Radioactive waste primarily containing sodium nitrate, nitrite, and hydroxide is stored in underground storage tanks at the Department of Energy (DOE) Savannah River Site (SRS). A comprehensive structural integrity program, which includes chemistry controls, is in place to preclude any consequential corrosion in these waste tanks to maintain safe storage of the waste. Sodium hydroxide and nitrite are used to inhibit nitrate-induced stress corrosion cracking and nitrate-induced pitting corrosion in the liquid phase of the waste tanks. Although these solution chemistry controls are in place, 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. Recent laboratory testing has not been successful in recreating vapor space cracking seen in the waste tanks. Subsequent testing is planned to better simulate tank conditions.
Radioactive waste is stored in underground storage tanks at the Department of Energy (DOE) Savannah River Site (SRS). The waste tanks, made of ASTM A285 steel, contain precipitated radionuclides and a liquid solution of primarily sodium nitrate, sodium nitrite, and sodium hydroxide. An assessment of the potential degradation mechanisms of the high level waste (HLW) tanks determined that nitrate-induced pitting corrosion and stress corrosion cracking were the two most significant degradation mechanisms. Specifically, nitrate-induced stress corrosion cracking was determined to be the principal degradation mechanism for the primary tank steel. Sodium hydroxide and nitrite are used to inhibit nitrate-induced stress corrosion cracking and nitrate-induced pitting corrosion in steel in contact with the liquid phase of the waste tanks. General corrosion is also prevented by the inhibitor levels specified for these two localized corrosion modes. The corrosion control program is based upon empirical data used to determine vulnerability to stress corrosion cracking and pitting at typical compositions of waste [1]. Stress Corrosion Cracking The waste solutions contain anions which can both cause or inhibit stress corrosion cracking (SCC). Nitrate or hydroxide may initiate SCC; however, the appropriate concentrations of one will inhibit cracking by the other. Nitrite that is present in the waste will also inhibit cracking [2]. Electrochemical polarization studies were performed to determine the initiator of the stress corrosion cracking. The studies showed that carbon steels are susceptible to nitrate SCC in potential ranges between -0.3 to 1.1 V vs. saturated calumel electrode (SCE), while hydroxide SCC occurs at potentials between -0.8 to -1.0V (SCE) [3]. In addition hydroxide SCC only occurs at temperatures in excess of 100°C, whereas nitrate SCC may occur at lower temperatures [4]. Given that the open-circuit potential measured in several waste tanks is between -0.44 to - 0.064 V and the temperature of the waste is generally less than 100°C, the observed cracking was determined to be caused by the nitrate [5]. The acidic crack tip leads to a more anodic open circuit potential and stimulation of the cathodic reactions.
