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ENVIRONMENTAL ISSUES ASSOCIATED WITH PURE D-T FUSION In assessing the environmental implications of D-T fusion power, we shall consider the following topics: radioactivity in effluents under normal operating conditions; reactor safety and waste disposal; thermal effects; and resource requirements. Further remarks on alternative fuel cycles are also included. ROUTINE RELEASES Under normal operating conditions, the primary radiological concern appears to be the escape of tritium to the environment. Current esti- mates suggest that a l000-MWe fusion power plant would have a tritium inventory of about 3 to 30 kg, or about 30 million to 300 million curies. Because tritium permeates most metals at high temperatures, it can diffuse through containment walls, fluid piping, and heat exchanger tubing used for the blanket and its related systems. There are two primary paths by which tritium might eventually escape to the environment during normal operating conditions: through the blanket containment walls and fluid piping into the surrounding atmosphere, or through the coolant system into the steam cycle via the coolant-system heat-exchanger tube walls. There are, as yet, no generally applicable standards concerning tritium release to the environment; nor can we say what the future limita- tions on tritium releases from fusion reactors should be. However, if light water fission reactor guidelines are applied to fusion reactors, the tritium leakage rate would have to be limited to a level of ~ l0 curries per day. The attainment of such a leakage rate seems technolog- ically feasible; however, a major objective of fusion reactor technology will have to be the demonstration of adequate tritium containment at acceptable costs. REACTOR SAFETY AND WASTE DISPOSAL The magnitude and characteristics of the radioactive inventories induced by neutron interactions in the structural material of the blanket are major considerations in assessing D-T fusion reactor safety and radio- active waste disposal. The magnitude of induced structural and blanket 24
25 radioactivity in a D-T fusion reactor is dependent upon the choice of material. For the wide range of materials and blanket designs being considered, the level of induced activity at equilibrium is generally in the range of about l billion to 10 billion curies for a l000-MWe plant, clearly a significant level. To reduce the level of induced radioactivity, materials R&D programs on low-activation materials such as graphite, silicon, carbide, high-purity aluminum, and titanium are being conducted by the Electric Power Research Institute (EPRI). How- ever, the level of activity by itself is not a meaningful measure of the technological problems posed by radioactive inventories; the associated nuclear afterheat, the time-dependent behavior of the acti- vation products, and mechanisms for their release ?!?o need exmination. At shutdown, the afterheat power density in the fuel of advanced fission reactors is anticipated to be at least one to two orders of magnitude greater than that expected in the structural components of fusion reactor blankets, even without the use of low-activation mate- rials. For example, liquid metal fast breeder reactor (LMFBR) fuel would have an afterheat power density of ~ l00 watts per cubic centi- meter while the first wall of a fusion reactor would have an afterheat power density no greater than ~ l watt per cubic centimeter. The con- clusion is that afterheat removal will be less of a problem in fusion reactors than in fission reactors. Moreover, it appears that the engineered safety features necessary to limit biological impact in the event of an accident may have to satisfy less strinapnt requirements in fusion reactor design than in fission reactor design. This observation does not mean that fusion reactors will necessarily be safer than fission reactors, but rather that the technology and engineering neces- sary to achieve a given level of safety may prove less difficult and costly for fusion reactors. A number of materials are candiates for fusion reactor structure. These include refractory (high-temperature) alloys based on niobium, molybdenum and vanadium, as well as conventional iron- and nickel-base alloys. Calculations suggest that, for most engineering alloys, long- term solutions to waste disposal similar to those sought for radioactive wastes from fission reactors may be required. On the other hand, the use of materials such as vanadium-base alloys might allow recycling of the blanket structure following a relatively short cooling period (of the order of l0 years). Should the low-activation materials mentioned previously prove practical, the decay period would be reduced even further. Within this context, it is possible that a fusion-power economy might not require the long-term radioactive waste disposal associated with fission power. This might represent a major societal advantage of fusion power relative to fission power. However, the mechanical behavior of vanadium alloys in a fusion reactor environment is unknown and there exists no industry (mining or fabrication) capable of producing large amounts of vanadium at present.
