Application of Minor Actinides as Burnable Poisons in Sodium Fast Reactors

Abstract

The reduction of the initial excess reactivity in fast reactor cores would enhance their inherent safety level as it would reduce the impact of a control rod withdrawal accident and lower the requirements on the absorption ability of control rods design. Compensation for burn-up reactivity loss is considered as a possible solution to limit initial excess reactivity. Minor actinides challenge the long-term nuclear waste management. Minor actinides can be transmuted from "absorber" isotopes to "fissile" isotopes, which show the possibility of their application as burnable poisons. Two loading modes of minor actinides as burnable poisons are considered in this paper: the first one, denominated homogenous mode, mixes minor actinides with all the fuel and the second one, denominated hybrid mode, packages minor actinides in independent pins in the fuel assemblies. The content of americium or neptunium in these two designs is considered with regards to current technological feasibility, including burn-up, cladding stress, decay heat and the neutron source of the assemblies considered here. Both these two modes are able to compensate for the reactivity loss of an industrial power core and thus reduce excess reactivity at the beginning of cycle. The application of these designs in the cores with higher reactivity loss will be considered in next step work. The impact of minor actinides loading on the core characteristics, including power distribution, material balance and feedback coefficient, are considered from the assembly level to the core level. The hybrid mode shows better management feasibility while the use of neptunium exhibits lower impacts on the current fuel recycling.