elements made of particle beds maintained between cylindrical porous barriers (see Fig- 7). Conceptual design studies performed on this concept demonstrated the advantage of thermal over fast reactors in terms of uranium inventory (12 kg for the ZrH moderated concept), but also revealed the great complexity of the internal architecture of the reactor (due to the need to specifically cool the ZrH significantly below 550°C in order to prevent its decomposition) and the requirement for permeation barriers to confine the hydrogen within the moderator. A reactor concept more directly derived from industrial gas cooled reactors, using hexagonal prismatic fuel elements consisting of a graphite matrix containing 45% BISO particles was found not to be competitive in terms of mass (500 kg penalty for the reactor block). Mass Evaluation The various subsystem design studies resulted in the system mass summaries given in Table II. The total system masses range from 1880 to 2320 kg, the differences between the systems operating at both ends of the heat source temperature range covered by the study (650 to 1125°C) amounting to 440 kg only. This quite small difference stems from the fact that, for a nuclear turboelectric SPS of the 20 kWe class, the subsystems primarily affected by temperature or system efficiency levels, namely the heat exchangers and the radiator, only account for 14 to 27% of the total system mass, the reactor and shielding sizing being mainly dictated by criticality and dose attenuation criteria respectively, and the mass of the other
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