Background
In assessing the potential for global and sustainable energy production in the long term it is clear that the diminishing availability and rising cost of energy based on carbon combined with the increased emphasis on low environmental impact energy sources generally, emphasizes the notion that nuclear fusion is one of very few candidates for the large-scale carbon-free production of base-load power.
Fusion has many potential attractions:
- Abundant fuel
- Intrinsically safe
- No production of CO2 or atmospheric pollutants
- "Clean nuclear stove" producing relatively short-lived waste.
On November 21, 2006, the seven participants formally agreed to fund the creation of a nuclear fusion reactor.[1] The program is anticipated to last for 30 years – 10 for construction, and 20 of operation. ITER was originally expected to cost approximately €5billion, but the rising price of raw materials and changes to the initial design may see that amount triple.[2] The reactor is expected to take 10 years to build with completion scheduled for 2018.[3] Site preparation has begun in Cadarache, France and procurement of large components has started.[4]
ITER is designed to produce approximately 500 MW of fusion power sustained for up to 1,000 seconds[5] (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 m3 reactor chamber. Although ITER is expected to produce (in the form of heat) 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.[6]
ITER was originally an acronym for International Thermonuclear Experimental 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 Latin,[7] reflecting ITER's potential role in harnessing nuclear fusion as a peaceful power source.
[edit]Timeline
| Date | Event |
|---|---|
| 2006-11-21 | Seven participants formally agreed to fund the creation of a nuclear fusion reactor.[1] |
| 2008 | Construction start[3] |
| 2018 | Predicted: Planned completion[3] |
| 2038 | Predicted: End of project |
[edit]Objectives
ITER's mission is to demonstrate the feasibility of fusion power, and prove that it can work without negative impact.[8] Specifically, this includes:
- To momentarily produce ten times more thermal energy from fusion heating than is supplied by auxiliary heating (a Q value of 10).
- To produce a steady-state plasma with a Q value greater than 5.
- To maintain a fusion pulse for up to 480 seconds.
- 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).
[edit]Reactor overview
- See also: Nuclear fusion
When deuterium and tritium fuse, two nuclei come together to form a helium nucleus (an alpha particle), and a high-energy neutron.
- 21D + 31T → 42He + 10n + 17.6 MeV
While in fact nearly all stable isotopes lighter on the periodic table than iron-56 and nickel-62, which have the highest nuclear binding energy, 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 tunnelingpast 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 (see Maxwell-Boltzmann distribution). 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 a magnetic fieldexperiences 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 (see neutron flux). 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 generating system is not of scientific interest, so instead the heat will be extracted and disposed of.
[edit]History
ITER began in 1985 as a collaboration between the then Soviet Union, the European Union (through EURATOM), the USA, and Japan. Conceptual and engineering design phases led to an acceptable, detailed design in 2001, underpinned by US$650 million worth of research and development by the "ITER Parties" to establish its practical feasibility. These parties (with the Russian Federation replacing the Soviet Union and with the USA opting out of the project in 1999 and returning in 2003) were joined in negotiations on the future construction, operation and decommissioning of ITER by Canada (who then terminated their participation at the end of 2003), the People's Republic of China and the Republic of Korea. India officially became part of ITER on 6 December 2005.
On 28 June 2005, it was officially announced that ITER will be built in the European Union in Southern France. The negotiations that led to the decision ended in a compromise between the EU and Japan, in that Japan was promised 20% of the research staff on the French location of ITER, as well as the head of the administrative body of ITER. In addition, another research facility for the project will be built in Japan, and the European Union has agreed to contribute about 50% of the costs of this institution.[9]
On 21 November 2006, an international consortium signed a formal agreement to build the reactor.[10]
On 24 September 2007, the People's Republic of China became the seventh party who had deposited the ITER Agreement to the IAEA.
On 24 October 2007, the ITER Agreement entered into force and the ITER Organization legally came into existence.
ITER will run in parallel with a materials test facility, the International Fusion Materials Irradiation Facility (IFMIF), which will develop materials suitable for use in the extreme conditions that will be found in future fusion power plants. Both of these will be followed by a demonstration power plant, DEMO, which would generate electricity. DEMO would be the first to produce electric energy for commercial use.
