2020 Fusion Version 18 Crackedl
Leigha Keplinger
Tritium fuel cannot be fully replenished. The deuterium-tritium reaction is favored by fusion developers because its reactivity is 20 times higher than a deuterium-deuterium fueled reaction, and the former reaction is strongest at one-third the temperature required for deuterium-only fusion. In fact, an approximately equal mixture of deuterium and tritium may be the only feasible fusion fuel for the foreseeable future. While deuterium is readily available in ordinary water, tritium scarcely exists in nature, because this isotope is radioactive with a half-life of only 12.3 years. The main source of tritium is fission nuclear reactors.
If adopted, deuterium-tritium based fusion would be the only source of electrical power that does not exploit a naturally occurring fuel or convert a natural energy supply such as solar radiation, wind, falling water, or geothermal. Uniquely, the tritium component of fusion fuel must be generated in the fusion reactor itself.
But there is a major difficulty: The lithium blanket can only partly surround the reactor, because of the gaps required for vacuum pumping, beam and fuel injection in magnetic confinement fusion reactors, and for driver beams and removal of target debris in inertial confinement reactors. Nevertheless, the most comprehensive analyses indicate that there can be up to a 15 percent surplus in regenerating tritium. But in practice, any surplus will be needed to accommodate the incomplete extraction and processing of the tritium bred in the blanket.
The second category of parasitic drain is the power needed to control the fusion plasma in magnetic confinement fusion systems (and to ignite fuel capsules in pulsed inertial confinement fusion systems). Magnetic confinement fusion plasmas require injection of significant power in atomic beams or electromagnetic energy to stabilize the fusion burn, while additional power is consumed by magnetic coils helping to control location and stability of the reacting plasma. The total electric power drain for this purpose amounts to at least six percent of the fusion power generated, and the electric power required to pump the blanket coolant is typically two percent of fusion power. The gross electric power output can be 40 percent of the fusion power, so the circulating power amounts to about 20 percent of the electric power output.
In inertial confinement fusion and hybrid inertial/magnetic confinement fusion reactors, after each fusion pulse, electric current must charge energy storage systems such as capacitor banks that power the laser or ion beams or imploding liners. The demands on circulating power are at least comparable with those for magnetic confinement fusion.
Radiation damage and radioactive waste. To produce usable heat, the neutron streams carrying 80 percent of the energy from deuterium-tritium fusion must be decelerated and cooled by the reactor structure, its surrounding lithium-containing blanket, and the coolant. The neutron radiation damage in the solid vessel wall is expected to be worse than in fission reactors because of the higher neutron energies. Fusion neutrons knock atoms out of their usual lattice positions, causing swelling and fracturing of the structure. Also, neutron-induced reactions generate large amounts of interstitial helium and hydrogen, forming gas pockets that lead to additional swelling, embrittlement, and fatigue. These phenomena put the integrity of the reaction vessel in peril.
The problem of neutron-degraded structures may be alleviated in fusion reactor concepts where the fusion fuel capsule is enclosed in a one-meter thick liquid lithium sphere or cylinder. But the fuel assemblies themselves will be transformed into tons of radioactive waste to be removed annually from each reactor. Molten lithium also presents a fire and explosion hazard, introducing a drawback common to liquid-metal cooled fission reactors.
Materials scientists are attempting to develop low-activation structural alloys that would allow discarded reactor materials to qualify as low-level radioactive waste that could be disposed of by shallow land burial. Even if such alloys do become available on a commercial scale, very few municipalities or counties are likely to accept landfills for low-level radioactive waste. There are only one or two repositories for such waste in every nation, which means that radioactive waste from fusion reactors would have to be transported across the country at great expense and safeguarded from diversion.
Nuclear weapons proliferation. The open or clandestine production of plutonium 239 is possible in a fusion reactor simply by placing natural or depleted uranium oxide at any location where neutrons of any energy are flying about. The ocean of slowing-down neutrons that results from scattering of the streaming fusion neutrons on the reaction vessel permeates every nook and cranny of the reactor interior, including appendages to the reaction vessel. Slower neutrons will be readily soaked up by uranium 238, whose cross section for neutron absorption increases with decreasing neutron energy.
