Application of Minor Actinides as Burnable Poisons in Sodium Fast Reactors

The reduction of the initial excess reactivity in fast reactor cores will enhance the inherent safety level of the cores as it does reduce the impact of control rod withdrawal (CRW) accidents. Compensation for burnup reactivity loss by means of burnable poison (BP) is considered as a possible solution to limit initial excess reactivity. Minor actinides (MAs) challenge long-term nuclear waste management, and they can be transmuted from absorber isotopes to fissile isotopes, which allows them to play the role of BPs.

Two loading modes of MAs as BPs are considered in this paper: The so-called homogeneous transmutation mode mixes MAs with the fuel, and the so-called hybrid transmutation mode packs MAs in independent pins in the fuel assemblies. The content of americium or neptunium in these two modes is considered with regard to current technological feasibility, including burnup, cladding stress, decay heat, and the neutron source of the assemblies considered here. Both of these modes are able to compensate for the reactivity loss of a 3600-MW(thermal) fast reactor and thus reduce excess reactivity at the beginning of cycle.

The impact of MA loading on the core characteristics, including power distribution, material balance, and feedback coefficient, is considered from the assembly level to the core level. The hybrid mode shows better management feasibility while the use of neptunium exhibits a lower impact on the current fuel recycling. Finally, the core behavior during a CRW transient is evaluated, which shows that the core loaded with BPs exhibits better safety performance in CRW transients due to their lower initial excess reactivity.