Nuclear Fusion

The nuclear fusion (known as sometimes thermonuclear ) is, with the fission, one of the two principal types of nuclear reactions applied. One should not confuse nuclear fusion with the Core fusion of a nuclear reactor which is a Nuclear accident particularly frightening.

Nuclear fusion is a process where two atomic nuclei are assembled to form a heavier core. The fusion of light cores releases from enormous quantities of energy coming from attraction between the nucleons due to the strong Interaction (cf energy binding).

This reaction is with work in the Sun and some star S of our Univers.

An interest of nuclear fusion is to be able to produce theoretically much more energy, with mass of " combustible" equalize, that fission. Moreover, the oceans naturally contain sufficient deuterium to make it possible to feed in energy planet during a few hundred millenia.

The products of fusion themselves (mainly of helium 4) are not radioactive, but when the reaction used emits fast neutrons, the latter can on the other hand transform the cores which capture them in isotopes being able the being.

In spite of the research tasks completed in the whole world the Years 1950, no application industrial of fusion to the energy production still led, apart from the military field with the Bombe H. There are however some other médiatisés uses, like the generators of neutrons used in particular for the detection of the explosives.

Mechanism of fusion

A nuclear fusion reaction requires that two atomic nuclei interpenetrate. It is necessary for that the cores overcome the repulsion due to their electric charges both positive (phenomenon known as of Coulomb barrier ). If one applied only the laws of traditional mechanics, the probability of obtaining the fusion of the cores would be very weak, because of the kinetic energy (corresponding to thermal agitation) extremely high necessary to the crossing of the barrier. However, the quantum Mécanique provides, which is checked in practice, that the Coulomb barrier can also be crossed by Tunnel effect, with weaker energies.

Energies necessary to fusion remain very high, corresponding to temperatures of several tens or even hundreds of million degrees according to the nature of the cores (see low: Plasmas of fusion). Within the Sun for example, the fusion of the hydrogen, which ends, by stages, to produce helium is carried out at temperatures of about 15 million degrees Kelvin, but diagrams of reaction different from those studied for the energy production of fusion on Earth. In certain more massive stars, higher temperatures allow the fusion of heavier cores.

When small cores amalgamate, the resulting core is found in an unstable state and must return in a stable condition of weaker energy, into ejecting one or more particles (Photon, Neutron, Proton, core of Hélium, according to the type of reaction), surplus energy is divided between the core and the emitted particles, in the form of kinetic energy. So that fusion is profitable énergétiquement, it is necessary that produced energy is higher than the power consumption for maintenance of the reactions and by thermal losses about the external middle. In the engines with fusion, it is thus necessary to avoid any contact between the medium of reaction and materials of the environment, which one carries out by an immaterial containment.

Whenever no about stable state exists, it can be impossible to cause the fusion of two cores (example: ⁴He + ⁴He).

The reactions of fusion which release the most energy are those which imply the lightest cores. Thus the cores of Deuterium (a Proton and a Neutron) and of Tritium (a proton and two neutrons) are implied in the following reactions:

Deuterium + Deuterium → Helium 3 + Neutron
Deuterium + Deuterium → Tritium + Proton
Deuterium + Tritium → Helium 4 + neutron
Deuterium + Helium 3 → Helium 4 + proton

These are the reactions which are studied in laboratory during experiments of controlled fusion.

Controlled fusion

There exist various conceivable processes making it possible to manage to confine the medium of reaction to produce reactions fusion nuclear, in particular the Fusion by magnetic containment and the Fusion by inertial confinement. None of them still led to industrial results for the production of electrical energy.

Another application of fusion, the production of neutrons, in particular for the detection of the explosives, for a long time arrived at the industrial stage.

fusion by magnetic containment:

the Tokamak S, where one confines a gas mixture of Isotopes of hydrogen thanks to a magnetic field produced by reels and an induced current circulating in plasma (examples: Torus Supra, ITER)
Stellarator S, where containment is entirely ensured by the reels (example: Wendelstein 7-X)
machines with Trap with magnetic mirrors, which could also be used for the space propulsion

fusion by inertial confinement:

machines with Inertial confinement by Laser, where a microball of isotopes is irradiated by powerful lasers (example: Laser Megajoule)
machines with axial Constriction (or Z-pinch), where a pastille of isotopes is compressed by impulses of Rays-X (example: Z machine (more than 2 billion degrees reached!) Laboratories Sandia). The conditions of fusion were obtained in March 2006 in a “Z-machine” with axial containment. Work started on the design of an experimental reactor with impulse using this principle.

