The low void worth core design (‘CFV’) based on an axially heterogeneous geometry

Abstract Since many years, various core designs have been proposed to improve the safety of sodium fast reactors. The macroscopic parameters that are the sodium void worth and the Doppler constant were focusing attention because they are the easiest way to evaluate the core behavior in case of transient, even if this simplified approach has been improved by the use of quasi-static transient evaluations (A,B,C or K,G,H coefficients). To reduce the positive sodium void worth, different designs have been proposed by different countries: by using axially or radially heterogeneous core, addition of sodium plenum, addition of fertile layers,… The CFV (Low Void worth Core) is based on these past designs and propose a combination of different specific geometries, precisely defined, to highly reduce the sodium void worth to a negative value. The main performances of the oxide core are conserved, compared to a similar homogeneous core. Each specificity of the CFV is studied: height of the fissile zone, height and position of the fertile layer, height of the sodium plenum, thickness of the fissile plugs, need of a boron upper protection and the use of different fissile heights depending on the radius. The paper presents the inequalities which governs the CFV geometry. The physical phenomena are described and explained. The different safety issues are also discussed: decoupling risk linked to the insertion of the fertile layer, the elementary feedback coefficients, the simplified approach of the A,B,C coefficients and a calculation of a Loss of Flow transient with the MACARENA system code.