Additional disadvantages shared with fission reactors. Tritium will be dispersed on the surfaces of the reaction vessel, particle injectors, pumping ducts, and other appendages. Corrosion in the heat exchange system, or a breach in the reactor vacuum ducts could result in the release of radioactive tritium into the atmosphere or local water resources. Tritium exchanges with hydrogen to produce tritiated water, which is biologically hazardous. Most fission reactors contain trivial amounts of tritium (less than 1 gram) compared with the kilograms in putative fusion reactors. But the release of even tiny amounts of radioactive tritium from fission reactors into groundwater causes public consternation.
In addition, there are the problems of coolant demands and poor water efficiency. A fusion reactor is a thermal power plant that would place immense demands on water resources for the secondary cooling loop that generates steam, as well as for removing heat from other reactor subsystems such as cryogenic refrigerators and pumps. Worse, the several hundred megawatts or more of thermal power that must be generated solely to satisfy the two classes of parasitic electric power drain places additional demand on water resources for cooling that is not faced by any other type of thermoelectric power plant. In fact, a fusion reactor would have the lowest water efficiency of any type of thermal power plant, whether fossil or nuclear. With drought conditions intensifying in sundry regions of the world, many countries could not physically sustain large fusion reactors.
Numerous alternative coolants for the primary heat-removal loop have been studied for both fission and fusion reactors, and one-meter thick liquid lithium walls may be essential for inertial confinement fusion systems to withstand the impulse loading. However, water has been used almost exclusively in commercial fission reactors for the last 60 years, including all of those presently under construction worldwide. This circumstance indicates that implementing any substitute for water coolant such as helium or liquid metal will be impractical in magnetic confinement fusion systems.
Another intractable operating expense is the 75-to-100 megawatts of parasitic electric power consumed continuously by on-site supporting facilities that must be purchased from the regional grid when the fusion source is not operating.
Multiple recurring expenses include the replacement of radiation-damaged and plasma-eroded components in magnetic confinement fusion, and the fabrication of millions of fuel capsules for each inertial confinement fusion reactor annually. And any type of nuclear plant must allocate funding for end-of-life decommissioning as well as the periodic disposal of radioactive wastes.
It is inconceivable that the total operating costs of a fusion reactor would be less than that of a fission reactor, and therefore the capital cost of a viable fusion reactor must be close to zero (or heavily subsidized) in places where the operating costs alone of fission reactors are not competitive with the cost of electricity produced by non-nuclear power, and have resulted in the shutdown of nuclear power plants.
The point of ITER and the other fusion reactors is the POTENTIAL for clean energy. The problems you raise, (to a large extent) can be mitigated. Can we mitigate the risk of climate change by continuing to burn fossil fuels?
No. Then it is imperative we keep looking.
Great read. Was wondering however, what about aneutronic fusion using only helium 3? Should produce no where near the neutron problem and electricity is produced from the alpha particles generated instead of heating water. Yes, I understand we would have to mine helium 3 from the moon and gas giants or possibly use a mix of helium 3 and deuterium. Anyway, would love to read your thoughts on that.
Great piece, Dr. Jassby. Objective and clearly you have some experience in the field (understatement). How about using Boron instead of Tritium, as some of the startups in the field are doing?
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Any update to this article in the context of HB11 laser confined & ignited fusion proponents? The dearth of radioactive and neutron byproducts appears resolve the bulk of concerns though plant efficiency is a pretty fundamental feature and metric.
The National Ignition Facility (NIF) was often presented to the public as a path to fusion power production but almost all fusion scientists admit that it is not a viable path. It has always been primarily funded as a thermonuclear weapons research tool. After many delays and over-cost funding it never achieved its primary goal of achieving a break-even fusion reaction. The designers admit that it likely never will. Its continued funding is propelled by the inertia of the billions of USD already sunk into the project.
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