Potential of Fusion

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The attractiveness of fusion is determined by its potential in the areas of fuel resources, cost, safety, environment, plant size, and products. In the competitive energy marketplace envisioned for the twenty-first century, fusion must be attractive in every area to succeed (Perkins, 1998). These areas are discussed below.

Fuel resources of deuterium are effectively unlimited. Water, by weight, contains 75 times as much energy as gasoline for the deuterium-deuterium reaction. (This corresponds to 10 billion years of primary energy at 1990 "burn rate" of 11 TW (1012 W), which is virtually unlimited.) For deuterium-tritium, assuming that half of the 17.6 MeV of the reaction is due to deuterium, water, by weight, contains 330 times as much fusion energy as the chemical energy in gasoline. Lithium, the source of tritium for first-generation fusion power plants, is significantly more abundant in the earth's crust than are the fission fuels, uranium or thorium. It is also about 50 times more abundant than uranium in seawater. The energy resources for several energy candidates are listed in Table 8.2. More detailed data are available in Perkins (1998).

Arthur W. Molvik and John L. Perkins Table 8.2 Lifetime of candidate energy resources

Candidate

Lifetime (years)

Breeder fission (238U + 232Th) Solar electric Natural gas (Coal)

1000s ^ million's* Unlimited 50 ^ 1000s* (200 - 500)

10000s ^ 100 million* Unlimited

Notes:

* Additional occurrences which are believed to exist but for which extraction technology does not yet exist and/or may not be economically viable, e.g. U, Li in seawater, gas-hydrates in clathrates. Courtesy of the University of California, Lawrence Livermore National Laboratory, and the Department of Energy under whose auspices the work was performed.

Costs of fusion power are estimated to be in the $0.04-0.10/kWh for 1000 MWe size plants (Delene, 1994; Delene et al., 2000; Krakowski, 1995; Najmabadi, 1998). Doubling the plant size to 2000 MWe can reduce the COE (cost-of-electricity) further by about 25% due to economy-of-scale (Logan et al., 1995). The lower range is competitive with other advanced forms of energy, with the exception of today's natural gas without carbon sequestration. However, at present, the lower cost range is populated only by more innovative concepts that involve more unknowns than does the conventionally-engineered tokamak concept which is predicted to occupy the higher cost range. An increasing number of fusion researchers are identifying cost as a go no-go issue (Delene, 1994; Delene et al., 2000; Krakowski, 1995). They are achieving some success in reducing the costs with innovative concepts, by simplifying old concepts or devising cheaper manufacturing techniques to reduce capital costs, and by introducing concepts that reduce the down time for periodic maintenance - for example, by using a flowing liquid that is continuously replaced for the tritium-generating lithium-containing blankets rather than needing to shut down a power plant to replace solid blanket modules (Moir, 1994, 1996,1997). As development of fusion energy continues, the uncertainties that limit our confidence in present cost estimates will be reduced.

Most fusion power plant concepts are passively safe. If we show that they meet the "no-evacuation in case of an accident" requirements, as expected, then we could site them in highly populated areas and tap the market for low-

grade heat for space-heating applications. Since the amount of fuel within a fusion power plant will not sustain operation for more than a few seconds at most, there is minimal stored nuclear energy available to trigger severe accidents.

The strong safety characteristics can be maintained by care in design. For example, all other sources of stored energy (chemical, magnetic, thermal) must be contained so that they cannot drive an accident; and materials need to be chosen that minimize activation of the power plant. It is particularly important to minimize the volatile hazardous materials that could be released in an accident. The materials forming the containment vessels must themselves be shielded from radiation to prevent degradation of their properties. With careful and thorough design and construction, radioactive releases are not expected to be a problem. While first generation power plants using deuterium-tritium must contain the tritium, the reaction products are not radioactive. Low activation also minimizes after-heat issues and the need for emergency cooling systems. These issues address local safety in the vicinity of a power plant.

