subsystems (power conditioning), being quasi independent of the heat-source temperature. It is worth noting that despite radical differences in technical options between the direct conversion and the liquid metal cooled systems, masses decrease about proportionally to the turbine inlet temperature: the increased mass of the epithermal gas- cooled reactor compensating for the mass savings afforded by the direct cycle: no primary heat exchanger, and no thawing phase at start-up. Consequently, gas cooled moderated reactors, discarded initially because temperature and lifetime constraints were felt to be incompatible with the use of ZrH or LiH, are being further evaluated. The aforementioned masses relate to systems equipped with a single BRU. Using a dual Brayton converter to improve reliability was found to induce a affordable mass penalty ranging from 200 to 270 kg, depending on the heat source temperature. Power Growth Potential The incentive to consider a turboelectric nuclear SPS using advanced technologies for high heat source temperatures, marginal at 20 kWe, gradually becomes more significant as the power level is increased (see Table III). At the 50 kWe level, the mass difference between the two extreme systems reaches 1180 kg, compared to 430 kg at 20 kWe. Moreover, 30 kWe is the maximum potential growth of a LMFBR derivative system equipped with a non-deployable radiator launched by Ariane V (available area: 140 m2). This limit is extended to 70 kWe for HTGR derivative systems.
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