Tokamak

[Audio] Welcome to Asher Flanagan's presentation on Tokamaks..

[Audio] What is a tokamak? The prerequisite knowledge for understanding tokamaks is knowledge about nuclear fusion. Unlike fission, fusion involves smaller atomic nuclei being forced together under such extreme force and/or pressure that the Coulombic repulsion between them is overcome. This allows them to approach each other to such proximity that the Strong Nuclear force begins to dominate the interaction. At this point the two nuclei are combined into a larger nuclei. This process does occur in nature, but only in the cores of stars and other similarly extreme environments. The quintessential example of this process is the fusion of two hydrogen isotopes: Deuterium and Tritium, pictured here on the right. When these two isotopes fuse, the products are an alpha particle (a.k.a. a helium nucleus), a neutron, and a gamma ray. Both the neutron and gamma ray bear significant kinetic energy, which can be harnessed and converted eventually to electrical energy that can be sent to a power grid. This process is pictured here on the left. Aside from the fundamental physical interactions being the opposite of what occurs inside a fission reactor, the process of harvesting heat from a fusion reactor and converting it to electrical energy is very similar to how a fission reactor does it..

[Audio] One type of fusion reactor is called a Tokamak. The world first learned of Tokamaks thanks to a research paper titled "Stability and Heating of Plasmas in Toroidal Chambers," that was submitted by Soviet scientists for the Second Atoms for Peace conference, held in Geneva Switzerland in October 1958. This was the birth of one of the most popular methods of realizing controlled nuclear fusion: Magnetic Confinement. The basic principle is one uses powerful magnetic fields to confine a plasma, generated by injecting the reactant gases into a vacuum chamber and then heating them, within a reactor. As plasma is a state of matter defined by the removal of electrons from their host atomic nuclei, what you are left with is a sea of positively and negatively charged particles. Both are readily influenced by externally applied magnetic fields, which can be deliberately modulated to encourage the particles to collide in the manner that causes fusion. The products, aside from the alpha particle, are a different story. Neutrons have no electromagnetic charge, and thus they can completely ignore magnetic fields. When neutrons are produced by fusion, they bear significant kinetic energy, so in short, they slam right into the internal walls of the tokamak at considerable speeds. The cumulative effect of these collisions results in heating of the reactor walls, heat that can be harnessed and siphoned away by the working fluid. The definitive feature of a tokamak is its toroidally-shaped reactor (a.k.a. doughnut-shaped) which has proven very successful at confining generated plasmas..

[Audio] What are the core components of a tokamak? The first, and arguably most important, are the magnets used to confine the generated plasmas. ITER, a multinational fusion research project, features several magnet components based on superconducting materials. Manufactured from niobium-tin or niobium-titanium, the magnets become superconducting when cooled with supercritical helium in the range of 4 Kelvin (-269 °C). Eighteen "D"-shaped toroidal field magnets, pictured on the left, placed around the vacuum vessel produce a magnetic field whose primary function is to confine the plasma particles. The toroidal field coils are designed to produce a total magnetic energy of 41 gigajoules and a maximum magnetic field of 11.8 tesla. Toroidal field coils are wound in "double pancakes"—layers of spiraled conductor embedded in radial plates and encased in large stainless-steel structures. The poloidal magnets, on the other hand, feature six ring-shaped poloidal field coils situated outside of the toroidal field magnet structure to shape the plasma and contribute to its stability by "pinching" it away from the walls. Remember that fusion only occurs naturally in the cores of stars, so the plasma must be heated to extreme temperatures in the hundreds of millions of degrees Celsius. No material on earth could withstand such temperatures, so it is vital the plasmas be kept far from the reactor walls to avoid damaging them..

[Audio] The central solenoid, pictured on the left, is the "backbone" of ITER's magnet system, allowing a powerful current to be induced in the ITER plasma and maintained during long plasma pulses. The further encourages the plasma to be influenced by the externally applied magnetic fields. Eighteen superconducting correction coils, pictured on the right, inserted between the toroidal and poloidal field coils will compensate for field errors caused by geometrical deviations due to manufacturing and assembly tolerances..

[Audio] The vacuum vessel is a hermetically sealed steel container that houses the fusion reactions and acts as a first safety containment barrier. In its doughnut-shaped chamber, or torus, the plasma particles spiral around continuously without touching the walls. The vacuum vessel provides a high-vacuum environment for the plasma, improves radiation shielding and plasma stability, acts as the primary confinement barrier for radioactivity, and provides support for in-vessel components such as the blanket and the divertor. Cooling water circulating through the vessel's double steel walls will remove the heat generated during operation. Forty-four openings, or ports, in the vacuum vessel provide access for remote handling operations, diagnostics, heating, and vacuum systems. Neutral beam injection, firing hydrogen isotopes into the plasma in order to heat it, will take place at equatorial level, for example, while on the lower level, five ports will be used for divertor cassette replacement and four for vacuum pumping. Approximately 55 percent of the space between the double walls of the vacuum vessel will be occupied by in-wall shielding in the form of modular blocks, weighing up to 500 kg each. Made of borated and ferromagnetic stainless steel, the blocks will provide shielding from neutron radiation for components situated outside of the vacuum vessel (such as the magnets) and contribute to plasma performance by limiting a type of perturbation known as toroidal field ripple..

