International Journal of Advanced Network, Monitoring and Controls Volume 04, No.04, 2019 Nuclear Fusion Power – An Overview of History, Present and Future Stewart C. Prager Department of Physics University of Wisconsin – Madison Madison, WI 53706, USA E-mail: [email protected] Summary—Fusion power offers the prospect of an allowing the nuclei to fuse together. Such conditions almost inexhaustible source of energy for future can occur when the temperature increases, causing the generations, but it also presents so far insurmountable ions to move faster and eventually reach speeds high engineering challenges. The fundamental challenge is to enough to bring the ions close enough together. The achieve a rate of heat emitted by a fusion plasma that nuclei can then fuse, causing a release of energy. exceeds the rate of energy injected into the plasma. The main hope is centered on tokamak reactors and II. FUSION TECHNOLOGY stellarators which confine deuterium-tritium plasma In the Sun, massive gravitational forces create the magnetically. right conditions for fusion, but on Earth they are much Keywords-Fusion Energy; Hydrogen Power; Nuclear Fusion harder to achieve. Fusion fuel – different isotopes of hydrogen – must be heated to extreme temperatures of I. INTRODUCTION the order of 50 million degrees Celsius, and must be Today, many countries take part in fusion research kept stable under intense pressure, hence dense enough to some extent, led by the European Union, the USA, and confined for long enough to allow the nuclei to Russia and Japan, with vigorous programs also fuse. The aim of the controlled fusion research underway in China, Brazil, Canada, and Korea. program is to achieve 'ignition', which occurs when Initially, fusion research in the USA and USSR was enough fusion reactions take place for the process to linked to atomic weapons development, and it become self-sustaining, with fresh fuel then being remained classified until the 1958 Atoms for Peace added to continue it. Once ignition is achieved, there is conference in Geneva. Following a breakthrough at the net energy yield – about four times as much as with Soviet tokamak, fusion research became 'big science' in nuclear fission. According to the Massachusetts the 1970s. But the cost and complexity of the devices Institute of Technology (MIT), the amount of power involved increased to the point where international co- produced increases with the square of the pressure, so operation was the only way forward. doubling the pressure leads to a fourfold increase in energy production. Fusion powers the Sun and stars as hydrogen atoms fuse together to form helium, and matter is converted With current technology, the reaction most readily into energy. Hydrogen, heated to very high feasible is between the nuclei of the two heavy forms temperatures changes from a gas to plasma in which (isotopes) of hydrogen – deuterium (D) and tritium (T). Each D-T fusion event releases 17.6 MeV (2.8 x 10- the negatively-charged electrons are separated from the 12 joule, compared with 200 MeV for a U-235 fission positively-charged atomic nuclei (ions). Normally, a fusion is not possible because the strongly repulsive and 3-4 MeV for D-D fusion). On a mass basis, the D- electrostatic forces between the positively charged T fusion reaction releases over four times as much nuclei prevent them from getting close enough together energy as uranium fission. Deuterium occurs naturally to collide and for fusion to occur. However, if the in seawater (30 grams per cubic metre), which makes it conditions are such that the nuclei can overcome the very abundant relative to other energy resources. electrostatic forces to the extent that they can come Tritium occurs naturally only in trace quantities within a very close range of each other, then the (produced by cosmic rays) and is radioactive, with a attractive nuclear force (which binds protons and half-life of around 12 years. Usable quantities can be neutrons together in atomic nuclei) between the nuclei made in a conventional nuclear reactor, or in the will outweigh the repulsive (electrostatic) force, present context, bred in a fusion system from lithium. DOI: 10.21307/ijanmc-2019-064 1 International Journal of Advanced Network, Monitoring and Controls Volume 04, No.04, 2019 b Lithium is found in large quantities (30 parts per III. MAGNETIC CONFINEMENT million) in the Earth's crust and in weaker In magnetic confinement fusion (MCF), hundreds concentrations in the sea. of cubic metres of D-T plasma at a density of less than In a fusion reactor, the concept is that neutrons a milligram per cubic metre are confined by a magnetic generated from the D-T fusion reaction will be field at a few atmospheres pressure and heated to absorbed in a blanket containing lithium which fusion temperature. surrounds the core. The lithium is then transformed Magnetic fields are ideal for