Nuclear Fusion

Nuclear fusion is the process by which stars produce energy. It involves the fusion of hydrogen nuclei to produce helium. This process transforms the mass of the nuclei into energy, according to the famous equation E=mc², very efficiently —in fact, it is the most efficient process to produce energy in the universe—, and it does not generate radioactive waste, as helium is not radioactive. It is the opposite process to nuclear fission, used in the present nuclear plants, where a heavy nucleus —uranium— is divided into two lighter ones, generating long-life radioactive waste. To fuse together two hydrogen nuclei, they need to collide at a very high speed. This can be done, in principle, by using particle accelerators; however, this way, there is no net generation of energy as the number of hydrogen nuclei accelerated is very small and we use more energy for the acceleration than is produced by the fusion. To obtain a net production of fusion energy, we need a large quantity of nuclei colliding among them at high speed. This can be obtained by heating hydrogen at very high temperatures —from tens to hundreds of millions of degrees—. In this condition, electrons are separated from the nuclei; it is the fourth state of the matter called plasma and stars are made up of it.

It is not easy to produce and keep hydrogen plasma at high temperatures as gases naturally expand when heated and energy is lost due to conduction and convection. In the stars, the mass of a star provides the strength to compress the hot gas, it is compressed by its own weight and the huge dimensions guarantee small heat losses. To reproduce the conditions of plasma of a star in the Earth, it is necessary to use other physical processes to compress the plasma and provide thermal insulation; this is achieved by using magnetic fields. As plasma is made up of electrically charged constituents (- electrons and + nuclei), they are forced to move orbiting around the magnetic field lines, following the electromagnetism laws. When the lines of the magnetic field have the suitable configuration —a toroid, a doughnut shape surface— nuclei and electrons are confined by it and they keep their temperature (see video 1). The electromagnetic forces also compress the plasma preventing it to expand and get cold.http://www.youtube.com/embed/aVeu49PPjNo?rel=0&enablejsapi=1&wmode=opaqueVídeo 1 – Toroid

The aim of the ITER project is to prove the scientific and technical feasibility of nuclear fusion as an almost infinite and non-pollutant energy source. It is now under construction in the South of France by an international consortium. The ITER project is now the largest scientific project in construction worldwide. Their members are the European Union —that contributed 45% of the budget—, China, Korea, India, Japan, Russia and the United States, which represent over half of the world population and the 80% of the global GDP. ITER uses a Tokamak magnetic field configuration that can contain very hot hydrogen plasma of a volume of 800,000,000 litres (see Figure 1).

Figure 1 –  ITER Tokamak nuclear fusion experimental reactor

The first Tokamak was developed by soviet scientists in the 60’s-70’s. Its magnetic configuration provides the best thermal insulation and compression forces to get plasma at the suitable temperature and pressure needed to produce nuclear fusion energy. At ITER, it is expected to obtain plasma at hundreds of million degrees —ten times hotter than the solar core— and magnetic fields 100,000 times larger than the one of the Earth. Also, 15,000,000 Ampere currents will flow through the plasma, which are 10,000,000 times higher than the ones found in regular electrical appliances —fridge, dishwasher, etc.—. To generate these electric currents and magnetic fields, ITER is equipped with huge coils made up of special cables —superconductors— that are kept at -269 Celsius degrees —nearly the lowest temperature in the universe— to keep their electric resistance at very low levels. So, this way, the quantity of energy used to generate the magnetic fields necessary to produce fusion energy is reduced. The reactor will be kept at temperatures 10 times higher that the solar core, and, at the same time, at the lowest temperature in the universe, just separated by few metres, fact that makes us realize the huge insulation power of magnetic fields.

ITER is designed to produce 500MW of fusion power; for that, plasma needs to be heated using 50MW power. It means it will produce ten times more energy than it consumes whilst heating plasma at hundreds of million degrees. The fuel that will be used at ITER is composed of two types of heavy hydrogen: deuterium and tritium that will be fused to produce helium and a neutron (see video 2). Deuterium is the most stable form of heavy hydrogen, it was formed in the first moments after the origin of the universe, after the Big-Bang, and it is found in a 0.015% proportion in the Earth natural hydrogen, for example, in sea water. Tritium is an even heavier form of hydrogen, but it is unstable, decaying into helium (He³) in 13 years, so it does not naturally occur on Earth. ITER will use tritium reserves from fission reactors that operate using heavy water —water with deuterium—.

Video 2 – Deuterium-Tritium Reaction

http://www.youtube.com/watch?v=1n8OPDRsupw&t=29s

In future fusion reactors, tritium will be produced in the same reactor using lithium —an abundant element exclusively used in the production of electrical batteries— and neutrons generated by the fusion reaction. The lithium in the reactor walls will absorb the energy of the neutrons liberated during the fusion reaction, increasing the temperature of the coolant fluid, that will be used to produce water vapour and therefore electrical energy in the future (see Figure 2), in a similar way to the currently existing thermal and nuclear plants. One of the aims of ITER is to prove the technology to produce tritium from lithium in a small-scale using what is calledTest Blanket Modules.

Figure 2 – Future nuclear fusion electric plant

The construction of ITER has represented a huge challenge with respect to infrastructure (see Figure 3 or Video 3) and the technology necessary for the different components of the reactor. For example, to build the magnetic coils it was necessary to multiply the global production of the materials needed for the cables by ten and the preparation of these cables took 8 years of joint production, in factories in the European Union, China, Korea, Japan, Russia and the United States. On the other side, the design and manufacture of the parts that protect the vessel of the reactor from the plasma power flow have required an outstanding technological design. Even though magnetic fields produce a thermal insulation from the plasma surplus, they are not perfect, and the walls of the fusion reactor undergo power flows of uncommon magnitudes. To protect the walls, they have developed parts made up of wolfram and cooled with high-pressure water that can withstand power flows of up to 20MW by square metre, which can be compared with the power flow in the surface of the Sun —60MW by square metre—, and higher than the one that withstand spaceships when entering the Earth’s atmosphere.

Video 3 – ITER Project buildings and infrastructure

http://www.youtube.com/embed/L0k42jvit8I?rel=0&enablejsapi=1&wmode=opaque

The construction of the main elements of the ITER reactor will finish in 2024. Then, in 2025, several tests will be carried out to test that their operation is as expected. After this phase, other auxiliary components will be added to the reactor in two periods between 2026-28 and 2030-31. Each period will be followed by stages of experimental operation of the reactor using conventional hydrogen and helium, all this without generating fusion energy. Finally, after gathering the experience to operate the reactor in these two phases, deuterium and tritium will be introduced in the reactor in 2036 and the research stage on fusion energy production will start. This stage will last at least one decade and will help us to optimize the fusion energy production, both in magnitude and in length; other operational aspects will be researched and developed at ITER to be applied to the next generation of fusion reactors whose construction will start in the second half of the decade 2030.

The scientific and technological knowledge generated during the construction and exploitation of ITER is shared among all the countries that are members of the project, as an exemplary reflection of the global collaborative spirit that guided the fusion nuclear research from its origins in the second half of the 20^th century.