[Audio] The 440 blanket modules that completely cover the inner walls of the vacuum vessel protect the steel structure and the superconducting toroidal field magnets from the heat and high-energy neutrons produced by the fusion reactions. As the neutrons are slowed in the blanket, their kinetic energy is transformed into heat energy and collected by the water coolant. In a fusion power plant, this energy will be used for electrical power production. All modules have a detachable first wall that directly faces the plasma and removes the plasma heat load, and a main shield block that is designed for neutron shielding. Due to its unique physical properties (low plasma contamination, low fuel retention), beryllium has been chosen as the element to cover the first wall. The rest of the blanket modules will be made of high-strength copper and stainless steel. ITER will be the first fusion device to operate with an actively cooled blanket. The cooling water—injected at 4 MPa and 70 °C—is designed to remove up to 736 MW of thermal power. During later stages of ITER operation, some of the blanket modules will be replaced with specialized modules to test materials for tritium breeding concepts. A future fusion power plant producing large amounts of power will be required to breed all of its own tritium. ITER will test this essential concept of tritium self-sustainment. The first wall panels are the detachable, front-facing elements of the blanket that are designed to withstand the heat flux from the plasma. These highly technological components are made of beryllium tiles bonded with a copper alloy and 316L (N) stainless steel. The shield blocks (the heavier of the two elements) provide nuclear shielding for the vacuum vessel and coil systems as well as support for the first wall panels. Cooling water will run to and from the shield blocks through manifolds and branch pipes to remove the high heat load expected during ITER operation..

[Audio] Situated at the bottom of the vacuum vessel, the divertor extracts heat and ash produced by the fusion reaction, minimizes plasma contamination, and protects the surrounding walls from thermal and neutronic loads. Each of the divertor's 54 "cassette assemblies" has a supporting structure in stainless steel and three plasma-facing components: the inner and outer vertical targets and the dome. The cassette assemblies also host a number of diagnostic components for plasma control and physics evaluation and optimization. The inner and outer vertical targets are positioned at the intersection of magnetic field lines where particle bombardment will be particularly intense in ITER. As the high-energy plasma particles strike the vertical targets, their kinetic energy is transformed into heat and the heat is removed by active water cooling. The heat flux sustained by the ITER divertor vertical targets is estimated at 10 MWm² (steady state) and 20 MWm² (slow transients). Tungsten, with the highest melting point of all the metals, has been chosen as the armor material following an international R&D effort, encouraging experimental results, and successful prototype testing. The plasma-facing components of the ITER divertor will be exposed to a heat load that is ten times higher than that of a spacecraft re-entering Earth's atmosphere..

[Audio] The ITER cryostat—the largest stainless-steel high-vacuum pressure chamber ever built (16,000 m³)—provides the high vacuum, ultra-cool environment for the ITER vacuum vessel and the superconducting magnets. Nearly 30 meters wide and as many in height, the internal diameter of the cryostat (28 meters) has been determined by the size of the largest components its surrounds: the two largest poloidal field coils. Manufactured from stainless steel, the cryostat weighs 3,850 tons. Its base section—1,250 tons—will be the single largest load of ITER Tokamak assembly. The cryostat has 23 penetrations to allow access for maintenance as well as over 200 penetrations—some as large as four meters in size—that provide access for cooling systems, magnet feeders, auxiliary heating, diagnostics, and the removal of blanket sections and parts of the divertor. Large bellows situated between the cryostat and the vacuum vessel will allow for thermal contraction and expansion in the structures during operation. The structure will have to withstand a vacuum pressure of 1 x 10 -4 Pa; the pump volume is designed for 8,500 m³..

[Audio] Unfortunately, an important current limitation of fusion must also be discussed: no fusion reactor anywhere, ever, has so far demonstrated net positive electrical energy output. While important breakthroughs have been made, such as that by the National Ignition Facility, a facility part of the US Department of Energy's Lawrence Livermore National Laboratory, that was a demonstration of net positive thermal energy, not electrical energy. In terms of sending power to an electrical grid, controlled nuclear fusion, and by extension tokamaks, remain theoretical for now. But perhaps ITER can be the first tokamak to prove it can be done..

[Audio] Thank you for your time. This has been Asher Flanagan's presentation on tokamaks for Dr. Farfan's Fundamentals of Nuclear Engineering, Section 02, Spring 2023..