procedure for Na- and Li-cooled system; to the ZrH and permeation barriers technologies for the gas-cooled thermal reactor; to the specific large kernel and thin coating fuel particle; and to the qualification of candidate superalloys and innovative reactor concepts for the HTGR derivative systems. Though the present state of European know-how in the technologies considered for the three candidate systems is quite different, development costs were found to be within a 15% range, due to the fact that the cost of the assumed long ground tests is nearly independent of the system technology. Similarly, lead times appeared comparable, with a small disadvantage for the UN/Li/MoRe system. However, the LMFBR and HTGR derivative systems remain clearly favoured when minimizing the development risks is a concern. More significant, the study positively concluded on the possibility to meet the deadline of 2006 for the launch of the flight prototype of the LMFBR derivative system, provided an active programme be started in 1990. Also, nuclear SPS appeared quite competitive, in terms of cost, with advanced photovoltaic solar SPS at the 20 kWe power level, the development cost of a LMFBR derivative system being nearly equal to the recurrent cost of a GaAs-Ni/H solar SPS. Conclusion Design studies have been performed on three reference design concepts for 20 kWe turboelectric power systems covering a wide range of reactor temperatures and relevant technologies. The selection criteria for two of the three concepts were to make use of the available technologies from the liquid metal fast breeder and high temperature gas- cooled reactors not only to minimize the cost of developing new technologies, but also to reduce the uncetainties associated with deploying new technologies with no flight experience. However, a third concept incorporates more advanced refractory alloys and fuel technologies. All considered candidate 20 kWe systems appear to exhibit about equivalent mass performances: 1900 to 2350 kg, which, moreover, are competitive with those of GaAs cell-Ni/H battery photovoltaic solar SPS of the same power level (2250 kg), recurrent cost of which is at least five times that of a nuclear SPS. No significant advantage is found for the advanced technology very high temperature UN/Li/MoRe-1125°C system at the 20 kWe power level. Differences have been identified between the LMFBR and HTGR derivative systems, relating to operating constraints, launch safety, reliability, extrapolation potential and development risk. These conclusions validate a development strategy of a first European 20 kWe nuclear space power source based on available or near-term technologies for minimizing the development risks. A major effort is currently underway to more precisely assess the candidate 20 kWe LMFBR versus various HTGR derivative power systems. This will be the basis for deciding on design options and the development strategy for a first 20 kWe nuclear Brayton system for European space missions. The next phase of the project, subject to commitment in 1989, will be dedicated to detailed design studies of the selected system.
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