Operating Constraints Start-up, normal, incidental and accidental transients have been investigated by computer simulation (see Figs. 8-10). All systems, provided they are properly designed, exhibit comparable (and acceptable) operating constraints, both at start-up and restart. For the liquid metal cooled system, a partial loss of flow accident (EM 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 can passively radiate their afterheat following a total loss of flow accident (see Fig. 10). The main difference in operating constraints relates 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, 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
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