26 THERMAL EFFECTS The D-T fusion reaction releases energy in two formsâneutron and charged- particle energy. The neutron energy, which eventually is manifested as heat within the blanket, would be recovered by a thermal energy-conversion system. The charged-particle energy could also be recovered as heat by a thermal energy-conversion system. However, direct recovery of this portion of the energy may also be possible. Because only about 20 per- cent of the energy released in D-T fusion appears as charged-particle energy, the impact of such direct energy conversion is marginal in a D-T fusion power economy. For example, consider a case in which a thermal energy-conversion system of 40 percent, efficiency is used for recovery of the neutron energy and a direct energy-conversion system of 70 percent efficiency is used for recovery of the charged-particle energy. The overall recovery efficiency for such a system would be 46 percent, about l5 percent higher than that for the thermal conversion system itself. However, fusion reactors inherently require input power to establish the fuel conditions necessary for fusion power production. Therefore, a frac- tion of the gross electrical output of the plant must be recirculated to sustain the fusion process. The amount of recirculating power required in fusion power plants can be appreciable. Thus, even when direct energy conversion is assumed for a portion of the fusion energy release, the overall plant efficiency (that is, the ratio of the net electrical power output to nuclear power release) of current D-T fusion reactor concepts is comparable to the overall plant efficiencies of fossil and fission power plants (i.e., in the vicinity of 30 to 40 percent), and there are corresponding thermal effluents as well. RESOURCE REQUIREMENTS The use of deuterium and lithium for fusion power should result in less stringent environmental impacts and economic constraints than those associated with the fuel requirements in fission power plants. On the other hand, the resource requirements for building the fusion reactor plant seem to be considerably more extensive. This is because estimated nuclear power densities in fusion reactor blankets, based on present understanding of permissible first-wall loading (2MW/square meter), are inherently lower (by one to two orders of magnitude) than those in fis- sion reactor cores, which means that, for a given power generation level, a fusion reactor blanket will required significantly more structural material. For example, the stainless steel required for the nuclear island of the UWMAK-II Tokamak fusion reactor design, is about 5 to l0 times greater than that required for the nuclear island of a comparable liquid metal fast fission breeder reactor. By a factor of 2, UWMAK-II improvements attained might be in the B and in the first-wall perfor- mance. Although there do not appear to be any resource limitations to prevent fusion power development, the environmental and economic implications of the resource requirements must be examined carefully. For example, use of materials such as niobium, chromium, nickel, manga- nese and helium presents unique resource requirements for fusion power.
27 The nature of the problem could be reduced substantially if low-activation, recyclable material could be developed. The principal use of helium in a fusion reactor will be in the cooling of superconducting magnets. On the basis of current reactor studies, the helium requirements for magnet cooling in fusion reactors would be ~ 0.05 metric tonnes per MWe. If helium were also used as a blanket coolant, the helium requirement would be increased by ~ 0.0l metric tonnes per MWe. Even if the present-value price of helium were to increase l00- fold relative to current helium prices (i.e., if we were forced to recover helium from the atmosphere), the .associated increase in the capital cost of fusion power would only be on the order of a few percent. Therefore, the long-term implications of our current helium policies do not suggest any serious obstacles to implementating fusion power. ALTERNATIVE FUSION FUELS Several environmental drawbacks are commonly attributed to D-T fusion power. First, it produces substantial amounts of neutrons that result in induced radioactivity within the reactor structure, and it requires the handling of the radioisotope tritium. Second, only about 20 percent of the fusion energy yield appears in the form of charged particles, limiting the extent to which direct energy conversion techniques might be applied. Finally, the use of D-T fusion power is limited by lithium resources, which are less abundant than deuterium resources. As men- tioned earlier, this limitation is on the time scale of l000 years. These drawbacks of D-T fusion fuel cycle have led to a number of alternative proposals, fusion-power reactors based only on deuterium being one possibility, the D- He fuel cycle being another. The goals of such proposals are to (l) reduce the magnitude of neutron production, as well as the need to handle large amounts of tritium, (2) produce more fusion power in the form of charged particles, and (3) reduce the system's dependence on lithium resources. It has also been suggested that materials with higher atomic numbers (such as lithium, beryllium, and boron)* be used as fusion fuels to provide power that is essentially free of neutrons and tritium and that releases all of the fusion energy in the form of charged particles. Many of the materials problems and reactor maintenance problems discussed earlier might be reduced or eliminated through the use of such fuel cycles. Although such alterna- tives to D-T fusion power appear very attractive, there are at least two important caveats: l. Of all the fusion fuels under current consideration, the deuterium- tritium fuel mixture requires the lowest value of nx by at least one order of magnitude and the lowest fusion temperatures by at least a *An often-cited reaction is: p + 1*B = 3 ⢠uHe +8.7 MeV, which has a peak cross-section of approximately 0.9 barns at 675 keV, and is to be compared with the D-T peak cross-section of about 5 barns at l00 keV.
28 factor of .5. When the plasma requirements for significant power genera- tion are compared with the anticipated plasma performance of current approaches to fusion power, it is apparent that fusion power must initially be based on a deuterium-tritium fuel economy. 2. The maximum attainable fusion power density is about one to two orders of magnitude greater with the D-T fuel mixture than with any of the alternative fuel mixtures. Thus, even if adequate confinement and heating could be achieved for implementing alternative fuels, such sys- tems would face an economic disadvantage relative to the D-T system on the basis of power density. Nonetheless, we would like to hold out the hope that at some future date technology may advance to the point where the use of such ideal fusion fuels will become practical.
