Principles of Fusion Energy Generation The fusion reaction between deuterium and tritium releases huge amounts of energy. Fusion of a fraction of a gram of material (about 1/30th of a gram) releases the amount of energy contained in two barrels of oil. This is one essential aspect of an ideal energy source. There is no possibility of more fusion energy being released than planned, because only the desired amount of fuel is injected and subjected to the demanding technical processes that will bring the fuel to the very special conditions that will initiate the reaction. These special conditions, and the processes to bring the fuel to them, are described in summary form below. Principles of Fusion Fusion energy is based on combining the nuclei of light elements to form nuclei of heavier elements. This releases energy proportional to the difference between the sum of the mass-energies of the fusing nuclei and mass-energies of the product particles, according to E=mc2. In the current instance, our fuel (the fusing nuclei) will be the two heavy isotopes of hydrogen. We will produce ordinary helium as our product isotope along with an energetic neutron, as shown in Figure T-1. Figure T-1. Reactants and
products in the Fusion reaction The
isotopes of hydrogen have specific names, unlike the isotopes of other
elements, namely deuterium and tritium.
Deuterium(2H) is naturally present in all water and thus seawater is our
primary source of fuel. Tritium(3H), the
other component of fuel in a fusion power source, is of very low abundance in
nature. This is in consequence of
tritium being an unstable isotope with a relatively short half-life, 12.3
years. Tritium to start-up the first of our fusion systems will come from
stores extracted from fission power plants, where it serves no useful purpose
and is unwanted. Containment of tritium is virtually the sole radiological
safety issue for fusion power. The difficulty of achieving zero release of
tritium in fission power plants comes from having water both in contact with
the core and to drive steam turbines. Fusion does not have this challenge, and
zero release is a practical goal. Although an external source of tritium is needed to start our operations, we will Conditions Required for FusionThe essential challenge of fusion is that the fuel nuclei are electrically charged with the same “sign”. The electrostatic (Coulombic) force pushes the nuclei apart, opposing their coming close enough together for the nuclear force to take over and cause fusion to occur. To get past this “Coulomb barrier”, the nuclei must be on a collision course at very high speed. This means the fuel must be raised to extreme temperatures. Temperatures on the order of 100 million degrees are needed to fuse the lightest Figure T-2. Ignition and burn parameter space The National Ignition Facility (NIF) operated by the Lawrence Livermore National Heavy Ion Accelerator DriverAccelerator technology, under development for most of a century, has beenessential to mankind’s deepening understanding of nature. By the early 1970's, the storied development of “atom smashers” had achieved the ability to accelerate electrons, protons and heavy ions to very high energy – much higher than the energy per “particle” required in the accelerator drivers needed to initiate fusion. (See Figure T-3.) It is important for practical fusion power, as planned by FPC, that accelerator technology is a mature technology. Once the desired ion source is identified and the desired particle energy is specified, engineers can design and build a machine to supply the needed number of particles (ionized heavy ions in HIF) in the desired (very short) time window. In our case, this requires a linear accelerator that is several miles long. Linear accelerators are a known technology, similar to that used in hundreds of accelerator facilities around the world. The short stopping distance of heavy ions (i.e., comparable to dimension of the fuel pellet) allows each ion to be high-energy, many billions of “electron-Volts, or eV.” (Note the eV units on the vertical axis in Figure T-3. M is for mega or millions, G for giga or billions, etc.) Since Power = Volts x Amps, raising the particle energy translates to reducing the particle current (beam current). While the technology to attain high energy per particle was well established by 1975, the big discovery of that year was that rearrangement of the known accelerator hardware could provide the robust ignition beams needed to get into the high gain regime that is shown in Figure T-2. The same spirit of innovation and creativity that led to the steady advances shown in Figure T-3 would achieve the total beam current needed for the beam to have Power = Energy x Current ~ 1 PW (a petaWatt is one million billion Watts); this is the canonical instantaneous power requirement for igniting inertial confinement fusion. Heavy ion accelerator drivers have another special advantage for reliable ignition. This is the ability to achieve the two-step process called “fast ignition”. More efficient than using the same process that compresses the fuel to ignite a central “hot spot”, in fast ignition a separate, very sharp pulse (high peak-power and less than 1/10 the duration of the compression process) is used to ignite only the desired mass of fuel after it has been compressed. The “fast ignited” fuel sets off the rest of the fuel much like a blasting cap sets off a stick of dynamite. The great importance of this feature of FPC’s driver (also a feature of the Russian design) is that the required fuel compression has been within the state of the art for some years already, as noted previously. which has been crucial in achieving new discoveries by the steadily increased capabilities. The capabilities needed for HIF accelerator-drivers