conditions from an initial thermal fuel loading of 4410 W. The RTG mass is about 56 kg (Fig. 8). Each converter is 114.0 cm long and 42.2 cm in diameter. It contains 572 silicon-germanium thermoelectric couples (unicouples) surrounded by multi-layers of insulation to reduce heat loss. Each unicouple assembly is attached to an aluminum outer case by sealing screws inserted through the case wall. The heat source supplies the thermal energy to the thermoelectric converter (Fig. 9). The heat source is a set of rectangular modules, each 9.4 cm X 9.65 cm X 5.3 cm overall, weighing about 1.45 kg, with a thermal output of about 233 W at launch. Each module contains four plutonium dioxide fuel pellets, encapsulated in iridium. Pairs of these fuel units are encased in a graphite impact shell, which is enclosed in an insulation system of ‘carbon-bonded carbon-fiber' graphite, and finally in a graphite reentry aeroshell made out of a carbon-impregnated fine-weave pierced fabric. Eighteen of these modules provide the heat for each of the two generators on Galileo and one on Ulysses. Figure 10 indicates the interrelationship of performance and safety parameters which are involved in the design process [2]. RTG Component Characteristics In selecting a radioisotope for use in space applications, DOE conducted a screening process of potentially usable radioisotopes. The criteria included specific power, halflife, availability, gamma radiation, radiation hazard and chemical form. Plutonium-238 dioxide was selected because it has the following characteristics:
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