Instalación destinada a la producción de energía mediante la fusión nuclear. Tras más de 60 años de investigación en este campo, se ha logrado mantener una reacción controlada, si bien aún no es energéticamente rentable.
La mayor dificultad se halla en soportar la enorme presión y temperatura que requiere una fusión nuclear (que sólo es posible encontrar de forma natural en el núcleo de una estrella). Además este proceso requiere una enorme inyección de energía inicial (aunque luego se podría automantener ya que la energía desprendida es mucho mayor).
Actualmente existen dos líneas de investigación, el confinamiento inercial y el confinamiento magnético.
El confinamiento inercial consiste en contener la fusión mediante el empuje de partículas o de rayos láser proyectados contra una partícula de combustible, que provocan su ignición instantánea.
Los dos proyectos más importantes a nivel mundial son el NIF (National Ignition Facility) en Estados Unidos y el LMJ") (Laser Mega Joule) en Francia.
El confinamiento magnético consiste en contener el material a fusionar en un campo magnético mientras se le hace alcanzar la temperatura y presión necesarias. El hidrógeno a estas temperaturas alcanza el estado de plasma "Plasma (estado de la materia)").
Los primeros modelos magnéticos, americanos, conocidos como Stellarator generaban el campo directamente en un reactor toroidal, con el problema de que el plasma se filtraba entre las líneas del campo.
Los ingenieros rusos mejoraron este modelo dando paso al Tokamak en el que un arrollamiento") de bobina primario inducía el campo sobre el plasma, aprovechando que es conductor, y utilizándolo de hecho como un arrollamiento secundario. Además la resistencia eléctrica del plasma lo calentaba.
El mayor reactor de este tipo, el JET (toro "Toro (matemáticas)") europeo conjunto) ha conseguido condiciones de fusión nuclear con un factor Q>0,7. Esto significa que el ratio entre la energía generada por fusión y la requerida para sostener la reacción es de 0.7. Para que la reacción se auto sostenga deben alcanzarse parámetros superiores a Q>1 y más aún para su viabilidad económica. El primer objetivo debe ser alcanzado con el proyecto ITER y el segundo con DEMO.
Se ha comprometido la creación de un reactor aún mayor, el ITER uniendo el esfuerzo internacional para lograr la fusión. Aun en el caso de lograrlo seguiría siendo un reactor experimental y habría que construir otro prototipo para probar la generación de energía, el llamado proyecto DEMO.
Possible fuels for nuclear fusion reactors
The optimal reaction to produce energy by fusion is that of deuterium and tritium due to their high effective section. It is also, for this reason, the most used in experimental tests. The reaction is the following:
Obtaining deuterium is not difficult since it is a stable and abundant element that formed in large quantities in the primordial soup of particles (see Big Bang). In water, one part in every 6,500 contains deuterium instead of hydrogen, which is why it is considered that there is an inexhaustible reserve of deuterium. In a self-sustained reactor the deuterium-tritium reaction would generate energy and neutrons. Neutrons are the negative part of the reaction and must be controlled since the neutron capture reactions in the walls of the reactor or in any atom of the reactant can induce radioactivity. In fact, neutrons, given enough time, can weaken the structure of the container itself with the consequent risk of dangerous cracks occurring. For this, there are moderators and neutron shields such as heavy water, beryllium, sodium or carbon as moderators widely used in fission plants, or boron and cadmium, used as products that completely stop neutrons by absorbing them. If you want to make a really clean reactor, you will have to look for other formulas. A double solution has been proposed to the problem of neutrons and that of tritium abundance. Tritium is not found in nature as it is unstable so it must be manufactured. To obtain it, you can resort to fission plants, where it can be generated by the activation of hydrogen contained in water, or by bombarding lithium, an abundant material in the Earth's crust, with neutrons.
There are two stable isotopes of lithium, lithium-6 and lithium-7, the latter being much more abundant. Unfortunately, the reaction that absorbs neutrons is the one that occurs with lithium-6, the least abundant. All this does not prevent many neutrons from hitting the walls of the reactor itself with the subsequent production of radioactive atoms. Despite this, one of the proposals for ITER is to coat the walls with lithium-6 which would stop a good part of the neutrons to produce more tritium. Due to all these problems, other reactions with a high effective section but cleaner are being investigated. One of the most promising is that of deuterium plus helium-3.
The problem in this reaction lies in the smaller effective section with respect to that of deuterium-tritium and in the obtaining of helium-3, which is the rarest isotope of said element. Protons do not entail as much danger as neutrons since they will not be easily captured by atoms due to the Coulomb barrier that they must cross, something that does not happen with neutrally charged particles such as neutrons. Additionally, a proton can be manipulated using electromagnetic fields. A solution to obtain helium-3 artificially would be to incorporate, in the reactor itself, the deuterium-deuterium reaction.
The problem is that, once again, we get a residual neutron, which brings us back to the neutron problem. Perhaps the key was obtaining natural helium-3, but this is extremely rare on Earth. It must be taken into account that the little natural helium-3 that is produced by radioactivity tends to escape from our dense atmosphere. The curious thing is that this isotope is abundant on the Moon. It is spread across its surface and comes from the solar wind that for billions of years has bathed the naked lunar surface with its ionized particles. This lunar helium could be, in the future, the key to fusion reactors.
Meanwhile, research is being carried out on materials that, although activated, only give rise to isotopes with a short half-life, so by letting these materials rest for a short period, they could be considered conventional (non-radioactive) waste. The main problem, in any case, would continue to be the difficulty of keeping the core frame in good condition without it deteriorating and having to be changed every so often.