Nuclear Fusion Applications - The Tokamak Reactor
KJM3900 project assignment spring 2015
Written by Bárbara Maria Teixeira Costa Peluzo
In this essay, concepts involving nuclear fusion have been introduced. After that, the main challenges and experiments related to nuclear fusion reactors have been discussed, focusing on the Magnetic Confinement Reactor – Tokamak.
Nuclear reactions are chemical phenomena in which nuclei may either combine or break into two or more products. In fusion we have nuclei combining into bigger ones, while in fission there is the opposite. What will indicates which reaction the nucleus will prefer is mainly its energy and mass: atoms tend to have the lowest possible energy and, for some mass numbers, we have even lower quantities of energy (Fig. 1). In addition, it is most observed fusion in lighter nuclei and fission in heavy ones (also, fusion is often used to make new elements, but they are very unstable and lasts for milliseconds or even less time). Fission may happens spontaneously in our planet, but fusion occurs naturally only in stars and its main reaction is hydrogen forming helium [1,2].
Hydrogen has 3 main isotopes observed:
1H, the most stable and abundant; 2H, called deuterium (D) and 3H, tritium (T). As mentioned before, inside stars, like the Sun, there is free protons (which can be considered hydrogen cations - H+), which react, producing helium. The mechanism, known as proton cycle, is explained in Eqs. 1, 2 and 3.
In the first step, two protons form a deuterium nucleus. It is important to observe that one of the protons turn into a neutron, otherwise the product would not be deuterium (in fact, a nucleus with only two protons does not exist). After that, a third proton reacts with the previous product and forms the isotope 3He. Then, two nuclei of 3He connect, producing both 4He and two protons (which will continue the reactions) .
Experimental and theoretical data show this mechanism as impossible to occur in laboratory conditions, because of its activation energy: Due to electrostatic repulsion between protons (Coulomb barrier), the nuclei need very high kinetic energy, what can only be reached by increasing the temperature to values about 108 K. In addition, the proton cycle demands a higher density of protons in order to maintain the reaction. Then, the reaction would take longer time and become impractical. In labs, the most probable fusion would already begin with D (deuteron cycle) (Eqs. 4, 5 and 6) :
Deuteron cycle still needs high temperatures to happen and this is the main reason why there is no power plants working by nuclear fusion (the amount of energy released is much higher than the uranium fission). The reactional mixture (deuterium in the gaseous state) need to be heated until ~108 K, transforming in what is called plasma: a state of matter in which particles are charged and have high kinetic energy. Due to the temperature needed, the reactor used must be safe and thermally stable, otherwise it would not keep the reaction, offering risk of explosion and danger [1,2].
Several techniques are been developing in order to make this reaction possible for energy source. Some of them try to contour the temperature question while others are about cold nuclear fusion.
NUCLEAR FUSION EXPERIMENTS
An example about how fusion may happen in lower temperatures is the reaction catalysed by negative muons, particles very similar to electrons but with mass about 200 times higher. They are added to the mixture of D and T, replace the electrons in the molecules and catalyse the fusion in low temperatures. This experiment was run in CERN several times, with different parameters, in order to find a high efficiency. Until now, the energy cost does not make it worth while [4,5].
Other experiments maintain the high temperature and aim to confine the plasma without touching the reactor walls. Even though, the reactor still must be constructed from a thermal stable material (i.e. a material which supports high temperatures), since the "atmosphere" around the plasma will reach high temperatures as well.
Fusion in deuterium plasma has been observed in laboratories using clusters activated by lasers, known as inertial confinement (Fig. 2). In this technique, 1 mg spheres containing D and T have their surface vaporized and the core compressed. Density and temperature are strongly increased, making the spheres to explode. Again, a good efficiency has not been achieved yet [6,7].
The most promising reactor for nuclear fusion works by magnetic confinement: since plasma is charged, magnetic fields may be used in order to interact with the charged particles and keep them "floating". The magnetic field is created by external electric currents and high energy particles. This reactor, which has a "doughnut shape" design (called toroidal design), has first been designed in Russia in the 1950’s and it is called Tokamak (Fig. 3).
The toroidal configuration provides symmetry for the reactor, making the heat and magnetic field uniform in all the plasma. In a first moment, the tube (which must be evacuated) is filled with the mixture in gaseous phase in high pressures (about 300 Pascal), what ensures the density needed – 1014 particles/cm³. Just after the heating, the gas turns into plasma immediately. Besides electrical current, fast neutral atoms, microwaves and alpha particles are used in order to reach and maintain the desired temperature in all the plasma .
H or D ions are accelerated and neutralized outside the Tokamak. Once they are not charged anymore, they cross the magnetic field without interacting with it and exchange heat with the plasma. Electromagnetic waves with microwaves frequency may heat the plasma by being absorbed by it. Finally, one of the fusion products, α particles (4He, see eq. 6), which have high energy, can exchange this energy with the plasma [7,9,10].
The Tokamak reactor structure (Fig. 4) consists of a vessel with several layers. The first one (1) is known as first wall and it is made of blanket boxes in which most of the fusion takes place. This region receives around 2 MW/m² of energy, therefore it is necessary a second layer of protection in case of any damage. The numbers 2 and 3 indicate the divertors: plates in which impurities and non-desirable ions are redirected out of the plasma (e.g. the neutral atoms and α particles after heat exchange). Removing process works by both magnetic fields and vacuum. These plates play a significant role in the entire process, since about 15% of the fusion material is pulled out of the inner chamber. In addition, they need to support even higher temperatures (the energy power is about 10 – 20 MW/m² in the divertors region). Different types of materials, like tungsten alloys, are being studied in order to improve and make this structure safer.
After the first successful experiment with Tokamak, many other reactors were installed and improved around the world. Although the magnetic confinement seems to be a good method, these reactors are not safe in totality: as mentioned before, the plasma reaches very high temperatures and any danger related to melting must be eliminated. For now, some new reactors based on the Tokamak concept are being developed in order to reach high efficiency. ITER (International Thermonuclear Experimental Reactor), a collaboration between nations like Japan, USA, Russia and EU, is an example [7,12].
The demand for energy is increasing all over the world. Fossil fuels are limited and pollutes the atmosphere. The nuclear energy is a promising option, although present some unsolved questions, like the radioactive waste.
Thus, the energy production by nuclear fusion may be the solution for the energy crisis. The magnetic confinement theory is able to make the nuclear fusion a reality, but it still needs to be improved.
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