confining a plasma into tritium (which is used to fuel the reactor) and because the electrical charges on the separated ions and helium. The blanket must be thick enough (about 1 electrons mean that they follow the magnetic field lines. metre) to slow down the high-energy (14 MeV) The aim is to prevent the particles from coming into neutrons. The kinetic energy of the neutrons is contact with the reactor walls as this will dissipate their absorbed by the blanket, causing it to heat up. The heat heat and slow them down. The most effective magnetic energy is collected by the coolant (water, helium or Li- configuration is toroidal, shaped like a doughnut, in Pb eutectic) flowing through the blanket and, in a which the magnetic field is curved around to form a fusion power plant, this energy will be used to generate closed loop. For proper confinement, this toroidal field electricity by conventional methods. If insufficient must have superimposed upon it a perpendicular field tritium is produced, some supplementary source must component (a poloidal field). The result is a magnetic be employed such as using a fission reactor to irradiate field with force lines following spiral (helical) paths heavy water or lithium with neutrons, and extraneous that confine and control the plasma. tritium creates difficulties with handling, storage and transport. There are several types of toroidal confinement system, the most important being tokamaks, The difficulty has been to develop a device that can stellarators and reversed field pinch (RFP) devices. heat the D-T fuel to a high enough temperature and confine it long enough so that more energy is released In a tokamak, the toroidal field is created by a series through fusion reactions than is used to get the reaction of coils evenly spaced around the torus-shaped reactor, going. While the D-T reaction is the main focus of and the poloidal field is created by a system of attention, long-term hopes are for a D-D reaction, but horizontal coils outside the toroidal magnet structure. this requires much higher temperatures. A strong electric current is induced in the plasma using a central solenoid, and this induced current also In any case, the challenge is to apply the heat to contributes to the poloidal field. In a stellarator, the human needs, primarily generating electricity. The helical lines of force are produced by a series of coils energy density of fusion reactions in gas is very much which may themselves be helical in shape. Unlike less than for fission reactions in solid fuel, and as noted tokamaks, stellarators do not require a toroidal current the heat yield per reaction is 70 times less. Hence to be induced in the plasma. RFP devices have the thermonuclear fusion will always have a much lower same toroidal and poloidal components as a tokamak, power density than nuclear fission, which means that but the current flowing through the plasma is much any fusion reactor needs to be larger and therefore stronger and the direction of the toroidal field within more costly, than a fission reactor of the same power the plasma is reversed. output. In addition, nuclear fission reactors use solid fuel which is denser than thermonuclear plasma, so the In tokamaks and RFP devices, the current flowing energy released is more concentrated. Also the neutron through the plasma also serves to heat it to a energy from fusion is higher than from fission – 14.1 temperature of about 10 million degrees Celsius. MeV instead of about 2 MeV, which presents Beyond that, additional heating systems are needed to significant challenges regarding structural materials. achieve the temperatures necessary for fusion. In stellarators, these heating systems have to supply all At present, two main experimental approaches are the energy needed. being studied: magnetic confinement and inertial confinement. The first method uses strong magnetic The tokamak (toroidalnya kamera ee magnetnaya fields to contain the hot plasma. The second involves katushka–torus-shaped magnetic chamber) was compressing a small pellet containing fusion fuel to designed in 1951 by Soviet physicists Andrei Sakharov extremely high densities using strong lasers or particle and Igor Tamm. Tokamaks operate within limited beams. parameters outside which sudden losses of energy confinement (disruptions) can occur, causing major thermal and mechanical stresses to the structure and 2 International Journal of Advanced Network, Monitoring and Controls Volume 04, No.04, 2019 walls. Nevertheless, it is considered the most A completely different concept, the 'Z-pinch' (or promising design, and research is continuing on 'zeta pinch'), uses a strong electrical current in a plasma various tokamaks around the world. to generate X-rays, which compress a tiny D-T fuel cylinder. Research is also being carried out on several types of stellarator. Lyman Spitzer devised and began work V.