Analysis of the impacts of homogeneous minor actinides loading in low void effect sodium fast reactor cores

Abstract

Minor actinides transmutation is a solution to decrease the long-term radiotoxicity of nuclear waste and limit their short-term decay heat. In the homogenous approach, minor actinides are mixed in the fuel and loaded in the core in order to turn them into fission products. This leads to a neutron spectrum hardening in the core, which has a negative impact on the core integral feedback coefficients such as the Doppler Effect or the sodium void worth. Analysis of these impacts was used in the past to establish limit on maximal minor actinides loading in a reactor core. Low-void cores (CFV in French) have been recently developed by CEA to achieve negative sodium void worth by adding axial heterogeneities in the core layout in the form of an upper sodium plenum and an inner fertile blanket. In this paper, the impacts of minor actinides loading in such a core are analyzed and a comparison is carried out with earlier homogeneous core designs. In a first time, the impacts on integral feedbacks coefficients are evaluated, and in a second time, the perturbations of the core behavior during selected representative transients are analyzed. It is shown that even if the impacts on integral coefficients are similar between the two core designs, the low-void core behavior during loss-of-flow transient is not negatively impacted by minor actinides loading. For reactivity insertion transient, the two core designs behave similarly. It can be concluded that considering heterogeneous cores, the use of integral coefficients is insufficient to characterize the impact of minor actinides loading on the core for loss-of-flow transient. However, the impacts of minor actinides loading on the Doppler integral feedback coefficient can be used as reliable estimator for the modification of the core behavior during a reactivity insertion transient.