The nuclear SPS is launched with its payload inside the radiator. Once in orbit, the payload is moved away from the SPS at the end of a deployable boom. The reactor is operated at constant power, and the BRU at constant speed, the unused energy being discharged to a resistive load. This is intended to ensure a high reliability by minimizing the number of control actuators and temperature and pressure cyclings. All the features result in a typical system layout illustrated in Fig. 3. It should be noted that, for a first comparison, the conversion systems are equipped with a single BRU and with an armored tube and fin gas-cooled radiator, and that the extension of the comparison to systems using dual BRU’s and heat pipes radiators is already under way as a consequence of preliminary reliability, design and operational behaviour assessments. Specific Design Features The systems compared differ mainly in the technologies considered for their nuclear reactor, which are: The LMFBR derivative or UO2/Na/SS-650°C system is powered by a sodium (or sodium-potassium) cooled fast spectrum reactor, with UO2 fuel rods and 316 stainless steel structures operated at a peak temperature of 923 K (650°C). This system has the significant advantage of benefitting from the technologies widely developed in Europe (especially in France) for the terrestrial liquid metal fast breeder reactors (Superphenix 1500 MWe power station). This system however suffers a priori from its moderate heat source temperature
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