A "fast track" plan to a commercial fusion power plant has been sketched out.[11] This scenario, which assumes that ITER continues to demonstrate that the tokamak line of magnetic confinement is the most promising for power generation, anticipates a full-scale power plant coming on-line in 2050, potentially leading to a large-scale adoption of fusion power over the following thirty years.
[edit]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. They have been tested at a record 80 kA. Other lower field ITER magnets (PF and CC) will use niobium-titanium.
[edit]Location
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The process of selecting a location for ITER was long and drawn out. The most likely sites were Cadarache in Provence-Alpes-Côte-d'Azur, France and Rokkasho, Aomori, Japan. Additionally, Canada announced a bid for the site in Clarington in May 2001, but withdrew from the race in 2003. Spain also offered a site at Vandellòs on 17 April 2002, but the EU decided to concentrate its support solely behind the French site in late November 2003. From this point on, the choice was between France and Japan.
On 3 May 2005, the EU and Japan agreed to a process which would settle their dispute by July.
At the final meeting in Moscow on 28 June 2005, the participating parties agreed on the site in Cadarache in Provence-Alpes-Côte-d'Azur, France.
Construction of the ITER complex began in 2008, while assembly of the tokamak itself is scheduled to begin in the year 2011.[4]
[edit]Participants
Currently there are seven parties participating in the ITER program: the European Union through the legally distinct organisationEURATOM, India, Japan, People's Republic of China, Russia, South Korea, and the United States of America (USA).[4] The host member, and hence the member contributing most of the costs, is the EU (through its Fusion for Energy organisation). However Japan is also a privileged partner (see History).
Canada was previously a full member, but has since pulled out due to a lack of funding from the Federal government. The lack of funding also resulted in Canada withdrawing from its bid for the ITER site in 2003.
It was announced that participants in the ITER will consider Kazakhstan's offer to join the program.[12]
ITER's work is supervised by ITER Council, which has the authority to appoint senior staff, amend regulations, decide on budgeting issues, and allow additional states or organizations to participate in ITER[13]. ITER Council's chairman is Evgeny Velikhov, initiator of ITER project[14].
[edit]Funding
As of 13 July 2010, the total price of constructing the experiment is expected to be in excess of € 15 billion.[15] Only a year earlier that estimate was € 10 billion.[16] Prior to that, the proposed costs for ITER were € 5 billion for the construction and € 5 billion for maintenance and the research connected with it during its 35 year lifetime. At the June 2005 conference in Moscow the participating members of the ITER cooperation agreed on the following division of funding contributions: 50% by the hosting member, the European Union and 10% by each non-hosting member. According to sources at the ITER meeting at Jeju, Korea, the six non-host partners will now contribute 6/11th of the total cost — a little over half — while EU will put in the rest. As for the industrial contribution, China, India, Korea, Russia, and the U.S. will contribute 1/11th each, Japan 2/11th, and EU 4/11th.[17]
Although Japan's financial contribution as a non-hosting member is 1/11th of the total, the EU agreed to grant it a special status so that Japan will provide for 2/11th of the research staff at Cadarache and be awarded 2/11th of the construction contracts, while the European Union's staff and construction components contributions will be cut from 5/11th to 4/11th.
[edit]Criticism
Jan Vande Putte of Greenpeace International said that "Governments should not waste our money on a dangerous toy which will never deliver any useful energy". "Instead, they should invest in renewable energy which is abundantly available, not in 2080 but today."[18]
A French association including about 700 anti-nuclear groups, Sortir du nucléaire (Get Out of Nuclear Energy), claimed that ITER was a hazard because scientists did not yet know how to manipulate the high-energy deuterium and tritium hydrogen isotopes used in the fusion process.[19]
The ITER project confronts numerous technically challenging issues. French physicist Sébastien Balibar, director of research at the CNRS, said, "We say that we will put the sun into a box. The idea is pretty. The problem is, we don't know how to make the box".[20][21]
A technical concern is that the 14 MeV neutrons produced by the fusion reactions will damage the materials from which the reactor is built.[22] Research is in progress to determine how and/or if reactor walls can be designed to last long enough to make a commercial power plant economically viable in the presence of the intense neutron bombardment. The damage is primarily caused by high energy neutrons knocking atoms out of their normal position in the crystal lattice. A related problem for a future commercial fusion power plant is that the neutron bombardment will induce radioactivity in the reactor itself[citation needed]. Maintaining and decommissioning a commercial reactor may thus be difficult and expensive. Another problem is that superconducting magnets are damaged by neutron fluxes. A new special research facility is planned for this activity, IFMIF.