have been present since 1975 Completing the exciting dimensions of the discovery of HIF in 1975 were the Reaction ChambersThe space in which the fusion reactions take place is called a reaction chamber.Three factors influence its design. First, the chamber needs to hold a good vacuum to enable the heavy ions from the accelerator system to reach the fuel pellet and to provide a secure containment vessel for the capture of the tritium that is generated after the reaction takes place. Second, the chamber must be able to withstand the pressure generated by the fusion reaction. And third, the reaction chamber must contain a liquid that can be heated to a high temperature as part of the energy extraction process. All of this has been designed on paper, and unique hardware has been demonstrated, but an integrated fusion system has, of course, never been built. Reaction chambers for ICF experiments have been built at numerous installations worldwide, the largest of which is part of the National Ignition Facility at the Lawrence Livermore National Laboratory. There is a fourth factor that must also be considered in the design of the reaction chamber. As stated earlier, the neutrons produced by the fusion reaction carry 80% of the reaction’s energy. The energy must be captured as thermal energy, for downstream conversion to electricity and other energy products, and the neutrons must be prevented from degrading the structural properties of the chamber materials. FPC’s chamber concept accomplishes all the required missions, and much more. The numerous advantages of the chamber’s configuration include a unique combination of long chamber life and the high temperatures in working fluid that are needed for efficient energy conversion. Ultimately, the set of advantages results in very large economic benefits. (Also see chamber summary .) Figure T-4.1 StarPower Reaction Chamber ready for ignition A cutaway view of the interior of FPC’s fusion chamber is shown conceptually in Figure T-4.2 Microseconds after ignition Nanoseconds
after the fusion reactions take place, the lithium sphere has
vaporized and an expanding sphere of lithium plasma (ionized gas) is now
present. As this plasma expands, it cools by radiation (photon flow), expansion, and by interaction with lithium droplets and sprays throughout the chamber. (Figure T-4.2) Vaporization of the lithium droplets strongly damps the pressure of the expanding While the hot lithium vapor is condensing at the ends of the chamber, and before The vapor pressure of lithium at 100 oC above its melting point is very low, which Figure T-5 Schematic of the Heat Exchange Process, illustrating use for electricity generation Production of Synthetic FuelsWe generally think that fusion power plants will be sources of electricity. FPC'sStarPower System (SPS) will also produce electricity, but electricity will not be the only product, and may not be the primary product. The availability of very high temperature heat permits the consideration of another process of energy conversion and that is the direct production of hydrogen from the thermal disassociation of water. The known, and well researched, Sulfur–Iodine process and the high temperature electrolysis process both produce hydrogen at about 50% efficiency. Hydrogen is the key component in the production of synthetic fuels and can be produced from SPS systems at a cost comparable to the cost of production from reforming of natural gas. The hydrogen can be used directly as a fuel source or it can be combined with carbon to form various hydrocarbons including gasoline, kerosene (jet fuel), or diesel oil. Since these latter fuels support the existing transportation infrastructure, we intend to focus our hydrogen output on production of these synthetic fuel products. The source of carbon can be biomass, or coal, or the CO2 in the atmosphere. Evaluation of the technology for extracting CO2 from the atmosphere by the Los Alamos National Laboratory concluded that this technology is now mature enough that it is viable as a source of CO2. Since we have to dispose of waste heat in any case, why not use the waste heat to drive a cooling tower that processes a large volume of air and extracts the CO2 from it? Carbon from an atmospheric source is slightly more expensive than carbon from coal, but the use of atmospheric carbon means that the liquid fuel we produce would become carbon neutral – the carbon comes from the atmosphere and returns to the atmosphere again when the fuel is consumed. The processes for conversion of hydrogen and CO2 to form synfuels is a mature industrial process that has been used by Exxon-Mobil in the past and is currently being used by Sasol to produce liquid fuels in plants in South Africa and Arabia. Until now, the constraint for the production of synthetic fuels has been the availability of an inexpensive source of hydrogen. The heat from an SPS will remedy this shortcoming and make the synthesis of liquid fuel possible at a cost less than the current cost of the same fuel from a crude oil feedstock. Development of Direct Conversion TechnologyFusion power promises to be much more efficient than conventional electricalpower sources. A fission power plant converts about one third of the energy in the fuel to useful power sent to the customer as electricity. This is because the Carnot (ideal) efficiency is necessarily low since the energy conversion process for fission is constrained by a low ratio of the input and output temperatures. The ambient temperature of the environment outside the plant sets the output temperature, but the input temperature in fission plants is constrained by the limits imposed on the materials of the pressure vessel. The efficiency of energy conversion in other fusion systems encounters similar temperature