FBR code designs leading to low sodium void worth

A better intrinsic safety of fast reactors relies to some extent on the reduction of sodium voiding reactivity effects. This paper recollects physical explanations about void reactivity effects and their variations, according to simple typical design changes, so as to point out the most promising directions to investigate to ensure the best void reactivity reductions. But out of equivalently void reactivity reducing changes, those of best economical characteristics must be sorted out; only fuel cycle costs will be taken here into account, the parameter taken as representative of fuel cycle costs being the core reactivity loss rate, directly correlated with the fuel elements residence time. Finally, to make this study profitable, the preceding results will be used to get alternative European Fast Reactor (EFR) core designs with low sodium void worth. Fixed parameters will be here EFR thermal output and, as far as possible, core radial bulk