Rather than having a large plasma current, a stellarator uses a twisted array of helical windings around the torus to create the poloidal field externally. As a result, it does not need to generate a plasma current, which means it is capable of steady-state operation rather than needing a pulse of energy from the central solenoid. However, stellarators have more complicated geometry and are more difficult to build because of these additional windings. The approach is thought to offer considerable promise, and though past stellarators have encountered more problems with plasma confinement than tokamaks, the technology continues to draw research attention.
The Wendelstein 7-X, an advanced stellarator that went online in Germany in also the largest so far is studying how well stellarators can contain energy and reach fusion conditions. No current device has been able to generate more fusion power than the heating energy required to start the reaction. Scientists measure this assessment with a value known as fusion gain expressed as the symbol Q , which is the ratio of fusion power to the input power required to maintain the reaction.
Fusion power plants will need to achieve Q values well above 10 to be economic. The many potential benefits of fusion as an energy source are the reason it has long been viewed as an ideal method of generation.
The fuel—isotopes of hydrogen—is readily available, and the only by-product is helium. Like a gas, coal, or fission plant, a fusion plant could operate around the clock, yet without producing any harmful emissions or long-lived radioisotopes. The risk of accidents with a fusion plant is very limited—if containment is lost, the fusion reaction simply stops. Though fusion is not risk-free, no explosions or wide-scale releases of energy are possible.
Getting to practical generation has been the key challenge. After more than 60 years of research in magnetic confinement fusion, most of the remaining impediments to fusion energy are those of engineering rather than science, though there are still important physics questions being investigated.
Plasma Confinement. Confining a fusion plasma inside a magnetic field is a bit like squeezing water inside a balloon. Differences in pressure, temperature, and density can cause the fields to balloon outward or spring a leak. Researchers have been able to confine fusion plasmas long enough to generate fusion reactions for many years.
However, the quality of plasma confinement—defined as the time required to lose energy to the vessel walls—is a key element in the cost-effectiveness of a hypothetical fusion power plant. This confinement time needs to be long enough to allow sufficient plasma energy to circulate in the confined region so that confined ions are kept hot enough to maintain an appropriate level of fusion. Current devices have managed confinement times of about 0. Recent studies have identified confinement quality as the most important factor for reducing capital costs, because it has a direct impact on the necessary size of the tokamak as well as other critical elements of the plant, such as the handling of heat and particle loads.
Further research is necessary to develop higher-quality confinement solutions that would reduce these costs. Though high-temperature superconducting materials, which can generate much stronger magnetic fields, have created some excitement in the fusion community, it is not yet known how well these will perform in operation, and studies have suggested that the choice of magnet technology may have relatively little impact on cost-effectiveness.
Tokamak Materials. The neutron radiation produced by DT fusion is an order of magnitude more energetic than that produced by nuclear fission.
In addition, the helium generated by the reaction, as well as excess heat and other impurities in the plasma, must be removed on an ongoing basis during operation. This exhaust path will be subject to extremely high temperatures and particle bombardment. No materials currently exist that can be confidently relied upon to survive these conditions over the life of a commercial power plant.
Developing them is an active area of research, with work exploring new alloys, better materials, and even liquid surfaces and candidate solutions. Better understanding of how these materials behave in the reactor environment and their interaction with fusion performance is necessary. Breeding Tritium. Deuterium is relatively abundant in nature, and sufficient supplies can be extracted from seawater.
Tritium, however, is a radioactive isotope with a half-life of only Though it exists naturally, it is far too rare to recover usefully from natural sources, and useable amounts must be manufactured.
Current methods rely on extraction from the coolant in heavy-water reactors or bombardment of lithium targets in light-water reactors. A single MW fusion power plant is expected to require about 50 kilograms kg of tritium fuel per year. Thus, fusion power plants will need a method to breed tritium in situ. Fortunately, the fusion reaction itself offers a potential means to do so.
Placing a lithium blanket around the tokamak would generate tritium and further heat as the fusion neutrons are captured by the lithium nuclei and spontaneously transition to tritium.
