B24 Mass budget

In the following we consider contributions to mass due to various components of the fusion reactor.

B.2.4.1 Radiator

Waste power is produced by the neutron power deposited in the blanket, by radiation, and by auxiliary systems. Energy is radiated to space following the Stefan-Boltzmann law

where e is the radiator emissivity; a is the Stefan-Boltzmann constant; rR is the radiator temperature; and Srad is the radiator surface. It is apparent from the above expression that, at fixed Prad, the radiator surface decreases as the radiator temperature increases. As shown in [Roth, 1989], the radiator temperature that minimizes the radiator mass in the limiting case of an ideal Carnot efficiency (v = l - rR/rH)

corresponds to 3/4 of the temperature TH in the blanket/exhaust system. This estimate yields low values of the conversion efficiency (^ = 25%). Present structural material limits do not allow going beyond TH « 300°C. The use of advanced materials (e.g., SiC/SiC) could achieve TH « 1,000°C. If prad is the mass per unit surface (measured in kilograms per square meter) of the radiator, the radiator mass Mrad is linked to fusion power by the following expression obtained by combining Equations (B.9) and (B.11) and using Carnot cycle efficiency

Equation (B.12) determines the specific power associated with the radiator. A "reasonable" value is 5 kW of rejected power for each kilogram of radiator mass, corresponding to prad = 1.5kg/m2 and a radiating temperature of 600 K. These numbers tend to be on the conservative side, as modern heat exchangers can be built that have specific weights of order 0.01 kg/kW to 0.15kg/kW.

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