Development of a Multi-technique Characterization Portfolio for Stainless Steels Exposed to Magnox Reprocessing Liquors

Abstract

AISI Type 304 stainless steel coupons have been exposed to a simulant aqueous environment representative of the Magnox Reprocessing Plant (MRP) at Sellafield, UK. The experiments were performed for extended time periods (up to 420 days) at elevated temperatures to develop a comprehensive understanding of the extent, nature, and depth of contamination for pipework and vessels in Magnox spent nuclear fuel reprocessing environments. This will inform upcoming decommissioning work which represents a major post-operational challenge. Previous relevant literature has focused on developing fundamental understanding of contamination mechanisms of stainless steels in simplistic, single-element systems, which lack elements of industrial relevance. Contamination behavior is expected to be drastically different in these more complex environments. A characterization portfolio has been developed to enable detailed assessment of corrosion and contamination behavior in acidic reprocessing environments. Solution, surface, and depth analysis determined that uptake was dominated by the elements present in highest concentrations within the environment, namely, Mg, Nd, and Cs. Most contaminants were incorporated into a relatively thin surface oxide layer (<100 nm) in metal oxide form, although there were some exceptions (Cs and Sr). Grain boundary etching was present despite very low corrosion rates (3 μm year −1 ). As a result of this lack of corrosion, diffusion of contaminants beyond the immediate surface (10− 20 nm) did not occur, evidenced through depth profiling. As a result of these findings, surface-based decontamination techniques minimizing excess secondary waste generation can be further developed in order to reduce the environmental and economic burden associated with decommissioning activities.