(iii) component and system modelling for system design point optimization and analysis of normal and accidental system behaviour; (iv) reliability studies; (v) a small experimental programme including in- and out-of-pile screening tests of candidate structural materials (superalloys and refractory alloys), compatibility tests of MoRe/Li and UN/MoRe/Li systems, preliminary fabricability tests of parts of critical components, and first comparative tests of the behaviour of Na and NaK cooled LMFBR derivative reactor mockups during immersion in water; (vi) sensitivity studies of the specific mass to system specifications; (vii) evaluation of design alternatives; (viii) elaboration of a development and qualification programme, and evaluation of the required lead time and development cost, with the assistance of the industry (Novatome/Framatome). —Feasibility study of a 20 kWe thermoelectric nuclear SPS heated by a LMFBR derivative nuclear source [12]. —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. Alternatively, 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 determining the technology, the design options, and the development strategy for 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. Specifications for the Study The three 20 kWe systems studied here are powering a satellite assumed to perform an earth observation mission in a heliosynchronous orbit at an altitude of 1012 km. SPS and payload, launched together, should fit within the X H = 4.6 X 17 m cargo bay of Ariane V (see Fig. 2). 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 Mrad on the payload plane located 20m from the reactor, with a shielded cone angle of 30°. The reactor design and the sizing of the auxilliary 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
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