Fusion is also attractive on the global safety issue: nuclear weapons proliferation - which can be minimized by ensuring that fissionable materials are absent. If, however, the world is using a significant amount of fission energy, then a fission-fusion hybrid power plant - consisting of a fissionable blanket surrounding a fusion core - could be a highly effective fission breeder (Maniscalco et al. ,1981). Each hybrid power plant could supply fuel for several non-breeder fission power plants. Preventing nuclear weapons proliferation would then be more difficult than in a pure fusion scenario, but similar to the safeguards needed for fission. Fusion, as well as fission, power plants should be operated under international supervision to prevent proliferation.

Fusion is environmentally attractive due to no carbon dioxide or other greenhouse gas emission, small land use compared with renewables, and non-radioactive reaction products. The major issue to be addressed is the activation level, volume, and lifetime of the power plant chamber components under bombardment by 14 MeV primary neutrons from deuterium-tritium fusion, plus the scattered neutrons that are degraded in energy. The use of thick (0.5 to 1 m) liquid walls or jets, composed of low-atomic-number materials, is one method that reduces activation to low levels and eliminates the damage suffered by solid materials from atomic displacements under neutron bombardment. Liquid walls also promise significant reductions in the volume of activated waste due to operation at higher power density, which allows smaller wall area (Moir, 1997). A second method, recycling of waste, promises further reduction in waste volumes. A third method, development work on long-lived and low-activation solid materials, is also important for two reasons. First, some solid materials are exposed to neutron bombardment either because the materials penetrate the liquid walls or because they are visible through gaps in the liquid walls. Second, thick liquid walls may not be compatible with all fusion concepts. For this reason it is important to develop the facilities to irradiate materials with a fusion spectrum of neutrons (a continuous distribution of neutron energies up to, but not exceeding, 14 MeV). Such facilities are needed to measure the damage and activation of candidate materials, in order to develop long-lifetime low-activation materials and concepts (see, e.g. Perkins et al., 2000).

The industrial ecology concept of recycling materials and components, rather than disposing of them (Frosch, 1992), is beginning to be applied to fusion power plants. Extensive recycling requires the use of relatively low-activation materials; "low-activation" in this context meaning that remote-handling and accident containment and cleanup are feasible during reprocessing; or better yet, that activation is so low that hands-on manipulations are feasible during reprocessing. It is not necessary to separate all activated materials during reprocessing; for example, replacement components for use in radioactive environments could use activated materials. Wherever possible, highly activated materials with inconveniently long half-lives would be separated for specialized uses or disposal. Recycling has the potential of diverting and re-using much of the waste stream, greatly reducing the amount of waste that requires disposal.

The high power density of fusion requires little land for the plant, compared with the requirements for the various forms of renewable energy. The space occupied can be made even smaller if we bury the larger elements deep enough for roads and agriculture to exist above it. This has been done at major high-energy physics accelerators, such as the Stanford Linear Accelerator (SLAC) near Palo Alto, California, and the European Laboratory for Particle Physics (CERN) accelerator complexes near Geneva, Switzerland. Such dual use of land will become increasingly important as population growth results in an increased need to use all the available land for agriculture and minimize that devoted to energy generation.

Electricity and heat are the primary products of fusion power plants. The heat can have direct value in industrial processes, or space heating for cities -if safety constraints can be met as we think is possible. Locating fusion power plants near the largest electricity and heat consumers will reduce costs by minimizing electrical transmission lines in addition to increasing the income through the sale of heat. Other products are also possible. The power can be applied to generating hydrogen or other synthetic fuels for efficient long distance power transmission through pipelines and for portable needs such as transportation, but competing in this area requires very low COE (Logan et al., 1995). Like fission, fusion is appropriate for recharging hydropower or other energy storage during off-peak demand periods. Radioisotope production and transmutation of fission waste would take advantage of the intense flux of neutrons available, but would also raise issues that need to be addressed: nuclear proliferation and possible additional activation of the fusion plant.

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