Nuclear fusion is a nuclear reaction through which two light nuclei of atoms, usually hydrogen and its isotopes (deuterium and tritium), are combined forming a heavier nucleus. This binding is usually accompanied by the emission of particles (in case of deuterium nuclei one neutron is emitted). This nuclear fusion reaction releases or absorbs a lot of energy in the form of gamma rays and kinetic energy of the emitted particles.This large amount of energy transforms matter to a plasma state.
The nuclear fusion reactions can emit or absorb energy. If the cores to merge have a lower mass than iron, energy is released. Conversely, if the atomic nuclei that fuse are heavier than iron, the nuclear reaction absorbs energy.
The nuclear fusion and the fusion of reactor core is different, which refers to the melting of the reactor core of a nuclear power plant due to overheating caused by poor cooling. During the Fukushima nuclear accident, this term was frequently used.
Nuclear fusion in nature
Besides of the Sun, stars are constantly experimenting nuclear fusion reactions. The light and heat that we're feeling from the Sun is the result of these nuclear fusion reactions: hydrogen nuclei collide and fuse resulting in a heavier helium nucleus releasing a tremendous amount of energy. The released energy reaches Earth in the form of electromagnetic radiation.
The forces of gravity in the universe generate perfect conditions for nuclear fusion.
A nuclear fusion reactions are also called thermonuclear reactions due to the high temperatures they experience. The temperature os the sun is close to 15 million degrees Celsius.
Technical requirements for nuclear fusion
To perform nuclear fusion reactions, the following requirements must be present:
- Get a very high temperature to separate the electron from the nucleus, approaching this another beating electrostatic repulsion forces. The gaseous mass consisting in the electrons and free atoms is called highly ionized plasma.
- Confinement to keep the plasma at elevated temperature for the minimum time required.
- Sufficient plasma density to which the cores are close to each other and can generate the nuclear fusion reactions.
Confinement nuclear fusion
Conventional landfills are used in nuclear fission reactors are not possible due to the high temperatures of the plasma must endure. For this reason, we have developed two important methods of confinement:
- Nuclear inertial confinement fusion (ICF): it consists in creating such a dense medium particles that do not have chance to escape without hitting each other. A small sphere composed of deuterium and tritium is hit by a laser beam, causing thus their implosion. Then, it becomes denser and explodes under the effect of the nuclear fusion reaction.
- Nuclear magnetic confinement fusion (MCF): the electrically charged plasma particles are trapped and confined through the action of a magnetic field space. The most developed device has a toroidal shape and it's called Tokamak.
Nuclear fusion reactions
The atomic elements normally used in nuclear fusion reactions are Hydrogen and its isotopes: the deuterium (D) and tritium (T). The major fusion reactions include:
D + D -> 3He + n + 3.2 MeV Merging Two Deuterium nuclei a Helium nucleus consisting of two protons and one neutron, releasing a 3.2 MeV neutron and obtaining energy.
D + D -> T + p + 4.03 MeV
Merging Two Deuterium nuclei, a nucleus of tritium, we obtain a proton and 4.03 MeV.
These reactions require kinetic energy of the nuclei necessary for the reactants to approach cores, overcoming then the forces of electrostatic repulsion. It's necessary to heat the gas to very high temperatures, as it is supposed tthat it takes place in the center of stars.
The prerequisite of a nuclear fusion reactor is confining this plasma to temperature, high density and at the right time, in order to enable the occurrence sufficient nuclear fusion reactions, preventing the particles to escape for a net gain energy. This energy gain depends on the energy needed to heat and confine the plasma, and it's less than the energy released by nuclear fusion reactions. Generally, for every milligram of deuterium-tritium we can obtain 335 MJ.
Fuel used for nuclear fusion reactions
For nuclear fusion reactions we need light nuclei. Basically deuterium and tritium, which are two isotopes of hydrogen.
Deuterium is an isotope of hydrogen stable consisting of a proton and neutron. They subsist in the water, one atom per 6500 atoms of hydrogen. It means that in seawater there's a concentration of 34 grams for every cubic meter of water. The energy content of deuterium is so high that the energy of deuterium can get a liter of seawater and it's equivalent to the energy you can get 250 liters of oil.
Therefore, considering that three quarters of the planet is covered by water, nuclear fusion is considered as an inexhaustible source of energy.
The other element used in nuclear fusion is tritium, the isotope of stable or radioactive atom of hydrogen. It's composed of a proton and two neutrons by beta emission decays relatively quickly. While tritium is scarce in nature, it can be generated by neutron capture reactions with the isotopes of lithium. Lithium is an abundant material in the earth's crust and in seawater.
Historical evolution and future projects on nuclear fusion
The origins of nuclear fusion are located around 1929 when Houtemans Atkinson showed the possibility of obtaining energy from fusion reactions. However, the most important concepts of nuclear fusion and its actual application have been developed since 1942 with the work of H. Bethe, E. Fermi, E. R. Oppenheimer and Teller, among others. The Sherwood project showed the first technological advances that helped to develop the concept of magnetic confinement, yielding the first designs: z-pinch, and stellarator magnetic mirrors.
