Reactors that aim to sustain the breed & burn (B&B) mode of operation at minimum discharge burnup require excellent neutron economy. Minimum-burnup B&B cores are generally large and feature low neutron leakage probability and a hard neutron spectrum. While highly promising fuel cycles can be achieved with such designs, the very same features are pushing the limits of the core's ability to passively respond safely to unprotected accidents. Low leakage minimum-burnup sodium-cooled B&B cores have a large positive coolant void-worth and coolant temperature reactivity coefficient. In this study, the applicability of major approaches for fast reactor void-worth reduction is evaluated specifically for B&B cores. The design, shuffling scheme and performance of a new metallic-fueled, sodium-cooled minimum burnup B&B core, used as basis for the void-worth reduction analysis, is presented. The analysis shows that reactivity control systems based on passive ~6Li injection during temperature excursions are the only option able to provide negative void-worth without significantly increasing the minimum burnup required for sustaining the B&B mode of operation. A new type of lithium expansion module (LEM) system was developed specifically for B&B cores and its effect on core performance is presented.
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