Rebecca Harms, Green/EFA member of the European Parliament's Committee on Industry, Research and Energy, said: "In the next 50 years nuclear fusion will neither tackle climate change nor guarantee the security of our energy supply." Arguing that the EU's energy research should be focused elsewhere, she said: "The Green/EFA group demands that these funds be spent instead on energy research that is relevant to the future. A major focus should now be put on renewable sources of energy." French Green party lawmaker Noël Mamèreclaims that more concrete efforts to fight present-day global warming will be neglected as a result of ITER: "This is not good news for the fight against the greenhouse effect because we're going to put ten billion euros towards a project that has a term of 30-50 years when we're not even sure it will be effective."[23]
A number of fusion researchers working on non-tokamak systems, such as Robert Bussard and Eric Lerner, have been critical of ITER for diverting funding that they believe could be used for their potentially more reasonable and/or cost effective fusion power plant designs.[24][25] Criticisms levied often revolve around claims of the unwillingness by ITER researchers to face up to potential problems (both technical and economic) due to the dependence of their jobs on the continuation of tokamak research.[24] An informal overview of the last decade of work was presented at the 57th International Astronautical Congress in October 2006.[26]
[edit]Response to criticism
Proponents believe that much of the ITER criticism is misleading and inaccurate, in particular the allegations of the experiment's "inherent danger." The stated goals for a commercial fusion power station design are that the amount of radioactive waste produced be hundreds of times less than that of a fission reactor, that it produces no long-lived radioactive waste, and that it is impossible for any fusion reactor to undergo a large-scale runaway chain reaction. This is because direct contact with the walls of the reactor would contaminate the plasma, cooling it down immediately and stopping the fusion process. Besides which, the amount of fuel planned to be contained in a fusion reactor chamber (one half gram of deuterium/tritium fuel[4]) is only enough to sustain the reaction for an hour at maximum,[27] whereas a fission reactor usually contains several years' worth of fuel.[28] In case of accident (or intentional act of terrorism) a fusion reactor releases far less radioactive pollution than an ordinary fission nuclear plant. Proponents note that large-scale fusion power — if it works — will be able to produce reliable electricity on demand and with virtually zero pollution (no gaseous CO2 / SO2 / NOx by-products are produced).
According to researchers at a demonstration reactor in Japan, a fusion generator should be feasible in the 2030s and no later than the 2050s. Japan is pursuing its own research program with several operational facilities exploring different aspects of practicability.[29]
In the United States alone, electricity accounts for US$210 billion in annual sales.[30] Asia's electricity sector attracted US$93 billion in private investment between 1990 and 1999.[31]These figures take into account only current prices. With petroleum prices widely expected to rise, political pressure on carbon production, and steadily increasing demand, these figures will undoubtedly also rise as known oil reserves are depleted (see Peak oil). Proponents contend that an investment in research now should be viewed as an attempt to earn a far greater future return for the economy.[citation needed] Also, worldwide investment of less than US$1 billion per year into ITER is not incompatible with concurrent research into other methods of power generation, which in 2007 totaled US$16.9 billion.[32]
Contrary to criticism, proponents of ITER assert that there are significant employment benefits associated with the project. ITER will provide employment for hundreds of physicists, engineers, material scientists, construction workers and technicians in the short term, and if successful, will lead to a global industry of fusion-based power generation[citation needed].
Supporters of ITER emphasize that the only way to convincingly prove ideas for withstanding the intense neutron flux is to experimentally subject materials to that flux — one of the primary missions of ITER and the IFMIF,[4] and both facilities will be of vital importance to the effort due to the differences in neutron power spectra between a real D-T burning plasma and the spectrum to be produced by IFMIF.[33] The purpose of ITER is to explore the scientific and engineering questions surrounding fusion power plants, such that it may be possible to build one intelligently in the future. It is nearly impossible to get satisfactory theoretical results regarding the properties of materials under an intense energetic neutron flux, and burning plasmas are expected to have quite different properties from externally heated plasmas.[citation needed] The point has been reached, according to supporters, where answering these questions about fusion reactors by experiment (via ITER) is an economical research investment, given the monumental potential benefit.
