Consequently, gas cooled moderated reactors, preliminarily discarded because temperature and lifetime constraints were felt difficultly compatible with the use of ZrH or LiH, are presently 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 an affordable mass penalty ranging from 200 to 270 kg, depending on the heat source temperature. 3.7 Power Growth Potential The incentive to consider turboelectric nuclear SPS using advanced technologies for high heat source temperature, nearly marginal at 20 kWe, becomes gradually more significant as the power level is increased (see table 3). 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. 3.8 Operating Constraints Start-up, normal, incidental and accidental transients have been investigated by computer simulation (see Fig. 8, 9, 10) All systems, provided they are properly designed, exhibit comparable (and acceptable) operating constraints, but at start-up and restart. For the liquid metal cooled system, a partial loss of flow accident (E.M. pump failure) at full power can be mastered by the control system (see Fig. 9) and does not require a reactor shut-down. Furthermore, all systems prove capable to passively remove their afterheat following a total loss of flow accident (see Fig. 10). The main difference in operating constraints relate to the start-up and restart phases of sodium or lithium cooled systems, where the thawing of the reactor and primary circuit (launched frozen) is required before running up the Brayton rotating unit. According to preliminary (partially experimental) studies, a combined nuclear and electric thawing appears feasible (should not endanger the integrity of the primary circuit structures), but penalizing in terms of energy consumption and mass of the auxiliary power units. The use of NaK (melting point -12°C) instead of sodium for the LMFBR derivative system should reduce this drawback without degrading its performance. However, other considerations might militate against this choice: safety considerations (reactor transport and handling on Earth, launch abort followed by fallout in water), corrosion, and production of gases under irradiation.
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