The ARIES program in the USA is the leading group in designing fusion reactors. Originally started by Robert W. Conn in the 1980s at the University of Wisconsin and the University of California (UC) Los Angeles, it is now headed by Farrokh Najmabadi at UC San Diego. Throughout the years, new ARIES designs have been made as new physics has been discovered. The designs are not only for tokamaks; stellarators and laser-fusion reactors have also been covered. The latest designs, ARIES-AT for advanced tokamaks and ARIES-ST for spherical tokamaks, inspired the FDF proposals described above. Practical considerations such as public accep­tance, reliability as a power source, and economic competitiveness pervade the studies. The designs are very detailed. They optimize the physics parameters, such as the shape of the plasma and the neutron wall loading. They also optimize the engineering details, such as how to replace blankets and how to join conductors to make the joints more radiation resistant. As new physics and new technology became available, the reactors ARIES I, II, … to ARIES-RS (reversed shear) and

ARIES-AT (advanced tokamak) evolved to become smaller and cheaper. This is shown in Fig. 9.35. We see that as fusion physics advanced from left to right in each group of bars, the size of the tokamak, the magnetic field, and the current — drive power could be decreased while increasing the neutron production. This is due to the great increase in plasma beta that the designers thought would be possible. The recirculating power fraction is the power used to run the power plant; the rest can be sold. It dropped from 29 to 14%. The thermal efficiency in the latest design breaks the 40% Carnot-cycle barrier by the use of a Brayton cycle. Finally, we see that the COE is expected to be halved from 100 to 50 per kWh with advanced tokamaks.

ARIES-AT is shown in Fig. 9.36. Unlike existing tokamaks, this reactor design has space at the center for remote maintenance and replacement of parts. The philosophy in reactor design is to assume that the physics and technology advancements that are in sight will actually be developed and, on that basis, optimize a reactor that will be acceptable to industry and the public. It is not known whether high-temperature superconductors will be available on a large scale, but this would simplify the reactor. The blankets will be of the DCLL variety, and it is predicted that the Pb-Li can reach 1,100°C without heating the SiC walls above 1,000°C. This high temperature is the key to the high thermal efficiency. For easier maintenance and better availability, the blankets are made in three layers, two of which will last the life

of the reactor. Only the first layer, along with the divertor, has to be changed out every five years. Sectors are removed horizontally and transported by rail in a hot corridor to a hot cell for processing. Shutdowns are estimated to take four weeks.

Turbocharging and supercharging in automobiles are terms that are well known to the public. Airplanes engines are turbocharged. Modern power plants use thermody­namic cycles that have higher efficiency than the classic Carnot cycle. The ARIES-AT reactor will use one of these called a Brayton cycle. The hot helium from the tokamak blanket is passed through a heat exchanger to heat helium that goes to electricity­generating turbines. The two helium loops are isolated from each other because the tokamak helium can contain contaminants like tritium. The turbine also runs with cooler helium at a different flow rate. The Brayton cycle precompresses the helium three times before it goes into helium turbines. The heat of the helium coming out of the turbines is recovered in coolers that cool the helium before it is compressed. It is this system that achieves the 59% thermal efficiency of the ARIES-AT design.

ARIES-AT will produce 1,755 MW of fusion power, 1,897 MW of thermal power, and 1,136 MW of electricity. The radioactive waste generated will be only 30 m3 per year or 1,270 m3 after 50 years. The plant will run for 40 of those years if availability is 80%. Ninety percent of this waste is of low-grade radioactivity; the rest needs to be stored for only 100 years. No provisions for public evacuation are necessary, and workers are not exposed to risks higher than in other power plants. The COE from ARIES-AT is compared with other sources in Fig. 9.37. We see that electricity from fusion is not expected to be extravagant.

Europeans have also made reactor models in their Power Plant Conceptual Studies (PPCS) [26]. Figure 9.38 is a diagram of the tokamak in those designs. As with the ARIES studies, Models A, B, C, and D in PPCS (Fig. 9.39) trace the evolu­tion of the design with advances in fusion physics and technology, with Model D using the most speculative assumptions. All these models produce about 1.5 GW of

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Natural gas Coal Nuclear Wind Fusion

(intermittent) (ARIES-AT)

Fig. 9.37 Estimated year 2020 cost of electricity in US cents per kilowatt-hour from different power sources [graph adapted from [25], but original data are from the Snowmass Energy Working group and the US Energy Information Agency (yellow ellipses)]. The red range is the cost if a $100/ton carbon tax is imposed. The fusion range is for different size reactors; larger ones have lower cost

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coolant manifolds (d i

8 upper ports (f)

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8 central ports (g)

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Fig. 9.38 Drawing of tokamak in Power Plant Conceptual Studies in Europe [26] electricity, but they are smaller and use less power with gains in knowledge. The recirculating fraction and thermal efficiency of Model D matches that of ARIES-AT. Safety and environmental issues were carefully considered. The cost estimates are given in Fig. 9.40, also in US cents per kWh. The difference between the wholesale price of electricity and that available to consumers is clearly shown. It is seen that fusion compares favorably with the most economical sources, wind and hydro.

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R (m) Fusion Bootstrap Wall load Current (MA) Recirc. Frac. Therm. Effic. power fraction (MW/m2)