ITER instance of tokamak
Reactor overview
When deuterium and tritium fuse, two nuclei come together to form a helium nucleus (an alpha particle), and a high-energy neutron.
21D + 31T → 42He + 10n + 17.59 MeVWhile nearly all stable isotopes lighter on the periodic table than iron-56 and nickel-62, which have the highest binding energy per nucleon, will fuse with some other isotope and release energy, deuterium and tritium are by far the most attractive for energy generation as they require the lowest activation energy (thus lowest temperature) to do so, while producing among the most energy per unit weight.All proto- and mid-life stars radiate enormous amounts of energy generated by fusion processes. Mass for mass, the deuterium–tritium fusion process releases roughly three times as much energy as uranium-235 fission, and millions of times more energy than a chemical reaction such as the burning of coal. It is the goal of a fusion power station to harness this energy to produce electricity.
Activation energies (in most fusion systems this is the temperature required to initiate the reaction) for fusion reactions are generally high because the protons in each nucleus will tend to strongly repel one another, as they each have the same positive charge. A heuristic for estimating reaction rates is that nuclei must be able to get within 100 femtometers (1 × 10−13 meter) of each other, where the nuclei are increasingly likely to undergo quantum tunneling past the electrostatic barrier and the turning point where the strong nuclear force and the electrostatic force are equally balanced, allowing them to fuse. In ITER, this distance of approach is made possible by high temperatures and magnetic confinement. ITER uses cooling equipment like a cryopump to cool the magnets to close to absolute zero. High temperatures give the nuclei enough energy to overcome their electrostatic repulsion (see Maxwell–Boltzmann distribution). For deuterium and tritium, the optimal reaction rates occur at temperatures higher than 100 million °C. At ITER, the plasma will be heated to 150 million °C (about ten times the temperature at the core of the Sun) by ohmic heating (running a current through the plasma). Additional heating is applied using neutral beam injection (which cross magnetic field lines without a net deflection and will not cause a large electromagnetic disruption) and radio frequency (RF) or microwave heating.At such high temperatures, particles have a large kinetic energy, and hence velocity. If unconfined, the particles will rapidly escape, taking the energy with them, cooling the plasma to the point where net energy is no longer produced. A successful reactor would need to contain the particles in a small enough volume for a long enough time for much of the plasma to fuse.
In ITER and many other magnetic confinement reactors, the plasma, a gas of charged particles, is confined using magnetic fields. A charged particle moving through a magnetic field experiences a force perpendicular to the direction of travel, resulting in centripetal acceleration, thereby confining it to move in a circle or helix around the lines of magnetic flux. ITER will use four types of magnets to contain the plasma: a central solenoid magnet, poloidal magnets around the edges of the tokamak, 18 D-shaped toroidal-field coils, and correction coils.A solid confinement vessel is also needed, both to shield the magnets and other equipment from high temperatures and energetic photons and particles, and to maintain a near-vacuum for the plasma to populate. The containment vessel is subjected to a barrage of very energetic particles, where electrons, ions, photons, alpha particles, and neutrons constantly bombard it and degrade the structure. The material must be designed to endure this environment so that a power station would be economical. Tests of such materials will be carried out both at ITER and at IFMIF (International Fusion Materials Irradiation Facility).Once fusion has begun, high-energy neutrons will radiate from the reactive regions of the plasma, crossing magnetic field lines easily due to charge neutrality (see neutron flux). Since it is the neutrons that receive the majority of the energy, they will be ITER's primary source of energy output. Ideally, alpha particles will expend their energy in the plasma, further heating it.The inner wall of the containment vessel will have 440 blanket modules that are designed to slow and absorb neutrons in a reliable and efficient manner and therefore protect the steel structure and the superconducting toroidal field magnets.  At later stages of the ITER project, experimental blanket modules will be used to test breeding tritium for fuel from lithium-bearing ceramic pebbles contained within the blanket module following the following reactions:
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