The Brayton Rotating Units (BRU) are of the single shaft on gas bearings single stage radial compressor radial turbine RICE alternator type, and are now in a cylindrical vessel pressurized with the cycle gas at compressor inlet conditions. The radiator, of the armored pressure tubes and beryllium fins gas cooled type, is designed as an axisymetrical self-supporting structure which also transfers the SPS load to the launcher nose via the equipment room and an adaptation piece. Its allocated reliability is 0.975, and its gas tubes are armored accordingly. 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 system of typical layout illustrated in Fig. 2. It should be noted that, for a first comparison, the conversion systems are equipped with a single BRU and with an armored tubes and fins gas cooled radiator, but that however, the extension of the comparison to systems using dual BRU's and heat pipe radiators is already under way, as a consequence of preliminary reliability, design and operational behaviour assessments. 3.3 Specific Design Features The systems compared mainly differ in the technologies considered for their nuclear source, 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 923K (650°C). This system is considered for the significant advantage of benefiting 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 a priori suffers from its moderate heat source temperature which leads to unfavorable mass performance due to the need for a radiator of large area (100 m2). Moreover, unusual required lifetime (7 years) and temperature (650°C coolant reactor exit temperature compared to 550°C for SPX) conditions call for a qualification effort that should not be underestimated. • The HTGR derivative or UO2/Direct Cyde/Superalloy-850°C system is equipped with a gas cooled particle bed epithermal reactor (a ZrH moderated thermal reactor is also considered) with superalloy structures, driving a direct cycle with a turbine inlet temperature of 1120K (850°C).
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