The stress corrosion cracking (SCC) behavior of tank steel was investigated in a series of environments designed to simulate the chemistry of legacy nuclear weapons production waste being stored at the Hanford tank reservation. Tests consisted of both slow strain rate tests using tensile specimens and dynamic (increasing) stress intensity tests using compact tension specimens. Based on the work conducted, SCC occurs in two potential ranges depending on the main chemistry of the waste simulant (i.e., nitrate-based or carbonate-based). Though the results indicate that SCC in the tanks may be unlikely, diligent mitigation and corrosion potential monitoring efforts are still necessary.
The Hanford tank reservation contains approximately 50 million gallons of liquid legacy radioactive waste from cold war weapons production, which is stored in 177 underground storage tanks. Current plans call for eventual vitrification processing and ultimate disposal of the resulting waste glass logs at the Yucca Mountain Repository. The double shelled carbon steel storage tanks presently used for storage will continue in operation until the vitrification plant construction is finalized and waste processing operations completed. There are several different waste chemistry types that have been grouped according to their main constituents. All of the wastes tend to be highly alkaline in nature, typically with pH values greater than 10. Some have hydroxide concentrations in excess of 6 M. Under alkaline conditions, carbon steels will tend to be passive and undergo relatively slow, uniform corrosion. Under these passive conditions, however, carbon steels also can become susceptible to localized corrosion (e.g., pitting) and stress corrosion cracking (SCC) in the presence of certain aggressive constituents, such as chloride and nitrate. The original single shell storage tanks experienced stress corrosion cracking failures as a result of the presence of high concentrations of nitrate in the waste. Research at Hanford and Savannah River Laboratories (SRL) demonstrated that cracking could be prevented by maintaining a high pH of the waste (>13) and post-weld heat treatment of the tanks.1 Accordingly, all of the double shelled storage tanks were fabricated with stress relieved welds and chemistry controls were instituted to maintain the pH of the waste above 13-13.5 (as reflected as a minimum hydroxide concentration) in combination with a minimum nitrite concentration. At lower pH values, it was unclear if the relationships developed for higher alkaline conditions would still apply. Due to various chemical reactions taking place inside the tanks, the waste chemistry will tend to change over time, especially given the currently estimated 2023 time horizon anticipated for tank operations to continue. In addition, the present chemistries for some of the tank waste types are no longer in specification with respect to corrosion (e.g., maintaining pH levels above 13-13.5). As an additional complication, the wastes from different tanks can be intermixed to facilitate chemistry control efforts and waste transfers. Thus, there is concern within the Department of Energy (DOE) and regulatory bodies that tank integrity will be compromised given these changes and variations in chemistry. Furthermore, if tank integrity is potentially compromised, there is a need to define mitigation procedures.
