When fusion will come available as an energy source, what are its characteristics? Will it be clean and safe? What is the waste, and how large is a typical unit? In short: a fusion power plant will generate about 1000-1500 MWe. In the plant, lithium is turned into the intermediate fuel tritium, which is heated to 150 million degrees together with deuterium.
One of the products of the deuterium-tritium fusion is a helium nucleus. It carries 20% of the energy that is produced during the reaction, in the form of kinetic energy. Being electrically charged, the helium nucleus is confined by the magnetic field, and transfers its energy to the bulk of the deuterium and tritium fuel. In this way, the fuel is heated. If the device is sufficiently large, this process allows the temperature required for fusion to be obtained mostly by "self-heating". In that case the plasma is called a "burning plasma".
Besides a helium nucleus, each fusion reaction produces a neutron carrying 80% of the released fusion energy. As a neutron has no electric charge, it is not confined by the magnetic field and passes straight into a "blanket" lining the walls of the torus, where it reacts with lithium to produce tritium, and where their energy is absorbed and removed by a circulating cooling fluid. Just as in a conventional power plant, the heated fluid can be brought to a heat exchanger, where steam is generated. The steam is used to drive a turbine for the production of electricity. Alternatively, the high temperature could be used to generate hydrogen.
Fusion is a particularly attractive energy solution as it uses fuels that are abundant and available around the globe. The primary fuels used in fusion are deuterium and lithium. Deuterium is a hydrogen isotope, which can be readily extracted from water (there is around 33g of deuterium in every cubic metre of water), and lithium is an abundant light metal - for example used in lithium-batteries - from which tritium can be generated inside the reactor.
In a fusion reaction, the amount of energy released is about four million times as high as the amount of energy released in an ordinary chemical reaction, like the burning of coal. That enormous difference means that a fusion power plant only needs a very small quantity of fuel. To power a fusion power plant of 1000 MW (the size of a large coal-fired power plant) for a year, you need 250 kg of fusion fuel. A coal-fired power plant of the same size needs 2.7 million tons of coal every year! The lithium from one laptop battery, combined with the deuterium in 100 litres of water, can cover the electricity use of an average European citizen for 30 years.
In April 2005, EFDA has released the European Fusion Power Plant Conceptual Study (PPCS). The study defines four future fusion power plant models illustrative of a wider range of possibilities, spanning from near-term to very advanced, and addresses questions related to safety and environmental impact, economics, and development needs.
In the study, the costs of future fusion electricity were computed from detailed plant models using well-attested industrial costing techniques. Although cost estimates so far ahead are highly dependent on technological and financial assumptions, the study concluded that the cost of electricity from the models is in the range of estimates for the future costs from other renewable sources, obtained from the literature.
To remove a possible misunderstanding: the production of fusion reactions is not difficult, and can be accomplished with equipment that fits on top of a kitchen table. But such experiments always require the input of far more energy than is liberated by the fusion reactions. The challenge is not to produce fusion reaction, but to turn fusion into an energy source that generates more energy than it consumes.
Regularly, articles appear in the media about fusion in small experiments, but although fusion reactions can take place in such experiments, it will never become a real energy source. Even so, there are three different fusion energy concepts in addition to the tokamak: the stellarator, the spherical tokamak, and inertial confinement fusion.
A stellarator has a beautiful appearance. Delicately curved coils generate a magnetic field with a complex shape inside the plasma vessel, which is oddly shaped itself. The advantage of the stellarato is that thanks to the complex shape of the coils, no electrical current is required inside the plasma, which is the case in the tokamak. This has a number of technical advantages. Although stellarators are very promising, the research in this direction is still in an earlier phase as that for the large tokamak-based machines. In the German Greifswald, a large stellarator named Wendelstein-7X is currently being built, and in Japan the Large Helical Device is being constructed, as shown in the illustration.
Figure 1:On top the complex coils of a stellarator, below a
view inside the Japanese Large Helical Device. Source: Max-Planck-Institut
für Plasmaphysik (Germany) and NIFS (Japan)
Figure 2:The sperical tokamak MAST
in Great Britain.
Source: CCFE
A second promising line of research is the spherical tokamak: a tokamak that is sphere-shaped. This shape also has a number of technological advantages. The MAST-experiment (see illustration) in the British fusion research institute in Culham, is an example of a spherical tokamak.
Inertial Confinement fusion follows a very different concept. A small pellet of frozen fusion fuel is flash-irradiated from all sides with a number of extremely intense laser beams. The outer layer of the pellet is blown away, which causes the inner part of the pellet to be compressed with great force. The compression causes the temperature and density to rise to a level necessary for fusion reactions to occur.
The main challenge in inertial fusion is achieving a powerful and homogeneous irradiation of the pellet at a high repetition rate: about 10 - 20 pellets would have to be heated and burned per second in a fusion power plant based on this principle. This type of research is mostly carried out in the USA.