Design Directions of Optimized Reactivity Control Systems in Sodium Fast Reactors

International audienceIn sodium fast reactors, the control rods, a movable cluster of open pins with boron carbide as absorber, are almost the only approach to control reactivity. Boron-10 has a good absorption ability in fast spectrum and its enrichment can be adjusted to satisfy various requirements. However, boron carbide behavior under irradiation and the liability coming from the high initial core excess reactivity justify the optimization of reactivity control in Generation-IV fast reactors. This paper discusses the relationship between boron-10 enrichment in the control rods and reactivity management using a series of representative cores. Then, various alternatives designs to improve or supplement classical control rods are discussed and their feasibility is investigated. The results show that large cores with small power density have usually small reactivity loss and thus the effective control system design is limited by the shutdown function, e.g. the need to keep enough negative reactivity stored in the rods to stop the chain reaction at any time. In these cores, alternative absorbers can be considered for boron carbide substitution, among which hafnium hydride based materials are good candidates. Moreover, the limited introduction of moderating materials is also a potential solution to optimize the control rods in such fast reactors.The main constraints on the reactivity control system will be found for the cores with high reactivity loss, which are usually small modular or prototype cores. In these cores, control rods with high boron-10 enrichment are required for reactivity loss compensation. At the same time, the significant excess reactivity in the core will worsen the core behavior in case of inadvertent rod withdrawal for instance. The coupling of absorbing material and large quantities of moderating material enables the loading of burnable poisons in the core. Burnable poisons are able to share the reactivity loss compensation function with the control rods and thus enhance core inherent safety