Furthermore the main line of research—the tokamak—has been developed to the point that it is now possible to undertake the penultimate step in magnetic confinement plasma physics research—the investigation of ‘burning’ plasmas in which the vast majority of the heating is provided by the fusion event itself. A detailed engineering design has been developed for atokamak experiment which would explore burning plasma physics and integrate reactor relevant technology. In the tokamak research program, recent advances in controlling the internal configuration of the plasma have led to the achievement of substantially improved energy and pressure confinement in tokamaks—the so-called ‘advanced tokamak’ modes—which reduces the projected cost of electricity from tokamak reactors by a factor of two to a value only about 50% more than the projected cost of electricity from advanced light-water reactors. In parallel, progress in the development of advanced, low activation structural materials supports the promise of environmentally benign fusion reactors, and research into alternate confinement concepts is yielding promise of future improvements in confinement.[34] Finally, supporters point out that other potential replacements to the current use of fossil fuel sources have environmental issues of their own. Solar, wind, and hydroelectric power all have a relatively low power output per square kilometer compared to ITER's successor DEMO which, at 2000 MW,[35] should have an energy density that exceeds even large fission power plants.[36]
[edit]Assessment of the vacuum vessel
ITER has decided to ask AIB-Vinçotte International (an inspection organization located in Belgium and accredited by the French Nuclear Authorities ASN) to assess the confinement (vacuum) vessel, the 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 between 390 and 430 tonnes.[37] When all the shielding and port structures are included, this adds up to a total of 5,116 tonnes. Its external diameter will measure 19.4 metres (64 ft), the internal 6.5 metres (21 ft). Once assembled, the whole structure will be 11.3 metres (37 ft) 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 millimetres (2.4 in) 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 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.
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Main components in the ITER tokamak will be the superconducting toroidal and poloidal field coils, which magnetically confine, shape, and control the hot plasma inside the vacuum vessel. Image courtesy of ITER. |
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Full-scale model of a vacuum-vessel section was built to verify manufacturing processes. It has dimensional accuracy of ±3 mm. |
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A remote handling system will be needed to service radioactive components inside the vessel. Here, the robot transports a blanket test module. |
An experimental fusion reactor that may help quench the world's thirst for energy will soon be taking shape. The International Thermonuclear Experimental Reactor, or ITER, is part of a program that intends to demonstrate the feasibility of fusion energy for peaceful purposes.
The program involves China, the EU, Japan, Korea, Russia, Switzerland, and the U.S. Construction will begin in 2007 at a site in Cadarache, France, with the $5 billion plant expected to be up and running in 2016.
ITER's goal is not electricity generation, per se, but rather to serve as an experimental test bed for optimizing fusion processes and verifying the hardware and systems that will be needed in tomorrow's electricity-producing fusion-power plants. Up to now, research has only addressed the scientific feasibility of fusion, with little attention to how it can be turned into a practical power source. The design is based around a hydrogen plasma torus operating at over 100 million °C, and it should produce 500 MW of fusion power.
A range of experimental operating modes will address issues such as plasma confinement and stability and getting rid of impurities. Specific goals include demonstrating power amplification — generating up to 10 times more power than it consumes — and steady-state "burning" of plasma. The reactor will also test components such as heat shields and superconducting magnets needed to make fusion energy generation practical. If these efforts prove successful, the next step will be building a prototype plant to demonstrate reliable electricity generation using fusion power.
FUSION'S FUTUREExperts fear the combination of a growing population and higher living standards will considerably increase world electricity demand in coming years. At the same time, demands to reduce fossil fuel use for environmental and political reasons leads to the need for other energy options.
Fusion holds promise as an essentially unlimited energy source with manageable environmental impact. The physics and engineering of a fusion-power station are not completely understood. But, according to ITER officials, the basic principles have been worked out and no technical roadblocks have been identified that will stop its development into a viable source of electricity. But the unanswered questions facing fusion are how to optimize the process and make it economically viable.
For energy production, experts generally agree the most suitable reaction involves the two heavy isotopes of hydrogen, deuterium and tritium. A sufficiently high rate of fusion requires temperatures hot enough to separate electrons from atomic nuclei, changing the fuel to plasma hotter than the center of the sun. Deuterium-tritium fusion creates helium, neutrons, and energy. For continuous operation, helium must be constantly removed and new fuel fed to the plasma. Surrounding structures absorb the neutrons and transfer heat to turbogenerators to make electricity.
