• comparative evaluation of nuclear versus solar 20 kWe space power systems, aiming at identifying the potential advantages and handicaps of nuclear over solar sources at this power level [13] • elaboration of a strategy for the development of european nuclear space power sources. Various Candidate Technologies for 20 kWe Nuclear Brayton Power Systems Constraints related to the launching capacity of ARIANE V make the choice of very high temperature technologies (refractory alloys, ...) mandatory for nuclear sources of the 200 kWe class. On the contrary, considering a less ambitious power level of 20 kWe, which corresponds to the projected European needs for the years 2005, opens the way to more conventional nuclear technologies, such as those developed for the Liquid Metal Fast Breeder Reactors, or for the High Temperature Gas Cooled Reactors. As a basis for deciding the technology, the design options, and the development strategy of a first 20 kWe nuclear Brayton system for European space missions, a comparative evaluation study of three reference space nuclear power systems concepts corresponding to three potential technologies has been undertaken. 3.1 Specifications for the Study The three 20 kWe systems considered are powering a satellite assumed to perform an earth observation mission on a heliosynchronous orbit at an altitude of 1012 km. SPS and payload, launched together, should fit within the <I>xH = 4.6x17 m2 cargo bay of ARIANE V (see Fig. 3). They are designed for a 7 year lifetime with a 0.90 reliability goal. The fast neutrons and gamma dose requirements to be met by the radiation shield are 1013 n/cm2 (E > 1 MeV) and 0.5 Mrads on the payload plane located 20 m from the reactor, with a shielded cone angle of 30°. Fhe reactor design and the sizing of the auxiliary power units should be such as to allow multiple (> 10) restarts. Finally, the reactors are sized for a built-in reactivity of 6% at BOL, and are provided with a safety rods system designed for giving a subcriticality margin of 5% in case of hypothetical compaction or immersion accidents following launch abort. 3.2 Design Features Common to the Systems Compared All three reference systems use a Brayton cycle with recuperation for power conversion because it appeared from previous studies to be well suited to the needs (efficiency, reliability), and the less development demanding conversion technology in Europe (nevertheless, in order to confirm this choice, the performances and feasibility of a 20 kWe thermoelectric nuclear SPS are being assessed in parallel with this comparison [8]).
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