Nuclear power generates a significant part of the world's electricity and has much to offer for the future, not only for electric power generation but also for other applications such as hydrogen production (see Chapter 9) and water desalination. Over the years, the safety record in western-built reactors has been remarkable, and advanced reactor design promises to be even safer. With near-zero emissions, nuclear power has a clear advantage over fossil fuels with regard to air pollution and growing concerns about global climate change. Moreover, with most resources of uranium and thorium being located in stable areas of the world, stability in prices and production would also be greatly improved. The progressive introduction of breeder reactors should allow a better usage of the these resources and secure the world's energy supply far into the future, until better energy-generating technologies - perhaps even nuclear fusion - emerge.
128 | Chapter 8 Renewable Energy Sources and Atomic Energy Nuclear Fusion
The Sun, like innumerable other stars, is a giant thermonuclear reactor which obtains its energy from nuclear fusion reactions of small nuclei, primarily hydrogen, to produce larger ones, mostly the transformation of hydrogen into helium. The mass of the fused reaction products being less than that of the initial particles, the difference is converted into energy in accordance with Einstein's equation, e = mc2. This represents a tremendous amount of energy - about ten times greater than a typical fission reaction for the same mass of nuclear fuel. Since the positive electric charges of the nuclei provide strong repulsive forces, the energy of the particles, and consequently the temperature, must be very high for the fusion reaction to occur at a sufficient and sustained rate. In the center of the stars this is achieved by intense gravitational forces in a high temperature plasma (15 million °C). Plasma is (beside solid, liquid and gas) the fourth state of matter where the atoms nuclei and electrons are separated and move independently from each other. On Earth however, natural gravitational confinement is impossible. Thus, different technologies must be used to contain very energetic particles and prevent them to from escaping before reacting. The two techniques presently considered to achieve this goal are: (i) magnetic confinement, in which a very high-temperature plasma is contained by a strong magnetic field for suitable extended periods of time; and (ii) inertial confinement, where fusion is realized in a small concentrated volume of plasma heated and compressed extremely rapidly with high-energy lasers. For electricity production, magnetic confinement is presently the most advanced and favored option. Experimental studies have been conducted in several countries since the 1950s, but the real breakthrough came in 1968 when the Russians obtained for the first time a 10 million °C plasma in a so-called Tokamak ("Toroidalnaya Kamera c Magnitinymi Katushkami" or toroidal vacuum chamber and magnetic reel in English). Since then, Tokamaks became the reference, leading to the construction of large-scale fusion experiments in the 1980s: the Japanese JT-60, American TFTR, or European JET. A Tokamak is basically a donut-shaped cylinder in which a strong magnetic field is created. The plasma, being a combination of almost independent electrically charged electrons and ions, is trapped in this magnetic field and circulates in a circle in the tokamak. The temperature necessary to maintain a fusion reaction in such systems is in the order of 100 million °C. To achieve such staggering temperatures, the plasma is heated by the injection of highly energetic neutral particles which will also serve as fuel, and with electromagnetic waves at specific frequencies which transfer their energy to the plasma through antennas placed in the confinement chamber. The magnetic field contains the hot plasma far enough from the inner tokamaks wall to avoid its cooling and allow fusion to occur. The heat generated by the reaction is removed through heat exchangers placed in the reactor's wall. This heat is then used to produce steam and finally electricity via a turbine/generator system, much like in today's fossil fuels and nuclear power plants.
To be viable from an energetic standpoint and to reach the so-called break-even point, the energy produced by the fusion reaction must be at least equal to the amount of energy furnished to heat the system. Beyond break-even, the important task will be to create much more energy than is injected into the system and to establish the feasibility of nuclear fusion as an economically viable source of energy.
Currently, the best ratio of energy produced over energy injected into the system, named gain and represented by the symbol Q, has been obtained at the Joint European Torus (JET) with a Q = 0.65, close to break-even. ITER (International Thermonuclear Experimental Reactor), an international collaboration between Europe, Japan, Russia, China, India, United States and South Korea on fusion reaction, is expected to yield a gain between 5 and 10 (Fig. 8.23). With fusion reactions being more efficient the larger the installation, this next generation Tokamak, which will be built in Cadarache, France, will be much bigger than its predecessor. It constitutes the preliminary step to the construction of commercial reactors of even larger size with Q superior to 30.
On Earth, the most accessible and practical nuclear fusion reaction on which all efforts have been concentrated, is the one between deuterium (D) and tritium (T), the two heavier isotopes of hydrogen which offers by far the highest gain (Fig. 8.24). Deuterium is a stable element, abundant in water (33 g m-3) from which it can be extracted. Existing deuterium resources represent more than 10 billion years of annual world energy consumption! Tritium however, is a radioactive element which does not exist in nature and has to be prepared from lithium by neutron bombardment. Lithium resources are estimated at 2000 years, but can be extended to several million years if it is extracted from sea water. In existing experimental reactors, tritium is generated outside and subsequently injected into the plasma. In future systems however, tritium will be produced directly inside the thermonuclear reactor. The inner walls will be made of lithium-containing materials which, under bombardment of neutrons from the fusion reaction, will be transformed to tritium.
Fusion reactors are intrinsically safe because at any time only small amounts of fuel are present in the plasma chamber, and any uncontrolled perturbation will
immediately induce a temperature drop, leading to a cessation of fusion reactions. The risk for potential runaways is therefore eliminated. Like fission, fusion does not produce any air pollutant contributing to acid rains or greenhouse gases responsible for global climate change, and could thus mitigate the environmental risks associates with fossil fuel burning. None of the basic fuels for fusion, deuterium and lithium, nor the reaction product helium, is radioactive or toxic. Tritium, which will be entirely produced on site, decomposes to helium by emission of a low-energy beta ray and has a relatively short half-life (12.3 years). Its radioactivity is low, but reactors will have to take into account the permeation of this gas through matter. As in every system under intense high-energy particles flow, the materials constituting the reactor will be activated. However, a judicious choice of materials with rapidly decreasing activity will allow a minimization of the amounts of radioactive waste to short-lived, low-activity materials. The deuterium-deuterium fusion reaction involves lower radioactivity, but is less practical as it requires about an order of magnitude higher plasma pressure to produce the same power than the deuterium-tritium reaction. The reaction of 3He with deuterium generates even less radioactivity. 3He, however, is not available on Earth and must be prepared. Eventually, with increased knowledge and advanced technology both of these fusion reactions could be used in fusion reactors.
Nuclear fusion is still an energy source of the future. Much progress has already been accomplished, but extensive research and development will still be needed. With mega-projects such as ITER and its followers, however, there is little doubt that this technology will be made practical during the 21st century. Fusion, when compared to present sources of energy, offers numerous advantages: the fuels, deuterium and lithium, are widespread and virtually inexhaustible, obliterating energy security problems and resource-based conflicts. Fusion also has large-scale power-generating capacity with minimal environmental impacts, and is inherently safe. Water used as working fluid in heat exchangers to cool the reactor and produce electricity could also be replaced by liquid metals or helium to achieve higher operating temperatures (1000 °C) and allow, besides electric power generation, the production of hydrogen by thermochemical splitting of water, as has already been proposed with nuclear fission (see Chapter 9). Because of the high cost and complex technology of eventual fusion energy plants, there will most likely initially be only relatively few such large installations serving major power needs, with smaller and more dispersed atomic fission plants providing more decentralized electricity production. Advances in superconductive power transmission lines could, however, substantially help and alleviate present electric transmission limitations.
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