The process is inherently safe, say ITER designers. No chain reaction is involved and cutting off fuel rapidly extinguishes the plasma. Likewise, closing the exhaust poisonsthe plasma with impurities and brings fusion to a halt. Large heat-transfer surfaces and watercooled heat sinks in ITER maintain low temperatures and prevent components from melting. And leak-tight containment barriers, needed for the process to work, confine any contaminants.
Radioactivity is a concern, though not nearly at the levels found in fission reactors. Tritium has a half-life of 12.5 years (meaning only 0.4% of the radioactivity remains after 100 years). Components in the reaction chamber will become radioactive, but most will return to safe levels 50 to 100 years after shutdown.
MASSIVE MAGNETSThe heart of ITER is a tokamak, a toroidal device that uses electric currents and magnetic fields to confine and heat the 800 m 3 gaseous plasma. The tokamak also ensures that any plasma particles that strike the surroundings are at low energy and hit heat-tolerant components. Otherwise, the plasma could cool, generate impurities, and cause damage.
A superconducting-magnet system consists of 18 D-shape toroidal-field (TF) coils, six poloidal-field (PF) coils, and a central solenoid (CS) coil. (Poloidal field lines loop the short way around a torus, whereas toroidal field lines loop the long way.) In addition, saddle-shape correction coils outside the TF magnets compensate for field errors due to manufacturing inaccuracies and misalignments during assembly. They also help control plasma instabilities.
The coils are massive, and will store more energy at higher fields than any ever built. The CS coil, for instance, weighs 840 t and stands about 12 m high and 4 m in diameter. Each TF coil weighs 290 t and measures 14 m high 9 m wide. These coils use superconducting Nb3Sn in a cable-in-conduit configuration. For TF coils, some 1,100 wires about 0.7 mm in diameter twist together inside a 4-cm diameter metal tube 820-m long. Cryogenic circulation pumps send supercritical helium through the tube, around the wires, and down a central gap to cool the coils.
Nb3 Sn is brittle, so manufacturing is time consuming and expensive. Initially, separate Nb and Sn wires (in a copper matrix) are wound into shape, then joined during a 200-hr heat treatment at 650°C. Next, it is electrically insulated with a wrap of glass fiber and Kapton polyimide, structurally reinforced, filled with liquid epoxy resin, and cured. Finally, it's further reinforced in a steel housing.
PF coils are in a field region that permits less-expensive NbTi strands. The PF coils, however, link with many systems, making replacement difficult. So each carries redundant turns so "incipient" short circuits are detected and isolated before causing damage. As a further precaution, the coils can be rewound in place or replaced, although at a price in machine downtime.
Reactor operation will verify coil performance and reliability, as well as the integrity of joints, conductors, and insulation — and overall manufacturing and assembly processes.
REACTOR HOUSINGFusion takes place in a welded, double-walled stainless-steel reactor vessel with internal shield plates and ferromagnetic inserts to reduce magneticfield ripple. It maintains an ultrahigh vacuum required for the plasma to form. The vessel holds a range of components, including blanket modules that absorb heat and provide shielding; a divertor that removes helium and other impurities; and access port plugs for heating, fueling, monitoring, and remote maintenance.
The vessel and internal components absorb and reduce neutron energy to levels tolerable for the magnets and surrounding equipment. It also confines coolant leaks so that radioactive materials cannot spread outside the plant. Separate water circuits cool the blanket, divertor, and vacuum vessel.
A thermally shielded cryostat houses the reactor vessel and superconducting magnets, and maintains ultralow temperatures for superconductivity. The cryostat is a reinforced single-wall cylinder 24-m high and 28-m in diameter (volume ≈14,000 m3) which also serves as a second confinement barrier.
Cryostat vacuum minimizes convective heat transfer, and thermal shields — stainless-steel panels cooled by 80°K helium gas between the cold magnets and warm vessel — minimize radiative heat transfer.
A bioshield surrounds the cryostat and reactor vessel. This concrete structure reduces radiation to safe levels and lets personnel access equipment soon after the tokamak stops operating.
Components near the reacting plasma eventually become radioactive. If replacement becomes necessary, parts must be removed by a remote handling system, placed in casks, and transported to hot cells for repair or disposal. A rail-mounted vehicle in the plasma chamber will service blanket modules, and a cantilevered transporter will maintain the divertor and port plugs. It is estimated that a single blanket module will take 25 days to replace, and the complete blanket (approximately 440 modules) 9 months.
