VAPOR SPACE AND LIQUID/AIR INTERFACE CORROSION OF LOW CARBON STEEL IN COMPLEX RADIOACTIVE HIGH LEVEL RADIOACTIVE WASTE

摘要

Radioactive waste is stored in underground storage tanks at the Department of Energy (DOE) Savannah River Site (SRS). Recent experience has shown that steel not in contact with the bulk waste solution or slurry, but exposed to the “vapor space” above the bulk waste, may be vulnerable to the initiation and propagation of corrosion, including pitting and stress corrosion cracking. Experiments have now been completed in complex radioactive waste simulants, in addition to the prior experiments in simple nitrate, nitrite, hydroxide solutions. The solution chemistries tested included minor waste constituents in addition to the NaNO3, NaNO2, and NaOH which comprise the bulk of the high level radioactive waste. The additional constituents were the sodium salts of aluminate, carbonate, sulfate, phosphate, chloride, chromate, and fluoride; and nitrate salts of transition metals, typically found in the high level waste. The results indicate (1) Minor and isolated pitting is possible within crevices vapor space of the tanks that contain low levels of stagnant dilute solution for an extended period of time and (2) Liquid/air interfacial corrosion is possible in dilute stagnant solutions, particularly with high chloride concentrations. The testing continues to indicate that well-inhibited solutions are protective even within the vapor space and liquid/air interface. A comprehensive structural integrity program including in-service inspections and focused experiments is in place to ensure the continued safe operation of these tanks..