Plasmas of fusion

At the temperature to which fusion is likely to occur, the Matière is with the state of plasma. It is about a particular state of the raw material in which the atoms or molecules form an ionized gas.

One or more electron S of the electronic cloud which surrounds each core was torn off, leaving Ion S charged positively and free electrons, the unit being electrically neutral.

In a thermal plasma, the great agitation of the ions and electrons produce many collisions between the particles. So that these collisions are sufficiently violent and involve a fusion, three sizes intervene:

the temperature T ;
the density NR ;
the time of containment τ .

The Critère of Lawson establishes that the factor Nτ must reach a certain threshold to obtain the breakeven where the energy released by fusion is equal to energy spent. The ignition occurs then at a stage much higher of energy production (impossible to create today in the current engines). It is a question of the threshold from which the reaction is able car-to discuss. For the reaction deuterium + tritium, this threshold is of 10 14 s/cm ³.

Deutérium reaction + Tritium analyzes

The binding energy of the components comes from the force of strong nuclear interaction, one of the four fundamental forces of interaction of the universe.

However the energy investment to provide to obtain this connection is proportional to the product of the electric charges of the two involved cores. This is why the choice for fusion was made on the Deutérium and the Tritium, two heavy isotopes of the hydrogen, for which this product is worth 1.

Minimal energy to provide to obtain a fusion is of 4 keV (equivalent at a temperature of 40 million Kelvin degrees); the energy released from 17,6 MeV is then distributed for 80% in the emitted neutron and 20% in helium 4 product.

But energy necessary to reach the Criterion of Lawson and a sufficiently positive output is towards 10 keV is 100 million of degrees.

The reaction deuterium + tritium results in an emission of fast neutrons. These neutrons are impossible to confine electromagnetically because they have a null electric charge and cannot be captured using electromagnetic fields. They are thus likely to be captured by the cores of atoms of the wall of the enclosure, which they sometimes transmute into Isotope S radioactive (phenomenon of activation). Activation can be accompanied by production of helium cores, likely to weaken structural materials. It could complicate the industrial use of fusion, and makes the object of studies with various proposals for solutions (for example walls in composites, or specific alloys of Fer), but they require experimental studies difficult to realize in the short run. The reactions generating of the neutrons are thus not completely “clean”, but are however definitely less generating of waste that reactions of Nuclear fission, and the lifespan of this waste is quite lower than that of the radioactive products created in the power stations with Nuclear fission.

If deuterium is available naturally in great quantities in the oceans, but requires the installation of very complex methods to be extracted about it, the Tritium must be prepared artificially because it is only in very minor amount in the natural environment from its nature of radioactive Isotope at short duration of half-life (half disappears in 12,3 years).

Applications of fusion

If fusion could be used in the military field with the H-bombs, there does not exist yet of civil application of fusion for the electrical production. Only prototypes of study could be currently built.

University formations

After the decision taken into 2006 to establish the project ITER in France, several French institutions of higher education united in order to open a speciality of master " sciences of the fusion". This formation aims at training the future scientists and engineers, French or foreigners, who wish to invest themselves in the programs as well national as private concerning research on energy and fusion, in particular in the scientific and technical exploitation of large associated equipment. The formation is done through three courses: two relate primarily to the physics of fusion, either by magnetic containment, or by inertial confinement; a third course is with more technological contents.

Ten establishments, distributed on four sites of the French territory, are cohabilities to deliver this diploma, with lesson which takes place in parallel in these sites and during regrouping of the students in Cadarache and Bordeaux: Universities of Aix-Marseilles and INSTN in area PACA, University Bordeaux 1 in Aquitaine, University Nancy I, INPL in Lorraine and University Paris 6, Southern Paris University 11, Polytechnic school, INSTN for the Ile de France. Five schools of engineer are also associated: Supelec, Supoptique, the central School Paris, ENSAM, the central School of Marseilles.

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