In some of the currently developed Sodium Fast Reactor (SFR) designs the core configuration will be rearranged to include internal spent fuel storage positions, with the benefit of avoiding an external sodium pool. However, in-vessel storage of failed fuel pins can lead to a direct contact between coolant and fuel, i.e. in a scenario where the cladding was breached, leading to their potential interaction in case of oxide based fuel forms (UO2 and MOX). The reaction product of sodium and oxide fuel is generally denoted as Na3MO4 (where M=U, Pu), and characterized by unfavorable physical properties, which might result in fuel swelling and/or pulverization, with the consequence of fissile isotopes or fission products dissemination into the primary system. The understanding of the corrosion mechanism and kinetics between liquid sodium and oxide based nuclear fuel becomes then of prior importance for establishing the feasibility of an internal storage of failed fuel pins.
In this contribution, we will present the results of out-of-pile tests that we perform to provide a basic knowledge of defective fuel pin behavior in contact with liquid sodium. With the aim to determine the physical mechanism involved in the sodium-fuel reaction, we firstly focused on the behavior of UO2 corrosion by sodium.
In order to cover the internal storage scenario, isothermal experiments were performed inside capsules with stagnant liquid sodium and UO2 at 800°C. To establish the influence of the grain orientation on the growth process of the reaction product, well-oriented (<111> and <001>) single crystals were used and finally to extend the behavior to the fuel pellet, a polycrystalline UO2 sample was tested as well. The corrosion product was analyzed by XRD, SEM-EDX and RAMAN. It consists in a homogeneous layer, whose morphology depends on the crystallographic orientation of the UO2 corroded grain or single crystal. However, the thickness of the corrosion layer does not seem to depend on the crystallographic orientation of the UO2 corroded grain that was unexpected.
A tentative interpretation of UO2 corrosion by sodium is proposed. This interpretation will be used for further study on the modelling of MOX-sodium interaction. There the influence of Pu on the corrosion process will be addressed and added to the UO2-Na model.