ABSTRACT The stress corrosion cracking (SCC) behavior of A537 tank steel was investigated in a series of environments designed to simulate the chemistry of legacy nuclear weapons production waste. Tests consisted of both slow strain rate tests using tensile specimens and constant load tests using compact tension specimens. Based on the tests conducted, nitrite was found to be a strong SCC inhibitor. Based on the test performed and the tank waste chemistry changes that are predicted to occur over time, the risk for SCC appears to be decreasing since the concentration of nitrate will decrease and nitrite will increase.
INTRODUCTION 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.
Though 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 and to hydroxide concentrations in excess of 6M. 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 SRL demonstrated that cracking could be prevented by maintaining a high pH of the waste (>13) and postweld heat treatment of the tanks. 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). Thus, there is concern within DOE and regulatory bodies that tank integrity will be compromised given these changes in chemistry. Furthermore, if tank integrity is potentially compromised, there is a need to define mitigation procedures.