In 1961, J. Nuckolls (USA) and N. Basov (USSR) developed a technique that could be obtained by hugh compression in nuclear fusion reactions caused by the transfer of energy. Secret programs are well developed in the U.S. and Russia. Later, France followed this secret development. Other countries such as Germany, Japan, Italy and the U.S. (Rochester) developed open programs.
In 1965, Artsimovich presented the results of their research during the "2nd Conference on Controlled Fusion and Plasma" about the TOKAMAK (Toroidal Kamera magnetik) concept.
In Tokamak concept, the need to confine the plasma magnetic field is the result of the combination of a toroidal field, a poloidal field created by two toroidal coils, and a vertical field (created by a transformer). The plasma acts as the secondary part of a transformer through which current is induced heating it. Through the transformer primary circulates a variable current intensity.
In 1968, the Nobel N. Basov, Award reported obtaining ignition temperatures and the production of neutrons in nuclear fusion reactions using lasers. Thereafter, it could have a lot of equipment in construction and operation under the tokamak concept as: TFR (France), T-4 and T-11 (USSR), and Alcator Ormak (USA). Something similar to the T-10 (USSR), PLT (USA), ETA (GB), ASEDX (RFA) and Frascati (EURATOM-Italy) started to be built.
In the 70s he began the first series of publications on FCI (Nuclear Fusion by Inertial Confinement). In the U.S., the principal investigators were Brueckner, Nuckolls, and Clark Kidder. In Russia, Basov and his team showed the most advanced experiment, reaching nearly 3 million neutron implosion areas of CD2.
Based on this concept, there have been a lot of facilities that have enabled laser edge research on nuclear fusion. From them we can highlight: NOVA (40 kJ, EUUU), OMEGA (30 kJ), GEKKO-XII (10 kJ, Japan), Phebus (3 kJ, France), VOLCANO (UK), ISKRA-5 (Russia).
After these laser facilities have been developed, two major projects showed high profits: National Ignition Facility (NIF) in the U.S. and Laser megajoule (LMJ) in France.
But the laser is not the only device capable of producing implosions. The thatelectrons and beams of light and heavy ions are important candidates for inertial confinement nuclear fusion. The following projects are born with light ions: ANGARA and PROTO (Russia), PBFA PBFA-I and-II (USA).
Heavy ions in the absence of experiments have been unable to achieve accurate results, although they have made certain predictions by theoretical simulations such as those in the HIDIF Project (Heavy Ion Design of Ignition Facility) sponsored by several European laboratories and institutes and the Lawrence Berkeley Laboratory American.
In the 90s, TOKAMAK facilities as JET (EURATOM), TFTR (USA) and JT-60 (Japan) yielded some power. The first was the JET, with a mixture of D (90%) and T (10%) achieved in 1991, a power of 1.7 MW. Subsequently, in 1993, with a mixture of TFTR DT 50% came to 6 MW, reaching temperatures of 30 keV. The 29 MW heating was spent. Nowadays, the TFTR is closed. Since now, they have come to produce up to 12 MW in nuclear fusion reactions controlled for more than a second (JET, 1997) and surely the current technological advances will reach the commercial range of hundreds of MW.
The experimental research in FCM (Nuclear Magnetic Confinement Fusion) in Spain is concentrated in the CIEMAT (Centre for Energy, Environment and Technology), from 1983, operating the first nuclear fusion machine, the TJ-I Tokamak.
From this moment, research has progressed steadily. In 1994 the first device in nuclear fusion built entirely in Spain was presented: the Stellerator TJ-I upgrade, that was ceded in 1999 to the University of Kiel in enter the TJ-II operation.
The TJ-II was a major scientific leap from previous experiments, considered one of the three most advanced stellerators in the world with German Wendelstein 7-AS the Max Planck Institute in Munich and the Japanese LHD Nagoya University.
The draft nuclear magnetic confinement fusion: the ITER
The most advanced nuclear Magnetic Confinement Fusion project ITER is the International Thermonuclear Experimental Reactor prototype based on the Tokamak concept, and it's expected to reach ignition. After the good results obtained in JET, in 1990 they continued the program with a higher melting facility with the reactor, and they proved their auxiliaries facilities with no generation of electricity. In this project the European Union, Canada, USA, Japan and Russia were active participants.
Futuristic image of the research project ITER nuclear fusion
The goal is to determine the technical and economic feasibility of nuclear magnetic fusion to generate electricity, as well as found the precondition for building a commercial demonstration plant phase.
ITER is a technology project with a construction estimated in 10 years and at least 20 IP. Among the technologies used for its construction and subsequent operation and maintenance, it includes robotics, superconductivity, microwave, accelerators and control systems.
The ITER machine will not produce electricity, and some solutions to the problems need to be solved to make viable future fusion reactors. This ambitious research project will be finished in 2050.
The investments for construction are estimated in 5,000 million euros approxinately. Running costs will reach 5,300 million euros and decommissioning will reach 430 million euros. The country of location (France) should bear the costs of site preparation and construction of the building.