which leads to unfavourable mass performance due to the need for a large area radiator (100 m2). Moreover, unusual required lifetime (seven years) and temperature (650°C coolant reactor exit temperature compared to 550°X for SPX) conditions call for a qualification effort that should not be underestimated. The HTGR derivative or UO2/direct cycle/superalloys-850°C system is equipped with a gas-cooled particle bed epithermal reactor (a ZrH moderated thermal reactor is also considered) with superalloys structures, driving a direct cycle with a turbine inlet temperature of 1120 K (850°C). This system is intended to make full use of the superalloys and of the techniques developed for high temperature gas cooled reactors. When compared to the UO2/Na/SS-650°C system, the higher heat-source temperature afforded by the gas cooled reactor should favour the mass performance of this system by making it possible to consider smaller and lighter radiators. Nevertheless, the additional effort to qualify the structural material (Hasteloy X for instance), the reactor design concept, and the ZrH (for the thermal reactor) will be kept in mind. The UN/Li/MoRe-1120°C system adopts the technologies already considered for the reference 200 kWe SPS: a lithium-cooled fast spectrum reactor with clad UN pins and Mo Re alloy as structural material. A 1400 K (1125°C) reactor peak temperature was selected in a first comparison so as to fully exploit the performance of the MoRe. However, the mass advantage and the power growth potential expected from this very high temperature are to be weighted against the long term development of the refractory alloy and lithium technologies. Therefore, the consequences of designing this system for lower operating temperatures will be analysed. Design Points A near optimum design point was computed for the three candidate systems (see Table I), using a preliminary version of the nuclear SPS design and optimization code ‘Diogene’ (see code architecture in Fig. 4), the optimization criterion being the minimum mass under the constraints set by the integration of the SPS and its satellite with Ariane V. Among the optimized parameters are: primary coolant flow-rate and pressure drops, helium-xenon molecular weight, BRU rotational speed, compressor inlet pressure and temperature and pressure ratio, turbine inlet temperature, recuperator efficiency, primary heat exchanger, radiator and recuperator pressure drops, radiator tube number and fin thickness. Surprisingly, all examined systems exhibit the same overall efficiency (about 18%) despite quite different heat source temperatures. This is typical of low-power turbogenerators equipped with fast or epithermal nuclear reactors, whose size, mainly dictated by criticality considerations, is nearly independent of the system efficiency. Indeed, it appears more efficient, for the system mass viewpoint, to spend the potential efficiency gains provided by higher temperatures to increase the heat sink temperature and the pressure drops, and to reduce the recuperator effectiveness, all of this contributing to substantial mass savings on the radiator, recuperator and primary heat exchanger. Consequently, the three candidate SPS have comparable thermal power requirements for their heat sources: 115 kWth, and mainly differ by the radiator area which varies from 38 to more than 116 m2, between the high and moderate temperature systems.
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