OPERATIONA combination of radio-frequency and neutralbeam systems heats the plasma and drives internal currents necessary for long burn pulses. The plasma absorbs energy at specific frequencies from RF waves, which are transmitted by antennas in the port plugs. Neutral beams are high-energy deuterium atoms injected into the plasma.
Experiments with combinations of neutral beam and RF systems, with a total heating power exceeding 110 MW, will determine the best mix.
Steady-state operation requires active control of the plasma current in which the "bootstrap" effect generates a large percentage of current needed for fusion. Bootstrap current results from a complex interplay between particles in a tokamak. It is produced by the plasma itself, not an external source. Thus, bootstrap current can take the place of inductive current drive, reducing the input power needed for steady-state burning.
Plasma exhaust — deuterium, tritium, helium from fusion reactions, and impurities — leaves through vacuum pumping ports and is separated into its constituents. Deuterium and tritium are reinjected to the plasma, either as frozen pellets from the inboard plasma side for good penetration into the plasma core, or by gas injection at the top of the plasma. Impurities are removed as waste and helium recycled if sufficiently pure.
ITER's elongated plasma would be vertically unstable without active feedback control. For instance, one control loop adjusts the up-down asymmetry of outer PF coil voltages to minimize movement in the plasma current center. Magnetic diagnostic loops on the inner vacuum-vessel surface keep track of plasma currents. Another loop linked to the main PF coil voltages helps limit changes in six measured gaps between the plasma and wall.
More than 40 diagnostic systems will monitor the plasma. The seven main types are magnetic, neutron, optical/infrared, bolometric (heat radiation), spectroscopic, microwave, and plasma-facing-component. They are also divided into three categories: basic machine protection and control, advanced plasma control, and evaluation and physics studies.
A central supervisory control system (SCS) and local subcontrol systems handle control, data acquisition, and communications during normal operation. The SCS also monitors and ensures plant subsystems operate within proper limits.
ITER will occupy a site of about 100 acres and construction is expected to take seven years. It will operate for an estimated 21 years, followed by six years of decommissioning to remove radioactive materials.
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Magnetic Confinement: The ITER Example
The main parts of the ITER tokamak reactor are: Courtesy ITER ITER tokamak |
- Vacuum vessel - holds the plasma and keeps the reaction chamber in a vacuum
- Neutral beam injector (ion cyclotron system) - injects particle beams from the accelerator into the plasma to help heat the plasma to critical temperature
- Magnetic field coils (poloidal, toroidal) - super-conducting magnets that confine, shape and contain the plasma using magnetic fields
- Transformers/Central solenoid - supply electricity to the magnetic field coils
- Cooling equipment (crostat, cryopump) - cool the magnets
- Blanket modules - made of lithium; absorb heat and high-energy neutrons from the fusion reaction
- Divertors - exhaust the helium products of the fusion reaction
 Magnetic-confinement fusion process |
- The fusion reactor will heat a stream of deuterium and tritium fuel to form high-temperature plasma. It will squeeze the plasma so that fusion can take place.
- The power needed to start the fusion reaction will be about 70 megawatts, but the power yield from the reaction will be about 500 megawatts.
- The fusion reaction will last from 300 to 500 seconds. (Eventually, there will be a sustained fusion reaction.)
- The lithium blankets outside the plasma reaction chamber will absorb high-energy neutrons from the fusion reaction to make more tritium fuel. The blankets will also get heated by the neutrons.
- The heat will be transferred by a water-cooling loop to a heat exchanger to make steam.
- The steam will drive electrical turbines to produce electricity.
- The steam will be condensed back into water to absorb more heat from the reactor in the heat exchanger.
as i seen in the diagram... what is the medium between plasma and heat exchanger??
ReplyDeletedoes it can withstand the temperature..???
is there any possible medium of plasma and heat exchanger can mix with turbine medium/??
sivabalan s/o sanafhei raja
me083646
As i read the acticle above,i remmber the carnot cycle that i studied in the thermodyanmics class. In this case,the hot temperature reservoir is the plasma chamber, and the the low temperature reservoir is the heat exchanger.pls correct me if i'm wrong
ReplyDeletevaageesan ganeesan
macha15_swimfaster@yahoo.com