Technology solutions to capture this tritium during operation are in development. Power Generation. To be useful as a power plant, a fusion reactor obviously must generate electricity. Fusion researchers generally envision that heat from the tokamak will be used to drive turbine generators, but exactly how the heat off-take will function is still a matter requiring considerable engineering.
While in a sense this is the most conventional part of the power plant design, the technological challenges remain significant, as for high efficiency, the device must operate at high temperatures. Most current designs envision a helium loop that would extract heat from the lithium blanket, and either drive a turbine directly or generate steam in a secondary loop.
Fusion scientists realized some time ago that existing tokamaks are simply not large or powerful enough to reach burning plasma conditions. In order to resolve the design of a power plant, research at power-plant scale is necessary.
Thus, a long-term goal has been building a facility that would have the necessary capabilities. Under the agreement, all members have equal access to the technology developed, though each member funds only a portion of the cost. The U. Although initial work began in , it took until before an engineering design was agreed upon. This brought the coalition to seven groups comprising 35 nations, making ITER the largest multinational science project in history.
The current ITER agreement was signed in , and a location near Aix-en-Provence in southern France was selected as the site of the facility. However, there were considerable challenges in getting such a large project with so many members off the ground. Construction proceeded somewhat fitfully for several years and fell badly behind schedule. The delays and budget overruns drew concern from several quarters, particularly the appropriations committees in Congress. This February fish-eye view above the ITER tokamak chamber under construction gives an idea of the impressive scale of the device.
When complete, the tokamak will be nearly 13 meters across. In , Bernard Bigot, the former director of the French Atomic Energy Commission, was brought in to assume oversight of the project. This process causes smaller atomic nuclei, like the hydrogen isotope tritium, to fuse into larger ones, like helium atoms. This process of fusion generates large amounts of power and is also how the sun and other stars create energy.
Fusion is one of the cleanest potential power sources , as it uses hydrogen for energy and produces mostly helium, an inert gas, as a byproduct. It also creates additional hydrogen isotopes that can be radioactive, but they are mostly fed back into the fusion reaction. These byproducts are also fairly short-lived compared to the radioactive waste produced by nuclear fission plants and are confined entirely to the power plant. Several significant challenges have prevented the creation of commercial-scale fusion reactors.
Any reactor would need to be built out of material that can stand up to the intense heat of plasma, which would need to be kept at extremely high temperatures under massive pressure for months at a time. The radiation generated by fusion, while mostly contained to the reactor, can also weaken the plant over time — requiring extra maintenance.
While breakthroughs in fusion are made with some regularity, they tend to reveal that there's much about fusion power we still don't know. There are a few projects in development, however, that may tackle these challenges and launch operational fusion generators. The ITER project, established in in France , will be the largest plasma physics experiment in the world once it is up and running.
You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser or turn off compatibility mode in Internet Explorer. In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript. The Joint European Torus has started conducting experiments with tritium fuel. Nuclear fusion is the phenomenon that powers the Sun and, if physicists can harness it on Earth, it would be a source of almost limitless energy.
In December, researchers at the Joint European Torus JET started conducting fusion experiments with tritium — a rare and radioactive isotope of hydrogen. It is the first time since that researchers have done experiments in a tokamak with any significant amount of tritium. UK hatches plan to build world's first fusion power plant. In June, JET will begin fusing even quantities of tritium and deuterium, another isotope of hydrogen. It is this fuel mix that ITER will use in its attempt to create more power from a fusion reaction than is put in — something that has never before been demonstrated.
The reactor should heat and confine a plasma of deuterium and tritium such that the fusion of the isotopes into helium produces enough heat to sustain further fusion reactions. ITER will begin operations with low-power hydrogen reactions in But from , it will run on a mix of deuterium and tritium. The goal then was to hit peak power, and the facility succeeded in achieving a record ratio of power out to power in known as a Q value of 0. That record still stands today; 1 would be break-even.
0コメント