HiPER's demonstrator would thus be between 10 and 1000 times as powerful as any of these. In order to make a practical commercial power generator, the high-gain of a device like HiPER would have to be combined with a high-repetition rate laser and a target chamber capable of extracting the power. Additional areas of research for post-HiPER devices include practical methods to carry the heat out of the target chamber for power production, protecting the device from the neutron flux generated by the fusion reactions, and the production of tritium from
12.5) Assessing fusion power Fusion power plants has the potential to substantially reduce the environmental impacts of increasing world electricity demands since, like nuclear fission power, they would make negligible contributions acid rain or the greenhouse effect compared to fossil fuels. Fusion power could easily satisfy the energy needs associated with continued economic growth, given the ready availability of fuels. There would be no danger of a runaway fusion reaction as this is intrinsically impossible and any malfunction would result in a rapid shutdown of the plant. However, although fusion generates no radioactive fission products or transuranic elements, and the unburned gases can be treated on site, there would a short-term radioactive waste problem due to activation products. Some component materials will become radioactive during the lifetime of a nuclear reactor, due to bombardment with high-energy neutrons, and will eventually become radioactive waste. The volume of such waste would be similar to that due to activation products from a fission reactor. The radiotoxicity of these wastes would be relatively short-lived compared with the actinides (long-lived alpha-emitting transuranic isotopes) from a fission reactor. There are also other concerns, such as those first raised in 1973 by the American Association for the Advancement of Science (AAAS). These include the hazard arising f rom an accident to the magnetic system.
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The total energy stored in the magnetic field would be similar to that of an average lightning bolt (100 billion joules, equivalent to about 45 tonnes of TNT). Attention was also drawn to the possibility of a lithium fire. In contact with air or water, lithium burns spontaneously and could release many times that amount of energy. Safety of nuclear fusion is a major issue. But the AAAS was most concerned about the release of tritium into the environment. It is radioactive and very difficult to contain since it can penetrate concrete, rubber and some grades of steel. As an isotope of hydrogen it is easily incorporated into water, making the water itself weakly radioactive. With a half-life of 12.4 years, tritium remains a threat to health for over one hundred years after it is created, as a gas or in water. It can be inhaled, absorbed through the skin or ingested. Inhaled tritium spreads throughout the soft tissues and tritiated water mixes quickly with all the water in the body. The AAAS estimated that each fusion reactor could release up to 2x1012 Bequerels of tritium a day during operation through routine leaks, assuming the best containment systems—much more in a year than the Three Mile Island accident released altogether. Moreover, an accident would release even more. This is one reason why long-term hopes are for the deuterium-deuterium fusion process, dispensing with tritium. Materials research and development will play a major role in determining fusion's future viability due to the very high energetic neutron bombardment, thermal stress, and magnetic forces. At this point in time the economics of fusion power are largely unknown. The capital costs are likely to be large, given that a fusion power plant would be much larger in physical size and more complex than a conventional fission power plant. The levelized cost cost of electricity from current fission reactors is greater than that from fossil fuels and wind, so fusion must make significant progress on this front to compete in future electricity markets. Thus, while the scientific community has made enormous progress in our scientific understanding of fusion, as of yet there is no clearly identified route to an attractive commercial fusion power plant that will sell in the energy marketplace of the 21st century and beyond. While fusion power clearly has much to offer if and when the technology is eventually developed, the problems associated with it also need to be addressed if is to become a widely used future energy source.
13) Fusion advantages Fusion power would provide much more energy for a given weight of fuel than any technology currently in use, and the fuel itself (primarily deuterium) exists abundantly in the Earth's ocean: about 1 in 6500 hydrogen atoms in seawater is deuterium. Although this may seem a low proportion (about 0.015%), because nuclear fusion reactions are so much more energetic than chemical combustion and seawater is easier to access and more plentiful than fossil fuels, some experts estimate that fusion could supply the world's energy needs for millions of years. An important aspect of fusion energy in contrast to many other energy sources is that the cost of production is inelastic. The cost of wind energy, for example, goes up as the optimal locations are developed first, while further generators must be sited in less ideal conditions. With fusion energy, the production cost will not increase much, even if large numbers of plants are built. It has been suggested that even 100 times the current energy consumption of the world is possible. Some problems which are expected to be an issue in this century such as fresh water shortages can actually be regarded merely as problems of energy supply. For example, in desalination plants, seawater can be
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purified through distillation or reverse osmosis. However, these processes are energy intensive. Even if the first fusion plants are not competitive with alternative sources, fusion could still become competitive if large scale desalination requires more power than the alternatives are able to provide. Despite being technically non-renewable, fusion power has many of the benefits of long-term renewable energy sources (such as being a sustainable energy supply compared to presently-utilized sources and emitting no greenhouse gases) as well as some of the benefits of the much more limited energy sources as hydrocarbons and nuclear fission (without reprocessing). Like these currently dominant energy sources, fusion could provide very high power-generation density and uninterrupted power delivery (due to the fact that it is not dependent on the weather , unlike wind and solar power). In nutshell the advantages are; The non - radioactive part of the fuel is abundant on a worldwide scale and practically inexhaustible. The radioactive part of the fuel (tritium) is generated in the reactor itself and is burned producing He and neutrons. The ash (He) is safe and non- radioactive. The quantities of the fuel and ash are very small (a few hundreds of kilograms per year and reactor.) The biological hazards presented by fusion waste are, after 10 years, one thousand times smaller than those associated with fission waste.
