Nuclear fusion is a nuclear reaction in which two nuclei of light atoms join together to form another heavier nucleus. The atoms used are hydrogen isotopes (deuterium and tritium). During the fusion of two atoms, the composition of the nuclear force of the nuclei is modified and a large amount of energy is released or absorbed in the form of gamma rays and also the kinetic energy of the emitted particles.
The energy emitted is so great that it is possible for matter to enter a plasma state.
Nuclear fusion reactions can emit or absorb energy. If the nuclei that are going to fuse have a lower mass than iron, energy is released. On the contrary, if the atomic nuclei that fuse are heavier than iron, the nuclear reaction absorbs energy.
At the moment, it has not yet been possible to build any fusion reactor capable of producing electrical energy. However, several projects are currently being worked on - such as the ITER project in the south of France - with the aim of generating clean energy through fusion energy.
Examples of nuclear fusion: the Sun
Examples of nuclear fusion can be found in various situations, both in nature and in human-controlled applications. Some examples are:
- Fusion in the Sun: The Sun's main source of energy is nuclear fusion. At its core, hydrogen nuclei combine to form helium, releasing an immense amount of energy in the form of light and heat.
- Hydrogen Bomb: Hydrogen bombs, also known as thermonuclear bombs, use nuclear fusion to generate extremely powerful explosions. In these bombs, the fusion of hydrogen nuclei to form helium and other heavy elements releases explosive energy.
- Experimental fusion reactors: Experimental fusion reactors, such as the Tokamak and Stellarator, have been built to research and develop controlled nuclear fusion as an energy source. These devices create conditions similar to those in the Sun's core to achieve fusion.
Requirements of a fusion reaction
To carry out nuclear fusion reactions, the following requirements must be met:
Achieve a very high temperature to separate the electrons from the nucleus and make it approach another, overcoming the electrostatic repulsion forces. The gaseous mass composed of free electrons and highly ionized atoms is called plasma.
Confinement is necessary to keep the plasma at an elevated temperature for a minimum of time.
The density of the plasma must be sufficient so that the nuclei are close to each other and can generate nuclear fusion reactions.
Confinement for nuclear fusion
The conventional confinements used in nuclear fission reactors are not possible due to the high plasma temperatures they must endure. For this reason, two important confinement methods have been developed:
Nuclear fusion by inertial confinement (ICF): It consists of creating a medium so dense that the particles have almost no chance of escaping without colliding with each other.
Nuclear fusion by magnetic confinement (MCF): The electrically charged particles of the plasma are trapped in a reduced space by the action of a magnetic field. The most developed device has a toroidal shape and is called Tokamak.
How it works: Fusion reactions
The atomic elements normally used in nuclear fusion reactions are Hydrogen and its isotopes: Deuterium (D) and Tritium (T). The most important fusion reactions are:
D + T -> 4He + n + 17.6 MeV
By fusing a Deuterium nucleus with a Tritium nucleus, a Helium nucleus is obtained formed by two neutrons and two protons, releasing 1 neutron and 17.6 MeV of energy.
D + D -> 3He + n + 3.2 MeV
By fusing two Deuterium nuclei, a Helium nucleus formed by one neutron and two protons is obtained, releasing one neutron and 3.2 MeV of energy.
D + D --> T + p + 4.03 MeV
By fusing two Deuterium nuclei, a Tritium nucleus, a proton and 4.03 MeV of energy are obtained.
For these reactions to take place, the kinetic energy necessary to bring the nuclei that are going to fuse together must be supplied to the nuclei, thus overcoming the forces of electrostatic repulsion. To do this, the gas needs to be heated to very high temperatures, such as those assumed to occur in the centers of stars.
The requirement of any nuclear fusion reactor is to confine said plasma at a sufficiently high temperature and density and for just the right amount of time to allow enough nuclear fusion reactions to occur, preventing particles from escaping, to obtain a net profit. of energy.
This energy gain depends on the energy needed to heat and confine the plasma being less than the energy released by nuclear fusion reactions. In principle, 335 MJ can be obtained for each milligram of deuterium-tritium.
Nuclear fuel for fusion
Light nuclei are needed for nuclear fusion reactions. Basically, Deuterium and Tritium are used, which are two isotopes of hydrogen (the lightest element in the periodic table).
1. Deuterium
Deuterium is a stable isotope of hydrogen formed by a proton and a neutron. Its abundance in water is one atom for every 6,500 hydrogen atoms. This means that in seawater there is a concentration of 34 grams of deuterium per cubic meter of water.
The energy content of deuterium is so high that the energy that can be obtained from the deuterium of one liter of seawater is equivalent to the energy that can be obtained from 250 liters of oil.
Taking into account the abundance of deuterium in the oceans, this energy source can be considered renewable.
2. Tritium
The other element used in nuclear fusion, tritium, is the unstable or radioactive isotope of the hydrogen atom. It is composed of one proton and two neutrons and decays by beta emission relatively quickly.
Although tritium is scarce in nature, it can be generated by neutron capture reactions with Lithium isotopes. Lithium is an abundant material in the Earth's crust and in sea water.
Nuclear fusion research project
The most advanced project in nuclear fusion by magnetic confinement is the ITER (International Thermonuclear Experimental Reactor), a prototype based on the Tokamak reactor concept, and in which ignition is expected to be achieved.
Given the good results obtained in the Joint European Torus (JET) , in 1990 it was decided to continue the fusion program with a larger facility in which, in addition to the reactor, its auxiliary systems could be tested without yet generating electricity. The European Union, Canada, the USA, Japan and Russia participate in this project.
The objective is to determine the technical and economic feasibility of nuclear fusion by magnetic confinement to generate electricity, as a prior phase to the construction of a commercial demonstration facility.
Electrical energy will not be produced in the ITER machine, solutions to the problems that need to be solved to make future nuclear fusion reactors viable will be tested.