constraints. Their chamber designs perform similar roles, which lead to similar temperature constraints on their materials, which need to operate slightly above the temperature of the steam that will drive the turbines. In contrast, FPC’s chamber system produces very high temperature heat at the same time that the chamber operates at uniquely low temperatures. For initial operations, we base our revenue estimates on the conservative use of conventional electricity generation using steam turbine systems. However, the high temperatures phases that the lithium passes through in each fusion power sequence can be exploited to raise the efficiency by implementing advanced energy conversion techniques. The earliest of the advanced techniques to be implemented will be high temperature gas turbines, as illustrated in Figure T-5. By transferring the heat at the highest temperature, near the condensation temperature of lithium, to a gas such as helium, the efficiency would reach the higher levels that are a well-known advantage of combined-cycle power plants fired by natural gas. Exciting prospects for further efficiency increases in the future involve the direct conversion that becomes a possibility because of the existence of a plasma state during the first phase of the lithium’s cycle. (Figure T-6) Because the temperature of this plasma will be above 20,000 oC, 50 percent or more of the heat can be converted “off the top” before the lithium temperature decreases (by further expansion and mixing with lithium rain) to be compatible with the steel tubes of heat exchangers. Considering the ideal efficiency of a thermodynamic process starting at 20,000 oC, overall efficiencies as high as 90 percent are seen to be possible. This underscores the fact that releasing fusion energy will not be the end of the story, but the beginning of a continuous improvement process, as has been the history with steam power, internal combustion, and even batteries. Most remarkably, these prospects for developing extraordinarily high efficiencies, which depend on having extraordinarily high temperatures, are made possible by the design that keeps the temperature of the structural materials of FPC’s chamber at extraordinarily low levels. Materials and EngineeringHundreds of accelerators exist in the world. All are designed using ordinarymaterials that are readily available. The materials needed for our Heavy Ion Fusion accelerator will be no different than for these other machines. The accelerator will be longer than Stanford’s original SLAC accelerator (built in the1960s) or Germany’s UNILAC (built in the 1970s), but it will continue to use the latest versions of the same technology. The precision tunneling employed in the construction of large accelerators will be applied to the design and construction of the first SPS, as well as all of its successors. The ion source system is a unique element in the FPC design. It will use heavy isotopes rather than electrons or protons as used in most accelerators. But this technology is also mature. Heavy ion accelerators operate at numerous laboratories in the US, Europe, and Asia. Our system will be different in that we will use a multiplicity of single-isotope ion sources. This requires a supply of the specific isotopes of the elements we plan to use – xenon, tin, lead, and mercury – elements chosen first to achieve the requirements for fusion ignition and second for their abundance of isotopes within the mass range that we want to use. Most likely, preparation of these single isotope supplies will be special for our project. The materials and technologies used to construct the reaction chambers are common to many high-stress, high-temperature applications, such as in refineries, metal processing industries, and rocket engines. Because our chamber operates at temperatures lower than any other power plant based on heat, and because the hazards of radiation are very much lower (in kind and amount) than in fission power plants, we anticipate some relaxation of the expensive “N-stamp” required for fission systems. We expect to benefit, however, from the excellent steels and processes that have been developed for fission systems, including those for liquid metal heat exchangers. No special or 'magic' materials are needed. FPC's chamber does not face the problem posed by fusion’s high-energy neutrons for most of the other fusion chamber concepts. The protection of the system from the fusion neutrons is based on our unique use of the lithium needed to generate the tritium fuel. Our system for handling lithium benefits from experience that dates from early in the space program and from liquid metal fission systems, as well as from fusion energy development. Lithium was considered a rare and hazardous metal until recently. The use of lithium in batteries for electronic equipments, and even electric automobiles, has changed much of that. Safe handling and packaging processes have been developed and the metal is now in many household goods, from cameras to computers. Its supply has expanded as well and it is now produced in quantities needed to supply the start-up of several fusion power plants per year. For long-term operations, and for the build out of the fusion energy system worldwide, we will extract lithium from seawater. Modern engineering design is highly dependent on computational technology in roles from computer assisted drafting to detailed component modeling and manufacture. This project is no exception. We will design and model all aspects of the system in a computer prior to construction. Besides assuring that all components fit together, this will build the end-to-end model to simulate operation of the system and define flow of data for precise and safe system control. Copyright (C) 2011. All Rights Reserved |