14) Conclusion 14.1) How safe is Fusion? The following explanation focusses on magnetic confinement of deuterium-tritium-fuelled plasmas, such as those in ITER, but similar or even stronger arguments apply also to other fuel combinations and to laser fusion. a)
The fusion process is inherently safe. Leak-tight confinement barriers are essential to produce fusion reactions. Equipment failure quickly leads to plasma extinguishment.
b)
No chain reaction is involved and the reaction is thermally self-limiting. . There is no danger of a large jump in plasma power output, since normal operation is close to pressure limits which already maximise the number of fusion reactions that will occur. In ITER, because of experimental uncertainty, it is possible for the plasma to operate at somewhat (<1.2) higher power levels than planned, but these can be easily brought under control in a matter of seconds
c)
The fusion process is limited to a few seconds burn, without continuous refueling.
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Achieving low loss burn conditions is a delicate matter and requires many conditions to be satisfied - the failure or change of a single one enhances plasma energy losses and terminates the burn. Halting the fuelling quickly extinguishes the plasma. In ITER about 0.5 g of fuel is in the machine at any time, and the fuelling/exhaust rate is also about 0.5 g/s. Even if the exhaust fails, the plasma is quickly poisoned by impurities, and extinguishes. The power and energy densities in the reactor and plasma are low. The main sources of energy which can damage ITER are pressurised coolant, chemical reactions (e.g. of leaking coolant and hot materials, or of hydrogen and air), heat from the fusion reaction in the plasma, and magnetic energy in the coils. There are no large stores of chemicals or other energy sources able to cause powerful explosions. ITER is designed such that its hardware avoids the unexpected release from energy sources or mitigates the consequences of any such release to acceptable levels not only for the general public, to ensure the ultimate safety of the plant, but also for plant operators, to protect their investment. To help in these respects, ITER has large heat transfer surfaces and heat sinks which transfer and absorb energy, maintaining low temperatures and avoiding melting of components. The same will be true in a power reactor, but the margins needed for ITER should be able to be reduced, and the overall power density should be able to be increased. The reaction products are either absorbed in surrounding structural or tritium-breeding materials (neutrons), or are non-radioactive (helium). In ITER nearly all materials around the plasma are to shield the surrounding equipment, whereas in a power reactor the bulk will breed tritium from lithium-containing materials, ready to burn it in the plasma. Activated structural materials from neutron irradiation are not mobile except dust and corrosion products which form only a small fraction. The neutrons produce activated waste materials. Dust is formed by sputtering from high energy particles in the plasma hitting the surrounding material surfaces. Although not necessarily a problem itself, this dust can become contaminated with tritium. Coolant channels can become corroded, especially in high nuclear radiation fields, and the corrosion can dislodge and be freed if a coolant pipe breaks. In ITER the coolant chemical control system is capable of maintaining coatings of activated corrosion products well below 10 kg per loop, with less than 60 g as loose material or ions in the coolant (these limits are used in accident analysis). In a power reactor this aspect will be further optimized. Negligible operational environmental impact. The potential risk to the local environment is limited and is reduced as low as judged reasonably achievable by the independent nuclear regulator in the country concerned. Negligible long term environmental impact. Neither the provision of fuel or plant hardware, nor its removal after use, places an intolerable and uncertain burden on current or future generations.
14.2) fusion for the near future
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Thanks to JET and other fusion experiments, major progress has been made towards developing fusion as a viable energy source, concludes Dr Robinson. “It’s time to move on a number of fronts,” he says. “We now have a good understanding of how the materials used in fusion plants might behave although further tests in a dedicated facility are still required, and we understand how to deal with the short-lived waste products that fusion plants will produce.” These short-lived waste products are the parts of the machine that would be bombarded with neutrons from the fusion process, and so become radioactive. With careful material selection and recycling, these wastes would last for significantly less time than fission waste – 30 to 40 years compared with hundreds of years. Dr Robinson continues, “When external costs are included, we have calculated that fusion costs will be comparable with clean coal or any baseload renewable energy technologies – and if you take into account that there would be no need for environmental remediation with fusion, the economics of fusion become even more favourable.”
REFERENCES 1. Nuclear Fusion Power, Assessing fusion power. 2. Heindler and Kernbichler, Proc. 5th Intl. Conf. on Emerging Nuclear Energy Systems, 1989, pp. 177-82. See also Residual radiation from a p–11B reactor
3.
The first A-bomb shot dates back to July 16, 1945 in Alamogordo ( New Mexico desert), while the first civilian fission plant was connected to the electric power network on June 27, 1954 in Obninsk (Russia).
4.
The first H-bomb, Ivy Mike, was detonated on Eniwetok , an atoll of the Pacific Ocean, on November 1, 1952 (local time).
5.
Great Soviet Encyclopedia, 3rd edition, entry on "Токамак", available online here
6. Effects of Z-pinch were first experimented on in 1790 by Martinus van Marum in the Netherlands: see this Wikipedia article.
7. T. Hamacher and A.M. Bradshaw (October 2001). "Fusion as a Future Power Source: Recent Achievements and Prospects" (PDF). World Energy Council. Archived from the original on 2004-05-06.
8. ^ Energy for Future CenturiesPDF (150 KiB)
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