Long-Term Aqueous Contamination of Stainless Steel in Simulant Nuclear Reprocessing Environments

Stainless steels are contaminated with a wide range of radionuclides over time when exposed to the harsh acidic environments in nuclear reprocessing facilities. Existing research into the contamination of stainless steel with problematic radionuclides has predominantly focused on single-contaminant systems. Hence a more comprehensive understanding is a necessity for the development and optimisation of decontamination and decommissioning strategies, thus minimising waste volumes and associated costs. This work aims to understand contaminant uptake behaviour on AISI 304 stainless steel in a mixed element system (Cs, Sr) over extended time periods (up to 420 days) and the effects of temperature and surface finish on contaminant uptake over shorter time periods (up to 28 days). Steel coupons were immersed in 12 M HNO3 to accelerate corrosion with both caesium and strontium ions (1 mM) and maintained at 50 °C. The extent of uptake was assessed through solution analysis using ICP-OES, identifying that strontium uptake exceeded caesium’s in all tested scenarios, with maximum uptake values of 0.208 g m-2 and 0.170 g m-2 obtained at 50°C for strontium and caesium, respectively. Kinetic modelling confirmed that uptake followed pseudo-second order behaviour. Stainless steel coupons were characterised after exposure, which found grain dropping and bulk dissolution had occurred after 420 days, leading to reduced levels of surface uptake. X-ray Photoelectron Spectroscopy found evidence of caesium and strontium chromates formed via co-precipitation with chromium corrosion products, confirming observations by previous studies. Furthermore, depth profiling with LA-ICP-MS found evidence of contaminants diffusing into the bulk substrate (by 30 - 40 nm), which has significant implications for the development and optimisation of decontamination approaches which can minimise excess waste generation.